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Endophytic bacterial populations are well-positioned to provide benefits to their host plants such as nutrient acquisition and plant hormone level manipulation. Actinorhizal plants such as alders are well known for their microbial symbioses that allow them to colonize harsh environments whether natural or anthropized. Although the nitrogen-fixing actinobacterium Frankia sp. is the main endophyte found in alder root nodules, other bacterial genera, whose roles remain poorly defined, inhabit this niche. In this study, we isolated a diverse panel of non- Frankia nodular endophytes (NFNE). Some NFNE were isolated from alders grown from surface-sterilized seeds and maintained in sterile conditions, suggesting these may have been seed-borne. In vitro testing of 24 NFNE revealed some possessed putative plant growth promotion traits. Their genomes were also sequenced to identify genes related to plant growth promotion traits. This study highlights the complexity of the alder nodular microbial community. It paves the way for further understanding of the biology of nodules and could help improve land reclamation practices that involve alders.

In the present study, we report the in vitro interactions between Frankia sp. ACN10a and non- Frankia nodular endophytes (NFNE) isolated from alder. The supernatant of NFNE grown in nitrogen-replete medium had neutral or negative effects on Frankia growth; none had a stimulatory effect. Inhibitory effects were observed for supernatants of some NFNE, notably Micromonospora, Pseudomonas, Serratia and Stenotrophomonas isolates. However, some NFNE– Frankia coculture supernatants could stimulate Frankia growth when used as a culture medium supplement. This was observed for supernatants of Frankia cocultured with Microvirga and S treptomyces isolates. In nitrogen-limited conditions, cocultures of Frankia with some NFNE, including some rhizobia and Cytobacillus, resulted in higher total biomass than Frankia-only cultures, suggesting cooperation, while other NFNE were strongly antagonistic. Microscopic observation of cocultures also revealed compromised Frankia membrane integrity, and some differentiation into stress resistance-associated morphotypes such as sporangia and reproductive torulose hyphae (RTH). Furthermore, the coculture of Frankia with Serratia sp. isolates resulted in higher concentrations of the auxinic plant hormone indole-3-acetic acid and related indolic compounds in the culture supernatant. This study sheds new light on the breadth of microbial interactions that occur amongst bacteria that inhabit the understudied ecological niche of the alder nodule.

The transfer of mobile genetic elements between bacteria is the main cause of the spread of antibiotic resistance genes. While biofilms are the predominant bacterial lifestyle both in the environment and in clinical settings, their impact on the propagation of mobile genetic elements is still poorly understood.

Alzheimer’s disease (AD) is the most common form of dementia and aging is the most common risk factor for developing the disease. The etiology of AD is not known but AD may be considered as a clinical syndrome with multiple causal pathways contributing to it. The amyloid cascade hypothesis, claiming that excess production or reduced clearance of amyloid-beta (Aβ) and its aggregation into amyloid plaques, was accepted for a long time as the main cause of AD. However, many studies showed that Aβ is a frequent consequence of many challenges/pathologic processes occurring in the brain for decades. A key factor, sustained by experimental data, is that low-grade infection leading to production and deposition of Aβ, which has antimicrobial activity, precedes the development of clinically apparent AD. This infection is chronic, low grade, largely clinically silent for decades because of a nearly efficient antimicrobial immune response in the brain. A chronic inflammatory state is induced that results in neurodegeneration. Interventions that appear to prevent, retard or mitigate the development of AD also appear to modify the disease. In this review, we conceptualize further that the changes in the brain antimicrobial immune response during aging and especially in AD sufferers serve as a foundation that could lead to improved treatment strategies for preventing or decreasing the progression of AD in a disease-modifying treatment.

Horizontal gene transfer (HGT) by integrative and conjugative elements (ICEs) is an important mechanism in the spread of antibiotic resistance genes. However, little is known about the spatiotemporal dynamic of ICEs propagation in bacterial biofilms, which are multicellular structures ubiquitous in the natural and clinical environment. Using a fluorescently marked ICEBs1, we report here that its propagation in biofilms is favored in recipient cells expressing the biofilm matrix. Also, conjugation appears restricted to clusters of bacteria in a close neighborhood in which a high level of ICEBs1 transfer occurs. These conjugative clusters are heterogenously distributed in the biofilm, forming close to the air-biofilm interface. Importantly, we established that transconjugant cells are the main contributors to ICEBs1 propagation in biofilms. Our findings provide a novel spatiotemporal understanding of ICEs propagation in biofilms, which should have an important role in our understanding of horizontal gene transfer in relevant settings.ImportanceThe transfer of mobile genetic elements between bacteria is the main cause of the spread of antibiotic resistance genes. While biofilms are the predominant bacterial lifestyle both in the environment and in clinical settings, their impact on the propagation of mobile genetic elements is still poorly understood. In this study, we examined the spatiotemporal propagation of the well-characterized integrative and conjugative element (ICE) ICEBs1. Using the Gram-positive B. subtilis, we observed that the main actors of ICEBs1 propagation in biofilms are the newly formed transconjugants that allow rapid transfer of ICEBs1 to new recipients. Our study provides a better understanding of the spatiotemporal dynamic of conjugative transfer in biofilms.

Bacillus subtilis and Bacillus velezensis are used in organic agriculture as an alternative to chemical pesticides to fight against phytopathogen organisms. These Gram-positive soil-dwelling bacteria are able to resist harsh conditions and survive by differentiating into endospores. Few studies have examined how bacterial populations change on plants over time, and whether they remain active or enter a dormant state. Nonetheless, these characteristics are strikingly important to determine the usage of B. subtilis and B. velezensis and their efficacy in environmental conditions. Here, we investigated the population dynamics of B. subtilis NCIB3610 and B. velezensis QST713 when applied as spores on different ornamental plants. We report that on all the plants studied (Echinacea purpurea ‘Salsa red’, Echinacea purpurea ‘Fatal attraction’, and Lavandula angustifolia ‘Hidecote blue’), spores rapidly germinated and colonized the rhizoplane, maintaining a relatively low proportion of spores in the population over time, whereas the bacterial population on the leaves rapidly declined. Bacteria in the surrounding soil did not germinate and persisted as spores. Taken together, these results suggest that only cells found at the rhizosphere remain metabolically active to allow the formation of a lasting relationship with the plant, making possible beneficial effects from the inoculated bacteria.

Earthworms are newcomers to South-Eastern Canada, as they were unable to survive the last glaciation period that ended about 11,000 years ago. Since their introduction by Europeans over recent centuries, these exotic earthworm species have substantially affected pedological processes and soil functions. For example, a recent study in the province of Quebec found that earthworms invading native sugar maple (Acer saccharum Marsh.) forests could potentially increase soil nitrous oxide (N2O) emissions by increasing denitrification rates. However, the underlying microbial mechanisms driving the production of this greenhouse gas via denitrification remain unclear. This led us to conduct field and laboratory studies in order to explore whether earthworms preferentially promote bacterial and/or fungal denitrification pathways. We measured earthworm abundance and collected surface mineral soil samples from 38 sugar maple forests, half of which were earthworm-free. In each soil sample, we measured fungal, bacterial and total microbial biomass by substrate-induced respiration, we measured fungal, bacterial and total denitrification by acetylene inhibition, and we quantified the abundance bacterial (nirK, nirS and nosZ) and fungal (P450nor) denitrifying genes by qPCR. Earthworm abundance correlated positively with bacterial as well as fungal biomass, but did not affect the bacterial-to-fungal biomass ratio. Accordingly, bacterial-mediated and fungal-mediated denitrification rates both increased with the abundance of earthworms. However, earthworm abundance correlated positively with the specific bacterial denitrification rate (SBDR = (bacterial-mediated denitrification rate) ÷ (bacterial biomass)), but not with the specific fungal denitrification rate (SFDR = (fungal-mediated denitrification rate) ÷ (fungal biomass)). Moreover, qPCR analyses showed a positive correlation between earthworm abundance and the proportion of all bacterial denitrifying genes in the microbial population, but no such effect on fungal denitrifying genes. Taken collectively, our results suggest that earthworms may increase N2O emissions in sugar maple forest soils by preferentially promoting the bacterial-mediated denitrification pathway.

Riboswitches are cis-acting regulatory RNA biosensors that rival the efficiency of those found in proteins. At the heart of their regulatory function is the formation of a highly specific aptamer–ligand complex. Understanding how these RNAs recognize the ligand to regulate gene expression at physiological concentrations of Mg2+ ions and ligand is critical given their broad impact on bacterial gene expression and their potential as antibiotic targets. In this work, we used single-molecule FRET and biochemical techniques to demonstrate that Mg2+ ions act as fine-tuning elements of the amino acid-sensing lysC aptamer's ligand-free structure in the mesophile Bacillus subtilis. Mg2+ interactions with the aptamer produce encounter complexes with strikingly different sensitivities to the ligand in different, yet equally accessible, physiological ionic conditions. Our results demonstrate that the aptamer adapts its structure and folding landscape on a Mg2+-tunable scale to efficiently respond to changes in intracellular lysine of more than two orders of magnitude. The remarkable tunability of the lysC aptamer by sub-millimolar variations in the physiological concentration of Mg2+ ions suggests that some single-aptamer riboswitches have exploited the coupling of cellular levels of ligand and divalent metal ions to tightly control gene expression.

Background Alzheimer’s disease (AD) is the most common neurodegenerative disease ultimately manifesting as clinical dementia. Despite considerable effort and ample experimental data, the role of neuroinflammation related to systemic inflammation is still unsettled. While the implication of microglia is well recognized, the exact contribution of peripheral monocytes/macrophages is still largely unknown, especially concerning their role in the various stages of AD. Objectives AD develops over decades and its clinical manifestation is preceded by subjective memory complaints (SMC) and mild cognitive impairment (MCI); thus, the question arises how the peripheral innate immune response changes with the progression of the disease. Therefore, to further investigate the roles of monocytes/macrophages in the progression of AD we assessed their phenotypes and functions in patients at SMC, MCI and AD stages and compared them with cognitively healthy controls. We also conceptualised an idealised mathematical model to explain the functionality of monocytes/macrophages along the progression of the disease. Results We show that there are distinct phenotypic and functional changes in monocyte and macrophage populations as the disease progresses. Higher free radical production upon stimulation could already be observed for the monocytes of SMC patients. The most striking results show that activation of peripheral monocytes (hyperactivation) is the strongest in the MCI group, at the prodromal stage of the disease. Monocytes exhibit significantly increased chemotaxis, free radical production, and cytokine production in response to TLR2 and TLR4 stimulation. Conclusion Our data suggest that the peripheral innate immune system is activated during the progression from SMC through MCI to AD, with the highest levels of activation being in MCI subjects and the lowest in AD patients. Some of these parameters may be used as biomarkers, but more holistic immune studies are needed to find the best period of the disease for clinical intervention.

Bacterial biofilms are involved in chronic infections and confer 10 to 1000 times more resistance to antibiotics, leading to treatment failure and complications. When transitioning from a planktonic lifestyle to biofilms, certain Gram-positive bacteria are likely to modulate several cellular pathways including central carbon metabolism, primary biosynthesis pathways and production of secondary metabolites. These metabolic adaptations might play a crucial role in biofilm formation by Gram-positive pathogens such as Staphylococcus aureus and Enterococcus faecalis. Here, we performed a transcriptomic approach to identify cellular pathways that might be similarly regulated during biofilm formation in these bacteria. Different strains and biofilm-inducing media were used to identify a set of regulated genes that are common and independent of the environment or accessory genomes analysed. The gene set enrichment analysis of the transcriptome of four different strains of Gram-positive bacteria identified biosynthesis of secondary metabolites, biosynthesis of antibiotics and purine biosynthesis as three commonly upregulated pathways in biofilm. Our approach did not highlight downregulated pathways during biofilm formation that were common to S. aureus and E. faecalis. Of the three upregulated pathways, the de novo IMP biosynthesis pathway constitutes a promising target of cellular adaptation during biofilm formation. Gene deletions in this pathway, particularly purN, purL, purQ, purH and purM significantly impaired biofilm formation of S. aureus.ImportanceBiofilms are often involved in nosocomial infections and can cause serious chronic infections if not treated properly. Current anti-biofilm strategies rely on antibiotic usage, but they have a limited impact because of the biofilm’s intrinsic resistance to drugs. Metabolism remodelling likely plays a central role during biofilm formation, but it is still unclear if these cellular adaptations are shared between strains and species. Using comparative transcriptomics of different strains of Staphylococcus aureus and Enterococcus faecalis, we identified a core of commonly regulated genes during biofilm formation. Interestingly, we observed that the de novo IMP biosynthesis was systematically upregulated during biofilm formation. This pathway could constitute an interesting new anti-biofilm target to increase the host spectrum, drug efficiency and prevent resistance evolution. These results are also relevant to a better understanding of biofilm physiology.

Iron (Fe) is one of the most important micronutrients for most life forms on earth. While abundant in soil, Fe bioavailability in oxic soil is very low. Under environmental conditions, bacteria need to acquire sufficient Fe to sustain growth while limiting the energy cost of siderophore synthesis. Biofilm formation might mitigate this Fe stress, since it was shown to accumulate Fe in certain Gram-negative bacteria and that this Fe could be mobilized for uptake. However, it is still unclear if, and to what extent, the amount of Fe accumulated in the biofilm can sustain growth and if the mobilization of this local Fe pool is modulated by the availability of environmental Fe (i.e., Fe outside the biofilm matrix). Here, we use a nondomesticated strain of the ubiquitous biofilm-forming soil bacterium Bacillus subtilis and stable Fe isotopes to precisely evaluate the origin of Fe during growth in the presence of tannic acid and hydroxides, used as proxies for different environmental conditions. We report that this B. subtilis strain can accumulate a large quantity of Fe in the biofilm, largely exceeding Fe associated with cells. We also report that only a fraction of biofilm-bound Fe is available for uptake in the absence of other sources of Fe in the vicinity of the biofilm. We observed that the availability of environmental Fe modulates the usage of this pool of biofilm-bound Fe. Finally, our data suggest that consumption of biofilm-bound Fe relates to the efficacy of B. subtilis to transport Fe from the environment to the biofilm, possibly through siderophores. IMPORTANCE Recent pieces of evidence suggest that Fe bound to the biofilm could assume at least two important functions, a local source of Fe for uptake and a support to extracellular metabolism, such as extracellular electron transfer. Our results show that B. subtilis can use biofilm-bound Fe for uptake only if it does not compromise Fe homeostasis of the biofilm, i.e., maintains a minimum Fe concentration in the biofilm for extracellular purposes. We propose a theoretical framework based on our results and recent literature to explain how B. subtilis manages biofilm-bound Fe and Fe uptake in response to environmental Fe availability. These results provide important insights into the management of biofilm-bound and environmental Fe by B. subtilis in response to Fe stress.

Plant growth promoting rhizobacteria (PGPR) have enormous potential for solving some of the myriad challenges facing our global agricultural system. Intense research efforts are rapidly moving the field forward and illuminating the wide diversity of bacteria and their plant beneficial activities. In the development of better crop solutions using these PGPR, producers are including multiple different species of PGPR in their formulations in a ‘consortia’ approach. While the intention is to emulate more natural rhizomicrobiome systems, the aspect of bacterial interactions has not been properly regarded. By using a tri-trophic model of Medicago truncatula A17 Jemalong, its nitrogen (N)-fixing symbiont Sinorhizobium meliloti Rm8530 and the PGPR Bacillus subtilis UD1022, we demonstrate indirect influences between the bacteria affecting their plant growth promoting activities. Co-cultures of UD1022 with Rm8530 significantly reduced Rm8530 biofilm formation and downregulated quorum sensing (QS) genes responsible for symbiotically active biofilm production. This work also identifies the presence and activity of a quorum quenching lactonase in UD1022 and proposes this as the mechanism for non-synergistic activity of this model ‘consortium’. These interspecies interactions may be common in the rhizosphere and are critical to understand as we seek to develop new sustainable solutions in agriculture.

Alzheimer’s disease (AD) is the most prevalent dementia in the world. Its cause(s) are presently largely unknown. The most common explanation for AD, now, is the amyloid cascade hypothesis, which states that the cause of AD is senile plaque forma- tion by the amyloid β peptide, and the formation of neurofibrillary tangles by hyperphosphorylated tau. A second, burgeoning theory by which to explain AD is based on the infection hypothesis. Much experimental and epidemiological data support the involvement of infections in the development of dementia. According to this mechanism, the infection either directly or via microbial virulence factors precedes the formation of amyloid β plaques. The amyloid β peptide, possessing antimicrobial properties, may be beneficial at an early stage of AD, but becomes detrimental with the progression of the disease, concomi- tantly with alterations to the innate immune system at both the peripheral and central levels. Infection results in neuroinflam- mation, leading to, and sustained by, systemic inflammation, causing eventual neurodegeneration, and the senescence of the immune cells. The sources of AD-involved microbes are various body microbiome communities from the gut, mouth, nose, and skin. The infection hypothesis of AD opens a vista to new therapeutic approaches, either by treating the infection itself or modulating the immune system, its senescence, or the body’s metabolism, either separately, in parallel, or in a multi-step way., Basque Government under the grant “Artificial Intelligence in BCAM number EXP. 2019/00432”

Secondary metabolites have an important impact on the biocontrol potential of soil-derived microbes. In addition, various microbe-produced chemicals have been suggested to impact the development and phenotypic differentiation of bacteria, including biofilms. The non-ribosomal synthesized lipopeptide ofBacillus subtilis, surfactin, has been described to impact the plant promoting capacity of the bacterium. Here, we investigated the impact of surfactin production on biofilm formation ofB. subtilisusing the laboratory model systems; pellicle formation at the air-medium interface and architecturally complex colony development, in addition to plant root-associated biofilms. We found that the production of surfactin byB. subtilisis not essential for pellicle biofilm formation neither in the well-studied strain, NCIB 3610, nor in the newly isolated environmental strains, but lack of surfactin reduces colony expansion. Further, plant root colonization was comparable both in the presence or absence of surfactin synthesis. Our results suggest that surfactin-related biocontrol and plant promotion inB. subtilisstrains are independent of biofilm formation.

Iron acquisition is of fundamental importance for microorganisms, since this metal is generally poorly bioavailable under natural conditions. In the environment, most bacteria are found tightly packed within multicellular communities named biofilms. Here, using the soil Gram-positive bacterium Bacillus subtilis, we show that biofilm formation and the production of siderophores, i.e., small molecules specifically binding metals, are both essential to ensure Fe uptake from the medium and maintain cellular Fe homeostasis. The biofilm matrix appears to play an important role favoring the efficient usage of siderophores. Taken together, our results demonstrate a close link between biofilm formation and iron acquisition in B. subtilis, allowing a better comprehension of how bacteria can cope with metal limitation under environmental conditions., Iron (Fe) is the most important metal in biology. Despite its abundance, Fe is mostly present as a ferric form in soils, strongly limiting its bioavailability. To overcome the challenge of Fe acquisition, many microorganisms produce siderophores to retrieve Fe from natural sources. Another ubiquitous feature of bacteria in natural environments is biofilm formation. Previous studies showed that external Fe strongly influenced biofilm formation in several bacteria, suggesting that this microenvironment plays a mechanistic role in micronutrient acquisition for bacteria. Here, we applied a complementary set of analytical methods and deletion mutants to evaluate the role of biofilm formation, siderophore production, and their interaction in Fe homeostasis in Bacillus subtilis. We observed that Fe homeostasis, i.e., active growth at a constant intracellular Fe concentration, requires both siderophore production and biofilm formation. Also, we report that in B. subtilis, both biofilm formation and siderophore production are required to achieve active Fe acquisition from the medium and to sustain normal growth. Furthermore, we provide evidence that the formation of biofilm slightly enhances the kinetics of Fe complexation by catechol siderophores and markedly improves siderophore use efficiency. These results provide new perspectives on the mechanism underlying siderophore-based acquisition of Fe in biofilm-forming bacteria. IMPORTANCE Iron acquisition is of fundamental importance for microorganisms, since this metal is generally poorly bioavailable under natural conditions. In the environment, most bacteria are found tightly packed within multicellular communities named biofilms. Here, using the soil Gram-positive bacterium Bacillus subtilis, we show that biofilm formation and the production of siderophores, i.e., small molecules specifically binding metals, are both essential to ensure Fe uptake from the medium and maintain cellular Fe homeostasis. The biofilm matrix appears to play an important role favoring the efficient usage of siderophores. Taken together, our results demonstrate a close link between biofilm formation and iron acquisition in B. subtilis, allowing a better comprehension of how bacteria can cope with metal limitation under environmental conditions.

Methicillin-resistant Staphylococcus aureus (MRSA) is a leading cause of deadly hospital-acquired infections. The discovery of anti- Staphylococcus antibiotics and new classes of drugs not susceptible to the mechanisms of resistance shared among bacteria is imperative. We recently showed that tomatidine (TO), a steroidal alkaloid from solanaceous plants, possesses potent antibacterial activity against S. aureus small-colony variants (SCVs), the notoriously persistent form of this bacterium that has been associated with recurrence of infections. Here, using genomic analysis of in vitro -generated TO-resistant S. aureus strains to identify mutations in genes involved in resistance, we identified the bacterial ATP synthase as the cellular target. Sequence alignments were performed to highlight the modified sequences, and the structural consequences of the mutations were evaluated in structural models. Overexpression of the atpE gene in S. aureus SCVs or introducing the mutation found in the atpE gene of one of the high-level TO-resistant S. aureus mutants into the Bacillus subtilis atpE gene provided resistance to TO and further validated the identity of the cellular target. FC04-100, a TO derivative which also possesses activity against non-SCV strains, prevents high-level resistance development in prototypic strains and limits the level of resistance observed in SCVs. An ATP synthesis assay allowed the observation of a correlation between antibiotic potency and ATP synthase inhibition. The selectivity index (inhibition of ATP production by mitochondria versus that of bacterial ATP synthase) is estimated to be >10 5 -fold for FC04-100.

Biofilms are ubiquitous communities of tightly associated bacteria encased in an extracellular matrix. Bacillus subtilis has long served as a robust model organism to examine the molecular mechanisms of biofilm formation, and a number of studies have revealed that this process is regulated by several integrated pathways. In this Review, we focus on the molecular mechanisms that control B. subtilis biofilm assembly, and then briefly summarize the current state of knowledge regarding biofilm disassembly. We also discuss recent progress that has expanded our understanding of B. subtilis biofilm formation on plant roots, which are a natural habitat for this soil bacterium.

Colonization of plant roots by Bacillus subtilis is mutually beneficial to plants and bacteria. Plants can secrete up to 30% of their fixed carbon via root exudates, thereby feeding the bacteria, and in return the associated B. subtilis bacteria provide the plant with many growth-promoting traits. Formation of a biofilm on the root by matrix-producing B. subtilis is a well-established requirement for long-term colonization. However, we observed that cells start forming a biofilm only several hours after motile cells first settle on the plant. We also found that intact chemotaxis machinery is required for early root colonization by B. subtilis and for plant protection. Arabidopsis thaliana root exudates attract B. subtilis in vitro, an activity mediated by the two characterized chemoreceptors, McpB and McpC, as well as by the orphan receptor TlpC. Nonetheless, bacteria lacking these chemoreceptors are still able to colonize the root, suggesting that other chemoreceptors might also play a role in this process. These observations suggest that A. thaliana actively recruits B. subtilis through root-secreted molecules, and our results stress the important roles of B. subtilis chemoreceptors for efficient colonization of plants in natural environments. These results demonstrate a remarkable strategy adapted by beneficial rhizobacteria to utilize carbon-rich root exudates, which may facilitate rhizobacterial colonization and a mutualistic association with the host., IMPORTANCE Bacillus subtilis is a plant growth-promoting rhizobacterium that establishes robust interactions with roots. Many studies have now demonstrated that biofilm formation is required for long-term colonization. However, we observed that motile B. subtilis mediates the first contact with the roots. These cells differentiate into biofilm-producing cells only several hours after the bacteria first contact the root. Our study reveals that intact chemotaxis machinery is required for the bacteria to reach the root. Many, if not all, of the B. subtilis 10 chemoreceptors are involved in the interaction with the plant. These observations stress the importance of root-bacterium interactions in the B. subtilis lifestyle.

Protein secretion is a complex process that can be modulated by folding factors in the endoplasmic reticulum (ER), such as calnexin, a highly-conserved molecular chaperone involved in quality control. In Schizosaccharomyces pombe, calnexin (Cnx1p) is essential for cell viability. The calnexin/Cnx1p determinants required for viability have been mapped within the last 123 residues of its C-terminus. To better understand the role(s) of calnexin/Cnx1p in secretion, we screened for cnx1 mutants 'super-secreting' cellulase. We identified ss14_cnx1, a mutant secreting 10-fold higher levels of the glycoprotein cellulase than the wild-type strain. While cellulase did not interact with ss14_Cnx1p, the ratio of secreted activity/quantity for this enzyme was not affected, suggesting that the quality control of folding in the ER was adequate in the mutant strain. Surprisingly, the ss14_Cnx1p mutant is composed of the 160 N-terminal amino acids of the mature molecule, thus this mutant defines a novel calnexin/Cnx1p region supporting Sz. pombe viability. Interestingly, like viable mutants spanning the last 52 aa of calnexin/Cnx1p, the 160 N-terminal residues encoded by ss14_cnx1 also forms a complex with the essential BiP chaperone. These results reveal the so far unidentified importance of the N-terminal region of calnexin/Cnx1p.

Calnexin is a molecular chaperone playing key roles in protein folding and the quality control of this process in the endoplasmic reticulum. We, and others, have previously demonstrated that cnx1+, the gene encoding the calnexin homologue in Schizosaccharomyces pombe, is essential for viability. We show that a particular cnx1 mutant induces a novel mechanism allowing the survival of S. pombe cells in the absence of calnexin/Cnx1p. Calnexin independence is dominant in diploid cells and is inherited in a non-Mendelian manner. Remarkably, this survival pathway, bypassing the necessity for calnexin, can be transmitted by transformation of cell extracts into a wild-type naive strain, thus implicating a non-chromosomal factor. Nuclease and UV treatments of cells extracts did not obliterate transmission of calnexin independence by transformation. However, protease digestion of extracts did reduce the appearance of calnexin-independent cells, indicating that a protein element is required for calnexin-less viability. We discuss a model in which this calnexin-less survival mechanism would be activated and perpetuated by a protein component acting as a genetic element.

Recent studies showed that plant roots influence the composition of the bacterial population present in their rhizosphere. This observation implies that roots can affect their own colonization by various bacteria, and notably by plant growth-promoting rhizobacteria (PGPR). Since the plant-beneficial activities depend largely on the presence of PGPR on the root, it appears that the root's capacity to stimulate its own colonization is a great advantage. The exudates secreted by the root contain molecules acting as chemoattractants for many PGPR, and these molecules sometimes act specifically to attract certain cognate bacteria. In some cases, such as infection by a pathogen, the root can also secrete a higher amount of an attractant to recruit a higher quantity of PGPR. The roots exudates can also serve as a carbon source, providing energy and nutrient to the bacteria to multiply in the rhizosphere. Importantly, roots can influence biofilm formation and thus colonization of the roots. Various molecules are involved in this process such as malate, plant polysaccharides and quorum-sensing mimics. In addition, root-produced molecules can affect various cellular pathways, via modulation of transcription, translation or both. Many of these plant-stimulated pathways provide beneficial activities, such as antimicrobials and auxins production. Research on this topic is at its beginning, and most of the molecules and bacterial receptors involved are still to be found.

Bacillus subtilis is a plant-beneficial Gram-positive bacterium widely used as a biofertilizer. However, relatively little is known regarding the molecular processes underlying this bacterium's ability to colonize roots. In contrast, much is known about how this bacterium forms matrix-enclosed multicellular communities (biofilms) in vitro. Here, we show that, when B. subtilis colonizes Arabidopsis thaliana roots it forms biofilms that depend on the same matrix genes required in vitro. B. subtilis biofilm formation was triggered by certain plant polysaccharides. These polysaccharides served as a signal for biofilm formation transduced via the kinases controlling the phosphorylation state of the master regulator Spo0A. In addition, plant polysaccharides are used as a source of sugars for the synthesis of the matrix exopolysaccharide. The bacterium's response to plant polysaccharides was observed across several different strains of the species, some of which are known to have beneficial effects on plants. These observations provide evidence that biofilm genes are crucial for Arabidopsis root colonization by B. subtilis and provide insights into how matrix synthesis may be triggered by this plant.

Galactose is a common monosaccharide that can be utilized by all living organisms via the activities of three main enzymes that make up the Leloir pathway: GalK, GalT, and GalE. In Bacillus subtilis, the absence of GalE causes sensitivity to exogenous galactose, leading to rapid cell lysis. This effect can be attributed to the accumulation of toxic galactose metabolites, since the galE mutant is blocked in the final step of galactose catabolism. In a screen for suppressor mutants restoring viability to a galE null mutant in the presence of galactose, we identified mutations in sinR, which is the major biofilm repressor gene. These mutations caused an increase in the production of the exopolysaccharide (EPS) component of the biofilm matrix. We propose that UDP-galactose is the toxic galactose metabolite and that it is used in the synthesis of EPS. Thus, EPS production can function as a shunt mechanism for this toxic molecule. Additionally, we demonstrated that galactose metabolism genes play an essential role in B. subtilis biofilm formation and that the expressions of both the gal and eps genes are interrelated. Finally, we propose that B. subtilis and other members of the Bacillus genus may have evolved to utilize naturally occurring polymers of galactose, such as galactan, as carbon sources., IMPORTANCE Bacteria switch from unicellular to multicellular states by producing extracellular matrices that contain exopolysaccharides. In such aggregates, known as biofilms, bacteria are more resistant to antibiotics. This makes biofilms a serious problem in clinical settings. The resilience of biofilms makes them very useful in industrial settings. Thus, understanding the production of biofilm matrices is an important problem in microbiology. In studying the synthesis of the biofilm matrix of Bacillus subtilis, we provide further understanding of a long-standing microbiological observation that certain mutants defective in the utilization of galactose became sensitive to it. In this work, we show that the toxicity observed before was because cells were grown under conditions that were not propitious to produce the exopolysaccharide component of the matrix. When cells are grown under conditions that favor matrix production, the toxicity of galactose is relieved. This allowed us to demonstrate that galactose metabolism is essential for the synthesis of the extracellular matrix.

In fission yeast, the ER-residing molecular chaperone calnexin is normally essential for viability. However, a specific mutant of calnexin that is devoid of chaperone function (Deltahcd_Cnx1p) induces an epigenetic state that allows growth of Schizosaccharomyces pombe without calnexin. This calnexin-independent (Cin) state was previously shown to be mediated via a non-chromosomal element exhibiting some prion-like features. Here, we report the identification of a gene whose overexpression induces the appearance of stable Cin cells. This gene, here named cif1(+) for calnexin-independence factor 1, encodes an uncharacterized nucleolar protein. The Cin cells arising from cif1(+) overexpression (Cin(cif1) cells) are genetically and phenotypically distinct from the previously characterized Cin(Deltahcd_cnx1) cells, which spontaneously appear in the presence of the Deltahcd_Cnx1p mutant. Moreover, cif1(+) is not required for the induction or maintenance of the Cin(Deltahcd_cnx1) state. These observations argue for different pathways of induction and/or maintenance of the state of calnexin independence. Nucleolar localization of Cif1p is required to induce the Cin(cif1) state, thus suggesting an unexpected interaction between the vital cellular role of calnexin and a function of the nucleolus.

In the yeast Schizosaccharomyces pombe, the molecular chaperone calnexin (Cnx1p) has been shown to be essential for viability. However, we recently reported that, under certain circumstances, S. pombe cells are able to survive in the absence of calnexin/Cnx1p, indicating that an inducible pathway can complement the calnexin/Cnx1p essential function(s). This calnexin-independent state (Cin) is transmitted by a nonchromosomal proteinaceous element exhibiting several prion-like properties. To assess to what extent the Cin state compensates for the absence of calnexin/Cnx1p, the Cin strain was further characterized. Cin cells exhibited cell-wall defects, sensitivity to heat shock, as well as higher secretion levels of a model glycoprotein. Together, these results indicate that the Cin state does not compensate for all calnexin/Cnx1p functions. Reintroduction of plasmid-borne cnx1(+) partially rescued most but not all of the phenotypes displayed by Cin cells. Interestingly, Cin cells in stationary phase exhibited increased levels of caspase activation, and this phenotype was not suppressed by the reintroduction of cnx1(+), suggesting that cells in the Cin state are subjected to a stress other than the absence of calnexin/Cnx1p.