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Cyclic diguanylate (c-di-GMP) is a near-ubiquitous signaling molecule that regulates the motility-to-sessility transition in many bacterial species. Among the phenotypes influenced by c-di-GMP are biofilm formation, motility, cell cycle, and virulence. The hallmark phenotypes regulated by c-di-GMP—biofilm formation and motility—are key determinants of host–bacterial interactions. A large body of research has identified the roles of c-di-GMP in regulating phenotypes in culture. While numerous studies have investigated roles for c-di-GMP during the establishment and maintenance of pathogenic host–bacterial associations, considerably less attention has been devoted to defining the roles of c-di-GMP during beneficial and commensal associations. This review describes the known roles of c-di-GMP in regulating phenotypes that contribute to host colonization, with a focus on knowledge gaps and future prospects for examining c-di-GMP during beneficial colonization. [ABSTRACT FROM AUTHOR]

Abstract Background The gut microbiota and their hosts profoundly affect each other’s physiology and evolution. Identifying host-selected traits is crucial to understanding the processes that govern the evolving interactions between animals and symbiotic microbes. Current experimental approaches mainly focus on the model bacteria, like hypermutating Escherichia coli or the evolutionary changes of wild stains by host transmissions. A method called atmospheric and room temperature plasma (ARTP) may overcome the bottleneck of low spontaneous mutation rates while maintaining mild conditions for the gut bacteria. Results We established an experimental symbiotic system with gnotobiotic bee models to unravel the molecular mechanisms promoting host colonization. By in vivo serial passage, we tracked the genetic changes of ARTP-treated Snodgrassella strains from Bombus terrestris in the non-native honeybee host. We observed that passaged isolates showing genetic changes in the mutual gliding locus have a competitive advantage in the non-native host. Specifically, alleles in the orphan mglB, the GTPase activating protein, promoted colonization potentially by altering the type IV pili-dependent motility of the cells. Finally, competition assays confirmed that the mutations out-competed the ancestral strain in the non-native honeybee gut but not in the native host. Conclusions Using the ARTP mutagenesis to generate a mutation library of gut symbionts, we explored the potential genetic mechanisms for improved gut colonization in non-native hosts. Our findings demonstrate the implication of the cell mutual-gliding motility in host association and provide an experimental system for future study on host-microbe interactions. Video Abstract

Background: The gut microbiota and their hosts profoundly affect each other's physiology and evolution. Identifying host-selected traits is crucial to understanding the processes that govern the evolving interactions between animals and symbiotic microbes. Current experimental approaches mainly focus on the model bacteria, like hypermutating Escherichia coli or the evolutionary changes of wild stains by host transmissions. A method called atmospheric and room temperature plasma (ARTP) may overcome the bottleneck of low spontaneous mutation rates while maintaining mild conditions for the gut bacteria. Results: We established an experimental symbiotic system with gnotobiotic bee models to unravel the molecular mechanisms promoting host colonization. By in vivo serial passage, we tracked the genetic changes of ARTP-treated Snodgrassella strains from Bombus terrestris in the non-native honeybee host. We observed that passaged isolates showing genetic changes in the mutual gliding locus have a competitive advantage in the non-native host. Specifically, alleles in the orphan mglB, the GTPase activating protein, promoted colonization potentially by altering the type IV pili-dependent motility of the cells. Finally, competition assays confirmed that the mutations out-competed the ancestral strain in the non-native honeybee gut but not in the native host. Conclusions: Using the ARTP mutagenesis to generate a mutation library of gut symbionts, we explored the potential genetic mechanisms for improved gut colonization in non-native hosts. Our findings demonstrate the implication of the cell mutual-gliding motility in host association and provide an experimental system for future study on host-microbe interactions. ArN8Xjm3N6Dubzus4DCSQm Video Abstract [ABSTRACT FROM AUTHOR]

ABSTRACT Group B Streptococcus (GBS) is a pathobiont responsible for invasive infections in neonates and the elderly. The transition from a commensal to an invasive pathogen relies on the timely regulation of virulence factors. In this study, we characterized the role of the SaeRS two-component system in GBS pathogenesis. Loss-of-function mutations in the SaeR response regulator decrease virulence in mouse models of invasive infection by hindering the ability of bacteria to persist at the inoculation site and to spread to distant organs. Transcriptome and in vivo analysis reveal a specialized regulatory system specifically activated during infection to control the expression of only two virulence factors: the PbsP adhesin and the BvaP secreted protein. The in vivo surge in SaeRS-regulated genes is complemented by fine-tuning mediated by the repressor of virulence CovRS system to establish a coordinated response. Constitutive activation of the SaeRS regulatory pathway increases PbsP-dependent adhesion and invasion of epithelial and endothelial barriers, though at the cost of reduced virulence. In conclusion, SaeRS is a dynamic, highly specialized regulatory system enabling GBS to express a restricted set of virulence factors that promote invasion of host barriers and allow these bacteria to persist inside the host during lethal infection.IMPORTANCEGroup B Streptococcus (or GBS) is a normal inhabitant of the human gastrointestinal and genital tracts that can also cause deadly infections in newborns and elderly people. The transition from a harmless commensal to a dangerous pathogen relies on the timely expression of bacterial molecules necessary for causing disease. In this study, we characterize the two-component system SaeRS as a key regulator of such virulence factors. Our analysis reveals a specialized regulatory system that is activated only during infection to dynamically adjust the production of two virulence factors involved in interactions with host cells. Overall, our findings highlight the critical role of SaeRS in GBS infections and suggest that targeting this system may be useful for developing new antibacterial drugs.

Stochastic bottlenecks encountered during infection can have a disproportionately large influence on successful parasite colonization. The relationship between parasite infection of discrete tissue niches and long-term persistence or transmission is not fully understood. Cellular barcoding combined with spatial proteomics and transcriptomics has the potential to connect gene expression phenotypes with infection outcomes during colonization. Protozoan pathogens such as Plasmodium spp., Leishmania spp., Toxoplasma gondii , and Trypanosoma spp. are often associated with high-mortality, acute and chronic diseases of global health concern. For transmission and immune evasion, protozoans have evolved diverse strategies to interact with a range of host tissue environments. These interactions are linked to disease pathology, yet our understanding of the association between parasite colonization and host homeostatic disruption is limited. Recently developed techniques for cellular barcoding have the potential to uncover the biology regulating parasite transmission, dissemination, and the stability of infection. Understanding bottlenecks to infection and the in vivo tissue niches that facilitate chronic infection and spread has the potential to reveal new aspects of parasite biology. [ABSTRACT FROM AUTHOR]

Group B Streptococcus (GBS) is a pathobiont responsible for invasive infections in neonates and the elderly. The transition from a commensal to an invasive pathogen relies on the timely regulation of virulence factors. In this study, we characterized the role of the SaeRS two-component system in GBS pathogenesis. Loss-of-function mutations in the SaeR response regulator decrease virulence in mouse models of invasive infection by hindering the ability of bacteria to persist at the inoculation site and to spread to distant organs. Transcriptome and in vivo analysis reveal a specialized regulatory system specifically activated during infection to control the expression of only two virulence factors: the PbsP adhesin and the BvaP secreted protein. The in vivo surge in SaeRS-regulated genes is complemented by fine-tuning mediated by the repressor of virulence CovRS system to establish a coordinated response. Constitutive activation of the SaeRS regulatory pathway increases PbsP-dependent adhesion and invasion of epithelial and endothelial barriers, though at the cost of reduced virulence. In conclusion, SaeRS is a dynamic, highly specialized regulatory system enabling GBS to express a restricted set of virulence factors that promote invasion of host barriers and allow these bacteria to persist inside the host during lethal infection., Importance: Group B Streptococcus (or GBS) is a normal inhabitant of the human gastrointestinal and genital tracts that can also cause deadly infections in newborns and elderly people. The transition from a harmless commensal to a dangerous pathogen relies on the timely expression of bacterial molecules necessary for causing disease. In this study, we characterize the two-component system SaeRS as a key regulator of such virulence factors. Our analysis reveals a specialized regulatory system that is activated only during infection to dynamically adjust the production of two virulence factors involved in interactions with host cells. Overall, our findings highlight the critical role of SaeRS in GBS infections and suggest that targeting this system may be useful for developing new antibacterial drugs., Competing Interests: C.B. acts as a scientific advisor for Scylla Biotech S.r.l. without receiving any compensation for this activity. G.T. is an employee of Scylla Biotech S.r.l. Scylla Biotech S.r.l did not provide funding for this study and had no role in its conduction. The remaining authors declare that the research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

The pathogenic bacterium Pseudomonas aeruginosa possesses high metabolic versatility, with its effectiveness to cause infections likely due to its well-regulated genetic content. P. aeruginosa PAO1 has at least six fadD paralogous genes, which have been implicated in fatty acid (FA) degradation and pathogenicity. In this study, we used mutagenesis and a functional approach in P. aeruginosa PAO1 to determine the roles of the fadD4 gene in acyclic terpene (AT) and FA assimilation and on pathogenicity. The results indicate that fadD4 encodes a terpenoyl-CoA synthetase utilized for AT and FA assimilation. Additionally, mutations in fadD paralogs led to the modification of the quorum-sensing las / rhl systems, as well as the content of virulence factors pyocyanin, biofilm, rhamnolipids, lipopolysaccharides (LPS), and polyhydroxyalkanoates. In a Caenorhabditis elegans in vivo pathogenicity model, culture supernatants from the 24-h-grown fadD4 single mutant increased lethality compared to the PAO1 wild-type (WT) strain; however, the double mutants fadD1/fadD2 , fadD1/fadD4 , and fadD2/fadD4 and single mutant fadD2 increased worm survival. A correlation analysis indicated an interaction between worm death by the PAO1 strain, the fadD4 mutation, and the virulence factor LPS. Fatty acid methyl ester (FAME) analysis of LPS revealed that a proportion of the LPS and FA on lipid A were modified by the fadD4 mutation, suggesting that FadD4 is also involved in the synthesis/degradation and modification of the lipid A component of LPS. LPS isolated from the fadD4 mutant and double mutants fadD1/fadD4 and fadD2/fadD4 showed a differential behavior to induce an increase in body temperature in rats injected with LPS compared to the WT strain or from the fadD1 and fadD2 mutants. In agreement, LPS isolated from the fadD4 mutant and double mutants fadD1/fadD2 and fadD2/fadD4 increased the induction of IL-8 in rat sera, but IL1-β cytokine levels decreased in the double mutants fadD1/fadD2 and fadD1/fadD4. The results indicate that the fadD genes are implicated in the degree of pathogenicity of P. aeruginosa PAO1 induced by LPS-lipid A, suggesting that FadD4 contributes to the removal of acyl-linked FA from LPS, rendering modification in its immunogenic response associated to Toll-like receptor TLR4. The genetic redundancy of fadD is important for bacterial adaptability and pathogenicity over the host. [ABSTRACT FROM AUTHOR]

The pathogenic bacterium Pseudomonas aeruginosa possesses high metabolic versatility, with its effectiveness to cause infections likely due to its well-regulated genetic content. P. aeruginosa PAO1 has at least six fadD paralogous genes, which have been implicated in fatty acid (FA) degradation and pathogenicity. In this study, we used mutagenesis and a functional approach in P. aeruginosa PAO1 to determine the roles of the fadD4 gene in acyclic terpene (AT) and FA assimilation and on pathogenicity. The results indicate that fadD4 encodes a terpenoyl-CoA synthetase utilized for AT and FA assimilation. Additionally, mutations in fadD paralogs led to the modification of the quorum-sensing las/rhl systems, as well as the content of virulence factors pyocyanin, biofilm, rhamnolipids, lipopolysaccharides (LPS), and polyhydroxyalkanoates. In a Caenorhabditis elegans in vivo pathogenicity model, culture supernatants from the 24-h-grown fadD4 single mutant increased lethality compared to the PAO1 wild-type (WT) strain; however, the double mutants fadD1/fadD2, fadD1/fadD4, and fadD2/fadD4 and single mutant fadD2 increased worm survival. A correlation analysis indicated an interaction between worm death by the PAO1 strain, the fadD4 mutation, and the virulence factor LPS. Fatty acid methyl ester (FAME) analysis of LPS revealed that a proportion of the LPS and FA on lipid A were modified by the fadD4 mutation, suggesting that FadD4 is also involved in the synthesis/degradation and modification of the lipid A component of LPS. LPS isolated from the fadD4 mutant and double mutants fadD1/fadD4 and fadD2/fadD4 showed a differential behavior to induce an increase in body temperature in rats injected with LPS compared to the WT strain or from the fadD1 and fadD2 mutants. In agreement, LPS isolated from the fadD4 mutant and double mutants fadD1/fadD2 and fadD2/fadD4 increased the induction of IL-8 in rat sera, but IL1-β cytokine levels decreased in the double mutants fadD1/fadD2 and fadD1/fadD4. The results indicate that the fadD genes are implicated in the degree of pathogenicity of P. aeruginosa PAO1 induced by LPS-lipid A, suggesting that FadD4 contributes to the removal of acyl-linked FA from LPS, rendering modification in its immunogenic response associated to Toll-like receptor TLR4. The genetic redundancy of fadD is important for bacterial adaptability and pathogenicity over the host.

Enterococcus faecalis is a major opportunistic bacterial pathogen of increasing clinical relevance. A substantial body of experimental evidence suggests that early biofilm formation plays a critical role in these infections, as well as in colonization and persistence in the GI tract as a commensal member of the microbiome in most terrestrial animals. Animal models of experimental endocarditis generally involve inducing mechanical valve damage by cardiac catheterization prior to infection, and it has long been presumed that endocarditis vegetation formation resulting from bacterial attachment to the endocardial endothelium requires some pre-existing tissue damage. Here we review both historical and contemporary animal model studies demonstrating the robust ability of E. faecalis to directly attach and form stable microcolony biofilms encased within a bacterially-derived extracellular matrix on the undamaged endovascular endothelial surface. We also discuss the morphological similarities when these biofilms form on other host tissues, including when E. faecalis colonizes the GI epithelium as a commensal member of the normal vertebrate microbiome - hiding in plain sight where it can serve as a source for systemic infection via translocation. We propose that these phenotypes may allow the organism to persist as an undetected infection in asymptomatic individuals and thus provide an infectious reservoir for later clinical endocarditis.

Enterococcus faecalis is a major opportunistic bacterial pathogen of increasing clinical relevance. A substantial body of experimental evidence suggests that early biofilm formation plays a critical role in these infections, as well as in colonization and persistence in the GI tract as a commensal member of the microbiome in most terrestrial animals. Animal models of experimental endocarditis generally involve inducing mechanical valve damage by cardiac catheterization prior to infection, and it has long been presumed that endocarditis vegetation formation resulting from bacterial attachment to the endocardial endothelium requires some pre-existing tissue damage. Here we review both historical and contemporary animal model studies demonstrating the robust ability of E. faecalis to directly attach and form stable microcolony biofilms encased within a bacterially-derived extracellular matrix on the undamaged endovascular endothelial surface. We also discuss the morphological similarities when these biofilms form on other host tissues, including when E. faecalis colonizes the GI epithelium as a commensal member of the normal vertebrate microbiome - hiding in plain sight where it can serve as a source for systemic infection via translocation. We propose that these phenotypes may allow the organism to persist as an undetected infection in asymptomatic individuals and thus provide an infectious reservoir for later clinical endocarditis. [ABSTRACT FROM AUTHOR]

Hydrophobins are morphogenetic, small secreted hydrophobic fungal proteins produced in response to changing development and environmental conditions. These proteins are important in the interaction between certain fungi and their hosts. In mutualistic ectomycorrhizal fungi several hydrophobins form a subclass of mycorrhizal-induced small secreted proteins that are likely to be critical in the formation of the symbiotic interface with host root cells. In this study, two genomes of the ectomycorrhizal basidiomycete Laccaria bicolor strains S238N-H82 (from North America) and 81306 (from Europe) were surveyed to construct a comprehensive genome-wide inventory of hydrophobins and to explore their characteristics and roles during host colonization. The S238N-H82 L. bicolor hydrophobin gene family is composed of 12 genes while the 81306 strain encodes nine hydrophobins, all corresponding to class I hydrophobins. The three extra hydrophobin genes encoded by the S238N-H82 genome likely arose via gene duplication and are bordered by transposon rich regions. Expression profiles of the hydrophobin genes of L. bicolor varied greatly depending on life stage (e.g. free living mycelium vs. root colonization) and on the host root environment. We conclude from this study that the complex diversity and range of expression profiles of the Laccaria hydrophobin multi-gene family have likely been a selective advantage for this mutualist in colonizing a wide range of host plants.

The Chinese white pine beetle Dendroctonus armandi (Coleoptera: Scolytinae) typically displays bivoltinism at altitudes below 1700 m in the Qinling Mountains, China. The periods of host colonization and larval overwintering are two important phases in the life cycle of bark beetles, as it is during these periods that they have to contend with host plant defences and periods of intense cold, respectively. Although during different seasons, the females and males of Chinese white pine beetles show varying tolerances to host plant terpenoids, the sex ratio and survival physiology condition of the two beetle generations are unknown. We investigated the sex ratio of individuals, and also examined the body mass, energy stores, and detoxication enzymes of males and females in each of the two generations in order to determine the overall population stability of each generation. We identified a female-biased sex ratio among adults in both generations. Furthermore, patterns of body mass, energy stores, and detoxication enzymes were found to differ between the two sexes and two seasons. Compared with the males, the females have a larger body mass and higher amounts of stored lipids, which are assumed to be adaptations designed to overcome host resistance and facilitate subsequent oviposition. [ABSTRACT FROM AUTHOR]

The genus Verticillium encompasses phytopathogenic species that cause vascular wilts of plants. In this review, we focus on Verticillium dahliae, placing emphasis on the controversy surrounding the elevation of a long-spored variant as a new species, recent advances in the analysis of compatible and incompatible interactions, highlighted by the use of strains expressing fluorescent proteins, and the genetic diversity among Verticillium spp. A synthesis of the approaches to explore genetic diversity, gene flow, and the potential for cryptic recombination is provided. Control of Verticillium wilt has relied on a panoply of chemical and nonchemical strategies, but is beset with environmental or site-specific efficacy problems. Host resistance remains the most logical choice, but is unavailable in most crops. The genetic basis of resistance to Verticillium wilt is unknown in most crops, as are the subcellular signaling mechanisms associated with Ve-mediated, race-specific resistance. Increased understanding in each of these areas promises to facilitate management of Verticillium wilts across a broad range of crops.

The genus Verticillium encompasses phytopathogenic species that cause vascular wilts of plants. In this review, we focus on Verticillium dahliae, placing emphasis on the controversy surrounding the elevation of a long-spored variant as a new species, recent advances in the analysis of compatible and incompatible interactions, highlighted by the use of strains expressing fluorescent proteins, and the genetic diversity among Verticillium spp. A synthesis of the approaches to explore genetic diversity, gene flow, and the potential for cryptic recombination is provided. Control of Verticillium wilt has relied on a panoply of chemical and nonchemical strategies, but is beset with environmental or site-specific efficacy problems. Host resistance remains the most logical choice, but is unavailable in most crops. The genetic basis of resistance to Verticillium wilt is unknown in most crops, as are the subcellular signaling mechanisms associated with Ve-mediated, race-specific resistance. Increased understanding in each of these areas promises to facilitate management of Verticillium wilts across a broad range of crops.

Currently our limited understanding of crop rhizosphere community assembly hinders attempts to manipulate it beneficially. Variation in root communities has been attributed to plant host effects, soil type, and plant condition, but it is hard to disentangle the relative importance of soil and host without experimental manipulation. To examine the effects of soil origin and host plant on root associated bacterial communities we experimentally manipulated four crop species in split-plot mesocosms and surveyed variation in bacterial diversity by Illumina amplicon sequencing. Overall, plant species had a greater impact than soil type on community composition. While plant species associated with different Operational Taxonomic Units (OTUs) in different soils, plants tended to recruit bacteria from similar, higher order, taxonomic groups in different soils. However, the effect of soil on root-associated communities varied between crop species: Onion had a relatively invariant bacterial community while other species (maize and pea) had a more variable community structure. Dynamic communities could result from environment specific recruitment, differential bacterial colonization or reflect broader symbiont host range; while invariant community assembly implies tighter evolutionary or ecological interactions between plants and root-associated bacteria. Irrespective of mechanism, it appears both communities and community assembly rules vary between crop species.

Staphylococcus aureus is highly adapted to colonization of the mammalian host. In humans the primary site of colonization is the epithelium of the nasal cavity. A major barrier to colonization is the resident microbiota, which have mechanisms to exclude S. aureus . As such, S. aureus has evolved mechanisms to compete with other bacteria, one of which is through secretion of proteinaceous toxins. S. aureus strains collectively produce a number of well-characterized Class I, II, and IV bacteriocins as well as several bacteriocin-like substances, about which less is known. These bacteriocins have potent antibacterial activity against several Gram-positive organisms, with some also active against Gram-negative species. S. aureus bacteriocins characterized to date are sporadically produced, and often encoded on plasmids. More recently the type VII secretion system (T7SS) of S. aureus has also been shown to play a role in interbacterial competition. The T7SS is encoded by all S. aureus isolates and so may represent a more widespread mechanism of competition used by this species. T7SS antagonism is mediated by the secretion of large protein toxins, three of which have been characterized to date: a nuclease toxin, EsaD; a membrane depolarizing toxin, TspA; and a phospholipase toxin, TslA. Further study is required to decipher the role that these different types of secreted toxins play in interbacterial competition and colonization of the host., Competing Interests: The authors declare no conflict of interests., (© The Author(s) 2024. Published by Oxford University Press on behalf of FEMS.)

There has been a proliferation of studies demonstrating an organism's health is influenced by its microbiota. However, factors influencing beneficial microbe colonization and the evolution of these relationships remain understudied relative to host–pathogen interactions. Vertically transmitted beneficial microbes are predicted to show high levels of specificity in colonization, including genotype matching, which may transpire through coevolution. We investigate how host and bacterial genotypes influence colonization of a core coevolved microbiota member in bumble bees. The hindgut colonizing Snodgrassella alvi confers direct benefits, but, as an early colonizer, also facilitates the further development of a healthy microbiota. Due to predominantly vertical transmission promoting tight evolution between colonization factors of bacteria and host lineages, we predict that genotype‐by‐genotype interactions will determine successful colonization. Germ‐free adult bees from seven bumble bee colonies (host genotypic units) were inoculated with one of six genetically distinct strains of S. alvi. Subsequent colonization within host and microbe genotypes combinations ranged from 0 to 100%, and an interaction between host and microbe genotypes determined colonization success. This novel finding of a genotype‐by‐genotype interaction determining colonization in an animal host‐beneficial microbe system has implications for the ecological and evolutionary dynamics of host and microbe, including associated host‐fitness benefits. [ABSTRACT FROM AUTHOR]

Currently our limited understanding of crop rhizosphere community assembly hinders attempts to manipulate it beneficially. Variation in root communities has been attributed to plant host effects, soil type, and plant condition, but it is hard to disentangle the relative importance of soil and host without experimental manipulation. To examine the effects of soil origin and host plant on root associated bacterial communities we experimentally manipulated four crop species in split-plot mesocosms and surveyed variation in bacterial diversity by Illumina amplicon sequencing. Overall, plant species had a greater impact than soil type on community composition. While plant species associated with different Operational Taxonomic Units (OTUs) in different soils, plants tended to recruit bacteria from similar, higher order, taxonomic groups in different soils. However, the effect of soil on root-associated communities varied between crop species: Onion had a relatively invariant bacterial community while other species (maize and pea) had a more variable community structure. Dynamic communities could result from environment specific recruitment, differential bacterial colonization or reflect broader symbiont host range; while invariant community assembly implies tighter evolutionary or ecological interactions between plants and root-associated bacteria. Irrespective of mechanism, it appears both communities and community assembly rules vary between crop species. [ABSTRACT FROM AUTHOR]

Cell walls are important interfaces of plant-fungal interactions, acting as robust physical and chemical barriers against invaders. Upon fungal colonization, plants deposit phenolics and callose at the sites of fungal penetration to prevent further fungal progression. Alterations in the composition of plant cell walls significantly impact host susceptibility. Furthermore, plants and fungi secrete glycan hydrolases acting on each other's cell walls. These enzymes release various sugar oligomers into the apoplast, some of which activate host immunity via surface receptors. Recent characterization of cell walls from plant-colonizing fungi has emphasized the abundance of β-glucans in different cell wall layers, which makes them suitable targets for recognition. To characterize host components involved in immunity against fungi, we performed a protein pull-down with the biotinylated β-glucan laminarin. Thereby, we identified a plant glycoside hydrolase family 81-type glucan-binding protein (GBP) as a β-glucan interactor. Mutation of GBP1 and its only paralog, GBP2 , in barley led to decreased colonization by the beneficial root endophytes Serendipita indica and S. vermifera , as well as the arbuscular mycorrhizal fungus Rhizophagus irregularis. The reduction of colonization was accompanied by enhanced responses at the host cell wall, including an extension of callose-containing cell wall appositions. Moreover, GBP mutation in barley also reduced fungal biomass in roots by the hemibiotrophic pathogen Bipolaris sorokiniana and inhibited the penetration success of the obligate biotrophic leaf pathogen Blumeria hordei. These results indicate that GBP1 is involved in the establishment of symbiotic associations with beneficial fungi—a role that has potentially been appropriated by barley-adapted pathogens. [Display omitted] • Barley GBP1 is a β-1,3-endoglucanase • GBPs contribute to the establishment of symbiotic associations in barley • Upon fungal colonization, gbp mutants display enhanced cell wall immune responses • Barley GBP mutation increases resistance to pathogen infection in roots and leaves Wanke et al. identify GBP1, a family 81 glycoside hydrolase, as a β-glucan interactor in barley. Mutating both barley GBP paralogs reduces colonization by beneficial root endophytes, arbuscular mycorrhizal fungi, and pathogens. These findings provide insights into the role of β-1,3-glucanases as compatibility factors in plant-fungal interactions. [ABSTRACT FROM AUTHOR]

Septoria canker is one of the major limiting factors in the use of hybrid poplar in agroforestry. The use of qPCR for quantification of fungal biomass in host tissue following infection can improve the speed and accuracy of both detection and phenotyping, potentially improving the management of this disease. This study reports the use of a multiplexed hydrolysis probe qPCR assay to detect and quantify the pathogen Sphaerulina musiva in artificially inoculated and naturally infected Populus trees. Species‐specific probes and primers were designed based on conserved gene sequences of S. musiva β‐tubulin and Populus spp. eukaryotic initiation factor IV (eIF4AII). Experiments were performed on greenhouse‐inoculated poplar stems at 1, 3 and 7 weeks post‐inoculation. The qPCR assay detected significant differences among clones and isolates in symptomless samples after 1 week. These differences were not apparent by visual phenotyping. The assay also detected differences among isolates, collected from a range of geographic locations, 3 weeks post‐inoculation. For naturally infected samples, the S. musiva detection rate ranged from 92% to 100%. The primers also proved robust for direct endpoint PCR‐based detection of S. musiva spores, with a minimum detection threshold of 10 spores. The primers proved to be sensitive and specific for S. musiva detection, making them a useful tool for diagnostics and comparisons among isolates and clones in the early stages of infection. [ABSTRACT FROM AUTHOR]

Summary: Vascular wilt caused by Verticillium dahliae is a destructive disease that represents a chronic economic problem for crop production worldwide. In this work, we characterized two new regulators of pathogenicity in this species. Vph1 (VDAG_06555) was identified in a candidate gene approach as a putative homologue of the transcription factor Ste12. Vhb1 (VDAG_08786), identified in a forward genetics approach, is similar to the homeobox transcription factor Htf1, reported as a regulator of conidiogenesis in several fungi. Deletion of vph1 did not affect vegetative growth, whereas deletion of vhb1 greatly reduced sporulation rates in liquid medium. Both mutants failed to induce Verticillium wilt symptoms. However, unlike Δvph1, Δvhb1 could be re‐isolated from the vascular system of some asymptomatic plants. Confocal microscopy further indicated that Δvph1 and Δvhb1 differed in their behaviour in planta; Δvph1 could not penetrate the root cortex, whereas Δvhb1 was impaired in its ability to colonize the xylem. In agreement with these observations, only Δvhb1 could penetrate cellophane paper. On cellophane, wild‐type and Δvhb1 strains produced numerous short branches with swollen tips, resembling the hyphopodia formed on root surfaces, contrasting with Δvph1, which generated unbranched long filaments without swollen tips. A microarray analysis showed that these differences in growth were associated with differences in global transcription patterns, and allowed us to identify a large set of novel genes potentially involved in virulence in V. dahliae. Ste12 homologues are known regulators of invasive growth, but Vhb1 is the first putative Htf1 homologue identified with a critical role in virulence. [ABSTRACT FROM AUTHOR]

The present scanning electron microscopy study describes the development of Lecanicillium muscarium, strain DAOM 198499, on the surface of diverse hosts, including Sphaerotheca fuliginea, a fungal host, and Macrosiphum euphorbiae and Aphidius nigripes, insect hosts. The hosts were sprayed with a conidial suspension of L. muscarium (107 conidia/ml). The specimens used in the SEM investigation were collected at particular periods after spraying and prepared for scanning using standard methods. Germination tubes developed twenty-four hours after applying L. muscarium conidia to each host. Hyphae were attached to the host by a thin mucilaginous matrix. Seventy-two hours after spraying, hyphae of S. fuliginea had collapsed and were encircled by the parasite, and primary sporulation of L. muscarium was observed. On the aphid host, colonization started with adherence of the conidia to the host cuticle, followed by conidial germination and growth of mycelium on the surface of the insect's integument. After 48 to 72 h, post colonization, the first sporulation was observed on the cuticle, particularly at articulations. The mode of parasitism of A. nigripes by this fungus was similar to that of the aphid. Development of L. muscarium was observed on both mummified aphids (containing the pupae of parasitoids) and adult parasitoids.

Destruction of citrus fruits by fungal pathogens during preharvest and postharvest stages can result in severe losses for the citrus industry. Antagonistic microorganisms used as biological agents to control citrus pathogens are considered alternatives to synthetic fungicides. In this study, we aimed to identify fungal pathogens causing dominant diseases on citrus fruits in a specialized citrus cultivation region of Vietnam and inspect soilborne Bacillus isolates with antifungal activity against these pathogens. Two fungal pathogens were characterized as Colletotrichum gloeosporioides and Penicillium digitatum based on morphological characteristics and ribosomal DNA internal transcribed spacer sequence analyses. Reinfection assays of orange fruits confirmed that C. gloeosporioides causes stem-end rot, and P. digitatum triggers green mold disease. By the heterologous expression of the green fluorescent protein (GFP) in C. gloeosporioides using Agrobacterium tumefaciens -mediated transformation, we could observe the fungal infection process of the citrus fruit stem-end rot caused by C. gloeosporioides for the first time. Furthermore, we isolated and selected two soilborne Bacillus strains with strong antagonistic activity for preventing the decay of citrus fruits by these pathogens. Molecular analyses of 16 S rRNA and gyrB genes showed that both isolates belong to B. velezensis . Antifungal activity assays indicated that bacterial culture suspensions could strongly inhibit C. gloeosporioides and P. digitatum , and shield orange fruits from the invasion of the pathogens. Our work provides a highly effective Bacillus -based preservative solution for combating the fungal pathogens C. gloeosporioides and P. digitatum to protect citrus fruits at the postharvest stages., Competing Interests: The authors declare no conflict of interest., (© 2023 The Authors.)

Bark beetles are associated with diverse communities of symbionts. Although fungi have received significant attention, we know little about how bacteria, and in particular their interactions with fungi, affect bark beetle reproduction. We tested how interactions between four bacterial associates, two symbiotic fungi, and two opportunistic fungi affect performance of mountain pine beetles ( Dendroctonus ponderosae) in host tissue. We compared beetle performance in phloem of its historical host, lodgepole pine ( Pinus contorta), and its novel host recently accessed through warming climate, jack pine ( Pinus banksiana). Overall, beetles produced more larvae, and established longer ovipositional and larval galleries in host tissue predominantly colonized by the symbiotic fungi, Grosmannia clavigera, or Ophiostoma montium than by the opportunistic colonizer Aspergillus and to a lesser extent, Trichoderma. This occurred in both historical and naïve hosts. Impacts of bacteria on beetle reproduction depended on particular fungus-bacterium combinations and host species. Some bacteria, e.g., Pseudomonas sp. D4-22 and Hy4T4 in P. contorta and Pseudomonas sp. Hy4T4 and Stenotrophomonas in P. banksiana, reduced antagonistic effects by Aspergillus and Trichoderma resulting in more larvae and longer ovipositional and larval galleries. These effects were not selective, as bacteria also reduced beneficial effects by symbionts in both host species. Interestingly, Bacillus enhanced antagonistic effects by Aspergillus in both hosts. These results demonstrate that bacteria influence brood development of bark beetles in host tissue. They also suggest that climate-driven range expansion of D. ponderosae through the boreal forest will not be significantly constrained by requirements of, or interactions among, its microbial associates. [ABSTRACT FROM AUTHOR]

To understand the growth-promoting and disease-inhibiting activities of plant growth-promoting rhizobacteria (PGPR) strains, the genomes of 12 Bacillus subtilis group strains with PGPR activity were sequenced and analyzed. These B. subtilis strains exhibited high genomic diversity, whereas the genomes of B. amyloliquefaciens strains (a member of the B. subtilis group) are highly conserved. A pairwise BLASTp matrix revealed that gene family similarity among Bacillus genomes ranges from 32 to 90%, with 2839 genes within the core genome of B. amyloliquefaciens subsp. plantarum. Comparative genomic analyses of B. amyloliquefaciens strains identified genes that are linked with biological control and colonization of roots and/or leaves, including 73 genes uniquely associated with subsp. plantarumstrains that have predicted functions related to signaling, transportation, secondary metabolite production, and carbon source utilization. Although B. amyloliquefaciens subsp. plantarum strains contain gene clusters that encode many different secondary metabolites, only polyketide biosynthetic clusters that encode difficidin and macrolactin are conserved within this subspecies. To evaluate their role in plant pathogen biocontrol, genes involved in secondary metabolite biosynthesis were deleted in a B. amyloliquefaciens subsp. plantarum strain, revealing that difficidin expression is critical in reducing the severity of disease, caused by Xanthomonas axonopodis pv. vesicatoria in tomato plants. This study defines genomic features of PGPR strains and links them with biocontrol activity and with host colonization. [ABSTRACT FROM AUTHOR]

Climatological variation and ecological perturbation have been pervasive drivers of faunal assembly, structure and diversification for parasites and pathogens through recurrent events of geographical and host colonization at varying spatial and temporal scales of Earth history. Episodic shifts in climate and environmental settings, in conjunction with ecological mechanisms and host switching, are often critical determinants of parasite diversification, a view counter to more than a century of coevolutionary thinking about the nature of complex host-parasite assemblages. Parasites are resource specialists with restricted host ranges, yet shifts onto relatively unrelated hosts are common during phylogenetic diversification of parasite lineages and directly observable in real time. The emerging Stockholm Paradigm resolves this paradox: Ecological Fitting (EF)--phenotypic flexibility and phylogenetic conservatism in traits related to resource use, most notably host preference--provides many opportunities for rapid host switching in changing environments, without the evolution of novel host-utilization capabilities. Host shifts via EF fuel the expansion phase of the Oscillation Hypothesis of host range and speciation and, more generally, the generation of novel combinations of interacting species within the Geographic Mosaic Theory of Coevolution. In synergy, an environmental dynamic of Taxon Pulses establishes an episodic context for host and geographical colonization. [ABSTRACT FROM AUTHOR]

Beneficial associations with bacteria are widespread across animals, spanning a range of symbiont localizations, transmission routes, and functions. While some of these associations have evolved into obligate relationships with permanent symbiont localization within the host, the majority require colonization of every host generation from the environment or via maternal provisions. Across the broad diversity of host species and tissue types that beneficial bacteria can colonize, there are some highly specialized strategies for establishment yet also some common patterns in the molecular basis of colonization. This review focuses on the mechanisms underlying the early stage of beneficial bacterium-invertebrate associations, from initial contact to the establishment of the symbionts in a specific location of the host's body. We first reflect on general selective pressures that can drive the transition from a free-living to a host-associated lifestyle in bacteria. We then cover bacterial molecular factors for colonization in symbioses from both model and nonmodel invertebrate systems where these have been studied, including terrestrial and aquatic host taxa. Finally, we discuss how interactions between multiple colonizing bacteria and priority effects can influence colonization. Taking the bacterial perspective, we emphasize the importance of developing new experimentally tractable systems to derive general insights into the ecological factors and molecular adaptations underlying the origin and establishment of beneficial symbioses in animals.

Abstract: Batrachochytrium dendrobatidis (B. dendrobatidis), a chytrid fungus, is one of the major contributors to the global amphibian decline. The fungus infects both tadpoles and adult amphibians. Tadpoles are infected in their keratinized mouthparts, and infected adults exhibit hyperkeratosis and loss of righting reflex. Infections of adults may result in death from cardiac arrest in susceptible species. Thyroid hormone plays a key role in amphibian metamorphosis. The occurrence of B. dendrobatidis in tadpoles during metamorphosis may result in exposure of the fungus to host morphogens including TH. This exposure may induce gene expression in the fungus contributing to invasion and colonization of the host. Here, we demonstrate movement of fungal zoospores toward TH. Additionally, expression of a subtilisin-like serine protease is up-regulated in B. dendrobatidis cells exposed to TH. A gene encoding this protease was cloned from B. dendrobatidis and expressed in Escherichia coli. The protein was partially purified and characterized. The similarity between subtilases of human dermatophytes and the B. dendrobatidis subtilisin-like serine protease suggests the importance of this enzyme in B. dendrobatidis pathogenicity. Cleavage of frog skin antimicrobial peptides (AMPs) by this B. dendrobatidis subtilisin-like serine protease suggests a role for this enzyme in fungal survival and colonization. [Copyright &y& Elsevier]

Background: Yersinia enterocolitica is a gastrointestinal foodborne pathogen found worldwide and which especially affects infants and young children. While different bioserotypes have been associated with varying pathogenicity, research on Y. enterocolitica is mainly conducted on the highly virulent mouse-lethal strains of biotype 1B and serotype O:8. We demonstrate here that two Y. enterocolitica bioserotype 1B/O:8 strains, 8081 and WA-314, display different virulence and fitness properties in a mouse model. In vivo co-infection experiments revealed that strain WA-314 overcomes strain 8081 in the colonization of spleen and liver. To trace the reasons of this incongruity, we present here the first high-quality sequence of the whole genome of strain WA-314 and compare it to the published genome of strain 8081. Results: Regions previously accepted as unique to strain 8081, like the YAPI and YGI-3 genomic islands, are absent from strain WA-314, confirming their strain-specificity. On the other hand, some fitness- and bacterial competitionassociated features, such as a putative colicin cluster and a xenobiotic-acyltransferase-encoding gene, are unique to strain WA-314. Additional acquisitions of strain WA-314 are seven prophage-like regions. One of these prophages, the 28-kb P4-like prophage YWA-4, encodes a PilV-like protein that may be used for adhesion to and invasion of the intestinal cells. Furthermore, a putative autotransporter and two type 1 fimbrial proteins of strain WA-314 show a sequence similarity <50% with the orthologous proteins in strain 8081. The dissimilar sequences of these proteins indicate possible different functions or interaction modes, reflecting the specific adhesion properties of Y. enterocolitica strains 8081 and WA-314 and thus the different efficiency of host colonization. Further important differences were found in two pYV plasmid-encoded virulence factors, YopM and YscP. The impact of these differences on virulence is discussed. Conclusions: Our study emphasizes that the virulence of pathogens can be increased, by acquiring new genes and/or improving the function of essential virulence proteins, resulting in permanently hyper-virulent strains. This work also highlights the importance of addressing genetic and phenotypic variations among closely related bacterial strains, even those belonging to the same bioserotype. [ABSTRACT FROM AUTHOR]

The banded elm bark beetle, Scolytus schevyrewi Semenov (Coleoptera: Scolytidae), native to Asia, was detected in the United States in 2003, and as of 2011 it is known to occur in 28 states and four Canadian provinces. S. schevyrewi infests the same elm (Ulmus spp.) hosts as the longestablished invasive, the European elm bark beetle, Scolytus multistriatus (Marsham). Information on the basic biology of S. schevyrewi in its native range is sparse; thus, we conducted laboratory studies and field studies in Colorado and Nevada. Comparisons of flight and behavioral responses were made with co-occurring S. multistriatus. When Siberian elm, Ulmus pumila L., cut logs (bolts) were allowed to be colonized by wild populations in the field, S. schevyrewi did not differ in emergence density from 10- versus 24-cm-diameter bolts. In the laboratory, S. schevyrewi readily colonized bolts of American elm, Ulmus americana L., but not Chinese elm, Ulmus parvifolia Jacq.; Siberian peashrnb, Caragana arborescens Lain.; a cherry, Prunusfontanesiana (Spach) C. K. Schneid.; or Russian olive, Elaeagnus angustifolia L. In Colorado, S. schevyrewi often landed on elm bolts between 12:00 p.m. and 4 p.m.; and near large elm trees, they were captured more frequently on sticky traps at 1.8 and 3.7 m aboveground than higher along the main stem. In Colorado/Nevada, S. schevyrewi initiated flight in April/March and ceased in October/September, whereas S. multistriatus initiated flight in April/May and ceased in October/September. In funnel trap flight assays of semiochemieals in Colorado or Nevada, S. schevyrewi had moderate responses, 3-10-fold greater than unbaited control traps, to Multilure (a commercial lure for S. multistriatus), 2-methyl-3-buten-2-ol (MB) + multistriatin, and MB + a plant extract that is included in a commercial formulation of Multilure. In contrast, S. multistriatus had a 226-259-fold greater response to Multilure than to the control. Both Scolytus species showed electroantennographic (EAG) responses to MB, racemic multistriatin, and (+)- and (-)- α-pinene, with the greatest sensitivity to multistriatin. S. schevyrewi was more responsive to ( - )-α-pinene than was S. multistriatus. [ABSTRACT FROM AUTHOR]

1 Sudden oak death is caused by the apparently introduced oomycete, Phytophthora ramorum. We investigated the role of bark and ambrosia beetles in disease progression in coast live oaks Quercus agrifolia. 2 In two Marin County, California sites, 80 trees were inoculated in July 2002 with P. ramorum and 40 were wounded without inoculation. Half of the trees in each group were sprayed with the insecticide permethrin [cyclopropanecarboxylic acid, 3-(2,2-dichloroethenyl)-2,2-dimethyl-(3-phenoxyphenyl) methyl ester] to prevent ambrosia and bark beetle attacks, and then were sprayed twice per year thereafter. After each treatment, sticky traps were placed on only the permethrin-treated trees. Beetles were collected periodically in 2003. 3 Inoculated trees accounted for 95% of all beetles trapped. The ambrosia beetles Monarthrum scutellare and Xyleborinus saxeseni and the western oak bark beetle Pseudopityophthorus pubipennis were the most abundant of the seven species trapped. 4 Permethrin treatment delayed initiation of beetle attacks and significantly reduced the mean number of attacks per tree. Beetles did not attack any wounded or noncankered inoculated trees. 5 Trees with larger cankers trapped more beetles early in the disease. Once permethrin lost effectiveness, the number of beetle entrance tunnels was a more reliable predictor of subsequent trap catch than was canker size. 6 Beetles were initially attracted to P. ramorum cankers in response to kairomones generated in the host-pathogen interaction. After beetles attacked the permethrin-treated trees, aggregation pheromones most probably were the principal factor in beetle colonization behaviour. [ABSTRACT FROM AUTHOR]

Abstract: Galerucella calmariensis (Coleoptera: Chrysomelidae) has become an effective biological control agent for purple loosestrife (Lythrum salicaria). A male-produced aggregation pheromone was recently identified in this mostly univoltine beetle, and attractiveness to both sexes was demonstrated in the spring, when overwintered adults reproduce. Nothing was known about whether the pheromone functioned for the summer generation, when some of the beetles may undergo a brief reproductive period before entering diapause. Our results showed that both sexes of the beetles responded strongly to synthetic pheromone during summer, and males collected in the field during early summer and held under the prevailing photoperiod were able to emit pheromone. These males mated readily with field-collected females, and fertile eggs were laid, indicating a reproductive role for the pheromone during early summer. Males emitted pheromone well only when fed L. salicaria. Adult beetles were able to persist on marginal hosts such Salix interior and Rosa spp., but almost no pheromone was detected. Emission ceased and resumed as host access changed, even after a long interruption. Attractiveness of L. salicaria foliage was investigated. Six “green-leaf” volatiles were identified that were readily sensed by beetle antennae, but a preliminary synthetic blend of these was not attractive in the field. A model is presented for pheromone-mediated host colonization after dispersal, and relationships among pheromone biology, diapause, photoperiod, and host quality are discussed. Practical uses for the pheromone are noted, and new information is given about longevity of pheromone lures. [Copyright &y& Elsevier]

Abstract: Interactions between the nematode Heterorhabditis bacteriophora isolate JPM4 and the fungus Metarhizium anisopliae, isolates LPP45 and LPP39, were studied during dual infections of Diatraea saccharalis. Mortality, production of infective juveniles (IJs) and production of conidia were evaluated. A positive effect was demonstrated for host mortality in duel infections of JPM4 and LPP39, causing 100% mortality with LT50 and LT95 values of 1.8 and 2.8 days, respectively. Higher values were seen when using the nematode or fungi individually. However, a combination of JPM4+LPP39 caused a significant reduction in IJ production. The results show that faster time to death, a moderately virulent fungal isolate could be combined with the nematode, however at the expense of IJ production. [Copyright &y& Elsevier]

1 Host tree terpenes can influence attraction of conifer-infesting bark beetles to their aggregation pheromones, and both synergistic and inhibitory effects have been reported. 2 We tested a gradient of ratios of (–)-α-pinene, the predominant monoterpene in Norway spruce, to the pheromone of Ips typographus, a major pest of Norway spruce. 3 Attraction of I. typographus increased as the release rate of (–)-α-pinene increased. The two highest (–)-α-pinene : pheromone ratios (526 : 1 and 2595 : 1) attracted twice as many I. typographus as pheromone alone, whereas low to intermediate ratios (56 : 1, 274 : 1) did not differ from pheromone alone. 4 Our results are in agreement with a proposed model, which suggests that bark beetles display unique response profiles to host terpenes depending on the physiological condition of the host trees that they typically colonize. Ips typographus, which is an aggressive species capable of colonizing and killing healthy trees, showed an increased attraction to monoterpene : pheromone ratios, and this may be high enough to inhibit attraction of less aggressive beetle species typically colonizing dead, dying or stressed trees. 5 Attraction of associates of I. typographus was also modified by (–)-α-pinene. Ips duplicatus, a competitor of I. typographus, showed increased attraction to the pheromone of I. typographus across all concentrations of (–)-α-pinene. [ABSTRACT FROM AUTHOR]

Leaf petioles of plum, coffee and sweet orange were examined using scanning electron microscopy (SEM). The presence of Xylella fastidiosa in the samples was confirmed by polymerase chain reaction and gel electrophoresis. The number of vessels colonized by X. fastidiosa was determined by SEM in petiole areas that were transversally sectioned under liquid nitrogen. The percentage of colonized vessels in petioles of coffee was higher than in petioles of plum and citrus whether trees were exhibiting mild symptoms (MS) or severe symptoms (SS). The percentage of vessels colonized varied from 10.9 (MS) to 38.0% (SS), 26 (MS) to 51.6% (SS), and 8 (MS) to 11.8% (SS) for plum, coffee and citrus, respectively, and did not vary by position within the petiole. Severity of symptoms consistently reflected higher proportion of colonized vessels in coffee and plum, but not in citrus. [ABSTRACT FROM AUTHOR]

Recently we have shown that a low (Rlow) and a high laboratory passage (Rhigh) of the poultry pathogen Mycoplasma gallisepticum prototype strain R differ markedly in their capability to invade non-phagocytic eukaryotic cells. In the present study the infection traits of these two mycoplasma passages were compared in an in vivo setting. After aerosol inoculation of chickens, M. gallisepticum was re-isolated from the inner organs of birds infected with Rlow, whereas no mycoplasma was recovered from the inner organs of birds infected with Rhigh. These results indicate that the two mycoplasma populations derived from strain R differ in their capacity to cross the mucosal barrier and suggest that cell invasion may play a major role in the observed systemic spreading of M. gallisepticum in its chicken host. [Copyright &y& Elsevier]

Abstract Background The invasion associated marker (iam) has been detected in the majority of invasive Campylobacter jejuni retrieved from humans. Furthermore, the detection of iam in C. jejuni isolated from two important hosts, humans and chickens, suggested a role for this marker in C. jejuni's colonization of multiple hosts. However, no data exist regarding the occurrence of this marker in C. jejuni isolated from non-poultry food-animals such as cattle, an increasingly important source for human infections. Since little is known about the genetics associated with C. jejuni's capability for colonizing physiologically disparate hosts, we investigated the occurrence of the iam in C. jejuni isolated from cattle and assessed the potential of iam-containing cattle and human isolates for chicken colonization and human cell invasion. Results Simultaneous RAPD typing and iam-specific PCR analysis of 129 C. jejuni isolated from 1171 cattle fecal samples showed that 8 (6.2%) of the isolates were iam-positive, while 7 (54%) of human-associated isolates were iam-positive. The iam sequences were mostly heterogeneous and occurred in diverse genetic backgrounds. All iam-positive isolates were motile and possessed important genes (cadF, ciaB, cdtB) associated with adhesion and virulence. Although certain iam-containing isolates invaded and survived in INT-407 cells in high numbers and successfully colonized live chickens, there was no clear association between the occurrence, allelic sequence, and expression levels of the iam and the aforementioned phenotypes. Conclusions We show that the prevalence of iam in cattle C. jejuni is relatively lower as compared to isolates occurring in humans and chickens. In addition, iam was polymorphic and certain alleles occur in cattle isolates that were capable of colonizing and invading chickens and human intestinal cells, respectively. However, the iam did not appear to contribute to the cattle-associated C. jejuni's potential for invasion and intracellular survival in human intestinal cells as well as chicken colonization.

The emergence of multidrug-resistant Escherichia coli ST131 is a major worldwide public health problem in humans. According to the "one health" approach, this study investigated animal reservoirs of ST131, their relationships with human strains, and the genetic features associated with host colonization. High-quality genomes originating from human, avian, and canine hosts were classified on the basis of their accessory gene content using pangenomic. Pangenomic clusters and subclusters were specifically and significantly associated with hosts. The functions of clustering accessory genes were mainly enriched in functions involved in DNA acquisition, interactions, and virulence (e.g., pathogenesis, response to biotic stimulus and interaction between organisms). Accordingly, networks of cooccurrent host interaction factors were significantly associated with the pangenomic clusters and the originating hosts. The avian strains exhibited a specific content in virulence factors. Rarely found in humans, they corresponded to pathovars responsible for severe human infections. An emerging subcluster significantly associated with both human and canine hosts was evidenced. This ability to significantly colonize canine hosts in addition to humans was associated with a specific content in virulence factors (VFs) and metabolic functions encoded by a new pathogenicity island in ST131 and an improved fitness that is probably involved in its emergence. Overall, VF content, unlike the determinants of antimicrobial resistance, appeared as a key actor of bacterial host adaptation. The host dimension emerges as a major driver of genetic evolution that shapes ST131 genome, enhances its diversity, and favors its dissemination. IMPORTANCE Until now, there has been no indication that the evolutionary dynamics of Escherichia coli ST131 may reflect independent and host-specific adaptation of this lineage outside humans. In contrast, the limited number of ST131 reports in animals supported the common view that it rather reflects a spillover of the human sector. This study uncovered a link between host, ST131 population structure, and virulence factor content which appeared to reflect adaptation to hosts. This study helps to better understand the reservoir of ST131, the putative transmission flux, associated risks and the evolutionary dynamics of this bacterial population and highlights a paradigm in which host colonization stands as a key ecological force of the ST131 evolution.

Host colonization by a pathogen requires proper sensing and response to local environmental cues, to ensure adaptation and continued survival within the host. The ionic milieu represents a critical potential source of environmental cues, and indeed, there has been extensive study of the interplay between host and pathogen in the context of metals such as iron, zinc, and manganese, vital ions that are actively sequestered by the host. The inherent non-uniformity of the ionic milieu also extends, however, to "abundant" ions such as chloride and potassium, whose concentrations vary greatly between tissue and cellular locations, and with the immune response. Despite this, the concept of abundant ions as environmental cues and key players in host-pathogen interactions is only just emerging. Focusing on chloride and potassium, this review brings together studies across multiple bacterial and parasitic species that have begun to define both how these abundant ions are exploited as cues during host infection, and how they can be actively manipulated by pathogens during host colonization. The close links between ion homeostasis and sensing/response to different ionic signals, and the importance of studying pathogen response to cues in combination, are also discussed, while considering the fundamental insight still to be uncovered from further studies in this nascent area of inquiry., (Copyright © 2021 American Society for Microbiology.)