Phenotypic switching in Candida glabrata accompanied by changes in expression of genes with deduced functions in copper detoxification and stress

The Candida glabrata Parent Strain Trap: How Phenotypic Diversity Affects Metabolic Fitness and Host Interactions

Reference strains improve reproducibility by standardizing observations and methodology, which has ultimately led to important insights into fungal pathogenesis. However, recent investigations have highlighted significant genotypic and phenotypic heterogeneity across isolates that influence genetic circuitry and virulence within a species. Candida glabrata is the second leading cause of candidiasis, a life-threatening infection, and undergoes extensive karyotype and phenotypic changes in response to stress. Much of the work conducted on this pathogen has focused on two sequenced strains, CBS138 (ATCC 2001) and BG2. Few studies have compared these strains in detail, but key differences include mating type and altered patterns of expression of EPA adhesins. In fact, most C. glabrata isolates and BG2 are MATa, while CBS138 is MATα. However, it is not known if other phenotypic differences between these strains play a role in our understanding of C. glabrata pathogenesis. Thus, we set out to characterize metabolic, cell wall, and host-interaction attributes for CBS138 and BG2. We found that BG2 utilized a broader range of nitrogen sources and had reduced cell wall size and carbohydrate exposure than CBS138, which we hypothesized results in differences in innate immune interactions and virulence. We observed that, although both strains were phagocytosed to a similar extent, BG2 replicated to higher numbers in macrophages and was more virulent during Galleria mellonella infection than CBS138 in a dose-dependent manner. Interestingly, deletion of SNF3, a major nutrient sensor, did not affect virulence in G. mellonella for BG2, but significantly enhanced larval killing in the CBS138 background compared to the parent strain. Understanding these fundamental differences in metabolism and host interactions will allow more robust conclusions to be drawn in future studies of C. glabrata pathogenesis. IMPORTANCE Reference strains provide essential insights into the mechanisms underlying virulence in fungal pathogens. However, recent studies in Candida albicans and other species have revealed significant genotypic and phenotypic diversity within clinical isolates that are challenging paradigms regarding key virulence factors and their regulation. Candida glabrata is the second leading cause of candidiasis, and many studies use BG2 or CBS138 for their investigations. Therefore, we aimed to characterize important virulence-related phenotypes for both strains that might alter conclusions about C. glabrata pathogenesis. Our study provides context for metabolic and cell wall changes and how these may influence host interaction phenotypes. Understanding these differences is necessary to support robust conclusions about how virulence factors may function in these and other very different strain backgrounds.

Nonidentical function of Upc2A binding sites in the Candida glabrata CDR1 promoter

Increased expression of the Candida glabrata CDR1 gene, encoding an ATP-binding cassette membrane transporter, is routinely observed in fluconazole-resistant isolates of this pathogenic yeast. CDR1 transcription has been well-documented to be due to activity of the Zn2Cys6 zinc cluster-containing transcription factor Pdr1. Gain-of-function mutations in the gene encoding this factor are the most commonly observed cause of fluconazole hyper-resistance in clinical isolates. We have recently found that the sterol-responsive transcription factor Upc2A also acts to control CDR1 transcription, providing a direct link between ergosterol biosynthesis and expression of Pdr1 target genes. While this earlier work implicated Upc2A as an activator of CDR1 transcription, our further analyses revealed the presence of a second Upc2A binding site that negatively regulated CDR1 expression. This Upc2A binding site designated a sterol-responsive element (SRE) was found to have significant lower affinity for Upc2A DNA-binding than the previously described SRE. This new SRE was designated SRE2 while the original, positively acting site was named SRE1. A mutant version of SRE2 prevented in vitro DNA-binding by recombinant Upc2A and, when introduced into the CDR1 promoter, caused decreased fluconazole susceptibility and increased CDR1 expression. This negative effect caused by loss of SRE2 was shown to be Pdr1 independent, consistent with the presence of at least one additional activator of CDR1 transcription. The ability of Upc2A to exert either positive or negative effects on gene expression resembles behavior of mammalian nuclear receptor proteins and reveals an unexpectedly complex nature for SRE effects on gene regulation.

Positive autoregulation and repression of transactivation are key regulatory features of the Candida glabrata Pdr1 transcription factor

Resistance to azole drugs, the major clinical antifungal compounds, is most commonly due to gain-of-function (GOF) substitution mutations in a gene called PDR1 in the fungal pathogen Candida glabrata. PDR1 encodes a zinc cluster-containing transcription factor. GOF forms of Pdr1 drive high level expression of downstream target gene expression with accompanying azole resistance. PDR1 has two homologous genes in Saccharomyces cerevisiae, called ScPDR1 and ScPDR3. This study provides evidence that the PDR1 gene in C. glabrata represents a blend of the properties found in the two S. cerevisiae genes. We demonstrated that GOF Pdr1 derivatives are overproduced at the protein level and less stable than the wild-type protein. Overproduction of wild-type Pdr1 increased target gene expression but to a lesser extent than GOF derivatives. Site-directed mutagenesis of Pdr1 binding sites in the PDR1 promoter provided clear demonstration that autoregulation of PDR1 is required for its normal function. An internal deletion mutant of Pdr1 lacking its central regulatory domain behaved as a hyperactive transcription factor that was lethal unless conditionally expressed. A full understanding of the regulation of Pdr1 will provide a new avenue of interfering with azole resistance in C. glabrata.

Evolution of regulatory networks in Candida glabrata: learning to live with the human host

The opportunistic human fungal pathogen Candida glabrata is second only to C. albicans as the cause of Candida infections and yet is more closely related to Saccharomyces cerevisiae. Recent advances in functional genomics technologies and computational approaches to decipher regulatory networks, and the comparison of these networks among these and other Ascomycete species, have revealed both unique and shared strategies in adaptation to a human commensal/opportunistic pathogen lifestyle and antifungal drug resistance in C. glabrata. Recently, several C. glabrata sister species in the Nakeseomyces clade representing both human associated (commensal) and environmental isolates have had their genomes sequenced and analyzed. This has paved the way for comparative functional genomics studies to characterize the regulatory networks in these species to identify informative patterns of conservation and divergence linked to phenotypic evolution in the Nakaseomyces lineage.

Aging: an emergent phenotypic trait that contributes to the virulence of Cryptococcus neoformans

The pathogenic fungus, Cryptococcus neoformans, is known to undergo phenotypic variation, which affects its virulence in the host. Recent investigations on C. neoformans cells in humans have validated the concept that phenotypic variation is present and relevant for the outcome of chronic cryptococcosis. The C. neoformans capsule is not the only trait that varies among strains. An emerging variant is the "old cell phenotype" generated when C. neoformans undergoes replicative aging. This phenotype, which other than larger size also exhibits a thickened cell wall, inhibits phagocytosis and killing by antifungals in vitro. In concert with the finding that old cells accumulate in vivo, this emergent trait could have significant impact on cryptococcal virulence and infection, and contribute to treatment failure.

Proteomic analysis of cell wall in four pathogenic species of Candida exposed to oxidative stress

In order for Candida species to adhere and colonize human host cells they must express cell wall proteins (CWP) and adapt to reactive oxygen species (ROS) generated by phagocytic cells of the human host during the respiratory burst. However, how these pathogens change the expression of CWP in response to oxidative stress (OSR) is not known. Here, fifteen moonlight-like CWP were identified that expressed differentially in four species of Candida after they were exposed to H2O2 or menadione (O2(-)). These proteins included: (i) glycolytic enzymes, such as glyceraldehyde-3-phosphate dehydrogenase (Gapdh), fructose-bisphosphate aldolase (Fba1), phosphoglycerate mutase (Gpm1), phosphoglycerate kinase (Pgk), pyruvate kinase (Pk) and enolase (Eno1); (ii) the heat shock proteins Ssb1 and Ssa2; (iii) OSR proteins such as peroxyredoxin (Tsa1), the stress protein Ddr48 (Ddr48) and glutathione reductase (Glr1); (iv) other metabolic enzymes such as ketol-acid reductoisomerase (Ilv5) and pyruvate decarboxylase (Pdc1); and (v) other proteins such as elongation factor 1-beta (Efb1) and the 14-3-3 protein homolog. RT-PCR revealed that transcription of the genes coding for some of the identified CWP are differentially regulated. To our knowledge this is the first report showing that moonlight-like CWP are the first line of defense of Candida against ROS, and that they are differentially regulated in each of these pathogens.

Population diversification in a yeast metabolic program promotes anticipation of environmental shifts

Detailed study of the dynamic response of yeast to combinations of sugars reveals an anticipatory population diversification strategy that allows rapid adaptation to shifts in environmental carbon source availability.

To survive in resource-limited and dynamic environments, microbial populations implement a diverse repertoire of regulatory strategies. These strategies often rely on anticipating impending environmental shifts, enabling the population to be prepared for a future change in conditions. It has long been known that cells optimize nutritional value from mixtures of carbon sources, for example glucose and galactose, by sequential activation of regulatory programs that allow for metabolizing the preferred carbon source first before metabolizing the secondary carbon source. Using automated flow-cytometry, we mapped the dynamical behavior of populations simultaneously presented with a large panel of different glucose and galactose concentrations. We show that, counter to expectations, in populations presented with glucose and galactose simultaneously, the galactose regulatory pathway is activated in a fraction of the cell population hours before glucose is fully consumed. We demonstrate that the size of this fraction of cells is tuned by the concentration of the two sugars. This population diversification may constitute a tradeoff between the benefit of rapid galactose consumption once glucose is depleted and the cost of expressing the galactose pathway.

Delineating the strategies by which cells contend with combinatorial changing environments is crucial for understanding cellular regulatory organization. When presented with two carbon sources, microorganisms first consume the carbon substrate that supports the highest growth rate (e.g., glucose) and then switch to the secondary carbon source (e.g., galactose), a paradigm known as the Monod model. Sequential sugar utilization has been attributed to transcriptional repression of the secondary metabolic pathway, followed by activation of this pathway upon depletion of the preferred carbon source. In this work, we demonstrate that although Saccharomyces cerevisiae cells consume glucose before galactose, the galactose regulatory pathway is activated in a fraction of the cell population hours before glucose is fully consumed. This early activation reduces the time required for the population to transition between the two metabolic programs and provides a fitness advantage that might be crucial in competitive environments.

Aging as an emergent factor that contributes to phenotypic variation in Cryptococcus neoformans

Cryptococcus neoformans, similar to other eukaryotes, undergoes replicative aging. Replicative life spans have been determined for clinical C. neoformans strains, and although they are a reproducible trait, life spans vary considerably among strains. C. neoformans has been proposed as an ideal model organism to investigate the contribution of replicative aging in a fungal pathogen population to emerging phenotypic variation during chronic cryptococcal infections. C. neoformans cells of advanced generational age manifest a distinct phenotype; specifically, a larger cell size, a thicker cell wall, drug resistance, as well as resistance to hydrogen peroxide-mediated killing. Consequently, old cells are selected in the host environment during chronic infection and aging could be an unanticipated mechanism of pathogen adaptation that contributes to persistent disease. Aging as a natural process of phenotypic variation should be further studied as it likely is also relevant for other eukaryotic pathogen populations that undergo asymmetric replicative aging.

Differential response of Candida albicans and Candida glabrata to oxidative and nitrosative stresses

Invasive candidiasis is associated with high mortality in immunocompromised and hospitalized patients. Candida albicans is the main pathological agent followed by Candida glabrata, Candida krusei, Candida parapsilosis, and Candida tropicalis. These pathogens colonize different host tissues in humans as they are able to neutralize the reactive species generated from nitrogen and oxygen during the respiratory burst. Among the enzymatic mechanisms that Candida species have developed to protect against free radicals are enzymes with antioxidant and immunodominant functions such as flavohemoglobins, catalases, superoxide dismutases, glutathione reductases, thioredoxins, peroxidases, heat-shock proteins, and enolases. These mechanisms are under transcriptional regulation by factors such as Cta4p, Cwt1p, Yap1p, Skn7p, Msn2p, and Msn4p. However, even though it has been proposed that all Candida species have similar enzymatic systems, it has been observed that they respond differentially to various types of stress. These differential responses may explain the colonization of different organs by each species. Here, we review the enzymatic mechanisms developed by C. albicans and C. glabrata species in response to oxidative and nitrosative stresses. Lack of experimental information for other pathogenic species limits a comparative approach among different organisms.

Analysis of promoter function in Aspergillus fumigatus

The filamentous fungus Aspergillus fumigatus is an important opportunistic pathogen that can cause high mortality levels in susceptible patient populations. The increasing dependence on antifungal drugs to control A. fumigatus has led to the inevitable acquisition of drug-resistant forms of this pathogen. In other fungal pathogens, drug resistance is often associated with an increase in transcription of genes such as ATP-binding cassette (ABC) transporters that directly lead to tolerance to commonly employed antifungal drugs. In A. fumigatus, tolerance to azole drugs (the major class of antifungal) is often associated with changes in the sequence of the azole target enzyme as well as changes in the transcription level of this gene. The target gene for azole drugs in A. fumigatus is referred to as cyp51A. In order to dissect transcription of cyp51A transcription and other genes of interest, we constructed a set of firefly luciferase reporter genes designed for use in A. fumigatus. These reporter genes can either replicate autonomously or be targeted to the pyrG locus, generating an easily assayable uracil auxotrophy. We fused eight different A. fumigatus promoters to luciferase. Faithful behaviors of these reporter gene fusions compared to their chromosomal equivalents were evaluated by 5' rapid amplification of cDNA ends (RACE) and quantitative reverse transcription-PCR (qRT-PCR) analysis. We used this reporter gene system to study stress-regulated transcription of a Hsp70-encoding gene, map an important promoter element in the cyp51A gene, and correct an annotation error in the actin gene. We anticipate that this luciferase reporter gene system will be broadly applicable in analyses of gene expression in A. fumigatus.

Bet hedging in yeast by heterogeneous, age-correlated expression of a stress protectant

A new experimental approach reveals a bet-hedging strategy in unstressed, clonal yeast cells, whereby they adopt a range of growth states that correlate with expression of a trehalose-synthesis regulator and predict resistance to future stress.

Genetically identical cells grown in the same culture display striking cell-to-cell heterogeneity in gene expression and other traits. A crucial challenge is to understand how much of this heterogeneity reflects the noise tolerance of a robust system and how much serves a biological function. In bacteria, stochastic gene expression results in cell-to-cell heterogeneity that might serve as a bet-hedging mechanism, allowing a few cells to survive through an antimicrobial treatment while others perish. Despite its clinical importance, the molecular mechanisms underlying bet hedging remain unclear. Here, we investigate the mechanisms of bet hedging in Saccharomyces cerevisiae using a new high-throughput microscopy assay that monitors variable protein expression, morphology, growth rate, and survival outcomes of tens of thousands of yeast microcolonies simultaneously. We find that clonal populations display broad distributions of growth rates and that slow growth predicts resistance to heat killing in a probabalistic manner. We identify several gene products that are likely to play a role in bet hedging and confirm that Tsl1, a trehalose-synthesis regulator, is an important component of this resistance. Tsl1 abundance correlates with growth rate and replicative age and predicts survival. Our results suggest that yeast bet hedging results from multiple epigenetic growth states determined by a combination of stochastic and deterministic factors.

Genetically identical cells grown in the same environment can display heterogeneity in their morphology, behavior, and composition of their cellular components. In some microorganisms, such cellular heterogeneity can underlie a phenomenon known as bet hedging because it enables some cells to survive in harsh environments, hence increasing the overall population fitness when environmental shifts are unpredictable. Bet hedging is likely to be an important strategy by which microbes infect humans and evade antimicrobial treatments, yet little is known of how cellular heterogeneity contributes to microbial survival. Here, we study the mechanisms underlying bet hedging in yeast. We find that populations of genetically identical yeast contain a broad distribution of growth rates and that slow growth predicts resistance to heat killing in a graded fashion. We identify several gene products that are likely to play a role in this bet-hedging strategy and confirm that Tsl1, a regulator of the production of the disaccharide trehalose, is an important component of acute stress resistance. Finally, we find that old age in cells correlates with a Tsl1-abundant, stress-resistant cell state. Our results suggest that trehalose synthesis is part of a complex and multifactorial mechanism that underlies bet hedging in yeast.

Significant differences in the cell-wall mannans from three Candida glabrata strains correlate with antifungal drug sensitivity

Candida glabrata is often the second or third most common cause of candidiasis after Candida albicans. C. glabrata infections are difficult to treat, often resistant to many azole antifungal agents and are associated with a high mortality rate in compromised patients. We determined the antigenic structure of the cell-wall mannoproteins from three C. glabrata strains, NBRC 0005, NBRC 0622 and NBRC 103857. (1)H NMR and methylation analyses of the acetolysis products of these mannoproteins showed a significant difference in the amount of the β-1,2-linked mannose residue and side-chain structure. The C. glabrata NBRC 103857 strain contained up to the triose side chains and the nonreducing terminal of the triose was predominantly the β-1,2-linked mannose residue. By contrast, the mannans of the two former strains possessed up to the tetraose side chains and the amount of the β-1,2-linked mannose residue was very low. Larger oligosaccharides than tetraose in the acetolysis products of these mannans were identified as incomplete cleavage fragments by analyzing methylation, (1)H NMR spectra and the α1-2,3 mannosidase degradation reaction. Resistance to the antifungal drugs itraconazole and micafungin was significantly different in these strains. Interestingly, the NBRC 103857 strain, which involved a large amount of the β-1,2-linked mannose residues, exhibited significant sensitivity to these antifungal drugs.

Genotypic, phenotypic, and proteomic characterization of Candida glabrata during sequential fluconazole exposure

AIM

Candida glabrata is a major pathogen in humans known to be intrinsically resistant to fluconazole. However, genotypic, phenotypic, and proteomic changes associated with reduced susceptibility to fluconazole are not properly understood. The aim of this study was to observe specific phenotypic, chromosomal, and proteomic alterations in a Candida glabrata strain sequentially exposed to fluconazole.

METHODS

Candida glabrata was exposed to increased concentrations of fluconazole in RPMI for 55 days. Phenotypic changes were evaluated using standard assays. Molecular/proteomic changes in C. glabrata were analyzed by contour-clamped homogeneous electric field electrophoresis, reverse transcription-polymerase chain reaction, and mass spectrometry.

RESULTS

  Candida glabrata demonstrated increased fluconazole resistance (>256 μg/mL), with extensive cross-resistance to ketoconazole (0.38-3.0 μg), itraconazole (8 to >32 μg), and voriconazole (0.125-1.5 μg). Morphologically dissimilar colonies on RPMI/fluconazole agar demonstrated variable chromosomal profiles compared with the control isolate. Stable chromosomal changes were associated with a significantly higher (P<0.05) mRNA level of the hemolysin gene compared with the control. Phenotypic switching on CuSO4 agar was associated with variable metallothionein mRNA transcription levels. The proteome analysis of a fluconazole-resistant offshoot demonstrated a total of 98 protein spots, 25 showing a twofold upregulation.

CONCLUSION

Fluconazole exposure initiates the chance evolution of a new colonizing population with specific virulence traits.

Regulation of the CgPdr1 transcription factor from the pathogen Candida glabrata

Candida glabrata is an opportunistic human pathogen that is increasingly associated with candidemia, owing in part to the intrinsic and acquired high tolerance the organism exhibits for the important clinical antifungal drug fluconazole. This elevated fluconazole resistance often develops through gain-of-function mutations in the zinc cluster-containing transcriptional regulator C. glabrata Pdr1 (CgPdr1). CgPdr1 induces the expression of an ATP-binding cassette (ABC) transporter-encoding gene, CgCDR1. Saccharomyces cerevisiae has two CgPdr1 homologues called ScPdr1 and ScPdr3. These factors control the expression of an ABC transporter-encoding gene called ScPDR5, which encodes a homologue of CgCDR1. Loss of the mitochondrial genome (ρ(0) cell) or overexpression of the mitochondrial enzyme ScPsd1 induces ScPDR5 expression in a strictly ScPdr3-dependent fashion. ScPdr3 requires the presence of a transcriptional Mediator subunit called Gal11 (Med15) to fully induce ScPDR5 transcription in response to ρ(0) signaling. ScPdr1 does not respond to either ρ(0) signals or ScPsd1 overproduction. In this study, we employed transcriptional fusions between CgPdr1 target promoters, like CgCDR1, to demonstrate that CgPdr1 stimulates gene expression via binding to elements called pleiotropic drug response elements (PDREs). Deletion mapping and electrophoretic mobility shift assays demonstrated that a single PDRE in the CgCDR1 promoter was capable of supporting ρ(0)-induced gene expression. Removal of one of the two ScGal11 homologues from C. glabrata caused a major defect in drug-induced expression of CgCDR1 but had a quantitatively minor effect on ρ(0)-stimulated transcription. These data demonstrate that CgPdr1 appears to combine features of ScPdr1 and ScPdr3 to produce a transcription factor with chimeric regulatory properties.

Antigenic and phenotypic variations in fungi

Mechanisms to vary the phenotypic characteristics of fungi are diverse and can be important for their life cycle. This review summarizes phenotypic variability in fungi and divides this phenomenon into three topics: (i) morphological transitions, which are environmentally induced and involve the entire fungal population, (ii) reversible phenotypic switching between different colony morphologies, which is restricted to a small fraction of the population, and (iii) antigenic variation of surface antigens, which can be immuno-dominant epitopes happens in individual fungal cells.

Dark brown is the more virulent of the switch phenotypes of Candida glabrata

Candida glabrata undergoes reversible, high-frequency core switching between phenotypes that include dark brown (DB), light brown (LB) and white (Wh). These phenotypes in turn can switch to the irregular wrinkle (IWr) phenotype. Natural isolates, however, express predominantly the DB phenotype, leading to the hypothesis that it has a colonization advantage over the other switch phenotypes. Using the mouse model of systemic infection, results are presented which support this hypothesis. DB has an advantage over other switch phenotypes in colonizing the two major target organs in the mouse model, the spleen and liver. A time-course study reveals that colonization of the major target organs occurs very rapidly (within 2 h) after host injection, and that the DB advantage for spleen and liver colonization is immediate. The DB advantage is maintained during clearing from spleen, liver and kidneys, and during delayed transient brain colonization. These results demonstrate that DB has a colonization advantage over other switch phenotypes, and that the switch phenotype expressed by a colonizing population therefore plays a fundamental role in virulence. It is therefore essential that switching be considered in both in vivo and in vitro studies of C. glabrata virulence.

Metallosensors, the ups and downs of gene regulation

In fungal cells, transcriptional regulatory mechanisms play a central role in both the homeostatic regulation of the essential metals iron, copper and zinc and in the detoxification of heavy metal ions such as cadmium. Fungi detect changes in metal ion levels using unique metallo-regulatory factors whose activity is responsive to the cellular metal ion status. New studies have revealed that these factors not only regulate the expression of genes required for metal ion acquisition, storage or detoxification but also globally remodel metabolism to conserve metal ions or protect against metal toxicity. This review focuses on the mechanisms metallo-regulators use to up- and down-regulate gene expression.

Changes in karyotype and azole susceptibility of sequential bloodstream isolates from patients with Candida glabrata candidemia

We examined the changes in genotypes and azole susceptibilities among sequential bloodstream isolates of Candida glabrata during the course of fungemia and the relationship of these changes to antifungal therapy. Forty-one isolates were obtained from 15 patients (9 patients who received antifungal therapy and 6 patients who did not) over periods of up to 36 days. The isolates were analyzed using pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) and tested for antifungal susceptibility to fluconazole, itraconazole, and voriconazole. PFGE typing consisted of electrophoretic karyotyping and restriction endonuclease analysis of genomic DNA by use of NotI (REAG-N). The 41 isolates yielded 23 different karyotypes and 11 different REAG-N patterns but only 3 MLST types. The sequential strains from each patient had identical or similar REAG-N patterns. However, they had two or three different karyotypes in 6 (40%) of 15 patients. The isolates from these six patients exhibited the same or similar azole susceptibilities, and five patients did not receive antifungal therapy. Development of acquired azole resistance in sequential isolates was detected for only one patient. For this patient, an isolate of the same genotype obtained after azole therapy showed three- or fourfold increases in the MICs of all three azole antifungals and exhibited increased expression of the CgCDR1 efflux pump. This study shows that karyotypic changes can develop rapidly among sequential bloodstream strains of C. glabrata from the same patient without antifungal therapy. In addition, we confirmed that C. glabrata could acquire azole resistance during the course of fungemia in association with azole therapy.

Role of a CUF1/CTR4 copper regulatory axis in the virulence of Cryptococcus neoformans

The study of regulatory networks in human pathogens such as Cryptococcus neoformans provides insights into host-pathogen interactions that may allow for correlation of gene expression patterns with clinical outcomes. In the present study, deletion of the cryptococcal copper-dependent transcription factor 1 (Cuf1) led to defects in growth and virulence factor expression in low copper conditions. In mouse models, cuf1Delta strains exhibited reduced dissemination to the brain, but no change in lung growth, suggesting copper is limiting in neurologic infections. To examine this further, a biologic probe of available copper was constructed using the cryptococcal CUF1-dependent copper transporter, CTR4. Fungal cells demonstrated high CTR4 expression levels after phagocytosis by macrophage-like J774.16 cells and during infection of mouse brains, but not lungs, consistent with limited copper availability during neurologic infection. This was extended to human brain infections by demonstrating CTR4 expression during C. neoformans infection of an AIDS patient. Moreover, high CTR4 expression by cryptococcal strains from 24 solid organ transplant patients was associated with dissemination to the CNS. Our results suggest that copper acquisition plays a central role in fungal pathogenesis during neurologic infection and that measurement of stable traits such as CTR4 expression may be useful for risk stratification of individuals with cryptococcosis.

Microbial cell individuality and the underlying sources of heterogeneity

Single cells in genetically homogeneous microbial cultures exhibit marked phenotypic individuality, a biological phenomenon that is considered to bolster the fitness of populations. Major phenotypes that are characterized by heterogeneity span the breadth of microbiology, in fields ranging from pathogenicity to ecology. The cell cycle, cell ageing and epigenetic regulation are proven drivers of heterogeneity in several of the best-known phenotypic examples. However, the full contribution of factors such as stochastic gene expression is yet to be realized.

Opaque cells signal white cells to form biofilms in Candida albicans

Upon homozygosis from a/alpha to a/a or alpha/alpha, Candida albicans must still switch from the 'white' to 'opaque' phenotype to mate. It was, therefore, surprising to discover that pheromone selectively upregulated mating-associated genes in mating-incompetent white cells without causing G1 arrest or shmoo formation. White cells, like opaque cells, possess pheromone receptors, although their distribution and redistribution upon pheromone treatment differ between the two cell types. In speculating about the possible role of the white cell pheromone response, it is hypothesized that in overlapping white a/a and alpha/alpha populations in nature, rare opaque cells, through the release of pheromone, signal majority white cells of opposite mating type to form a biofilm that facilitates mating. In support of this hypothesis, it is demonstrated that pheromone induces cohesiveness between white cells, minority opaque cells increase two-fold the thickness of majority white cell biofilms, and majority white cell biofilms facilitate minority opaque cell chemotropism. These results reveal a novel form of communication between switch phenotypes, analogous to the inductive events during embryogenesis in higher eukaryotes.