Iron deprivation inducesEFG1-mediated hyphal development inCandida albicanswithout affecting biofilm formation

The intricate link between iron, mitochondria and azoles in Candida species

Invasive fungal infections are rapidly increasing, and the opportunistic pathogenic Candida species are the fourth most common cause of nosocomial systemic infections. The current antifungal classes, of which azoles are the most widely used, all have shortcomings. Azoles are generally considered fungistatic rather than fungicidal, they do not actively kill fungal cells and therefore resistance against azoles can be rapidly acquired. Combination therapies with azoles provide an interesting therapeutic outlook and agents limiting iron are excellent candidates. We summarize how iron is acquired by the host and transported towards both storage and iron-utilizing organelles. We indicate whether these pathways alter azole susceptibility and/or tolerance, to finally link these transport mechanisms to mitochondrial iron availability. In this review, we highlight putative novel intracellular iron shuffling mechanisms and indicate that mitochondrial iron dynamics in relation to azole treatment and iron limitation is a significant knowledge gap.

Comparative transcriptional analysis of Candida auris biofilms following farnesol and tyrosol treatment

Candida auris is frequently associated with biofilm-related invasive infections. The resistant profile of these biofilms necessitates innovative therapeutic options, where quorum sensing may be a potential target. Farnesol and tyrosol are two fungal quorum-sensing molecules with antifungal effects at supraphysiological concentrations. Here, we performed genome-wide transcript profiling with C. auris biofilms following farnesol or tyrosol exposure using transcriptome sequencing (RNA-Seq). Since transition metals play a central role in fungal virulence and biofilm formation, levels of intracellular calcium, magnesium, and iron were determined following farnesol or tyrosol treatment using inductively coupled plasma optical emission spectrometry. Farnesol caused an 89.9% and 73.8% significant reduction in the calcium and magnesium content, respectively, whereas tyrosol resulted in 82.6%, 76.6%, and 81.2% decrease in the calcium, magnesium, and iron content, respectively, compared to the control. Genes involved in biofilm events, glycolysis, ergosterol biosynthesis, fatty acid oxidation, iron metabolism, and autophagy were primarily affected in treated cells. To prove ergosterol quorum-sensing molecule interactions, microdilution-based susceptibility testing was performed, where the complexation of farnesol, but not tyrosol, with ergosterol was impeded in the presence of exogenous ergosterol, resulting in a minimum inhibitory concentration increase in the quorum-sensing molecules. This study revealed several farnesol- and tyrosol-specific responses, which will contribute to the development of alternative therapies against C. auris biofilms.

IMPORTANCE

Candida auris is a multidrug-resistant fungal pathogen, which is frequently associated with biofilm-related infections. Candida-derived quorum-sensing molecules (farnesol and tyrosol) play a pivotal role in the regulation of fungal morphogenesis and biofilm development. Furthermore, they may have remarkable anti-biofilm effects, especially at supraphysiological concentrations. Innovative therapeutic approaches interfering with quorum sensing may be a promising future strategy against C. auris biofilms; however, limited data are currently available concerning farnesol-induced and tyrosol-related molecular effects in C. auris. Here, we detected several genes involved in biofilm events, glycolysis, ergosterol biosynthesis, fatty acid oxidation, iron metabolism, and autophagy, which were primarily influenced following farnesol or tyrosol exposure. Moreover, calcium, magnesium, and iron homeostasis were also significantly affected. These results reveal those molecular and physiological events, which may support the development of novel therapeutic approaches against C. auris biofilms.

Biocompatible carbon quantum dots as versatile imaging nanotrackers of fungal pathogen - Candida albicans

Aim: The development of carbon quantum dots (C-QDs) as nanotrackers to understand drug-pathogen interactions, virulence and multidrug resistance. Methods: Microwave synthesis of C-QDs was performed using citric acid and polyethylene glycol. Further, in vitro toxicity was evaluated and imaging applications were demonstrated in Candida albicans isolates. Results: Well-dispersed, ultra small C-QDs exhibited no cyto/microbial/reactive oxygen species-mediated toxicity and internalized effectively in Candida yeast and hyphal cells. C-QDs were employed for confocal imaging of drug-sensitive and -resistant cells, and a study of the yeast-to-hyphal transition using atomic force microscopy in Candida was conducted for the first time. Conclusion: These biocompatible C-QDs have promising potential as next-generation nanotrackers for in vitro and in vivo targeted cellular and live imaging, after functionalization with biomolecules and drugs.

Artemisinin May Disrupt Hyphae Formation by Suppressing Biofilm-Related Genes of Candida albicans: In Vitro and In Silico Approaches

This study aimed to assess the antifungal and antibiofilm efficacy of artemisinin against Candida (C.) species, analyze its impact on gene expression levels within C. albicans biofilms, and investigate the molecular interactions through molecular docking. The antifungal efficacy of artemisinin on a variety of Candida species, including fluconazole-resistant and -susceptible species, was evaluated by the microdilution method. The effect of artemisinin on C. albicans biofilm formation was investigated by MTT and FESEM. The mRNA expression of the genes related to biofilm was analyzed by qRT-PCR. In addition, molecular docking analysis was used to understand the interaction between artemisinin and C. albicans at the molecular level with RAS1-cAMP-EFG1 and EFG1-regulated genes. Artemisinin showed higher sensitivity against non-albicans Candida strains. Furthermore, artemisinin was strongly inhibitory against C. albicans biofilms at 640 µg/mL. Artemisinin downregulated adhesion-related genes ALS3, HWP1, and ECE1, hyphal development genes UME6 and HGC1, and hyphal CAMP-dependent protein kinase regulators CYR1, RAS1, and EFG1. Furthermore, molecular docking analysis revealed that artemisinin and EFG1 had the highest affinity, followed by UME6. FESEM analysis showed that the fluconazole- and artemisinin-treated groups exhibited a reduced hyphal network, unusual surface bulges, and the formation of pores on the cell surfaces. Our study suggests that artemisinin may have antifungal potential and showed a remarkable antibiofilm activity by significantly suppressing adhesion and hyphal development through interaction with key proteins involved in biofilm formation, such as EFG1.

Genetic Analysis of Candida albicans Filamentation by the Iron Chelator BPS Reveals a Role for a Conserved Kinase-WD40 Protein Pair

Candida albicans is a major human pathogenic fungus that is distinguished by its capability to switch from a yeast to a hyphal morphology under different conditions. Here, we analyze the cellular effects of high concentrations of the iron chelator bathophenanthroline disulfonate (BPS). BPS inhibits cellular growth by withholding iron, but when iron chelation is overcome by the addition of hemoglobin as an iron source, the cells resume growth as hyphae. The BPS hyphal induction pathway was characterized by identifying the hyphal-specific transcription factors that it requires and by a forward genetic screen for mutants that fail to form hyphae in BPS using a transposon library generated in a haploid strain. Among the mutants identified are the DYRK1-like kinase Yak1 and Orf19.384, a homolog of the DYRK1-associated protein WDR68/DCAF7. Orf19.384 nuclear localization depends on Yak1, similar to their mammalian counterparts. We identified the hyphal suppressor transcription factor Sfl1 as a candidate target of Yak1-Orf19.384 and show that Sfl1 modification is similarly affected in the yak1 and orf19.384 mutant strains. These results suggest that DYRK1/Yak1 and WDR68/Orf19.384 represent a conserved protein pair that regulates cell differentiation from fungi to animals.

Berberine inhibits Candida albicans growth by disrupting mitochondrial function through the reduction of iron absorption

AIMS

This study aimed to investigate whether berberine (BBR) can inhibit the iron reduction mechanism of Candida albicans, lowering the iron uptake of the yeast and perhaps having antimicrobial effects.

METHODS AND RESULTS

We determined that BBR may cause extensive transcriptional remodeling in C. albicans and that iron permease Ftr1 played a crucial role in this process through eukaryotic transcriptome sequencing. Mechanistic research showed that BBR might selectively inhibit the iron reduction pathway to lower the uptake of exogenous iron ions, inhibiting C. albicans from growing and metabolizing. Subsequent research revealed that BBR caused significant mitochondrial dysfunction, which triggered the process of mitochondrial autophagy. Moreover, we discovered that C. albicans redox homeostasis, susceptibility to antifungal drugs, and hyphal growth are all impacted by the suppression of this mechanism by BBR.

CONCLUSIONS

The iron reduction mechanism in C. albicans is disrupted by BBR, which disrupts mitochondrial function and inhibits fungal growth. These findings highlight the potential promise of BBR in antifungal applications.

The impact of iron and heme availability on the healthy human gut microbiome in vivo and in vitro

Responses of the indigenous human gut commensal microbiota to iron are poorly understood because of an emphasis on in vitro studies of pathogen iron sensitivity. In a study of iron supplementation in healthy humans, we identified gradual microbiota shifts in some participants correlated with bacterial iron internalization. To identify direct effects due to taxon-specific iron sensitivity, we used participant stool samples to derive diverse in vitro communities. Iron supplementation of these communities caused small compositional shifts, mimicking those in vivo, whereas iron deprivation dramatically inhibited growth with irreversible, cumulative reduction in diversity and replacement of dominant species. Sensitivity of individual species to iron deprivation in axenic culture generally predicted iron dependency in a community. Finally, exogenous heme acted as a source of inorganic iron to prevent depletion of some species. Our results highlight the complementarity of in vivo and in vitro studies in understanding how environmental factors affect gut microbiotas.

Functional Portrait of Irf1 (Orf19.217), a Regulator of Morphogenesis and Iron Homeostasis in Candida albicans

The alternate growth of Candida albicans between a unicellular yeast form and a multicellular hyphal form is crucial for its ability to cause disease. Interestingly, both morphological forms support distinct functions during proliferation in the human host. We previously identified ORF19.217 (C2_08890W_A), encoding a zinc-finger transcription factor of the C2H2 family, in a systematic screen of genes whose overexpression contributes to C. albicans' morphological changes. Conditional overexpression of ORF19.217 with the strong tetracycline-inducible promoter (P TET ) resulted in a hyperfilamentous phenotype. We examined growth of the orf19.217 knockout-mutant in different hypha-inducing conditions and found that the mutant still formed hyphae under standard hypha-inducing conditions. To further investigate the function of Orf19.217 in C. albicans, we combined genome-wide expression (RNA-Seq) and location (ChIP-Seq) analyses. We found that Orf19.217 is involved in regulatory processes comprising hyphal morphogenesis and iron acquisition. Comparative analysis with existing C. albicans hyphal transcriptomes indicates that Orf19.217-mediated filamentation is distinct from a true hyphal program. Further, the orf19.217 knockout-mutant did not show increased sensitivity to iron deprivation, but ORF19.217 overexpression was able to rescue the growth of a hap5-mutant, defective in a subunit of the CCAAT-complex, which is essential for iron acquisition. This suggested that Orf19.217 is involved in regulation of iron acquisition genes during iron deprivation and acts in a parallel pathway to the established CCAAT-complex. Interestingly, the orf19.217-mutant turned out to be defective in its ability to form filaments under iron-deficiency. Taken together our findings propose that the transcription factor Orf19.217 stimulates expression of the hyphal regulators EFG1 and BRG1 to promote filamentous growth under iron deprivation conditions, allowing the fungus to escape these iron-depleted conditions. The transcription factor therefore appears to be particularly important for adaptation of C. albicans to diverse environmental conditions in the human host. In regard to the newly identified functions, we have given the regulator the name Irf1, Iron-dependent Regulator of Filamentation.

Bypass of Dfi1 Regulation of Candida albicans Invasive Filamentation by Iron Limitation

Candida albicans filamentation, the ability to convert from oval yeast cells to elongated hyphal cells, is a key factor in its pathogenesis. Previous work has shown that the integral membrane protein Dfi1 is required for filamentation in cells grown in contact with a semisolid surface. Investigations into the downstream targets of the Dfi1 pathway revealed potential links to two transcription factors, Sef1 and Czf1. Sef1 regulates iron uptake and iron utilization genes under low-iron conditions, leading us to hypothesize that there exists a link between iron availability and contact-dependent invasive filamentation. In this study, we showed that Sef1 was not required for contact-dependent filamentation, but it was required for wild-type (WT) expression levels of a number of genes during growth under contact conditions. Czf1 is required for contact-dependent filamentation and for WT levels of expression of several genes. Constitutive expression and activation of either Sef1 or Czf1 individually in a dfi1 null strain resulted in a complete rescue of the dfi1 null filamentation defect. Because Sef1 is normally activated in low-iron environments, we embedded WT and dfi1 null cells in iron-free agar medium supplemented with various concentrations of ferrous ammonium sulfate (FAS). dfi1 null cells embedded in media with a low concentration of iron (20 μM FAS) showed increased filamentation in comparison to mutant cells embedded in higher concentrations of iron (50 to 500 μM). WT cells produced filamentous colonies in all concentrations. Together, the data indicate that Dfi1, Czf1, Sef1, and environmental iron regulate C. albicans contact-dependent filamentation.

IMPORTANCECandida albicans is an opportunistic pathogen responsible for a larger proportion of candidiasis and candidemia cases than any other Candida species. The ability of C. albicans cells to invade and cause disease is linked to their ability to filament. Despite this, there are gaps in our knowledge of the environmental cues and intracellular signaling that triggers the switch from commensal organism to filamentous pathogen. In this study, we identified a link between contact-dependent filamentation and iron availability. Over the course of tissue invasion, C. albicans cells encounter a number of different iron microenvironments, from the iron-rich gut to iron-poor tissues. Increased expression of Sef1-dependent iron uptake genes as a result of contact-dependent signaling will promote the adaptation of C. albicans cells to a low-iron-availability environment.

Transcriptional Control of Hypoxic Hyphal Growth in the Fungal Pathogen Candida albicans

The ability of Candida albicans, an important human fungal pathogen, to develop filamentous forms is a crucial determinant for host invasion and virulence. While hypoxia is one of the predominant host cues that promote C. albicans filamentous growth, the regulatory circuits that link oxygen availability to filamentation remain poorly characterized. We have undertaken a genetic screen and identified the two transcription factors Ahr1 and Tye7 as central regulators of the hypoxic filamentation. Both ahr1 and tye7 mutants exhibited a hyperfilamentous phenotype specifically under an oxygen-depleted environment suggesting that these transcription factors act as negative regulators of hypoxic filamentation. By combining microarray and ChIP-chip analyses, we have characterized the set of genes that are directly modulated by Ahr1 and Tye7. We found that both Ahr1 and Tye7 modulate a distinct set of genes and biological processes. Our genetic epistasis analysis supports our genomic finding and suggests that Ahr1 and Tye7 act independently to modulate hyphal growth in response to hypoxia. Furthermore, our genetic interaction experiments uncovered that Ahr1 and Tye7 repress the hypoxic filamentation via the Efg1 and Ras1/Cyr1 pathways, respectively. This study yielded a new and an unprecedented insight into the oxygen-sensitive regulatory circuit that control morphogenesis in a fungal pathogen.

Molecular and comparative genomic analyses reveal evolutionarily conserved and unique features of the Schizosaccharomyces japonicus mycelial growth and the underlying genomic changes

Fungal pathogens, from phytopathogenic fungus to human pathogens, are able to alternate between the yeast-like form and filamentous forms. This morphological transition (dimorphism) is in close connection with their pathogenic lifestyles and with their responses to changing environmental conditions. The mechanisms governing these morphogenetic conversions are still not fully understood. Therefore, we studied the filamentous growth of the less-known, non-pathogenic dimorphic fission yeast, S. japonicus, which belongs to an ancient and early evolved branch of the Ascomycota. Its RNA sequencing revealed that several hundred genes were up- or down-regulated in the hyphae compared to the yeast-phase cells. These genes belonged to different GO categories, confirming that mycelial growth is a rather complex process. The genes of transport- and metabolic processes appeared especially in high numbers among them. High expression of genes involved in glycolysis and ethanol production was found in the hyphae, while other results pointed to the regulatory role of the protein kinase A (PKA) pathway. The homologues of 49 S. japonicus filament-associated genes were found by sequence alignments also in seven distantly related dimorphic and filamentous species. The comparative genomic analyses between S. japonicus and the closely related but non-dimorphic S. pombe shed some light on the differences in their genomes. All these data can contribute to a better understanding of hyphal growth and those genomic rearrangements that underlie it.

Candida albicans requires iron to sustain hyphal growth

Candida albicans is an important opportunistic fungal pathogen of immunocompromised individuals. The ability to switch between yeast and hyphal growth forms is critical for its pathogenesis. Hyphal development in C. albicans requires two temporally linked regulations for initiation and maintenance. Here, we performed transcriptome sequencing (RNA-Seq) to analyze the transcriptional consequences for the two different phases of hyphal development. Genome-wide transcription profiling reveals that the sets associated with hyphal initiation were significantly enriched in genes for hyphal cell wall, biofilm matrix and actin polarization. In addition to hypha-specific genes, numerous genes involved in iron acquisition, such as FTR1 and SEF1, are highly induced specifically during sustained hyphal development even when additional free iron is supplied in the medium. Therefore, iron uptake genes are induced by signals that can support prolonged hyphal development in an iron-independent manner. The induction of iron acquisition genes during hyphal elongation was further confirmed by quantitative reverse transcription-PCR under various hypha-inducing conditions. Remarkably, preventing C. albicans from acquiring iron blocks BRG1 activation, leading to impaired hyphal maintenance, and ectopically expressed BRG1 can sustain hyphal development bypassing the requirement of iron. Our study elucidates an underlying mechanism of how multiple virulence factors are interconnected and are induced simultaneously during infection.

Interplay between transcriptional regulators and the SAGA chromatin modifying complex fine-tune iron homeostasis

The human fungal pathogen Candida albicans responds to iron deprivation by a global transcriptome reconfiguration known to be controlled by the transcriptional regulators Hap43 (also known as Cap2), Sef1, and the trimeric Hap2-Hap3-Hap5 complex. However, the relative roles of these regulators are not known. To dissect this system, we focused on the FRP1 and ACO1 genes, which are induced and repressed, respectively, under iron deprivation conditions. Chromatin immunoprecipitation assays showed that the trimeric HAP complex and Sef1 are recruited to both FRP1 and ACO1 promoters. While the HAP complex occupancy at the FRP1 promoter was Sef1-dependent, occupancy of Sef1 was not dependent on the HAP complex. Furthermore, iron deprivation elicited histone H3-Lys9 hyperacetylation and Pol II recruitment mediated by the trimeric HAP complex and Sef1 at the FRP1 promoter. In contrast, at the ACO1 promoter, the HAP trimeric complex and Hap43 promoted histone deacetylation and also limited Pol II recruitment under iron deprivation conditions. Mutational analysis showed that the SAGA subunits Gcn5, Spt7, and Spt20 are required for C. albicans growth in iron-deficient medium and for H3-K9 acetylation and transcription from the FRP1 promoter. Thus, the trimeric HAP complex promotes FRP1 transcription by stimulating H3K9Ac and Pol II recruitment and, along with Hap43, functions as a repressor of ACO1 by maintaining a deacetylated promoter under iron-deficient conditions. Thus, a regulatory network involving iron-responsive transcriptional regulators and the SAGA histone modifying complex functions as a molecular switch to fine-tune tight control of iron homeostasis gene expression in C. albicans.

Exploring Small Heat Shock Proteins (sHSPs) for Targeting Drug Resistance in Candida albicans and other Pathogenic Fungi

Fungal infections have predominantly increased worldwide that leads to morbidity and mortality in severe cases. Invasive candidiasis and other pathogenic fungal infections are a major problem in immunocompromised individuals and post-operative patients. Increasing resistance to existing antifungal drugs calls for the identification of novel antifungal drug targets for chemotherapeutic interventions. This demand for identification and characterization of novel drug targets leads to the development of effective antifungal therapy against drug resistant fungi. Heat shock proteins (HSPs) are important for various biological processes like protein folding, posttranslational modifications, transcription, translation, and protein aggregation. HSPs are involved in maintaining homeostasis of the cell. A subgroup of HSPs is small heat shock proteins (sHSPs), which functions as cellular chaperones. They are having a significant role in the many cellular functions like development, cytoskeletal organization, apoptosis, membrane lipid polymorphism, differentiation, autophagy, in infection recognition and are major players in various stresses like osmotic stress, pH stress, etc. Studies have shown that fungal cells express increased levels of sHSPs upon antifungal drug induced stress responses. Here we review the important role of small heat shock proteins (sHSPs) in fungal diseases and their potential as antifungal targets.

Loss of Arp1, a putative actin-related protein, triggers filamentous and invasive growth and impairs pathogenicity in Candida albicans

The polymorphous cellular shape of Candida albicans, in particular the transition from a yeast to a filamentous form, is crucial for either commensalism or life-threatening infections of the host. Various external or internal stimuli, including serum and nutrition starvation, have been shown to regulate filamentous growth primarily through two classical signaling pathways, the cAMP-PKA and the MAPK pathways. Genotoxic stress also induces filamentous growth, but through independent pathways, and little is known about negative regulation during this reversible morphological transition. In this study, we established that ARP1 in C. albicans, similar to its homolog in S. cerevisiae, has a role in nuclei separation and spindle orientation. Deletion of ARP1 generated filamentous and invasive growth as well as increased biofilm formation, accompanied by up-regulation of hyphae specific genes, such as HWP1, UME6 and ALS3. The filamentous and invasive growth of the ARP1 deletion strain was independent of transcription factors Efg1, Cph1 and Ume6, but was suppressed by deleting checkpoint BUB2 or overexpressing NRG1. Deletion of ARP1 impaired the colonization of Candida cells in mice and also attenuated virulence in a mouse model. All the data suggest that loss of ARP1 activates filamentous and invasive growth in vitro, and that it positively regulates virulence in vivo, which provides insight into actin-related morphology and pathogenicity in C. albicans.

Iron Metabolism, Pseudohypha Production, and Biofilm Formation through a Multicopper Oxidase in the Human-Pathogenic Fungus Candida parapsilosis

C. parapsilosis is the second or third most common opportunistic human-pathogenic Candida species, being responsible for severe fungal infections among immunocompromised patients, especially low-birth-weight infants (0 to 2 years of age). Among the major virulence factors that pathogenic fungi possess is the ability to compete with the host for essential micronutrients, including iron. Accessible iron is required for the maintenance of several metabolic processes. In order to obtain accessible iron from the host, pathogenic fungi have developed several iron acquisition and metabolic mechanisms. Although C. parapsilosis is a frequent cause of invasive candidiasis, little is known about what iron metabolic processes this fungus possesses that could contribute to the species’ virulent behavior. In this study, we identified the multicopper oxidase FET3 gene that regulates iron homeostasis maintenance and also plays important roles in the morphology of the fungus as well as in biofilm formation, two additional factors in fungal virulence.

Among all the essential micronutrients, iron plays an important role in mammalian biology. It is also essential for pathogens infecting mammalian hosts, including bacteria, fungi, and protozoans. As the availability of accessible iron is limited within the mammalian host, several human-pathogenic fungal pathogens, such as Candida albicans, Cryptococcus neoformans, Candida glabrata, and Aspergillus fumigatus, have developed various iron uptake mechanisms. Although Candida parapsilosis is the second or third most common non-albicans Candida species associated with systemic and superficial Candida infections in immunocompromised patients, the mechanisms of iron uptake and homoeostasis remain unknown in this fungus. In the current report, we show that a homologue of the multicopper oxidase gene FET3 is present in the genome of C. parapsilosis (CPAR2_603600) and plays a significant role in iron acquisition. We found that homozygous deletion mutants of CPAR2_603600 showed defects under low-iron conditions and were also sensitive to various stressors. Our results also revealed that the levels of pseudohypha formation and biofilm formation were reduced in the null mutants compared to the wild type. This phenotypic defect could be partially rescued by supplementation with excess iron in the growth medium. The expression levels of the orthologues of various iron metabolism-related genes were also altered in the mutants compared to the parental strain. In conclusion, our report describes the role of CPAR2_603600 in iron homoeostasis maintenance as well as morphology and biofilm formation regulation in this pathogenic fungus.

IMPORTANCEC. parapsilosis is the second or third most common opportunistic human-pathogenic Candida species, being responsible for severe fungal infections among immunocompromised patients, especially low-birth-weight infants (0 to 2 years of age). Among the major virulence factors that pathogenic fungi possess is the ability to compete with the host for essential micronutrients, including iron. Accessible iron is required for the maintenance of several metabolic processes. In order to obtain accessible iron from the host, pathogenic fungi have developed several iron acquisition and metabolic mechanisms. Although C. parapsilosis is a frequent cause of invasive candidiasis, little is known about what iron metabolic processes this fungus possesses that could contribute to the species’ virulent behavior. In this study, we identified the multicopper oxidase FET3 gene that regulates iron homeostasis maintenance and also plays important roles in the morphology of the fungus as well as in biofilm formation, two additional factors in fungal virulence.

Transcriptional responses of Candida glabrata biofilm cells to fluconazole are modulated by the carbon source

Candida glabrata is an important human fungal pathogen known to trigger serious infections in immune-compromised individuals. Its ability to form biofilms, which exhibit high tolerance to antifungal treatments, has been considered as an important virulence factor. However, the mechanisms involving antifungal resistance in biofilms and the impact of host niche environments on these processes are still poorly defined. In this study, we performed a whole-transcriptome analysis of C. glabrata biofilm cells exposed to different environmental conditions and constraints in order to identify the molecular pathways involved in fluconazole resistance and understand how acidic pH niches, associated with the presence of acetic acid, are able to modulate these responses. We show that fluconazole treatment induces gene expression reprogramming in a carbon source and pH-dependent manner. This is particularly relevant for a set of genes involved in DNA replication, ergosterol, and ubiquinone biosynthesis. We also provide additional evidence that the loss of mitochondrial function is associated with fluconazole resistance, independently of the growth condition. Lastly, we propose that C. glabrata Mge1, a cochaperone involved in iron metabolism and protein import into the mitochondria, is a key regulator of fluconazole susceptibility during carbon and pH adaptation by reducing the metabolic flux towards toxic sterol formation. These new findings suggest that different host microenvironments influence directly the physiology of C. glabrata, with implications on how this pathogen responds to antifungal treatment. Our analyses identify several pathways that can be targeted and will potentially prove to be useful for developing new antifungals to treat biofilm-based infections.

On Commensalism of Candida

Candida species are both opportunistic fungal pathogens and common members of the human mycobiome. Over the years, the main focus of the fungal field has been on understanding the pathogenic potential and disease manifestation of these organisms. Therefore, understanding of their commensal lifestyle, interactions with host epithelial barriers, and initial transition into pathogenesis is less developed. In this review, we will describe the current knowledge on the commensal lifestyle of these fungi, how they are able to adhere to and colonize host epithelial surfaces, compete with other members of the microbiota, and interact with the host immune response, as well as their transition into opportunistic pathogens by invading the gastrointestinal epithelium.

Copper-only superoxide dismutase enzymes and iron starvation stress in Candida fungal pathogens

Copper (Cu)-only superoxide dismutases (SOD) represent a newly characterized class of extracellular SODs important for virulence of several fungal pathogens. Previous studies of the Cu-only enzyme SOD5 from the opportunistic fungal pathogen Candida albicans have revealed that the active-site structure and Cu binding of SOD5 strongly deviate from those of Cu/Zn-SODs in its animal hosts, making Cu-only SODs a possible target for future antifungal drug design. C. albicans also expresses a Cu-only SOD4 that is highly similar in sequence to SOD5, but is poorly characterized. Here, we compared the biochemical, biophysical, and cell biological properties of C. albicans SOD4 and SOD5. Analyzing the recombinant proteins, we found that, similar to SOD5, Cu-only SOD4 can react with superoxide at rates approaching diffusion limits. Both SODs were monomeric and they exhibited similar binding affinities for their Cu cofactor. In C. albicans cultures, SOD4 and SOD5 were predominantly cell wall proteins. Despite these similarities, the SOD4 and SOD5 genes strongly differed in transcriptional regulation. SOD5 was predominantly induced during hyphal morphogenesis, together with a fungal burst in reactive oxygen species. Conversely, SOD4 expression was specifically up-regulated by iron (Fe) starvation and controlled by the Fe-responsive transcription factor SEF1. Interestingly, Candida tropicalis and the emerging fungal pathogen Candida auris contain a single SOD5-like SOD rather than a pair, and in both fungi, this SOD was induced by Fe starvation. This unexpected link between Fe homeostasis and extracellular Cu-SODs may help many fungi adapt to Fe-limited conditions of their hosts.

Magnesium deprivation affects cellular circuitry involved in drug resistance and virulence in Candida albicans

OBJECTIVES

Candida albicans has to struggle for the limited micronutrients present in the hostile host niche, including magnesium (Mg). The aim of this study was to examine the effect of Mg deprivation on drug resistance mechanisms and virulence traits of C. albicans.

METHODS

The drug susceptibility of C. albicans strain SC5314 was determined by broth microdilution and spot assay. Efflux pump activity was measured using the substrate rhodamine 6G. Membrane intactness was studied by propidium iodide influx, and ergosterol levels were determined by the alcoholic KOH method. Metabolic flexibility was examined by studying the activity of glyoxylate cycle enzymes. Virulence factors were assessed by yeast-to-hyphae transition, biofilm formation and cell adherence. An in vivo study was also performed in a Caenorhabditis elegans infection model.

RESULTS

Mg chelation leads to potentiation of membrane-targeting antifungals. The role of Mg on membrane homeostasis was explored and significant differences in ergosterol levels were found. Interestingly, it was also observed that Mg deprivation impedes the metabolic flexibility of C. albicans SC5314 by inhibiting glyoxylate cycle enzymes. Furthermore, Mg deprivation inhibited potential virulence traits, including morphological transition, biofilm formation and buccal epithelial cell adherence. All of the disrupted gene targets were validated by reverse transcription PCR. Lastly, enhanced survival of C. elegans infected with C. albicans SC5314 under Mg deprivation was observed.

CONCLUSION

In view of the restricted growth of C. albicans in a Mg-deficient environment, approaches could be utilised to boost the effectiveness of existing antifungals thereby improving the management of fungal infections.

The game theory of Candida albicans colonization dynamics reveals host status-responsive gene expression

Background

The fungal pathogen Candida albicans colonizes the gastrointestinal (GI) tract of mammalian hosts as a benign commensal. However, in an immunocompromised host, the fungus is capable of causing life-threatening infection. We previously showed that the major transcription factor Efg1p is differentially expressed in GI-colonizing C. albicans cells dependent on the host immune status. To understand the mechanisms that underlie this host-dependent differential gene expression, we utilized mathematical modeling to dissect host-pathogen interactions. Specifically, we used principles of evolutionary game theory to study the mechanism that governs dynamics of EFG1 expression during C. albicans colonization.

Results

Mathematical modeling predicted that down-regulation of EFG1 expression within individual fungal cells occurred at different average rates in different hosts. Rather than using relatively transient signaling pathways to adapt to a new environment, we demonstrate that C. albicans overcomes the host defense strategy by modulating the activity of diverse fungal histone modifying enzymes that control EFG1 expression.

Conclusion

Based on our modeling and experimental results we conclude that C. albicans cells sense the local environment of the GI tract and respond to differences by altering EFG1 expression to establish optimal survival strategies. We show that the overall process is governed via modulation of epigenetic regulators of chromatin structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-016-0268-1) contains supplementary material, which is available to authorized users.

Antifungal Action of Methylene Blue Involves Mitochondrial Dysfunction and Disruption of Redox and Membrane Homeostasis in C. albicans

Candida albicans is known to cause infections ranging from superficial and systemic in immunocompromised person. In this study, we explored that the antifungal action of Methylene blue (MB) is mediated through mitochondrial dysfunction and disruption of redox and membrane homeostasis against C. albicans. We demonstrated that MB displayed its antifungal potential against C. albicans and two clinical isolates tested. We also showed that MB is effective against two non- albicans species as well. Notably, the antifungal effect of MB seems to be independent of the major drug efflux pumps transporter activity. We explored that MB treated Candida cells were sensitive on non-fermentable carbon source leading us to propose that MB inhibits mitochondria. This sensitive phenotype was reinforced with the fact that sensitivity of Candida cells to MB could be rescued upon the supplementation of ascorbic acid, an antioxidant. This clearly suggests that disturbances in redox status are linked with MB action. We further demonstrated that Candida cells were susceptible to membrane perturbing agent viz. SDS which was additionally confirmed by transmission electron micrographs showing disruption of membrane integrity. Moreover, the ergosterol levels were significantly decreased by 66% suggesting lipid compositional changes due to MB. Furthermore, we could demonstrate that MB inhibits the yeast to hyphal transition in C. albicans which is one of the major virulence attribute in most of the hyphal inducing conditions. Taken together, the data generated from present study clearly establishes MB as promising antifungal agent that could be efficiently employed in strategies to treat Candida infections.

Adaptations of the Secretome of Candida albicans in Response to Host-Related Environmental Conditions

The wall proteome and the secretome of the fungal pathogen Candida albicans help it to thrive in multiple niches of the human body. Mass spectrometry has allowed researchers to study the dynamics of both subproteomes. Here, we discuss some major responses of the secretome to host-related environmental conditions. Three β-1,3-glucan-modifying enzymes, Mp65, Sun41, and Tos1, are consistently found in large amounts in culture supernatants, suggesting that they are needed for construction and expansion of the cell wall β-1,3-glucan layer and thus correlate with growth and might serve as diagnostic biomarkers. The genes ENG1, CHT3, and SCW11, which encode an endoglucanase, the major chitinase, and a β-1,3-glucan-modifying enzyme, respectively, are periodically expressed and peak in M/G₁. The corresponding protein abundances in the medium correlate with the degree of cell separation during single-yeast-cell, pseudohyphal, and hyphal growth. We also discuss the observation that cells treated with fluconazole, or other agents causing cell surface stress, form pseudohyphal aggregates. Fluconazole-treated cells secrete abundant amounts of the transglucosylase Phr1, which is involved in the accumulation of β-1,3-glucan in biofilms, raising the question whether this is a general response to cell surface stress. Other abundant secretome proteins also contribute to biofilm formation, emphasizing the important role of secretome proteins in this mode of growth. Finally, we discuss the relevance of these observations to therapeutic intervention. Together, these data illustrate that C. albicans actively adapts its secretome to environmental conditions, thus promoting its survival in widely divergent niches of the human body.

Iron-responsive chromatin remodelling and MAPK signalling enhance adhesion in Candida albicans

Recent cumulative data show that various transcription factors are recruited to the chromatin in an iron-responsive manner to affect diverse cellular functions in the pathogenic fungus Candida albicans. Here we identified groups of iron-responsive genes in C. albicans by chromatin remodelling analysis at gene promoters, using micrococcal nuclease (MNase) digestion followed by deep sequencing. Chromatin in the promoter regions of iron uptake and utilization genes showed repressed and active configuration, respectively, under iron-replete conditions. GO Term enrichment analysis of genes with differentially remodelled chromatin, in respective promoter locales, suggested that many genes involved in adhesion are also iron-responsive. C. albicans was observed to be more self-adherent (twofold increase) and formed higher biofilm mass (77% increase) in the presence of iron. Furthermore, we identified various known and novel adhesion-related genes with iron-dependent active chromatin profiles that are indicative of potential upregulation under iron-replete conditions. Transcription factor Cph1 that is activated upon Cek1 phosphorylation also showed an active chromatin profile under iron-replete conditions and cells showed iron-responsive Cek1 MAPK phosphorylation in the presence of iron. Thus, iron affects diverse biological functions by modulating chromatin profiles of large gene sets and by signalling through Cek1 MAPK in C. albicans.

Novel role of the Candida albicans ferric reductase gene CFL1 in iron acquisition, oxidative stress tolerance, morphogenesis and virulence

Ferric reductase catalyzes the reduction of ferric iron into ferrous iron and plays an essential role in high-affinity iron acquisition. In this study, we found that the cfl1Δ/Δ (orf19.1263) mutant was not defective in iron acquisition. However, deletion of CFL1 increased cellular iron accumulation by elevating surface ferric reductase activity in Candida albicans, revealing that there existed functional redundancy and/or a compensatory upregulation mechanism among ferric reductase genes. The absence of CFL1 resulted in increased expression levels of other alternative ferric reductase genes, including FRP1, CFL2 and FRE10. In addition, CFL1 played an important role in the response to different oxidative stresses. Further research revealed that the cfl1Δ/Δ mutant exhibited higher levels of both ROS production and SOD activity under oxidative conditions. Moreover, deletion of CFL1 led to a profound defect in filamentous development in an iron-independent manner at both 30 and 37 °C. The cfl1Δ/Δ mutant exhibited highly attenuated virulence and reduced fungal burdens in the mouse systemic infection model, indicating that CFL1 might be a potential target for antifungal drug development. In summary, our results provide new insights into the roles of ferric reductase gene in C. albicans.

Mitochondria influence CDR1 efflux pump activity, Hog1-mediated oxidative stress pathway, iron homeostasis, and ergosterol levels in Candida albicans

Mitochondrial dysfunction in Candida albicans is known to be associated with drug susceptibility, cell wall integrity, phospholipid homeostasis, and virulence. In this study, we deleted CaFZO1, a key component required during biogenesis of functional mitochondria. Cells with FZO1 deleted displayed fragmented mitochondria, mitochondrial genome loss, and reduced mitochondrial membrane potential and were rendered sensitive to azoles and peroxide. In order to understand the cellular response to dysfunctional mitochondria, genome-wide expression profiling of fzo1Δ/Δ cells was performed. Our results show that the increased susceptibility to azoles was likely due to reduced efflux activity of CDR efflux pumps, caused by the missorting of Cdr1p into the vacuole. In addition, fzo1Δ/Δ cells showed upregulation of genes involved in iron assimilation, in iron-sufficient conditions, characteristic of iron-starved cells. One of the consequent effects was downregulation of genes of the ergosterol biosynthesis pathway with a commensurate decrease in cellular ergosterol levels. We therefore connect deregulated iron metabolism to ergosterol biosynthesis pathway in response to dysfunctional mitochondria. Impaired activation of the Hog1 pathway in the mutant was the basis for increased susceptibility to peroxide and increase in reactive oxygen species, indicating the importance of functional mitochondria in controlling Hog1-mediated oxidative stress response. Mitochondrial phospholipid levels were also altered as indicated by an increase in phosphatidylserine and phosphatidylethanolamine and decrease in phosphatidylcholine in fzo1Δ/Δ cells. Collectively, these findings reinforce the connection between functional mitochondria and azole tolerance, oxidant-mediated stress, and iron homeostasis in C. albicans.

Novel insight into the expression and function of the multicopper oxidases in Candida albicans

Iron is an essential element required for most organisms. The high-affinity iron-uptake systems in the opportunistic pathogen Candida albicans are activated under iron-limited conditions and are also required for virulence. Here one component of high-affinity iron-uptake systems, the multicopper oxidase (MCO) genes, was characterized. We examined the expression of five MCO genes and demonstrated that CaFET3 and CaFET34 were the major MCO genes in response to iron deficiency. Complementation of the Saccharomyces cerevisiae fet3Δ mutant showed that CaFET34 could effectively rescue the growth phenotype in iron-limited medium. Deletion of CaFET33 and CaFET34 in C. albicans decreased cellular iron content and iron acquisition during iron starvation. However, the fet33Δ/Δ and fet34Δ/Δ mutants exhibited no obvious growth defect in solid iron-limited medium while the fet34Δ/Δ mutant showed a slight growth defect in liquid medium. Further analysis shows that other members of the five MCO genes, especially CaFET3, would compensate for the absence of CaFET33 and CaFET34. Furthermore, for the first time, we provide evidence that CaFET34 is implicated in hyphal development in an iron-independent manner and is required for C. albicans virulence in a mouse model of systemic infection. Together, our results not only expand our understanding about the expression of the MCO genes in C. albicans, but also provide a novel insight into the role of CaFET34 in iron metabolism, hyphal development and virulence.

Involvement of the mitogen activated protein kinase Hog1p in the response of Candida albicans to iron availability

BACKGROUND

Iron is an essential nutrient for almost all organisms, and generating iron limiting conditions for pathogens is one of the host defense strategies against microbial infections. Excess of iron can be toxic; therefore, iron uptake is tightly controlled. The high affinity iron uptake system of the opportunistic pathogenic yeast Candida albicans has been shown to be essential for virulence. Several transcription factors and regulators of iron uptake genes were identified, but the knowledge of signaling pathways is still limited. Gene expression profiling of the Δhog1 deletion mutant indicated an involvement of the mitogen activated protein (MAP) kinase Hog1p. However, the function of Hog1p in the response of C. albicans to iron availability was not studied in detail. Thus, we analyzed phenotypic and molecular responses of C. albicans to different iron concentrations particularly with respect to the activity of the Hog1p MAP kinase module.

RESULTS

We observed flocculation of yeast cells, when the iron ion concentration was equal to or higher than 5 μM. This phenotype was dependent on the MAP kinase Hog1p and the corresponding MAP kinase kinase Pbs2p. Moreover, high extracellular iron ion concentrations led to hyper-phosphorylation of Hog1p. We determined lower amounts of multicopper ferroxidase (MCFO) proteins and lower ferric reductase activity, when the iron ion concentration in the medium was increased. This effect was also observed for the Δhog1 mutant. However, the amounts of MCFO proteins and the cell surface ferric reductase activity were increased in the Δhog1 in comparison to wild type cells. This effect was independent of iron availability in growth media.

CONCLUSIONS

In C. albicans, the MAP kinase Hog1p is part of the network regulating the response of the organism to iron availability. Hog1p was transiently phosphorylated under high iron concentrations and was essential for a flocculent phenotype. Furthermore, deletion of HOG1 led to increased levels of components of the reductive iron uptake system in comparison to the wild-type, independent of iron concentrations in the media. However, the additional induction of this system by low iron concentrations was independent of HOG1.

Normal adaptation of Candida albicans to the murine gastrointestinal tract requires Efg1p-dependent regulation of metabolic and host defense genes

Although gastrointestinal colonization by the opportunistic fungal pathogen Candida albicans is generally benign, severe systemic infections are thought to arise due to escape of commensal C. albicans from the gastrointestinal (GI) tract. The C. albicans transcription factor Efg1p is a major regulator of GI colonization, hyphal morphogenesis, and virulence. The goals of this study were to identify the Efg1p regulon during GI tract colonization and to compare C. albicans gene expression during colonization of different organs of the GI tract. Our results identified significant differences in gene expression between cells colonizing the cecum and ileum. During colonization, efg1(-) null mutant cells expressed higher levels of genes involved in lipid catabolism, carnitine biosynthesis, and carnitine utilization than did colonizing wild-type (WT) cells. In addition, during laboratory growth, efg1(-) null mutant cells grew to a higher density than WT cells. The efg1(-) null mutant grew in depleted medium, while WT cells could grow only if the depleted medium was supplemented with carnitine, a compound that promotes the metabolism of fatty acids. Altered gene expression and altered growth capability support the ability of efg1(-) cells to hypercolonize naïve mice. Also, Efg1p was shown to be important for transcriptional responses to the stresses present in the cecum environment. For example, during colonization, SOD5, encoding a superoxide dismutase, was highly upregulated in an Efg1p-dependent manner. Ectopic expression of SOD5 in an efg1(-) null mutant increased the fitness of the efg1(-) null mutant cells during colonization. These data show that EFG1 is an important regulator of GI colonization.

Identification and functional characterization of mitochondrial carrier Mrs4 in Candida albicans

Iron is an essential nutrient required for the growth and metabolism in Candida albicans. Here, we for the first time identified Mrs4 as a new member of mitochondrial carrier family in C. albicans. Our experiments revealed that C. albicans Mrs4 (CaMrs4) is localized to the mitochondria and required for mitochondrial morphology. We found that CaMrs4 is required for cell growth, and the mrs4Δ/Δ mutant showed a more severe growth defect in iron deficiency. Deletion of MRS4 affected cellular iron content by altering the expression of iron regulon genes in C. albicans, such as AFT2, SMF3, FTR1 and ISU1. Candida albicans Aft2 factor functions as a negative regulator of MRS4 expression through the CACCC Aft-type sequence in a gene dose-dependent fashion. In addition, the mrs4Δ/Δ mutant exhibited hypersensitivity to oxidants and most metal ions, but decreased sensitivity to cobalt. Exogenous iron could suppress the sensitivity of the mrs4Δ/Δ mutant to oxidants and most metal ions, suggesting that the role of CaMrs4 is partially mediated by iron availability. Furthermore, deletion of MRS4 resulted in delayed filamentation under tested conditions. Taken together, these findings characterize a new mitochondrial carrier and provide a novel insight into the role of CaMrs4 in mitochondrial function.

Modulation of morphogenesis in Candida albicans by various small molecules

The pathogenic yeast Candida albicans, a member of the mucosal microbiota, is responsible for a large spectrum of infections, ranging from benign thrush and vulvovaginitis in both healthy and immunocompromised individuals to severe, life-threatening infections in immunocompromised patients. A striking feature of C. albicans is its ability to grow as budding yeast and as filamentous forms, including hyphae and pseudohyphae. The yeast-to-hypha transition contributes to the overall virulence of C. albicans and may even constitute a target for the development of antifungal drugs. Indeed, impairing morphogenesis in C. albicans has been shown to be a means to treat candidiasis. Additionally, a large number of small molecules such as farnesol, fatty acids, rapamycin, geldanamycin, histone deacetylase inhibitors, and cell cycle inhibitors have been reported to modulate the yeast-to-hypha transition in C. albicans. In this minireview, we take a look at molecules that modulate morphogenesis in this pathogenic yeast. When possible, we address experimental findings regarding their mechanisms of action and their therapeutic potential. We discuss whether or not modulating morphogenesis constitutes a strategy to treat Candida infections.

Antifungal curcumin induces reactive oxygen species and triggers an early apoptosis but prevents hyphae development by targeting the global repressor TUP1 in Candida albicans

In the present study, we have investigated the antifungal effects of a natural polyphenol, CUR (curcumin), against albicans and non-albicans species of Candida and have shown its ability to inhibit the growth of all the tested strains. The inhibitory effects of CUR were independent of the status of the multidrug efflux pump proteins belonging to either ABC transporter (ATP-binding cassette transporter) or MFS (major facilitator) superfamilies of transporters. By using a systemic murine model of infection, we established that CUR and piperine, when administered together, caused a significant fungal load reduction (1.4log10) in kidneys of Swiss mice. Additionally, CUR raised the levels of ROS (reactive oxygen species), which, as revealed by annexin V-FITC labelling, triggered early apoptosis in Candida cells. Coincident with the raised ROS levels, mRNAs of tested oxidative stress-related genes [CAP1 (Candida albicans AP-1), CaIPF7817 (putative NADH-dependent flavin oxidoreductase), SOD2 (superoxide dismutase 2), GRP2 (NADPH-dependent methyl glyoxal reductase) and CAT1 (catalase 1)] were also elevated. The growth inhibitory effects of CUR could be reversed by the addition of natural and synthetic antioxidants. Notably, independent of ROS status, polyphenol CUR prevented hyphae development in both liquid and solid hypha-inducing media by targeting the global suppressor TUP1 (thymidine uptake 1). Taken together, our results provide the first evidence that CUR acts as an antifungal agent, via generation of oxidative stress, and inhibits hyphae development by targeting TUP1.

Morphogenic regulator EFG1 affects the drug susceptibilities of pathogenic Candida albicans

This study shows that the morphogenic regulator EFG1 level affects the drug susceptibilities of Candida albicans when grown on solid growth media. The Deltaefg1 mutant showed sensitivity particularly to those drugs that target ergosterol or its metabolism. Efg1p disruption showed a gene-dosage effect on drug susceptibilities and resulted in enhanced susceptibility to drugs in the homozygous mutant as compared with the wild type, heterozygous and revertant strains. The enhanced sensitivity to drugs was independent of the status of ATP-binding cassette and MFS multidrug efflux pumps of C. albicans. The Deltaefg1 mutant displayed increased membrane fluidity that coincided with the downregulation of ERG11 and upregulation of OLE1 and ERG3, leading to enhanced passive diffusion of drugs. Interestingly, Deltaefg1 mutant cells displayed enhanced levels of endogenous ROS levels. Notably, the higher levels of ROS in the Deltaefg1 mutant could be reversed by the addition of antioxidants. However, the restoration of ROS levels did not reverse the drug sensitivities of the Deltaefg1 mutant. Taken together, we, for the first time, establish a new role to EFG1 in affecting the drug susceptibilities of C. albicans cells, independent of ROS and known drug efflux mechanisms.

Pseudomonas aeruginosa secreted factors impair biofilm development in Candida albicans

Signal-mediated interactions between the human opportunistic pathogens Pseudomonas aeruginosa and Candida albicans affect virulence traits in both organisms. Phenotypic studies revealed that bacterial supernatant from four P. aeruginosa strains strongly reduced the ability of C. albicans to form biofilms on silicone. This was largely a consequence of inhibition of biofilm maturation, a phenomenon also observed with supernatant prepared from non-clinical bacterial species. The effects of supernatant on biofilm formation were not mediated via interference with the yeast–hyphal morphological switch and occurred regardless of the level of homoserine lactone (HSL) produced, indicating that the effect is HSL-independent. A transcriptome analysis to dissect the effects of the P. aeruginosa supernatants on gene expression in the early stages of C. albicans biofilm formation identified 238 genes that exhibited reproducible changes in expression in response to all four supernatants. In particular, there was a strong increase in the expression of genes related to drug or toxin efflux and a decrease in expression of genes associated with adhesion and biofilm formation. Furthermore, expression of YWP1, which encodes a protein known to inhibit biofilm formation, was significantly increased. Biofilm formation is a key aspect of C. albicans infections, therefore the capacity of P. aeruginosa to antagonize this has clear biomedical implications.

Hypoxic adaptation by Efg1 regulates biofilm formation by Candida albicans

Hypoxia is encountered frequently by Candida albicans during systemic infection of the human host. We tested if hypoxia allows biofilm formation by C. albicans, which is a major cause of perseverance and antifungal resistance in C. albicans infections. Using an in vitro biofilm system, we unexpectedly discovered that several positive regulators of biofilm formation during normoxia, including Tec1, Ace2, Czf1, Och1, and Als3, had little or no influence on biofilm development during hypoxia, irrespective of the carbon dioxide level, indicating that C. albicans biofilm pathways differ depending on the oxygen level. In contrast, the Efg1 and Flo8 regulators were required for both normoxic and hypoxic biofilm formation. To explore the role of Efg1 during hypoxic and/or biofilm growth, we determined transcriptome kinetics following release of EFG1 expression by a system under transcriptional control of a doxycycline-inducible promoter. During hypoxia, Efg1 rapidly induced expression of all major classes of genes known to be associated with normoxic biofilm formation, including genes involved in glycolysis, sulfur metabolism, and antioxidative and peroxisome activities, as well as genes for iron uptake. The results suggest that hypoxic adaptation mediated by the Efg1 and Flo8 regulators is required even during normoxic biofilm development, while hypoxic biofilm formation in deep tissues or in organs may generate foci of C. albicans infections.