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1.  Genetic Control of Conventional and Pheromone-Stimulated Biofilm Formation in Candida albicans 
PLoS Pathogens  2013;9(4):e1003305.
Candida albicans can stochastically switch between two phenotypes, white and opaque. Opaque cells are the sexually competent form of C. albicans and therefore undergo efficient polarized growth and mating in the presence of pheromone. In contrast, white cells cannot mate, but are induced – under a specialized set of conditions – to form biofilms in response to pheromone. In this work, we compare the genetic regulation of such “pheromone-stimulated” biofilms with that of “conventional” C. albicans biofilms. In particular, we examined a network of six transcriptional regulators (Bcr1, Brg1, Efg1, Tec1, Ndt80, and Rob1) that mediate conventional biofilm formation for their potential roles in pheromone-stimulated biofilm formation. We show that four of the six transcription factors (Bcr1, Brg1, Rob1, and Tec1) promote formation of both conventional and pheromone-stimulated biofilms, indicating they play general roles in cell cohesion and biofilm development. In addition, we identify the master transcriptional regulator of pheromone-stimulated biofilms as C. albicans Cph1, ortholog of Saccharomyces cerevisiae Ste12. Cph1 regulates mating in C. albicans opaque cells, and here we show that Cph1 is also essential for pheromone-stimulated biofilm formation in white cells. In contrast, Cph1 is dispensable for the formation of conventional biofilms. The regulation of pheromone- stimulated biofilm formation was further investigated by transcriptional profiling and genetic analyses. These studies identified 196 genes that are induced by pheromone signaling during biofilm formation. One of these genes, HGC1, is shown to be required for both conventional and pheromone-stimulated biofilm formation. Taken together, these observations compare and contrast the regulation of conventional and pheromone-stimulated biofilm formation in C. albicans, and demonstrate that Cph1 is required for the latter, but not the former.
Author Summary
Candida albicans is the predominant fungal pathogen afflicting humans, where many infections arise due to its proclivity to form biofilms. Biofilms are complex multicellular communities in which cells exhibit distinct properties to those grown in suspension. They are particularly relevant in the development of device-associated infections, and thus understanding biofilm regulation and biofilm architecture is a priority. C. albicans has the ability to form different types of biofilms under different environmental conditions. Here, we compare the regulation of biofilm formation in conventional biofilms, for which a core transcriptional network has recently been identified, with pheromone-stimulated biofilms, which occur when C. albicans white cells are exposed to pheromone. Our studies show that several regulatory components control biofilm formation under both conditions, including the network transcriptional regulators Bcr1, Brg1, Rob1, and Tec1. However, other transcriptional regulators are specific to each model of biofilm development. In particular, we demonstrate that Cph1, the master regulator of the pheromone response during mating, is essential for pheromone-stimulated biofilm formation but is dispensable for conventional biofilms. These studies provide an in-depth analysis of the regulation of pheromone-stimulated biofilms, and demonstrate that both shared and unique components operate in different models of biofilm formation in this human pathogen.
PMCID: PMC3630098  PMID: 23637598
2.  Critical Role of Bcr1-Dependent Adhesins in C. albicans Biofilm Formation In Vitro and In Vivo 
PLoS Pathogens  2006;2(7):e63.
The fungal pathogen Candida albicans is frequently associated with catheter-based infections because of its ability to form resilient biofilms. Prior studies have shown that the transcription factor Bcr1 governs biofilm formation in an in vitro catheter model. However, the mechanistic role of the Bcr1 pathway and its relationship to biofilm formation in vivo are unknown. Our studies of biofilm formation in vitro indicate that the surface protein Als3, a known adhesin, is a key target under Bcr1 control. We show that an als3/als3 mutant is biofilm-defective in vitro, and that ALS3 overexpression rescues the biofilm defect of the bcr1/bcr1 mutant. We extend these findings with an in vivo venous catheter model. The bcr1/bcr1 mutant is unable to populate the catheter surface, though its virulence suggests that it has no growth defect in vivo. ALS3 overexpression rescues the bcr1/bcr1 biofilm defect in vivo, thus arguing that Als3 is a pivotal Bcr1 target in this setting. Surprisingly, the als3/als3 mutant forms a biofilm in vivo, and we suggest that additional Bcr1 targets compensate for the Als3 defect in vivo. Indeed, overexpression of Bcr1 targets ALS1, ECE1, and HWP1 partially restores biofilm formation in a bcr1/bcr1 mutant background in vitro, though these genes are not required for biofilm formation in vitro. Our findings demonstrate that the Bcr1 pathway functions in vivo to promote biofilm formation, and that Als3-mediated adherence is a fundamental property under Bcr1 control. Known adhesins Als1 and Hwp1 also contribute to biofilm formation, as does the novel protein Ece1.
The formation of biofilms (surface-attached microbial communities) on implanted medical devices such as catheters is a major cause of fungal and bacterial infections. Prior studies of the fungal pathogen Candida albicans have shown that the regulator Bcr1 is required for biofilm formation in vitro, but the mechanism through which it promotes biofilm formation and its significance for biofilm formation in vivo was uncertain. The authors demonstrate that Bcr1 is required for biofilm formation in vivo in a rat model of catheter-based infection. Manipulation of Bcr1 target genes through mutation and gene overexpression shows that the known surface adhesin Als3 has a pivotal role in biofilm formation and that adhesins Als1 and Hwp1 also contribute to biofilm formation. The results thus indicate that adherence is the key property regulated by Bcr1 and highlight a group of adhesins as potential therapeutic targets.
PMCID: PMC1487173  PMID: 16839200
3.  Ability of Candida albicans Mutants To Induce Staphylococcus aureus Vancomycin Resistance during Polymicrobial Biofilm Formation▿  
Candida albicans and Staphylococcus aureus form vigorous polymicrobial biofilms in serum, which may serve as the source of coinfection in patients. More importantly, S. aureus is highly resistant to vancomycin during polymicrobial biofilm formation, with no decreases in bacterial viability observed with up to 1,600 μg/ml drug. In these mixed-species biofilms, S. aureus preferentially associates with C. albicans hyphae, which express a variety of unique adhesins. We tested C. albicans mutants deficient in transcriptional regulators of morphogenesis (CPH1 and EFG1) and biofilm formation (BCR1) to investigate the role of hyphae in mediating polymicrobial biofilm formation. These mutants also have reduced expression of hypha-specific adhesins. The ability to form polymicrobial biofilms correlated with the ability to form hyphae in these mutants. However, only mutants that could adhere to the abiotic surface could induce S. aureus vancomycin resistance, regardless of the presence of hyphae. In examining factors that may mediate interspecies adhesion, we found that the C. albicans ALS family of adhesins (Als1 to Als7 and Als9) was not involved, and neither was the hypha-specific adhesin Hwp1. Therefore, polymicrobial biofilm formation and subsequent antibiotic resistance is a multifactorial process that may require a unique combination of fungal and/or bacterial adhesins.
PMCID: PMC2934986  PMID: 20566760
4.  Fungal Morphogenetic Pathways Are Required for the Hallmark Inflammatory Response during Candida albicans Vaginitis 
Infection and Immunity  2014;82(2):532-543.
Vulvovaginal candidiasis, caused primarily by Candida albicans, presents significant health issues for women of childbearing age. As a polymorphic fungus, the ability of C. albicans to switch between yeast and hyphal morphologies is considered its central virulence attribute. Armed with new criteria for defining vaginitis immunopathology, the purpose of this study was to determine whether the yeast-to-hypha transition is required for the hallmark inflammatory responses previously characterized during murine vaginitis. Kinetic analyses of vaginal infection with C. albicans in C57BL/6 mice demonstrated that fungal burdens remained constant throughout the observation period, while polymorphonuclear leukocyte (PMN), S100A8, and interleukin-1β levels obtained from vaginal lavage fluid increased by day 3 onward. Lactate dehydrogenase activity was also positively correlated with increased effectors of innate immunity. Additionally, immunodepletion of neutrophils in infected mice confirmed a nonprotective role for PMNs during vaginitis. Determination of the importance of fungal morphogenesis during vaginitis was addressed with a two-pronged approach. Intravaginal inoculation of mice with C. albicans strains deleted for key transcriptional regulators (bcr1Δ/Δ, efg1Δ/Δ, cph1Δ/Δ, and efg1Δ/Δ cph1Δ/Δ) controlling the yeast-to-hypha switch revealed a crucial role for morphogenetic signaling through the Efg1 and, to a lesser extent, the Bcr1 pathways in contributing to vaginitis immunopathology. Furthermore, overexpression of transcription factors NRG1 and UME6, to maintain yeast and hyphal morphologies, respectively, confirmed the importance of morphogenesis in generating innate immune responses in vivo. These results highlight the yeast-to-hypha switch and the associated morphogenetic response as important virulence components for the immunopathogenesis of Candida vaginitis, with implications for transition from benign colonization to symptomatic infection.
PMCID: PMC3911367  PMID: 24478069
5.  Alternative Mating Type Configurations (a/α versus a/a or α/α) of Candida albicans Result in Alternative Biofilms Regulated by Different Pathways 
PLoS Biology  2011;9(8):e1001117.
Similar multicellular structures can evolve within the same organism that may have different evolutionary histories, be controlled by different regulatory pathways, and play similar but nonidentical roles. In the human fungal pathogen Candida albicans, a quite extraordinary example of this has occurred. Depending upon the configuration of the mating type locus (a/α versus a/a or α/α), C. albicans forms alternative biofilms that appear similar morphologically, but exhibit dramatically different characteristics and are regulated by distinctly different signal transduction pathways. Biofilms formed by a/α cells are impermeable to molecules in the size range of 300 Da to 140 kDa, are poorly penetrated by human polymorphonuclear leukocytes (PMNs), and are resistant to antifungals. In contrast, a/a or α/α biofilms are permeable to molecules in this size range, are readily penetrated by PMNs, and are susceptible to antifungals. By mutational analyses, a/α biofilms are demonstrated to be regulated by the Ras1/cAMP pathway that includes Ras1→Cdc35→cAMP(Pde2—|)→Tpk2(Tpk1)→Efg1→Tec1→Bcr1, and a/a biofilms by the MAP kinase pathway that includes Mfα→Ste2→ (Ste4, Ste18, Cag1)→Ste11→Hst7→Cek2(Cek1)→Tec1. These observations suggest the hypothesis that while the upstream portion of the newly evolved pathway regulating a/a and α/α cell biofilms was derived intact from the upstream portion of the conserved pheromone-regulated pathway for mating, the downstream portion was derived through modification of the downstream portion of the conserved pathway for a/α biofilm formation. C. albicans therefore forms two alternative biofilms depending upon mating configuration.
Author Summary
Single-celled microbes can form biofilms, or aggregates of cells that adhere to one another on a surface, in response to many environmental factors. Like many microbial pathogens, the yeast Candida albicans can form biofilms that normally provide protective environments against antifungals, antibodies, and white blood cells, thus ensuring higher rates of survival in response to assault by drugs or the human immune system. We report that while a majority (around 90%) of C. albicans strains form traditional biofilms that are impermeable to molecules of low and high molecular weight, and that are impenetrable to white blood cells, a minority (around 10%) form biofilms that are both permeable and penetrable. Formation of the minority-type alternative biofilms is dictated by a change at a single genetic locus, the mating type locus. Homozygous a/a or α/α cells are mating-competent, whereas the heterozygous a/α cells are mating-incompetent. Cells of the mating-incompetent a/α genotype form the impermeable, traditional biofilm, whereas the mating-competent a/a or α/α genotype forms the permeable biofilm. The characteristics of a/a and α/α biofilms are consistent with a suggested role in mating by facilitating the transfer of hormone signals through the permeable biofilm. The two types of biofilm are also regulated by different signal transduction pathways: the a/α form by the Ras1/cAMP pathway, and the a/a or α/α forms by the MAP kinase pathway. Components of the latter pathway suggest that its downstream portion evolved from the a/α pathway. C. albicans, therefore, forms two superficially similar biofilms, exhibiting very different permeability characteristics, regulated by different signal transduction pathways, dictated by different mating type locus configurations, and serving quite different purposes in its life history.
PMCID: PMC3149048  PMID: 21829325
6.  The NDR/LATS Kinase Cbk1 Controls the Activity of the Transcriptional Regulator Bcr1 during Biofilm Formation in Candida albicans 
PLoS Pathogens  2012;8(5):e1002683.
In nature, many microorganisms form specialized complex, multicellular, surface-attached communities called biofilms. These communities play critical roles in microbial pathogenesis. The fungal pathogen Candida albicans is associated with catheter-based infections due to its ability to establish biofilms. The transcription factor Bcr1 is a master regulator of C. albicans biofilm development, although the full extent of its regulation remains unknown. Here, we report that Bcr1 is a phosphoprotein that physically interacts with the NDR kinase Cbk1 and undergoes Cbk1-dependent phosphorylation. Mutating the two putative Cbk1 phosphoacceptor residues in Bcr1 to alanine markedly impaired Bcr1 function during biofilm formation and virulence in a mouse model of disseminated candidiasis. Cells lacking Cbk1, or any of its upstream activators, also had reduced biofilm development. Notably, mutating the two putative Cbk1 phosphoacceptor residues in Bcr1 to glutamate in cbk1Δ cells upregulated the transcription of Bcr1-dependent genes and partially rescued the biofilm defects of a cbk1Δ strain. Therefore, our data uncovered a novel role of the NDR/LATS kinase Cbk1 in the regulation of biofilm development through the control of Bcr1.
Author Summary
C. albicans infections frequently involve the formation of biofilms on implanted devices such as indwelling catheters. These complex communities of surface-associated fungal cells embedded in a matrix of extracellular polysaccharides protect C. albicans from host defences and antifungal agents. In recent years, several genes involved in the development of biofilms of C. albicans have been identified. These studies have uncovered complex regulatory networks that control the activity of several transcription factors during different steps of biofilm development. Bcr1 is a transcription factor that plays a major role in this process and yet, its regulation has not been studied extensively. Here, we show that Bcr1 function in biofilm formation and virulence requires phosphorylation of threonine 191 and serine 556 by the NDR/LATS kinase Cbk1. Moreover, given that Cbk1 is also required for the onset and maintenance of hyphal growth, our study highlights this kinase as a pivotal regulator of several developmental programs that are essential for the biology and pathogenesis of C. albicans.
PMCID: PMC3349750  PMID: 22589718
7.  Towards non-invasive monitoring of pathogen–host interactions during Candida albicans biofilm formation using in vivo bioluminescence 
Cellular Microbiology  2013;16(1):115-130.
Candida albicans is a major human fungal pathogen causing mucosal and deep tissue infections of which the majority is associated with biofilm formation on medical implants. Biofilms have a huge impact on public health, as fungal biofilms are highly resistant against most antimycotics. Animal models of biofilm formation are indispensable for improving our understanding of biofilm development inside the host, their antifungal resistance and their interaction with the host immune defence system. In currently used models, evaluation of biofilm development or the efficacy of antifungal treatment is limited to ex vivo analyses, requiring host sacrifice, which excludes longitudinal monitoring of dynamic processes during biofilm formation in the live host. In this study, we have demonstrated for the first time that non-invasive, dynamic imaging and quantification of in vitro and in vivo C. albicans biofilm formation including morphogenesis from the yeast to hyphae state is feasible by using growth-phase dependent bioluminescent C. albicans strains in a subcutaneous catheter model in rodents. We have shown the defect in biofilm formation of a bioluminescent bcr1 mutant strain. This approach has immediate applications for the screening and validation ofantimycotics under in vivo conditions, for studying host–biofilm interactions in different transgenic mouse models and for testing the virulence of luminescent C. albicans mutants, hereby contributing to a better understanding of the pathogenesis of biofilm-associated yeast infections.
PMCID: PMC4204156  PMID: 23962311
8.  Comparative Phenotypic Analysis of the Major Fungal Pathogens Candida parapsilosis and Candida albicans 
PLoS Pathogens  2014;10(9):e1004365.
Candida parapsilosis and Candida albicans are human fungal pathogens that belong to the CTG clade in the Saccharomycotina. In contrast to C. albicans, relatively little is known about the virulence properties of C. parapsilosis, a pathogen particularly associated with infections of premature neonates. We describe here the construction of C. parapsilosis strains carrying double allele deletions of 100 transcription factors, protein kinases and species-specific genes. Two independent deletions were constructed for each target gene. Growth in >40 conditions was tested, including carbon source, temperature, and the presence of antifungal drugs. The phenotypes were compared to C. albicans strains with deletions of orthologous transcription factors. We found that many phenotypes are shared between the two species, such as the role of Upc2 as a regulator of azole resistance, and of CAP1 in the oxidative stress response. Others are unique to one species. For example, Cph2 plays a role in the hypoxic response in C. parapsilosis but not in C. albicans. We found extensive divergence between the biofilm regulators of the two species. We identified seven transcription factors and one protein kinase that are required for biofilm development in C. parapsilosis. Only three (Efg1, Bcr1 and Ace2) have similar effects on C. albicans biofilms, whereas Cph2, Czf1, Gzf3 and Ume6 have major roles in C. parapsilosis only. Two transcription factors (Brg1 and Tec1) with well-characterized roles in biofilm formation in C. albicans do not have the same function in C. parapsilosis. We also compared the transcription profile of C. parapsilosis and C. albicans biofilms. Our analysis suggests the processes shared between the two species are predominantly metabolic, and that Cph2 and Bcr1 are major biofilm regulators in C. parapsilosis.
Author Summary
Candida species are among the most common causes of fungal infection worldwide. Infections can be both community-based and hospital-acquired, and are particularly associated with immunocompromised individuals. Candida albicans is the most commonly isolated species and is the best studied. However, other species are becoming of increasing concern. Candida parapsilosis causes outbreaks of infection in neonatal wards, and is one of the few Candida species that is transferred from the hands of healthcare workers. C. parapsilosis, like C. albicans, grows as biofilms (cell communities) on the surfaces of indwelling medical devices like feeding tubes. We describe here the construction of a set of tools that allow us to characterize the virulence properties of C. parapsilosis, and in particular its ability to grow as biofilms. We find that some of the regulatory mechanisms are shared with C. albicans, but others are unique to each species. Our tools, based on selectively deleting regulatory genes, will provide a major resource to the fungal research community.
PMCID: PMC4169492  PMID: 25233198
9.  Role of Bcr1-Activated Genes Hwp1 and Hyr1 in Candida Albicans Oral Mucosal Biofilms and Neutrophil Evasion 
PLoS ONE  2011;6(1):e16218.
Candida albicans triggers recurrent infections of the oropharyngeal mucosa that result from biofilm growth. Prior studies have indicated that the transcription factor Bcr1 regulates biofilm formation in a catheter model, both in vitro and in vivo. We thus hypothesized that Bcr1 plays similar roles in the formation of oral mucosal biofilms and tested this hypothesis in a mouse model of oral infection. We found that a bcr1/bcr1 mutant did not form significant biofilm on the tongues of immunocompromised mice, in contrast to reference and reconstituted strains that formed pseudomembranes covering most of the tongue dorsal surface. Overexpression of HWP1, which specifies an epithelial adhesin that is under the transcriptional control of Bcr1, partly but significantly rescued the bcr1/bcr1 biofilm phenotype in vivo. Since HWP1 overexpression only partly reversed the biofilm phenotype, we investigated whether additional mechanisms, besides adhesin down-regulation, were responsible for the reduced virulence of this mutant. We discovered that the bcr1/bcr1 mutant was more susceptible to damage by human leukocytes when grown on plastic or on the surface of a human oral mucosa tissue analogue. Overexpression of HYR1, but not HWP1, significantly rescued this phenotype. Furthermore a hyr1/hyr1 mutant had significantly attenuated virulence in the mouse oral biofilm model of infection. These discoveries show that Bcr1 is critical for mucosal biofilm infection via regulation of epithelial cell adhesin and neutrophil function.
PMCID: PMC3026825  PMID: 21283544
10.  Expression of UME6, a Key Regulator of Candida albicans Hyphal Development, Enhances Biofilm Formation via Hgc1- and Sun41-Dependent Mechanisms 
Eukaryotic Cell  2013;12(2):224-232.
Biofilm formation is associated with the ability of Candida albicans, the major human fungal pathogen, to resist antifungal therapies and grow on tissues, catheters, and medical devices. In order to better understand the relationship between C. albicans morphology and biofilm formation, we examined biofilms generated in response to expression of UME6, a key filament-specific transcriptional regulator. As UME6 levels rise, C. albicans cells are known to transition from yeast to hyphae, and we also observed a corresponding increase in the level of biofilm formation in vitro. In addition to forming a biofilm, we observed that a C. albicans strain expressing constitutive high levels of UME6 promoted tissue invasion in a reconstituted human three-dimensional model of oropharyngeal candidiasis. Confocal microscopy indicated that both the top and bottom layers of the biofilm generated upon high-level constitutive UME6 expression consist primarily of hyphal cells. UME6-driven biofilm formation was reduced upon deletion of Hgc1, a cyclin-related protein important for hyphal development, as well as Sun41, a putative cell wall glycosidase. Constitutive high-level UME6 expression was also able to completely bypass both the filamentation and biofilm defects of a strain deleted for Efg1, a key transcriptional regulator of these processes. Finally, we show that both Sun41 and Efg1 affect the ability of UME6 to induce certain filament-specific transcripts. Overall, these findings indicate a strong correlation between increased C. albicans hyphal growth and enhanced biofilm formation and also suggest functional relationships between UME6 and other regulators of biofilm development.
PMCID: PMC3571304  PMID: 23223035
11.  Candida albicans Als3p is required for wild-type biofilm formation on silicone elastomer surfaces 
Microbiology (Reading, England)  2006;152(Pt 8):2287-2299.
Candida albicans ALS3 encodes a large cell-surface glycoprotein that has adhesive properties. Immunostaining of cultured C. albicans germ tubes showed that Als3p is distributed diffusely across the germ tube surface. Two-photon laser scanning microscopy of model catheter biofilms grown using a PALS3-green fluorescent protein (GFP) reporter strain showed GFP production in hyphae throughout the biofilm structure while biofilms grown using a PTPI1-GFP reporter strain showed GFP in both hyphae and yeast-form cells. Model catheter biofilms formed by an als3Δ/als3Δ strain were weakened structurally and had approximately half the biomass of a wild-type biofilm. Reintegration of a wild-type ALS3 allele restored biofilm mass and wild-type biofilm structure. Production of an Als3p-Agα1p fusion protein under control of the ALS3 promoter in the als3Δ/als3Δ strain restored some of the wild-type biofilm structural features, but not the wild-type biofilm mass. Despite its inability to restore wild-type biofilm mass, the Als3p-Agα1p fusion protein mediated adhesion of the als3Δ/als3Δ C. albicans strain to human buccal epithelial cells (BECs). The adhesive role of the Als3p N-terminal domain was further demonstrated by blocking adhesion of C. albicans to BECs with immunoglobulin reactive against the Als3p N-terminal sequences. Together, these data suggest that portions of Als3p that are important for biofilm formation may be different from those that are important in BEC adhesion, and that Als3p may have multiple functions in biofilm formation. Overexpression of ALS3 in an efg1Δ/efg1Δ strain that was deficient for filamentous growth and biofilm formation resulted in growth of elongated C. albicans cells, even under culture conditions that do not favour filamentation. In the catheter biofilm model, the ALS3 overexpression strain formed biofilm with a mass similar to that of a wild-type control. However, C. albicans cells in the biofilm had yeast-like morphology. This result uncouples the effect of cellular morphology from biofilm formation and underscores the importance of Als3p in biofilm development on silicone elastomer surfaces.
PMCID: PMC2583121  PMID: 16849795
12.  Regulatory Role of Glycerol in Candida albicans Biofilm Formation 
mBio  2013;4(2):e00637-12.
Biofilm formation by Candida albicans on medically implanted devices poses a significant clinical challenge. Here, we compared biofilm-associated gene expression in two clinical C. albicans isolates, SC5314 and WO-1, to identify shared gene regulatory responses that may be functionally relevant. Among the 62 genes most highly expressed in biofilms relative to planktonic (suspension-grown) cells, we were able to recover insertion mutations in 25 genes. Twenty mutants had altered biofilm-related properties, including cell substrate adherence, cell-cell signaling, and azole susceptibility. We focused on one of the most highly upregulated genes in our biofilm proles, RHR2, which specifies the glycerol biosynthetic enzyme glycerol-3-phosphatase. Glycerol is 5-fold-more abundant in biofilm cells than in planktonic cells, and an rhr2Δ/Δ strain accumulates 2-fold-less biofilm glycerol than does the wild type. Under in vitro conditions, the rhr2Δ/Δ mutant has reduced biofilm biomass and reduced adherence to silicone. The rhr2Δ/Δ mutant is also severely defective in biofilm formation in vivo in a rat catheter infection model. Expression profiling indicates that the rhr2Δ/Δ mutant has reduced expression of cell surface adhesin genes ALS1, ALS3, and HWP1, as well as many other biofilm-upregulated genes. Reduced adhesin expression may be the cause of the rhr2Δ/Δ mutant biofilm defect, because overexpression of ALS1, ALS3, or HWP1 restores biofilm formation ability to the mutant in vitro and in vivo. Our findings indicate that internal glycerol has a regulatory role in biofilm gene expression and that adhesin genes are among the main functional Rhr2-regulated genes.
Candida albicans is a major fungal pathogen, and infection can arise from the therapeutically intractable biofilms that it forms on medically implanted devices. It stands to reason that genes whose expression is induced during biofilm growth will function in the process, and our analysis of 25 such genes confirms that expectation. One gene is involved in synthesis of glycerol, a small metabolite that we find is abundant in biofilm cells. The impact of glycerol on biofilm formation is regulatory, not solely metabolic, because it is required for expression of numerous biofilm-associated genes. Restoration of expression of three of these genes that specify cell surface adhesins enables the glycerol-synthetic mutant to create a biofilm. Our findings emphasize the significance of metabolic pathways as therapeutic targets, because their disruption can have both physiological and regulatory consequences.
PMCID: PMC3622937  PMID: 23572557
13.  Alcohol Dehydrogenase Restricts the Ability of the Pathogen Candida albicans To Form a Biofilm on Catheter Surfaces through an Ethanol-Based Mechanism‡  
Infection and Immunity  2006;74(7):3804-3816.
Candida biofilms formed on indwelling medical devices are increasingly associated with severe infections. In this study, we used proteomics and Western and Northern blotting analyses to demonstrate that alcohol dehydrogenase (ADH) is downregulated in Candida biofilms. Disruption of ADH1 significantly (P = 0.0046) enhanced the ability of Candida albicans to form biofilm. Confocal scanning laser microscopy showed that the adh1 mutant formed thicker biofilm than the parent strain (210 μm and 140 μm, respectively). These observations were extended to an engineered human oral mucosa and an in vivo rat model of catheter-associated biofilm. Inhibition of Candida ADH enzyme using disulfiram and 4-methylpyrazole resulted in thicker biofilm (P < 0.05). Moreover, biofilms formed by the adh1 mutant strain produced significantly smaller amounts of ethanol, but larger amounts of acetaldehyde, than biofilms formed by the parent and revertant strains (P < 0.0001), demonstrating that the effect of Adh1p on biofilm formation is mediated by its enzymatic activity. Furthermore, we found that 10% ethanol significantly inhibited biofilm formation in vitro, with complete inhibition of biofilm formation at ethanol concentrations of ≥20%. Similarly, using a clinically relevant rabbit model of catheter-associated biofilm, we found that ethanol treatment inhibited biofilm formation by C. albicans in vivo (P < 0.05) but not by Staphylococcus spp. (P > 0.05), indicating that ethanol specifically inhibits Candida biofilm formation. Taken together, our studies revealed that Adh1p contributes to the ability of C. albicans to form biofilms in vitro and in vivo and that the protein restricts biofilm formation through an ethanol-dependent mechanism. These results are clinically relevant and may suggest novel antibiofilm treatment strategies.
PMCID: PMC1489753  PMID: 16790752
14.  Characterization of Biofilm Formation and the Role of BCR1 in Clinical Isolates of Candida parapsilosis 
Eukaryotic Cell  2014;13(4):438-451.
In Candida parapsilosis, biofilm formation is considered to be a major virulence factor. Previously, we determined the ability of 33 clinical isolates causing bloodstream infection to form biofilms and identified three distinct groups of biofilm-forming strains (negative, low, and high). Here, we establish two different biofilm structures among strains forming large amounts of biofilm in which strains with complex spider-like structures formed robust biofilms on different surface materials with increased resistance to fluconazole. Surprisingly, the transcription factor Bcr1, required for biofilm formation in Candida albicans and C. parapsilosis, has an essential role only in strains with low capacity for biofilm formation. Although BCR1 leads to the formation of more and longer pseudohyphae, it was not required for initial adhesion and formation of mature biofilms in strains with a high level of biofilm formation. Furthermore, an additional phenotype affected by BCR1 was the switch in colony morphology from rough to crepe, but only in strains forming high levels of biofilm. All bcr1Δ/Δ mutants showed increased proteolytic activity and increased susceptibility to the antimicrobial peptides protamine and RP-1 compared to corresponding wild-type and complemented strains. Taken together, our results demonstrate that biofilm formation in clinical isolates of C. parapsilosis is both dependent and independent of BCR1, but even in strains which showed a BCR1-independent biofilm phenotype, BCR1 has alternative physiological functions.
PMCID: PMC4000102  PMID: 24297446
15.  Arginine-Induced Germ Tube Formation in Candida albicans Is Essential for Escape from Murine Macrophage Line RAW 264.7▿ † 
Infection and Immunity  2009;77(4):1596-1605.
The opportunistic fungal pathogen Candida albicans is a part of the normal flora but it also causes systemic candidiasis if it reaches the bloodstream. Upon being phagocytized by macrophages, an important component of innate immunity, C. albicans rapidly upregulates a set of arginine biosynthetic genes. Arginine, urea, and CO2 induced hyphae in a density-dependent manner in wild-type, cph1/cph1, and rim101/rim101 strains but not in efg1/efg1 or cph1/cph1 efg1/efg1 strains. Arginase (Car1p) converts arginine to urea, which in turn is degraded by urea amidolyase (Dur1,2p) to produce CO2, a signal for hyphal switching. We used a dur1,2/dur1,2 mutant (KWN6) and the complemented strain, KWN8 (dur1,2/dur1,2::DUR1,2/DUR1,2) to study germ tube formation. KWN6 could not make germ tubes in the presence of arginine or urea but did in the presence of 5% CO2, which bypasses Dur1,2p. We also tested the effect of arginine on the interaction between the macrophage line RAW 264.7 and several strains of C. albicans. Arginine activated an Efg1p-dependent yeast-to-hypha switch, enabling wild-type C. albicans and KWN8 to escape from macrophages within 6 h, whereas KWN6 was defective in this regard. Additionally, two mutants that cannot synthesize arginine, BWP17 and SN152, were defective in making hyphae inside the macrophages, whereas the corresponding arginine prototrophs, DAY286 and SN87, formed germ tubes and escaped from macrophages. Therefore, metabolism of arginine by C. albicans controls hyphal switching and provides an important mechanism for escaping host defense.
PMCID: PMC2663133  PMID: 19188358
16.  Production of Tyrosol by Candida albicans Biofilms and Its Role in Quorum Sensing and Biofilm Development▿  
Eukaryotic Cell  2006;5(10):1770-1779.
Tyrosol and farnesol are quorum-sensing molecules produced by Candida albicans which accelerate and block, respectively, the morphological transition from yeasts to hyphae. In this study, we have investigated the secretion of tyrosol by C. albicans and explored its likely role in biofilm development. Both planktonic (suspended) cells and biofilms of four C. albicans strains, including three mutants with defined defects in the Efg 1 and Cph 1 morphogenetic signaling pathways, synthesized extracellular tyrosol during growth at 37°C. There was a correlation between tyrosol production and biomass for both cell types. However, biofilm cells secreted at least 50% more tyrosol than did planktonic cells when tyrosol production was related to cell dry weight. The addition of exogenous farnesol to a wild-type strain inhibited biofilm formation by up to 33% after 48 h. Exogenous tyrosol appeared to have no effect, but scanning electron microscopy revealed that tyrosol stimulated hypha production during the early stages (1 to 6 h) of biofilm development. Experiments involving the simultaneous addition of tyrosol and farnesol at different concentrations suggested that the action of farnesol was dominant, and 48-h biofilms formed in the presence of both compounds consisted almost entirely of yeast cells. When biofilm supernatants were tested for their abilities to inhibit or enhance germ tube formation by planktonic cells, the results indicated that tyrosol activity exceeds that of farnesol after 14 h, but not after 24 h, and that farnesol activity increases significantly during the later stages (48 to 72 h) of biofilm development. Overall, our results support the conclusion that tyrosol acts as a quorum-sensing molecule for biofilms as well as for planktonic cells and that its action is most significant during the early and intermediate stages of biofilm formation.
PMCID: PMC1595342  PMID: 16980403
17.  Hsp90 Governs Dispersion and Drug Resistance of Fungal Biofilms 
PLoS Pathogens  2011;7(9):e1002257.
Fungal biofilms are a major cause of human mortality and are recalcitrant to most treatments due to intrinsic drug resistance. These complex communities of multiple cell types form on indwelling medical devices and their eradication often requires surgical removal of infected devices. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. We previously established that in the leading human fungal pathogen, Candida albicans, Hsp90 enables the emergence and maintenance of drug resistance in planktonic conditions by stabilizing the protein phosphatase calcineurin and MAPK Mkc1. Hsp90 also regulates temperature-dependent C. albicans morphogenesis through repression of cAMP-PKA signalling. Here we demonstrate that genetic depletion of Hsp90 reduced C. albicans biofilm growth and maturation in vitro and impaired dispersal of biofilm cells. Further, compromising Hsp90 function in vitro abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal drugs, the azoles. Depletion of Hsp90 led to reduction of calcineurin and Mkc1 in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 levels led to a marked decrease in matrix glucan levels, providing a compelling mechanism through which Hsp90 might regulate biofilm azole resistance. Impairment of Hsp90 function genetically or pharmacologically transformed fluconazole from ineffectual to highly effective in eradicating biofilms in a rat venous catheter infection model. Finally, inhibition of Hsp90 reduced resistance of biofilms of the most lethal mould, Aspergillus fumigatus, to the newest class of antifungals to reach the clinic, the echinocandins. Thus, we establish a novel mechanism regulating biofilm drug resistance and dispersion and that targeting Hsp90 provides a much-needed strategy for improving clinical outcome in the treatment of biofilm infections.
Author Summary
Candida albicans and Aspergillus fumigatus are the most common causative agents of fungal infections worldwide. Both species can form biofilms on host tissues and indwelling medical devices that are highly resistant to antifungal treatment. Here we implicate the molecular chaperone Hsp90 as a key regulator of biofilm dispersion and drug resistance. Compromising Hsp90 function reduced biofilm formation of C. albicans in vitro and impaired dispersal of biofilm cells, potentially blocking their capacity to serve as reservoirs for infection. Further, compromise of Hsp90 function abrogated resistance of C. albicans biofilms to the most widely deployed class of antifungal, the azoles, both in vitro and in a mammalian model of catheter-associated candidiasis. Key drug resistance regulators were depleted upon reduction of Hsp90 levels in planktonic but not biofilm conditions, suggesting that Hsp90 regulates drug resistance through different mechanisms in these distinct cellular states. Reduction of Hsp90 markedly reduced levels of matrix glucan, a carbohydrate important for C. albicans biofilm drug resistance. Inhibition of Hsp90 also reduced resistance of A. fumigatus biofilms to the newest class of antifungal, the echinocandins. Thus, targeting Hsp90 provides a promising strategy for the treatment of biofilm infections caused by diverse fungal species.
PMCID: PMC3169563  PMID: 21931556
18.  The APSES transcription factor Efg1 is a global regulator that controls morphogenesis and biofilm formation in Candida parapsilosis 
Molecular Microbiology  2013;90(1):36-53.
Efg1 (a member of the APSES family) is an important regulator of hyphal growth and of the white-to-opaque transition in Candida albicans and very closely related species. We show that in Candida parapsilosis Efg1 is a major regulator of a different morphological switch at the colony level, from a concentric to smooth morphology. The rate of switching is at least 20-fold increased in an efg1 knockout relative to wild type. Efg1 deletion strains also have reduced biofilm formation, attenuated virulence in an insect model, and increased sensitivity to SDS and caspofungin. Biofilm reduction is more dramatic in in vitro than in in vivo models. An Efg1 paralogue (Efh1) is restricted to Candida species, and does not regulate concentric-smooth phenotype switching, biofilm formation or stress response. We used ChIP-seq to identify the Efg1 regulon. A total of 931 promoter regions bound by Efg1 are highly enriched for transcription factors and regulatory proteins. Efg1 also binds to its own promoter, and negatively regulates its expression. Efg1 targets are enriched in binding sites for 93 additional transcription factors, including Ndt80. Our analysis suggests that Efg1 has an ancient role as regulator of development in fungi, and is central to several regulatory networks.
PMCID: PMC3912905  PMID: 23895281
19.  Candida albicans Hyphal Formation and Virulence Assessed Using a Caenorhabditis elegans Infection Model ▿  
Eukaryotic Cell  2009;8(11):1750-1758.
Candida albicans colonizes the human gastrointestinal tract and can cause life-threatening systemic infection in susceptible hosts. We study here C. albicans virulence determinants using the nematode Caenorhabditis elegans in a pathogenesis system that models candidiasis. The yeast form of C. albicans is ingested into the C. elegans digestive tract. In liquid media, the yeast cells then undergo morphological change to form hyphae, which results in aggressive tissue destruction and death of the nematode. Several lines of evidence demonstrate that hyphal formation is critical for C. albicans pathogenesis in C. elegans. First, two yeast species unable to form hyphae (Debaryomyces hansenii and Candida lusitaniae) were less virulent than C. albicans in the C. elegans assay. Second, three C. albicans mutant strains compromised in their ability to form hyphae (efg1Δ/efg1Δ, flo8Δ/flo8Δ, and cph1Δ/cph1Δ efg1Δ/efg1Δ) were dramatically attenuated for virulence. Third, the conditional tet-NRG1 strain, which enables the external manipulation of morphogenesis in vivo, was more virulent toward C. elegans when the assay was conducted under conditions that permit hyphal growth. Finally, we demonstrate the utility of the C. elegans assay in a screen for C. albicans virulence determinants, which identified several genes important for both hyphal formation in vivo and the killing of C. elegans, including the recently described CAS5 and ADA2 genes. These studies in a C. elegans-C. albicans infection model provide insights into the virulence mechanisms of an important human pathogen.
PMCID: PMC2772404  PMID: 19666778
20.  Targeted Changes of the Cell Wall Proteome Influence Candida albicans Ability to Form Single- and Multi-strain Biofilms 
PLoS Pathogens  2014;10(12):e1004542.
Biofilm formation is an important virulence trait of the pathogenic yeast Candida albicans. We have combined gene overexpression, strain barcoding and microarray profiling to screen a library of 531 C. albicans conditional overexpression strains (∼10% of the genome) for genes affecting biofilm development in mixed-population experiments. The overexpression of 16 genes increased strain occupancy within a multi-strain biofilm, whereas overexpression of 4 genes decreased it. The set of 16 genes was significantly enriched for those encoding predicted glycosylphosphatidylinositol (GPI)-modified proteins, namely Ihd1/Pga36, Phr2, Pga15, Pga19, Pga22, Pga32, Pga37, Pga42 and Pga59; eight of which have been classified as pathogen-specific. Validation experiments using either individually- or competitively-grown overexpression strains revealed that the contribution of these genes to biofilm formation was variable and stage-specific. Deeper functional analysis of PGA59 and PGA22 at a single-cell resolution using atomic force microscopy showed that overexpression of either gene increased C. albicans ability to adhere to an abiotic substrate. However, unlike PGA59, PGA22 overexpression led to cell cluster formation that resulted in increased sensitivity to shear forces and decreased ability to form a single-strain biofilm. Within the multi-strain environment provided by the PGA22-non overexpressing cells, PGA22-overexpressing cells were protected from shear forces and fitter for biofilm development. Ultrastructural analysis, genome-wide transcript profiling and phenotypic analyses in a heterologous context suggested that PGA22 affects cell adherence through alteration of cell wall structure and/or function. Taken together, our findings reveal that several novel predicted GPI-modified proteins contribute to the cooperative behaviour between biofilm cells and are important participants during C. albicans biofilm formation. Moreover, they illustrate the power of using signature tagging in conjunction with gene overexpression for the identification of novel genes involved in processes pertaining to C. albicans virulence.
Author Summary
Candida albicans is the most prevalent human fungal pathogen. Its ability to cause disease relies, in part, on the formation of biofilms, a protective structure of highly adherent cells tolerant to antifungal agents and the host immune response. The biofilm is considered as a persistent root of infection, disseminating infectious cells to other locations. In this study, we performed large-scale phenotypic analyses aimed at identifying genes whose overexpression affects biofilm development in C. albicans. Our screen relied on a collection of 531 C. albicans strains, each conditionally overexpressing one given gene and carrying one specific molecular tag allowing the quantification of strain abundance in mixed-population experiments. Our results strikingly revealed the enrichment of strains overproducing poorly-characterized surface proteins called Pgas (Putative GPI-Anchored proteins), within a 531-strain-containing biofilm model. We show that these PGA genes differentially contribute to single-strain and multi-strain biofilm formation and are involved in specific stages of the biofilm developmental process. Taken together, our results reveal the importance of C. albicans cell surface proteins during biofilm formation and reflect the powerful use of strain barcoding in combination with gene overexpression to identify genes and/or pathways involved in processes pertaining to virulence of pathogenic microbes.
PMCID: PMC4263760  PMID: 25502890
21.  Divergent Targets of Candida albicans Biofilm Regulator Bcr1 In Vitro and In Vivo 
Eukaryotic Cell  2012;11(7):896-904.
Candida albicans is a causative agent of oropharyngeal candidiasis (OPC), a biofilm-like infection of the oral mucosa. Biofilm formation depends upon the C. albicans transcription factor Bcr1, and previous studies indicate that Bcr1 is required for OPC in a mouse model of infection. Here we have used a nanoString gene expression measurement platform to elucidate the role of Bcr1 in OPC-related gene expression. We chose for assays a panel of 134 genes that represent a range of morphogenetic and cell cycle functions as well as environmental and stress response pathways. We assayed gene expression in whole infected tongue samples. The results sketch a portrait of C. albicans gene expression in which numerous stress response pathways are activated during OPC. This one set of experiments identifies 64 new genes with significantly altered RNA levels during OPC, thus increasing substantially the number of known genes in this expression class. The bcr1Δ/Δ mutant had a much more limited gene expression defect during OPC infection than previously reported for in vitro growth conditions. Among major functional Bcr1 targets, we observed that ALS3 was Bcr1 dependent in vivo while HWP1 was not. We used null mutants and complemented strains to verify that Bcr1 and Hwp1 are required for OPC infection in this model. The role of Als3 is transient and mild, though significant. Our findings suggest that the versatility of C. albicans as a pathogen may reflect its ability to persist in the face of multiple stresses and underscore that transcriptional circuitry during infection may be distinct from that detailed during in vitro growth.
PMCID: PMC3416506  PMID: 22544909
22.  Dispersion as an Important Step in the Candida albicans Biofilm Developmental Cycle 
PLoS Pathogens  2010;6(3):e1000828.
Biofilms are dynamic microbial communities in which transitions between planktonic and sessile modes of growth occur interchangeably in response to different environmental cues. In the last decade, early events associated with C. albicans biofilm formation have received considerable attention. However, very little is known about C. albicans biofilm dispersion or the mechanisms and signals that trigger it. This is important because it is precisely C. albicans cells dispersed from biofilms that are the main culprits associated with candidemia and establishment of disseminated invasive disease, two of the gravest forms of candidiasis. Using a simple flow biofilm model recently developed by our group, we have performed initial investigations into the phenomenon of C. albicans biofilm dispersion, as well as the phenotypic characteristics associated with dispersed cells. Our results indicate that C. albicans biofilm dispersion is dependent on growing conditions, including carbon source and pH of the media used for biofilm development. C. albicans dispersed cells are mostly in the yeast form and display distinct phenotypic properties compared to their planktonic counterparts, including enhanced adherence, filamentation, biofilm formation and, perhaps most importantly, increased pathogenicity in a murine model of hematogenously disseminated candidiasis, thus indicating that dispersed cells are armed with a complete arsenal of “virulence factors” important for seeding and establishing new foci of infection. In addition, utilizing genetically engineered strains of C. albicans (tetO-UME6 and tetO-PES1) we demonstrate that C. albicans biofilm dispersion can be regulated by manipulating levels of expression of these key genes, further supporting the evidence for a strong link between biofilms and morphogenetic conversions at different stages of the C. albicans biofilm developmental cycle. Overall, our results offer novel and important insight into the phenomenon of C. albicans biofilm dispersion, a key part of the biofilm developmental cycle, and provide the basis for its more detailed analysis.
Author Summary
Candida albicans is the main causative agent of candidiasis, a difficult-to-treat infection that occurs mostly in severely immunosuppressed and other at-risk patients. Candidiasis is often associated with the formation of biofilms (attached microbial communities encapsulated within a protective matrix) on host surfaces and/or implantable medical devices, most notably intravascular catheters. In recent years, for C. albicans, the process of biofilm formation has received much attention. However, the same is not true for biofilm dispersion (the release of cells from the biofilm). This is important since these dispersed cells are responsible for the subsequent establishment of disseminated candidiasis at distal organs. Here we have taken advantage of a model of biofilm formation under conditions of flow recently described by our group to study and characterize the phenomenon of C. albicans biofilm dispersion. Rather than an end-stage process, our results indicate that dispersion occurs at all different stages of the biofilm developmental cycle and is influenced by nutritional and other physiochemical conditions. In addition, our findings provide initial insights into how this process is regulated at the molecular level. We also demonstrate that dispersed cells display distinct phenotypic properties that are associated with increased virulence, with important clinical repercussions.
PMCID: PMC2847914  PMID: 20360962
23.  Functional Mapping of the Candida albicans Efg1 Regulator▿ † 
Eukaryotic Cell  2008;7(5):881-893.
Efg1p is a key transcriptional regulator in Candida albicans which controls various aspects of morphogenesis and metabolism in this organism. Efg1p contains a central basic helix-loop-helix (bHLH) domain, flanked by sequences highly conserved in fungal APSES proteins, as well as polyglutamine stretches at the N- and C-terminal ends. A systematic deletion approach to specify functional domains of Efg1p revealed that the APSES domain is essential for morphogenesis of the normal yeast and true hyphal cell forms and that bHLH flanking sequences are needed for Efg1p stability. Additional C-terminal sequences were required for hypha formation on some inducing media, and most Efg1p sequences were needed for chlamydospore morphogenesis. Overexpression of EFG1 led to pseudohypha formation only if a functional APSES domain was present, while a switch from the opaque to the white cell type in addition depended on the presence of certain N- and C-terminal segments. Yeast two-hybrid experiments revealed that binding of Efg1p to its antagonist Czf1p required two regions outside of the APSES domain, which did not coincide with Efg1p sequences needed for its transcriptional repressor activity. Binding of the Flo8 transcription factor to Efg1p did not require the APSES domain but appeared to occur at two or more redundant domains. In contrast, DNA binding of Efg1p to an MluI cell cycle box (MCB) element solely required the APSES domain. Overall, these results suggest that functional domains of Efg1p are spread throughout most of its sequences, including the central APSES domain involved in DNA binding, as well as flanking regions required for various protein interactions and regulatory activities.
PMCID: PMC2394972  PMID: 18375615
24.  Function of Candida albicans Adhesin Hwp1 in Biofilm Formation 
Eukaryotic Cell  2006;5(10):1604-1610.
Hwp1 is a well-characterized Candida albicans cell surface protein, expressed only on hyphae, that mediates tight binding to oral epithelial cells. Prior studies indicate that HWP1 expression is dependent upon Bcr1, a key regulator of biofilm formation. Here we test the hypothesis that Hwp1 is required for biofilm formation. In an in vitro model, the hwp1/hwp1 mutant produces a thin biofilm that lacks much of the hyphal mass found in the hwp1/HWP1 reconstituted strain. In a biofilm cell retention assay, we find that the hwp1/hwp1 mutant is defective in retention of nonadherent bcr1/bcr1 mutant cells. In an in vivo rat venous catheter model, the hwp1/hwp1 mutant has a severe biofilm defect, yielding only yeast microcolonies in the catheter lumen. These properties of the hwp1/hwp1 mutant are consistent with its role as a hypha-specific adhesin and indicate that it is required for normal biofilm formation. Overexpression of HWP1 in a bcr1/bcr1 mutant background improves adherence in the in vivo catheter model. This finding provides additional support for the model that Hwp1 is critical for biofilm adhesion. Hwp1 is the first cell surface protein known to be required for C. albicans biofilm formation in vivo and is thus an excellent therapeutic target.
PMCID: PMC1595337  PMID: 17030992
25.  Candida albicans Biofilms Produce Antifungal-Tolerant Persister Cells▿  
Antimicrobial Agents and Chemotherapy  2006;50(11):3839-3846.
Fungal pathogens form biofilms that are highly recalcitrant to antimicrobial therapy. The expression of multidrug resistance pumps in young biofilms has been linked to increased resistance to azoles, but this mechanism does not seem to underlie the resistance of mature biofilms that is a model of in vivo infection. The mechanism of drug resistance of mature biofilms remains largely unknown. We report that biofilms formed by the major human pathogen Candida albicans exhibited a strikingly biphasic killing pattern in response to two microbicidal agents, amphotericin B, a polyene antifungal, and chlorhexidine, an antiseptic, indicating that a subpopulation of highly tolerant cells, termed persisters, existed. The extent of killing with a combination of amphotericin B and chlorhexidine was similar to that observed with individually added antimicrobials. Thus, surviving persisters form a multidrug-tolerant subpopulation. Interestingly, surviving C. albicans persisters were detected only in biofilms and not in exponentially growing or stationary-phase planktonic populations. Reinoculation of cells that survived killing of the biofilm by amphotericin B produced a new biofilm with a new subpopulation of persisters. This suggests that C. albicans persisters are not mutants but phenotypic variants of the wild type. Using a stain for dead cells, rare dark cells were visible in a biofilm after amphotericin B treatment, and a bright and a dim population were physically sorted from this biofilm. Only the dim cells produced colonies, showing that this method allows the isolation of yeast persisters. Given that persisters formed only in biofilms, mutants defective in biofilm formation were examined for tolerance of amphotericin B. All of the known mutants affected in biofilm formation were able to produce normal levels of persisters. This finding indicates that attachment rather than formation of a complex biofilm architecture initiates persister formation. Bacteria produce multidrug-tolerant persister cells in both planktonic and biofilm populations, and it appears that yeasts and bacteria have evolved analogous strategies that assign the function of survival to a small part of the population. In bacteria, persisters are dormant cells. It remains to be seen whether attachment initiates dormancy that leads to the formation of fungal persisters. This study suggests that persisters may be largely responsible for the multidrug tolerance of fungal biofilms.
PMCID: PMC1635216  PMID: 16923951

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