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Antimicrobial resistance has rendered many conventional therapeutic measures, such as antibiotics, ineffective. This makes the treatment of infections from pathogenic micro-organisms a major growing health, social, and economic challenge. Recently, nanomaterials, including two-dimensional (2D) materials, have attracted scientific interest as potential antimicrobial agents. Many of these studies, however, rely on the input of activation energy and lack real-world utility. In this work, we present the broad-spectrum antimicrobial activity of few-layered black phosphorus (BP) at nanogram concentrations. This property arises from the unique ability of layered BP to produce reactive oxygen species, which we harness to create this unique functionality. BP is shown to be highly antimicrobial toward susceptible and resistant bacteria and fungal species. To establish cytotoxicity with mammalian cells, we showed that both L929 mouse and BJ-5TA human fibroblasts were metabolically unaffected by the presence of BP. Finally, we demonstrate the practical utility of this approach, whereby medically relevant surfaces are imparted with antimicrobial properties via functionalization with few-layer BP. Given the self-degrading properties of BP, this study demonstrates a viable and practical pathway for the deployment of novel low-dimensional materials as antimicrobial agents without compromising the composition or nature of the coated substrate.

The fabrication of antimicrobial surfaces that exhibit enhanced activity toward a large variety of microbial species is one of the major challenges of our time. In fact, the negative effects associated with both bacterial and fungal infections are enormous, especially considering that many microbial species are developing resistance to known antibiotics. In this work, we show how a combination of a specific surface morphology and surface chemistry can create a surface that exhibits nearly 100% antimicrobial activity toward both Gram-negative and Gram-positive bacteria and fungal cells. Arrays of vertically aligned, oxygen-deficient zinc oxide (ZnO) nanowires grown on a substrate exhibit enhanced antimicrobial activity compared with surfaces containing either less defective nanowires or highly oxygen-deficient flat films. This synergistic effect between physical activity (morphology) and chemical activity (surface composition) has been shown to be responsible for the outstanding antimicrobial activity of our surfaces, especially toward notoriously resilient bacterial or fungal species. These findings provide a series of design rules for tuning the activities of antibacterial and antifungal nanomaterials. These rules constitute an excellent platform for the development of next-generation antimicrobial surfaces.

Background Fungal infections are common life-threatening diseases amongst immunodeficient individuals. Invasive fungal disease is commonly treated with an azole antifungal agent, resulting in selection pressure and the emergence of drug resistance. Antifungal resistance is associated with higher mortality rates and treatment failure, making the current clinical management of fungal disease very challenging. Clinical isolates from a variety of fungi have been shown to contain mutations in the MSH2 gene, encoding a component of the DNA mismatch repair pathway. Mutation of MSH2 results in an elevated mutation rate that can increase the opportunity for selectively advantageous mutations to occur, accelerating the development of antifungal resistance. Objectives To characterize the molecular mechanisms causing the microevolutionary emergence of antifungal resistance in msh2 mismatch repair mutants of Cryptococcus neoformans. Methods The mechanisms resulting in the emergence of antifungal resistance were investigated using WGS, characterization of deletion mutants and measuring ploidy changes. Results The genomes of resistant strains did not possess mutations in ERG11 or other genes of the ergosterol biosynthesis pathway. Antifungal resistance was due to small contributions from mutations in many genes. MSH2 does not directly affect ploidy changes. Conclusions This study provides evidence that resistance to fluconazole can evolve independently of ERG11 mutations. A common microevolutionary route to the emergence of antifungal resistance involves the accumulation of mutations that alter stress signalling, cellular efflux, membrane trafficking, epigenetic modification and aneuploidy. This complex pattern of microevolution highlights the significant challenges posed both to diagnosis and treatment of drug-resistant fungal pathogens.

Fungal infections are common life-threatening diseases amongst immunodeficient individuals. Invasive fungal disease is commonly treated with an azole antifungal agent, resulting in selection pressure and the emergence of drug resistance. Antifungal resistance is associated with higher mortality rates and treatment failure, making the current clinical management of fungal disease very challenging. Clinical isolates from a variety of fungi have been shown to contain mutations in theTo characterize the molecular mechanisms causing the microevolutionary emergence of antifungal resistance inThe mechanisms resulting in the emergence of antifungal resistance were investigated using WGS, characterization of deletion mutants and measuring ploidy changes.The genomes of resistant strains did not possess mutations inThis study provides evidence that resistance to fluconazole can evolve independently of

Biofilms are assemblages of microbial cells, extracellular polymeric substances (EPS), and other components extracted from the environment in which they develop. Within biofilms, the spatial distribution of these components can vary. Here we present a fundamental characterization study to show differences between biofilms formed by Gram-positive methicillin-resistant Staphylococcus aureus (MRSA), Gram-negative Pseudomonas aeruginosa, and the yeast-type Candida albicans using synchrotron macro attenuated total reflectance-Fourier transform infrared (ATR-FTIR) microspectroscopy. We were able to characterise the pathogenic biofilms’ heterogeneous distribution, which is challenging to do using traditional techniques. Multivariate analyses revealed that the polysaccharides area (1200–950 cm−1) accounted for the most significant variance between biofilm samples, and other spectral regions corresponding to amides, lipids, and polysaccharides all contributed to sample variation. In general, this study will advance our understanding of microbial biofilms and serve as a model for future research on how to use synchrotron source ATR-FTIR microspectroscopy to analyse their variations and spatial arrangements.

Iron is a key trace element important for many biochemical processes and its availability varies with the environment. For human pathogenic fungi iron acquisition can be particularly problematical because host cells sequester free iron as part of the acute-phase response to infection. Fungi rely on high-affinity iron uptake systems, such as reductive iron assimilation (RIA) and siderophore-mediated iron uptake (non-RIA). These have been extensively studied in pathogenic fungi that exist outside of host cells, but much less is known for intracellular fungal pathogens. Talaromyces marneffei is a dimorphic fungal pathogen endemic to Southeast Asia. In the host T. marneffei resides within macrophages where it grows as a fission yeast. T. marneffei has genes of both iron assimilation systems as well as a paralogue of the siderophore biosynthetic gene sidA, designated sidX. Unlike other fungi, deletion of sidA or sidX resulted in cell type-specific effects. Mutant analysis showed that T. marneffei yeast cells also employ RIA for iron acquisition, providing an additional system in this cell type that differs substantially from hyphal cells. These data illustrate the specialized iron acquisition systems used by the different cell types of a dimorphic fungal pathogen and highlight the complexity in siderophore-biosynthetic pathways amongst fungi.

Passaging of microbes in vitro can lead to the selection of microevolved derivatives with differing properties to their original parent strains. One well characterised instance is the phenotypic differences observed between the series of strains derived from the type strain of the human pathogenic fungus Cryptococcus neoformans. A second case was reported in the close relative Cryptococcus deneoformans, in which a well-studied isolate ATCC 24067 (52D) altered its phenotypic characteristics after in vitro passaging in different laboratories. One of these derivatives, ATCC 24067A, has decreased virulence and also exhibits a hypermutator phenotype, in which the mutation rate is increased compared to wild type. In this study, the molecular basis behind the changes in the lineage of ATCC 24067 was determined by next-generation sequencing of the parent and passaged strain genomes. This analysis resulted in the identification of a point mutation that causes a D270G amino acid substitution within the exonuclease proofreading domain of the DNA polymerase delta subunit encoded by POL3. Complementation with POL3 confirmed that this mutation is responsible for the hypermutator phenotype of this strain. Regeneration of the mutation in C. neoformans, to eliminate the additional mutations present in the ATCC 24067A genetic background, demonstrated that the hypermutator phenotype of the pol3(D270G) mutant causes rapid microevolution in vitro but does not result in decreased virulence. These findings indicate that mutator strains can emerge in these pathogenic fungi without conferring a fitness cost, but the subsequent rapid accumulation of mutations can be deleterious.

Fungal infections are an increasing public health problem, particularly in immunocompromised individuals. While these pathogenic fungi show polyphyletic origins with closely related non-pathogenic species, many undergo morphological transitions to produce pathogenic cell types that are associated with increased virulence. However, the characteristics of these pathogenic cells that contribute to virulence are poorly defined. Talaromyces marneffei grows as a non-pathogenic hyphal form at 25°C but undergoes a dimorphic transition to a pathogenic yeast form at 37°C in vitro and following inhalation of asexual conidia by a host. Here we show that this transition is associated with major changes in central carbon metabolism, and that these changes are correlated with increased virulence of the yeast form. Comprehensive metabolite profiling and 13C-labeling studies showed that hyphal cells exhibited very active glycolytic metabolism and contain low levels of internal carbohydrate reserves. In contrast, yeast cells fully catabolized glucose in the mitochondrial TCA cycle, and store excess glucose in large intracellular pools of trehalose and mannitol. Inhibition of the yeast TCA cycle inhibited replication in culture and in host cells. Yeast, but not hyphae, were also able to use myo-inositol and amino acids as secondary carbon sources, which may support their survival in host macrophages. These analyses suggest that T. marneffei yeast cells exhibit a more efficient oxidative metabolism and are capable of utilizing a diverse range of carbon sources, which contributes to their virulence in animal tissues, highlighting the importance of dimorphic switching in pathogenic yeast.

Penicillium marneffei is an opportunistic human pathogen endemic to Southeast Asia. At 25° P. marneffei grows in a filamentous hyphal form and can undergo asexual development (conidiation) to produce spores (conidia), the infectious agent. At 37° P. marneffei grows in the pathogenic yeast cell form that replicates by fission. Switching between these growth forms, known as dimorphic switching, is dependent on temperature. To understand the process of dimorphic switching and the physiological capacity of the different cell types, two microarray-based profiling experiments covering approximately 42% of the genome were performed. The first experiment compared cells from the hyphal, yeast, and conidiation phases to identify “phase or cell-state–specific” gene expression. The second experiment examined gene expression during the dimorphic switch from one morphological state to another. The data identified a variety of differentially expressed genes that have been organized into metabolic clusters based on predicted function and expression patterns. In particular, C-14 sterol reductase–encoding gene ergM of the ergosterol biosynthesis pathway showed high-level expression throughout yeast morphogenesis compared to hyphal. Deletion of ergM resulted in severe growth defects with increased sensitivity to azole-type antifungal agents but not amphotericin B. The data defined gene classes based on spatio-temporal expression such as those expressed early in the dimorphic switch but not in the terminal cell types and those expressed late. Such classifications have been helpful in linking a given gene of interest to its expression pattern throughout the P. marneffei dimorphic life cycle and its likely role in pathogenicity.

Discovering the genes underlying fundamental processes that enable cells to live and reproduce is a technical challenge, because loss of gene function in mutants results in organisms that cannot survive. This study describes a forward genetics method to identify essential genes in fungi, based on the propensity for Agrobacterium tumefaciens to insert T-DNA molecules into the promoters or 5' untranslated regions of genes and by placing a conditional promoter within the T-DNA. Insertions of the promoter of the GAL7 gene were made in the human pathogen Cryptococcus neoformans. Nine strains of 960 T-DNA insertional mutants screened grew on media containing galactose, but had impaired growth on media containing glucose, which suppresses expression from GAL7. T-DNA insertions were found in the homologs of IDI1, MRPL37, NOC3, NOP56, PRE3 and RPL17, all of which are essential in ascomycete yeasts Saccharomyces cerevisiae or Schizosaccharomyces pombe. Altering the carbon source in the medium provided a system to identify phenotypes in response to stress agents. The pre3 proteasome subunit mutant was further characterized. The T-DNA insertion and phenotype co-segregate in progeny from a cross, and the growth defect is complemented by the reintroduction of the wild type gene into the insertional mutant. A deletion allele was generated in a diploid strain, this heterozygous strain was sporulated, and analysis of the progeny provided additional genetic evidence that PRE3 is essential. The experimental design is applicable to other fungi and has other forward genetic applications such as to isolate over-expression suppressors or enhance the production of traits of interest.

Molecular genetic approaches typically detect recombination in microbes regardless of assumed asexuality. However, genetic data have shown the AIDS-associated pathogen Penicillium marneffei to have extensive spatial genetic structure at local and regional scales, and although there has been some genetic evidence that a sexual cycle is possible, this haploid fungus is thought to be genetically, as well as morphologically, asexual in nature because of its highly clonal population structure. Here we use comparative genomics, experimental mixed-genotype infections, and population genetic data to elucidate the role of recombination in natural populations of P. marneffei. Genome wide comparisons reveal that all the genes required for meiosis are present in P. marneffei, mating type genes are arranged in a similar manner to that found in other heterothallic fungi, and there is evidence of a putatively meiosis-specific mutational process. Experiments suggest that recombination between isolates of compatible mating types may occur during mammal infection. Population genetic data from 34 isolates from bamboo rats in India, Thailand and Vietnam, and 273 isolates from humans in China, India, Thailand, and Vietnam show that recombination is most likely to occur across spatially and genetically limited distances in natural populations resulting in highly clonal population structure yet sexually reproducing populations. Predicted distributions of three different spatial genetic clusters within P. marneffei overlap with three different bamboo rat host distributions suggesting that recombination within hosts may act to maintain population barriers within P. marneffei., Author Summary Fungal pathogen populations show patterns ranging from globally recombining to endemic and clonal. Among the most genetically and spatially restricted fungi is the highly clonal pathogen Penicillium marneffei, an endemic AIDS-associated pathogen in Southeast Asia. Previous studies have shown that P. marneffei has a pattern of extreme clonality despite the ability to disperse across wide distances and the presence of mating type genes that are required for sexual recombination. In this study we used genetic markers, comparative genomics, experimental data, and spatial models to determine the influence of sex on P. marneffei populations, and we found that although there was substantial evidence of sexual recombination, most of the recombination in natural populations was limited to sexual neighborhoods, amongst genetically similar and spatially close individuals. Based on the results of experiments and spatial models we found support for sex occurring in bamboo rats that are known to harbor P. marneffei and the pathogens sexual neighborhoods. Our study suggests that the high levels of effective clonality and endemicity found in P. marneffei may have more to do with specific host interactions than with an innate inability to generate population genetic diversity through sexual recombination.

Penicillium marneffei is an opportunistic pathogen of humans and displays a temperature dependent dimorphic transition. Like many fungi, exogenous DNA introduced by DNA mediated transformation is integrated randomly into the genome resulting in inefficient gene deletion and position-specific effects. To enhance successful gene targeting, the consequences of perturbing components of the non-homologous end joining recombination pathway have been examined. The deletion of the KU70 and LIG4 orthologs, pkuA and ligD, respectively, dramatically enhanced the observed homologous recombination frequency leading to efficient gene deletion. While ΔpkuA was associated with reduced genetic stability over-time, ΔligD represents a suitable recipient strain for downstream applications and combined with a modified Gateway™ system for the rapid generation of gene deletion constructs, this represents an efficient pipeline for characterizing gene function in P. marneffei.

P. marneffei has been established as an experimentally amenable system to study morphogenesis and pathogenicity. This paper describes the development of a number of tools, including numerous selectable markers, to expand the ease with which it can be genetically manipulated. Combined with strains engineered for homologous recombination of exogenous DNA, these tools facilitate efficient molecular genetic studies.

In order to cause disease fungal pathogens must be capable of evading or tolerating the host immune defence system. One commonly utilized evasion mechanism is the ability to continually reside within macrophages of the innate immune system and survive subsequent phagocytic destruction. For intracellular growth to occur, fungal pathogens which typically grow in a filamentous hyphal form in the environment must be able to switch growth to a unicellular yeast growth form in a process known as dimorphic switching. The cue to undergo dimorphic switching relies on the recognition of, and response to, the intracellular host environment. Two-component signalling systems are utilized by eukaryotes to sense and respond to changes in the external environment. This study has investigated the role of the hybrid histidine kinase components encoded by drkA and slnA, in the dimorphic pathogen Penicillium marneffei. Both SlnA and DrkA are required for stress adaptation but are uniquely required for different aspects of asexual development, hyphal morphogenesis and cell wall integrity. Importantly, slnA and drkA are both essential for the generation of yeast cells in vivo, with slnA required for the germination of conidia and drkA required for dimorphic switching during macrophage infection.

The global prevalence of fungal diseases has escalated in the last several decades. Currently, it is estimated that fungi infect 1.7 billion people annually and result in 1.5 million deaths every year1. Deaths due to fungal infections are increasing, with mortality often exceeding 50%, further increasing to 100% if treatment is delayed1. Despite these staggering figures, the contribution of fungal infections to the global burden of disease remains under-recognised. In Australia, over a 5-year period fungal infections cost Australia an estimated $583 million2. The median cost for one invasive fungal disease (IFD) is AU$30957, increasing to AU$80291 if the patient is admitted to an intensive care unit3. Treatment of fungal infections poses significant challenges due to the small number of safe and effective antifungal drugs available and emerging antifungal drug resistance. Resistance to every class of antifungal drugs has been described and for some drug classes is extremely common4,5.

This is the first study in a dimorphic fungal pathogen to investigate the role of a response regulator downstream of two-component signaling systems and its connection to the high-osmolarity glycerol pathway. This study will inspire further research into the downstream components of two-component signaling systems and their role during pathogenic growth., For successful infection to occur, a pathogen must be able to evade or tolerate the host’s defense systems. This requires the pathogen to first recognize the host environment and then signal this response to elicit a complex adaptive program in order to activate its own defense strategies. In both prokaryotes and eukaryotes, two-component signaling systems are utilized to sense and respond to changes in the external environment. The hybrid histidine kinases (HHKs) at the start of the two-component signaling pathway have been well characterized in human pathogens. However, how these HHKs regulate processes downstream currently remains unclear. This study describes the role of a response regulator downstream of these HHKs, sskA, in Talaromyces marneffei, a dimorphic human pathogen. sskA is required for asexual reproduction, hyphal morphogenesis, cell wall integrity, osmotic adaptation, and the morphogenesis of yeast cells both in vitro at 37°C and during macrophage infection, but not during dimorphic switching. Comparison of the ΔsskA mutant with a strain in which the mitogen-activated protein kinase (MAPK) of the high-osmolarity glycerol pathway (SakA) has been deleted suggests that SskA acts upstream of this pathway in T. marneffei to regulate these morphogenetic processes. This was confirmed by assessing the amount of phosphorylated SakA in the ΔsskA mutant, antifungal resistance due to a lack of SakA activation, and the ability of a constitutively active sakA allele (sakAF316L) to suppress the ΔsskA mutant phenotypes. We conclude that SskA regulates morphogenesis and osmotic stress adaptation in T. marneffei via phosphorylation of the SakA MAPK of the high-osmolarity glycerol pathway. IMPORTANCE This is the first study in a dimorphic fungal pathogen to investigate the role of a response regulator downstream of two-component signaling systems and its connection to the high-osmolarity glycerol pathway. This study will inspire further research into the downstream components of two-component signaling systems and their role during pathogenic growth.

The ability of pathogenic fungi to switch between a multicellular hyphal and unicellular yeast growth form is a tightly regulated process known as dimorphic switching. Dimorphic switching requires the fungus to sense and respond to the host environment and is essential for pathogenicity. This review will focus on the role of dimorphism in fungi commonly called thermally dimorphic fungi, which switch to a yeast growth form during infection. This group of phylogenetically diverse ascomycetes includes Talaromyces marneffei (recently renamed from Penicillium marneffei), Blastomyces dermatitidis (teleomorph Ajellomyces dermatitidis), Coccidioides species (C. immitis and C. posadasii), Histoplasma capsulatum (teleomorph Ajellomyces capsulatum), Paracoccidioides species (P. brasiliensis and P. lutzii) and Sporothrix schenckii (teleomorph Ophiostoma schenckii). This review will explore both the signalling pathways regulating the morphological transition and the transcriptional responses necessary for intracellular growth. The physiological requirements of yeast cells during infection will also be discussed, highlighting recent advances in the understanding of the role of iron and calcium acquisition during infection.

Talaromyces marneffei (Basionym: Penicillium marneffei) is a significant opportunistic fungal pathogen in patients infected with human immunodeficiency virus in Southeast Asia. T. marneffei cells have been shown to become melanized in vivo. Melanins are pigment biopolymers which act as a non-specific protectant against various stressors and which play an important role during virulence in fungi. The synthesis of the two most commonly found melanins in fungi, the eumelanin DOPA-melanin and the allomelanin DHN-melanin, requires the action of laccase enzymes. The T. marneffei genome encodes a number of laccases and this study describes the characterization of one of these, pbrB, during growth and development. A strain carrying a PbrB-GFP fusion shows that pbrB is expressed at high levels during asexual development (conidiation) but not in cells growing vegetatively. The pbrB gene is required for the synthesis of DHN-melanin in conidia and when deleted results in brown pigmented conidia, in contrast to the green conidia of the wild type.

The maize pathogen Ustilago maydis switches from budding to filamentous, dikaryotic growth in response to environmental signals including nutrient status, growth in the host, and the presence of mating pheromones. The filamentous dikaryon is capable of proliferating within host tissue to cause disease symptoms including tumors. The transition from yeast cells to hyphal filaments is regulated by a mitogen-activated protein kinase cascade and a cyclic-AMP-protein kinase A (PKA) pathway. Serial analysis of gene expression with PKA mutants identified orthologs of components of the PHO phosphate acquisition pathway as transcriptional targets of the PKA pathway, and these included genes for Pho84, an acid phosphatase, and the vacuolar transport chaperones Vtc1 and Vtc4. In Saccharomyces cerevisiae , Vtc4p is required during the fusion of inorganic-phosphate-containing vesicles to the vacuolar membrane and the consequent accumulation of phosphate stored as polyphosphate (polyP) in the vacuole. We found that deletion of vtc4 in U. maydis also reduced polyP stored in vacuoles. Intriguingly, Δ vtc4 mutants possessed a filamentous cellular morphology, in contrast to the budding, yeast-like growth of the wild-type parent. The Δ vtc4 mutants also displayed decreased symptom development and reduced proliferation in planta. The interaction with PKA signaling was further investigated by the generation of Δ vtc4 ubc1 double mutants. Deletion of vtc4 completely suppressed the multiple-budded phenotype of a Δ ubc1 mutant, indicating that polyP stores are essential for this PKA-induced trait. Overall, this study reveals a novel role for PKA-regulated polyP accumulation in the control of fungal morphogenesis and virulence.

The switch from budding to filamentous growth is a key aspect of invasive growth and virulence for the fungal phytopathogen Ustilago maydis . The cyclic AMP (cAMP) signaling pathway regulates dimorphism in U. maydis , as demonstrated by the phenotypes of mutants with defects in protein kinase A (PKA). Specifically, a mutant lacking the regulatory subunit of PKA encoded by the ubc1 gene displays a multiple-budded phenotype and fails to incite disease symptoms, although proliferation does occur in the plant host. A mutant with a defect in a catalytic subunit of PKA, encoded by adr1 , has a constitutively filamentous phenotype and is nonpathogenic. We employed serial analysis of gene expression to examine the transcriptomes of a wild-type strain and the ubc1 and adr1 mutants to further define the role of PKA in U. maydis . The mutants displayed changes in the transcript levels for genes encoding ribosomal proteins, genes regulated by the b mating-type proteins, and genes for metabolic functions. Importantly, the ubc1 mutant displayed elevated transcript levels for genes involved in phosphate acquisition and storage, thus revealing a connection between cAMP and phosphate metabolism. Further experimentation indicated a phosphate storage defect and elevated acid phosphatase activity for the ubc1 mutant. Elevated phosphate levels in culture media also enhanced the filamentous growth of wild-type cells in response to lipids, a finding consistent with PKA regulation of morphogenesis in U. maydis . Overall, these findings extend our understanding of cAMP signaling in U. maydis and reveal a link between phosphate metabolism and morphogenesis.

Ras and Rho GTPases have been examined in a wide variety of eukaryotes and play varied and often overlapping roles in cell polarization and development. Studies in Saccharomyces cerevisiae and mammalian cells have defined some of the central activities of these GTPases. However, these paradigms do not explain the role of these proteins in all eukaryotes. Unlike yeast, but like more complex eukaryotes, filamentous fungi have Rac-like proteins in addition to Ras and Cdc42. To investigate the unique functions of these proteins and determine how they interact to co-ordinately regulate morphogenesis during growth and development we undertook a genetic analysis of GTPase function by generating double mutants of the Rho GTPases cflA and cflB and the newly isolated Ras GTPase rasA from the dimorphic pathogenic fungus, Penicillium marneffei. P. marneffei growth at 25 degrees C is as multinucleate, septate, branched hyphae which are capable of undergoing asexual development (conidiation), while at 37 degrees C, uninucleate pathogenic yeast cells which divide by fission are produced. Here we show that RasA (Ras) acts upstream of CflA (Cdc42) to regulate germination of spores and polarized growth of both hyphal and yeast cells, while also exhibiting CflA-independent activities. CflA (Cdc42) and CflB (Rac) co-ordinately control hyphal cell polarization despite also having unique roles in regulating conidial germination and polarized growth of yeast cells (CflA) and polarized growth of conidiophore cell types and hyphal branching (CflB).

The opportunistic human pathogenic fungus Penicillium marneffei is dimorphic and is thereby capable of growth either as filamentous multinucleate hyphae or as uninucleate yeast cells which divide by fission. The dimorphic switch is temperature dependent and requires regulated changes in morphology and cell shape. Cdc42p is a Rho family GTPase which in Saccharomyces cerevisiae is required for changes in polarized growth during mating and pseudohyphal development. Cdc42p homologs in higher organisms are also associated with changes in cell shape and polarity. We have cloned a highly conserved CDC42 homolog from P. marneffei named cflA. By the generation of dominant-negative and dominant-activated cflA transformants, we have shown that CflA initiates polarized growth and extension of the germ tube and subsequently maintains polarized growth in the vegetative mycelium. CflA is also required for polarization and determination of correct cell shape during yeast-like growth, and active CflA is required for the separation of yeast cells. However, correct cflA function is not required for dimorphic switching and does not appear to play a role during the generation of specialized structures during asexual development. In contrast, heterologous expression of cflA alleles in Aspergillus nidulans prevented conidiation.

Penicillium marneffei is an emerging human-pathogenic fungus endemic to Southeast Asia. Like a number of other fungal pathogens, P. marneffei exhibits temperature-dependent dimorphic growth and grows in two distinct cellular morphologies, hyphae at 25°C and yeast cells at 37°C. Hyphae can differentiate to produce the infectious agents, asexual spores (conidia), which are inhaled into the host lung, where they are phagocytosed by pulmonary alveolar macrophages. Within macrophages, conidia germinate into unicellular yeast cells, which divide by fission. This minireview focuses on the current understanding of the genes required for the morphogenetic control of conidial germination, hyphal growth, asexual development, and yeast morphogenesis in P. marneffei .

In order to cause disease fungal pathogens must be capable of evading or tolerating the host immune defence system. One commonly utilized evasion mechanism is the ability to continually reside within macrophages of the innate immune system and survive subsequent phagocytic destruction. For intracellular growth to occur, fungal pathogens which typically grow in a filamentous hyphal form in the environment must be able to switch growth to a unicellular yeast growth form in a process known as dimorphic switching. The cue to undergo dimorphic switching relies on the recognition of, and response to, the intracellular host environment. Two-component signalling systems are utilized by eukaryotes to sense and respond to changes in the external environment. This study has investigated the role of the hybrid histidine kinase components encoded by drkA and slnA, in the dimorphic pathogen Penicillium marneffei. Both SlnA and DrkA are required for stress adaptation but are uniquely required for different aspects of asexual development, hyphal morphogenesis and cell wall integrity. Importantly, slnA and drkA are both essential for the generation of yeast cells in vivo, with slnA required for the germination of conidia and drkA required for dimorphic switching during macrophage infection.

The family of Ste20-related kinases is conserved from yeast to mammals and includes the p21 activated kinases (PAKs) and germinal centre kinases (GCKs). These kinases have been shown to be involved in signaling through mitogen activated protein kinase (MAPK) pathways and in morphogenesis through the regulation of cytokinesis and actin-dependent polarized growth. This review concentrates on the role of Ste20-related kinases in fungi where recent research has revealed roles for both PAKs and GCKs in the regulation of cytokinesis and in previously unidentified roles in promoting hyphal growth and differentiation of asexual development structures. In particular, the importance of PAKs during pathogenesis will be examined.

Cytokinesis is essential for proliferative growth but also plays equally important roles during morphogenesis and development. The human pathogen Penicillium marneffei is capable of dimorphic switching in response to temperature, growing in a multicellular filamentous hyphal form at 25°C and in a unicellular yeast form at 37°C. P. marneffei also undergoes asexual development at 25°C to produce multicellular differentiated conidiophores. Thus, P. marneffe i exhibits cell division with and without cytokinesis and division by budding and fission, depending on the cell type. The type II myosin gene, myoB , from P. marneffei plays important roles in the morphogenesis of these cell types. Deletion of myoB leads to chitin deposition defects at sites of cell division without perturbing actin localization. In addition to aberrant hyphal cells, distinct conidiophore cell types are lacking due to malformed septa and nuclear division defects. At 37°C, deletion of myoB prevents uninucleate yeast cell formation, instead producing long filaments resembling hyphae at 25°C. The Δ myoB cells also often lyse due to defects in cell wall biogenesis. Thus, MyoB is essential for correct morphogenesis of all cell types regardless of division mode (budding or fission) and defines differences between the different types of growth.

Pathogens have developed diverse strategies to infect their hosts and evade the host defense systems. Many pathogens reside within host phagocytic cells, thus evading much of the host immune system. For dimorphic fungal pathogens which grow in a multicellular hyphal form, a central attribute which facilitates growth inside host cells without rapid killing is the capacity to switch from the hyphal growth form to a unicellular yeast form. Blocking this transition abolishes or severely reduces pathogenicity. Host body temperature (37°C) is the most common inducer of the hyphal to yeast transition in vitro for many dimorphic fungi, and it is often assumed that this is the inducer in vivo. This work describes the identification and analysis of a new pathway involved in sensing the environment inside a host cell by a dimorphic fungal pathogen, Penicillium marneffei. The pakB gene, encoding a p21-activated kinase, defines this pathway and operates independently of known effectors in P. marneffei. Expression of pakB is upregulated in P. marneffei yeast cells isolated from macrophages but absent from in vitro cultured yeast cells produced at 37°C. Deletion of pakB leads to a failure to produce yeast cells inside macrophages but no effect in vitro at 37°C. Loss of pakB also leads to the inappropriate production of yeast cells at 25°C in vitro, and the mechanism underlying this requires the activity of the central regulator of asexual development. The data shows that this new pathway is central to eliciting the appropriate morphogenetic response by the pathogen to the host environment independently of the common temperature signal, thus clearly separating the temperature- and intracellular-dependent signaling systems., Author Summary Dimorphic fungal pathogens pose significant health and agricultural problems worldwide. These fungi have the capacity to switch between a multicellular hyphal growth form and a unicellular yeast growth form. Often one form is pathogenic, found in infected hosts, and the other is not. Many dimorphic fungal pathogens of humans produce the yeast form during infection and this form resides within host phagocytic immune cells, where it can tolerate killing by these cells and is not exposed to the acquired immune system. Inhibiting the pathogen's ability to switch growth forms has been shown to block pathogenicity. This study identifies a pathway used by the fungal pathogen to sense the host and switch to the appropriate growth form. This study provides new insights into the molecular mechanisms which are important for pathogenicity and may identify factors which can be targeted to block the ability of the pathogen to successfully reside within host cells.

Asexual spores (conidia) are the infectious propagules of many pathogenic fungi, and the capacity to sense the host environment and trigger conidial germination is a key pathogenicity determinant. Germination of conidia requires the de novo establishment of a polarised growth axis and consequent germ tube extension. The molecular mechanisms that control polarisation during germination are poorly understood. In the dimorphic human pathogenic fungus Penicillium marneffei, conidia germinate to produce one of two cell types that have very different fates in response to an environmental cue. At 25 °C, conidia germinate to produce the saprophytic cell type, septate, multinucleate hyphae that have the capacity to undergo asexual development. At 37 °C, conidia germinate to produce the pathogenic cell type, arthroconidiating hyphae that liberate uninucleate yeast cells. This study shows that the p21-activated kinase pakA is an essential component of the polarity establishment machinery during conidial germination and polarised growth of yeast cells at 37 °C but is not required for germination or polarised growth at 25 °C. Analysis shows that the heterotrimeric G protein α subunit GasC and the CDC42 orthologue CflA lie upstream of PakA for germination at both temperatures, while the Ras orthologue RasA only functions at 25 °C. These findings suggest that although some proteins that regulate the establishment of polarised growth in budding yeast are conserved in filamentous fungi, the circuitry and downstream effectors are differentially regulated to give rise to distinct cell types., Author Summary Many fungal infections are initiated by the entry of dormant fungal spores into their host. Once inside the host these dormant spores must reactivate (germinate) for the infection to proceed. Productive infections necessitate that the fungus grow and divide within the host, which makes understanding the mechanisms that control germination crucial to developing preventative or prophylactic treatments for fungal infections. The molecular mechanisms that control spore germination are poorly understood and studies of the opportunistic fungal pathogen Penicillium marneffei have shown that a group of highly conserved signalling and cell polarity factors, known as small GTPases, play important roles in germination and other aspects of morphogenesis. In this study we have shown that a downstream target of these small GTPases, a p21-activated kinase plays a crucial role in germination at the host temperature of 37 °C but not at 25 °C. This is the first component of germination, which shows temperature-dependent effects and provides insights into the different mechanisms used by fungal pathogens to infect their host or to grow saprophytically in non-host environments.

During infection, pathogens must utilise the available nutrient sources in order to grow while simultaneously evading or tolerating the host’s defence systems. Amino acids are an important nutritional source for pathogenic fungi and can be assimilated from host proteins to provide both carbon and nitrogen. The hpdA gene of the dimorphic fungus Penicillium marneffei, which encodes an enzyme which catalyses the second step of tyrosine catabolism, was identified as up-regulated in pathogenic yeast cells. As well as enabling the fungus to acquire carbon and nitrogen, tyrosine is also a precursor in the formation of two types of protective melanin; DOPA melanin and pyomelanin. Chemical inhibition of HpdA in P. marneffei inhibits ex vivo yeast cell production suggesting that tyrosine is a key nutrient source during infectious growth. The genes required for tyrosine catabolism, including hpdA, are located in a gene cluster and the expression of these genes is induced in the presence of tyrosine. A gene (hmgR) encoding a Zn(II)2-Cys6 binuclear cluster transcription factor is present within the cluster and is required for tyrosine induced expression and repression in the presence of a preferred nitrogen source. AreA, the GATA-type transcription factor which regulates the global response to limiting nitrogen conditions negatively regulates expression of cluster genes in the absence of tyrosine and is required for nitrogen metabolite repression. Deletion of the tyrosine catabolic genes in the cluster affects growth on tyrosine as either a nitrogen or carbon source and affects pyomelanin, but not DOPA melanin, production. In contrast to other genes of the tyrosine catabolic cluster, deletion of hpdA results in no growth within macrophages. This suggests that the ability to catabolise tyrosine is not required for macrophage infection and that HpdA has an additional novel role to that of tyrosine catabolism and pyomelanin production during growth in host cells., Author Summary Fungi that infect humans are a major health problem, especially for those with compromised immune systems. Many fungal infections are extremely difficult to cure and if left untreated are fatal. For successful infection to occur, the fungal pathogen must be able to grow by acquiring and utilising the available nutrient sources within the host whilst evading or tolerating the host’s defence systems. Expression profiling in several pathogenic fungal species has revealed that genes required for tyrosine catabolism are induced specifically in the pathogenic cell type at 37°C. As well as enabling the fungus to acquire carbon and nitrogen intermediates from proteins within the host, tyrosine is also an important precursor in the formation of two different types of melanin, which protects cells against the host’s defence systems. This study shows that the ability to catabolise tyrosine and produce tyrosine derived melanin is not required for the initial stages of fungal infection. However, a novel role for hpdA, which encodes the enzyme which catalyses the second step of tyrosine catabolism, was identified during growth in host cells.

Ustilago maydis is a ubiquitous pathogen of maize and a well-established model organism for the study of plant-microbe interactions. This basidiomycete fungus does not use aggressive virulence strategies to kill its host. U. maydis belongs to the group of biotrophic parasites (the smuts) that depend on living tissue for proliferation and development. Here we report the genome sequence for a member of this economically important group of biotrophic fungi. The 20.5-million-base U. maydis genome assembly contains 6,902 predicted protein-encoding genes and lacks pathogenicity signatures found in the genomes of aggressive pathogenic fungi, for example a battery of cell-wall-degrading enzymes. However, we detected unexpected genomic features responsible for the pathogenicity of this organism. Specifically, we found 12 clusters of genes encoding small secreted proteins with unknown function. A significant fraction of these genes exists in small gene families. Expression analysis showed that most of the genes contained in these clusters are regulated together and induced in infected tissue. Deletion of individual clusters altered the virulence of U. maydis in five cases, ranging from a complete lack of symptoms to hypervirulence. Despite years of research into the mechanism of pathogenicity in U. maydis, no 'true' virulence factors had been previously identified. Thus, the discovery of the secreted protein gene clusters and the functional demonstration of their decisive role in the infection process illuminate previously unknown mechanisms of pathogenicity operating in biotrophic fungi. Genomic analysis is, similarly, likely to open up new avenues for the discovery of virulence determinants in other pathogens.

Many of the genes required during polarised growth establishment and maintenance comprise a conserved core which is used to orchestrate global changes in morphology in a broad range of fungi, and therefore generate the various cell types produced by these organisms. The ability to utilize these conserved core components to elicit a variety of cell shapes in different organisms is most likely a result of differences in the specific modes of regulation of these proteins and their upstream activators, in addition to a diverse set of downstream effectors. The field is at an exciting stage, with the major players identified and characterised in a number of systems, and it will be interesting to see how these players are regulated and their activates recruited differently in different contexts to effect the various cell types and shapes which exist.

During maize infection, the fungal pathogen Ustilago maydis undergoes a dimorphic transition from budding, yeast-like cells to a filamentous dikaryon that proliferates in the host. This transition is regulated by mating and environmental signals. Septation is likely to be important in the growth of the infectious dikaryon because of the need to maintain specific cellular compartments during dikaryotic growth. Recently, we found that the transcript level for a septin gene was influenced by the conserved cyclic AMP (cAMP)/protein kinase A signaling pathway that participates in regulating dimorphism in U. maydis . In this study, we describe the detailed analysis of the function of this septin gene, designated sep3 , in the growth, development, and pathogenesis of U. maydis . We show that sep3 is required for normal cellular morphology and the division of budding haploid cells. The gene is also required for lipid-induced filamentous growth in culture but not during the formation of mating filaments on agar medium or in planta. Strains with a deletion in sep3 have a reduction in symptom development in maize, with filamentous cells in planta displaying morphological defects. In addition, sep3 influences the differentiation of hyphae into teliospores and the germination of these teliospores to produce the meiotic haploid progeny that complete the disease life cycle. Finally, the deletion of sep3 was found to influence the multiple budding phenotype of a mutant with a defect in the regulatory subunit of protein kinase A. This result is consistent with a link between sep3 and the control of morphogenesis by cAMP signaling. Overall, this study highlights the importance of regulating septation and changes in morphology during phytopathogenesis.

Rac proteins control polarized growth in many organisms but the specific function of these proteins remains undefined. In this study, we describe the cloning and functional characterization of a RAC homolog, cflB, from the dimorphic fungus Penicillium marneffei. P. marneffei produces asexual spores on complex structures (conidiophores)and switches between hyphal and yeast growth. CflB colocalizes with actin at the tips of vegetative hyphal cells and at sites of cell division. Deletion of cflB results in cell division (septation) and growth defects in both vegetative hyphal and conidiophore cell types such that cells become depolarized, exhibit inappropriate septation and the actin cytoskeleton is severely disrupted. This data suggests that Rac proteins play a crucial role in actin dependent polarized growth and division. The CDC42 ortholog in P. marneffei, cflA, controls vegetative hyphal and yeast growth polarization but does not affect asexual development. By contrast, CflB affects cellular polarization during asexual development and hyphal growth but not during yeast growth. This shows that these two GTPases have both overlapping and distinct roles during growth and development. RAC orthologs are not found in less morphologically complex eukaryotes such as Saccharomyces cerevisiae, suggesting that RAC genes might have evolved with increasing cellular complexity.