Glucose promotes stress resistance in the fungal pathogen Candida albicans

Chlorhexidine markedly potentiates the oxidants scavenging abilities of Candida albicans

The oxidant scavenging ability (OSA) of catalase-rich Candida albicans is markedly enhanced by chlorhexidine digluconate (CHX), polymyxin B, the bile salt ursodeoxycholate and by lysophosphatidylcholine, which all act as detergents facilitating the penetration of oxidants and their intracellular decomposition. Quantifications of the OSA of Candida albicans were measured by a highly sensitive luminol-dependent chemiluminescence assay and by the Thurman's assay, to quantify hydrogen peroxide (H2O2). The OSA enhancing activity by CHX depends to some extent on the media on which candida grew. The OSA of candida treated by CHX was modulated by whole human saliva, red blood cells, lysozyme, cationic peptides and by polyphenols. Concentrations of CHX, which killed over 95 % of Candida albicans cells, did not affect the cells' abilities to scavenge reactive oxygen species (ROS). The OSA of Candida cells treated by CHX is highly refractory to H2O2 (50 mM) but is strongly inhibited by hypochlorous acid, lecithin, trypan blue and by heparin. We speculate that similarly to catalase-rich red blood cells, Candida albicans and additional catalase-rich microbiota may also have the ability to scavenge oxidants and thus can protect catalase-negative anaerobes and facultative anaerobes cariogenic streptococci against peroxide and thus secure their survival in the oral cavity.

Ineffective Phosphorylation of Mitogen-Activated Protein Kinase Hog1p in Response to High Osmotic Stress in the Yeast Kluyveromyces lactis

When treated with a hyperosmotic stimulus, Kluyveromyces lactis cells respond by activating the mitogen-activated protein kinase (MAPK) K. lactis Hog1 (KlHog1) protein via two conserved branches, SLN1 and SHO1. Mutants affected in only one branch can cope with external hyperosmolarity by activating KlHog1p by phosphorylation, except for single ΔKlste11 and ΔKlste50 mutants, which showed high sensitivity to osmotic stress, even though the other branch (SLN1) was intact. Inactivation of both branches by deletion of KlSHO1 and KlSSK2 also produced sensitivity to high salt. Interestingly, we have observed that in ΔKlste11 and ΔKlsho1 ΔKlssk2 mutants, which exhibit sensitivity to hyperosmotic stress, and contrary to what would be expected, KlHog1p becomes phosphorylated. Additionally, in mutants lacking both MAPK kinase kinases (MAPKKKs) present in K. lactis (KlSte11p and KlSsk2p), the hyperosmotic stress induced the phosphorylation and nuclear internalization of KlHog1p, but it failed to induce the transcriptional expression of KlSTL1 and the cell was unable to grow in high-osmolarity medium. KlHog1p phosphorylation via the canonical HOG pathway or in mutants where the SHO1 and SLN1 branches have been inactivated requires not only the presence of KlPbs2p but also its kinase activity. This indicates that when the SHO1 and SLN1 branches are inactivated, high-osmotic-stress conditions activate an independent input that yields active KlPbs2p, which, in turn, renders KlHog1p phosphorylation ineffective. Finally, we found that KlSte11p can alleviate the sensitivity to hyperosmotic stress displayed by a ΔKlsho1 ΔKlssk2 mutant when it is anchored to the plasma membrane by adding the KlSho1p transmembrane segments, indicating that this chimeric protein can substitute for KlSho1p and KlSsk2p.

Dressed to impress: impact of environmental adaptation on the Candida albicans cell wall

Candida albicans is an opportunistic fungal pathogen of humans causing superficial mucosal infections and life‐threatening systemic disease. The fungal cell wall is the first point of contact between the invading pathogen and the host innate immune system. As a result, the polysaccharides that comprise the cell wall act as pathogen associated molecular patterns, which govern the host–pathogen interaction. The cell wall is dynamic and responsive to changes in the external environment. Therefore, the host environment plays a critical role in regulating the host–pathogen interaction through modulation of the fungal cell wall. This review focuses on how environmental adaptation modulates the cell wall structure and composition, and the subsequent impact this has on the innate immune recognition of C. albicans.

Community priming--effects of sequential stressors on microbial assemblages

Microbes in nature are exposed to complex environmental stressors which challenge their functioning or survival. Priming is the improved reaction of an organism to an environmental stressor following a preceding, often milder stress event. This phenomenon, also known as cross-protection, predictive response strategy or acquired stress resistance, is becoming an increasingly well-established research topic in microbiology, which has so far been examined from the perspective of a single organism or population. However, microbes in nature occur as part of communities; thus it is timely to highlight the need to also include this level beyond the individual species in studies of priming effects. We here introduce a conceptual framework for such studies at the level of the microbial assemblage and also chart a way forward for empirical and theoretical study. We illustrate some of the elements of our framework with a simple simulation model. Given the dynamic habitat of many microbes, incorporating priming is important for a more complete understanding of microbial community responses to stress.

Detection of inhibitors of Candida albicans Cdr transporters using a diS-C3(3) fluorescence

Candida albicans is a major cause of opportunistic and life-threatening, systemic fungal infections. Hence new antifungal agents, as well as new methods to treat fungal infections, are still needed. The application of inhibitors of drug-efflux pumps may increase the susceptibility of C. albicans to drugs. We developed a new fluorescence method that allows the in vivo activity evaluation of compounds inhibiting of C. albicans transporters. We show that the potentiometric dye 3,3′-dipropylthiacarbocyanine iodide diS-C₃(3) is pumped out by both Cdr1 and Cdr2 transporters. The fluorescence labeling with diS-C₃(3) enables a real-time observation of the activity of C. albicans Cdr1 and Cdr2 transporters. We demonstrate that enniatin A and beauvericin show different specificities toward these transporters. Enniatin A inhibits diS-C₃(3) efflux by Cdr1 while beauvericin inhibits both Cdr1p and Cdr2p.

Candida survival strategies

Only few Candida species, e.g., Candida albicans, Candida glabrata, Candida dubliniensis, and Candida parapsilosis, are successful colonizers of a human host. Under certain circumstances these species can cause infections ranging from superficial to life-threatening disseminated candidiasis. The success of C. albicans, the most prevalent and best studied Candida species, as both commensal and human pathogen depends on its genetic, biochemical, and morphological flexibility which facilitates adaptation to a wide range of host niches. In addition, formation of biofilms provides additional protection from adverse environmental conditions. Furthermore, in many host niches Candida cells coexist with members of the human microbiome. The resulting fungal-bacterial interactions have a major influence on the success of C. albicans as commensal and also influence disease development and outcome. In this chapter, we review the current knowledge of important survival strategies of Candida spp., focusing on fundamental fitness and virulence traits of C. albicans.

Effects of microcystin-LR on bacterial and fungal functional genes profile in rat gut

The short-term exposure to microcystin-LR (MC-LR, one of the most common and toxic variants generated by toxigenic cyanobacteria) induced gut dysfunction such as generation of reactive oxygen species, cell erosion and deficient intestinal absorption of nutrients. However, till now, little is known about its impact on gut microbial community, which has been considered as necessary metabolic assistant and stresses resistant entities for the host. This study was designed to reveal the shift of microbial functional genes in the gut of rat orally gavaged with MC-LR. GeoChip detected a high diversity of bacterial and fungal genes involved in basic metabolic processes and stress resistance. The results showed that the composition of functional genes was significantly changed in rat gut after one week of exposure to MC-LR, and we found some relatively enriched genes that are involved in carbon degradation including chitin, starch and limonene metabolism, and these genes were mainly derived from fungal and bacterial pathogens. In addition, we found large amounts of significantly enriched genes relevant to degradation of the specific carbon compounds, aromatics. The dysbiosis of bacterial and fungal flora gave an implication of pathogens invasion. The enriched gene functions could be linked to acute gastroenteritis induced by MC-LR.

A novel function for Hog1 stress-activated protein kinase in controlling white-opaque switching and mating in Candida albicans

Candida albicans is a commensal in heathy people but has the potential to become an opportunistic pathogen and is responsible for half of all clinical infections in immunocompromised patients. Central to understanding C. albicans behavior is the white-opaque phenotypic switch, in which cells can undergo an epigenetic transition between the white state and the opaque state. The phenotypic switch regulates multiple properties, including biofilm formation, virulence, mating, and fungus-host interactions. Switching between the white and opaque states is associated with many external stimuli, such as oxidative stress, pH, and N-acetylglucosamine, and is directly regulated by the Wor1 transcriptional circuit. The Hog1 stress-activated protein kinase (SAPK) pathway is recognized as the main pathway for adapting to environmental stress in C. albicans. In this work, we first show that loss of the HOG1 gene in A: / A: and α/α cells, but not A: /α cells, results in 100% white-to-opaque switching when cells are grown on synthetic medium, indicating that switching is repressed by the A1: /α2 heterodimer that represses WOR1 gene expression. Indeed, switching in the hog1Δ strain was dependent on the presence of WOR1, as a hog1Δ wor1Δ strain did not show switching to the opaque state. Deletion of PBS2 and SSK2 also resulted in C. albicans cells switching from white to opaque with 100% efficiency, indicating that the entire Hog1 SAPK pathway is involved in regulating this unique phenotypic transition. Interestingly, all Hog1 pathway mutants also caused defects in shmoo formation and mating efficiencies. Overall, this work reveals a novel role for the Hog1 SAPK pathway in regulating white-opaque switching and sexual behavior in C. albicans.

Glucose directly promotes antifungal resistance in the fungal pathogen, Candida spp

Effects of glucose on the susceptibility of antifungal agents were investigated against Candida spp. Increasing the concentration of glucose decreased the activity of antifungal agents; voriconazole was the most affected drugs followed by amphotericin B. No significant change has been observed for anidulafungin. Biophysical interactions between antifungal agents with glucose molecules were investigated using isothermal titration calorimetry, Fourier transform infrared, and (1)H NMR. Glucose has a higher affinity to bind with voriconazole by hydrogen bonding and decrease the susceptibility of antifungal agents during chemotherapy. In addition to confirming the results observed in vitro, theoretical docking studies demonstrated that voriconazole presented three important hydrogen bonds and amphotericin B presented two hydrogen bonds that stabilized the glucose. In vivo results also suggest that the physiologically relevant higher glucose level in the bloodstream of diabetes mellitus mice might interact with the available selective agents during antifungal therapy, thus decreasing glucose activity by complex formation. Thus, proper selection of drugs for diabetes mellitus patients is important to control infectious diseases.

Metabolism in fungal pathogenesis

Fungal pathogens must assimilate local nutrients to establish an infection in their mammalian host. We focus on carbon, nitrogen, and micronutrient assimilation mechanisms, discussing how these influence host-fungus interactions during infection. We highlight several emerging trends based on the available data. First, the perturbation of carbon, nitrogen, or micronutrient assimilation attenuates fungal pathogenicity. Second, the contrasting evolutionary pressures exerted on facultative versus obligatory pathogens have led to contemporary pathogenic fungal species that display differing degrees of metabolic flexibility. The evolutionarily ancient metabolic pathways are conserved in most fungal pathogen, but interesting gaps exist in some species (e.g., Candida glabrata). Third, metabolic flexibility is generally essential for fungal pathogenicity, and in particular, for the adaptation to contrasting host microenvironments such as the gastrointestinal tract, mucosal surfaces, bloodstream, and internal organs. Fourth, this metabolic flexibility relies on complex regulatory networks, some of which are conserved across lineages, whereas others have undergone significant evolutionary rewiring. Fifth, metabolic adaptation affects fungal susceptibility to antifungal drugs and also presents exciting opportunities for the development of novel therapies.

Stress adaptation in a pathogenic fungus

Candida albicans is a major fungal pathogen of humans. This yeast is carried by many individuals as a harmless commensal, but when immune defences are perturbed it causes mucosal infections (thrush). Additionally, when the immune system becomes severely compromised, C. albicans often causes life-threatening systemic infections. A battery of virulence factors and fitness attributes promote the pathogenicity of C. albicans. Fitness attributes include robust responses to local environmental stresses, the inactivation of which attenuates virulence. Stress signalling pathways in C. albicans include evolutionarily conserved modules. However, there has been rewiring of some stress regulatory circuitry such that the roles of a number of regulators in C. albicans have diverged relative to the benign model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. This reflects the specific evolution of C. albicans as an opportunistic pathogen obligately associated with warm-blooded animals, compared with other yeasts that are found across diverse environmental niches. Our understanding of C. albicans stress signalling is based primarily on the in vitro responses of glucose-grown cells to individual stresses. However, in vivo this pathogen occupies complex and dynamic host niches characterised by alternative carbon sources and simultaneous exposure to combinations of stresses (rather than individual stresses). It has become apparent that changes in carbon source strongly influence stress resistance, and that some combinatorial stresses exert non-additive effects upon C. albicans. These effects, which are relevant to fungus–host interactions during disease progression, are mediated by multiple mechanisms that include signalling and chemical crosstalk, stress pathway interference and a biological transistor.

Metabolism impacts upon Candida immunogenicity and pathogenicity at multiple levels

Metabolism is integral to the pathogenicity of Candida albicans, a major fungal pathogen of humans. As well as providing the platform for nutrient assimilation and growth in diverse host niches, metabolic adaptation affects the susceptibility of C. albicans to host-imposed stresses and antifungal drugs, the expression of key virulence factors, and fungal vulnerability to innate immune defences. These effects, which are driven by complex regulatory networks linking metabolism, morphogenesis, stress adaptation, and cell wall remodelling, influence commensalism and infection. Therefore, current concepts of Candida-host interactions must be extended to include the impact of metabolic adaptation upon pathogenicity and immunogenicity.

Mechanisms underlying the exquisite sensitivity of Candida albicans to combinatorial cationic and oxidative stress that enhances the potent fungicidal activity of phagocytes

Immune cells exploit reactive oxygen species (ROS) and cationic fluxes to kill microbial pathogens, such as the fungus Candida albicans. Yet, C. albicans is resistant to these stresses in vitro. Therefore, what accounts for the potent antifungal activity of neutrophils? We show that simultaneous exposure to oxidative and cationic stresses is much more potent than the individual stresses themselves and that this combinatorial stress kills C. albicans synergistically in vitro. We also show that the high fungicidal activity of human neutrophils is dependent on the combinatorial effects of the oxidative burst and cationic fluxes, as their pharmacological attenuation with apocynin or glibenclamide reduced phagocytic potency to a similar extent. The mechanistic basis for the extreme potency of combinatorial cationic plus oxidative stress—a phenomenon we term stress pathway interference—lies with the inhibition of hydrogen peroxide detoxification by the cations. In C. albicans this causes the intracellular accumulation of ROS, the inhibition of Cap1 (a transcriptional activator that normally drives the transcriptional response to oxidative stress), and altered readouts of the stress-activated protein kinase Hog1. This leads to a loss of oxidative and cationic stress transcriptional outputs, a precipitous collapse in stress adaptation, and cell death. This stress pathway interference can be suppressed by ectopic catalase (Cat1) expression, which inhibits the intracellular accumulation of ROS and the synergistic killing of C. albicans cells by combinatorial cationic plus oxidative stress. Stress pathway interference represents a powerful fungicidal mechanism employed by the host that suggests novel approaches to potentiate antifungal therapy.

The immune system combats infection via phagocytic cells that recognize and kill pathogenic microbes. Human neutrophils combat Candida infections by killing this fungus with a potent mix of chemicals that includes reactive oxygen species (ROS) and cations. Yet, Candida albicans is relatively resistant to these stresses in vitro. We show that it is the combination of oxidative plus cationic stresses that kills yeasts so effectively, and we define the molecular mechanisms that underlie this potency. Cations inhibit catalase. This leads to the accumulation of intracellular ROS and inhibits the transcription factor Cap1, which is critical for the oxidative stress response in C. albicans. This triggers a dramatic collapse in fungal stress adaptation and cell death. Blocking either the oxidative burst or cationic fluxes in human neutrophils significantly reduces their ability to kill this fungal pathogen, indicating that combinatorial stress is pivotal to immune surveillance.

Impact of glucose levels on expression of hypha-associated secreted aspartyl proteinases in Candida albicans

BACKGROUND

Ten secreted aspartyl proteinase (Sap) genes were identified in Candida albicans. The products of SAP genes are considered to be virulent factors of C. albicans that participated in causing mucocutaneous and systemic candidiasis in humans. Depending on environmental conditions, C. albicans may stay in yeast-form or convert into invasive hypha-form, and these issues may affect the expression of SAP genes. In this study we explored the component(s) of culture media that may affect the expression of hypha-associated SAP genes.

RESULTS

We demonstrate that glucose levels modulate both the hyphae development and the expression strength of hypha-associated SAP genes (SAP4-6). In contrast to high glucose concentration (2%), lower glucose level (0.1%) is more potent to promote hyphae development and to promptly elicit the expression of hypha-associated Sap proteins during yeast-to-hypha transition of C. albicans. Both Cph1-mediated MAP kinase cascade and Efg1-mediated cAMP/PKA pathway, although the latter seemed dominant, participate in convey the glucose signaling to regulate the expression of hypha-associated SAP genes and this glucose level effect may perform at very early stage of yeast-to-hypha transition. In addition, when C. albicans was co-cultured with THP-1 human monocytes, the engulfed C. albicans was developing hypha efficiently within 1 hr and the expression of hypha-associated Sap proteins could be detected on the distal surface of hyphae.

CONCLUSION

We propose that the glucose level of bloodstream (approximately 0.1%) may be facilitated for stimulation of C. albicans to develop invasive hypha-form and to elicit promptly production of high-level hypha-associated Sap proteins.

Aging and cell death in the other yeasts, Schizosaccharomyces pombe and Candida albicans

How do cells age and die? For the past 20 years, the budding yeast, Saccharomyces cerevisiae, has been used as a model organism to uncover the genes that regulate lifespan and cell death. More recently, investigators have begun to interrogate the other yeasts, the fission yeast, Schizosaccharomyces pombe, and the human fungal pathogen, Candida albicans, to determine if similar longevity and cell death pathways exist in these organisms. After summarizing the longevity and cell death phenotypes in S. cerevisiae, this mini-review surveys the progress made in the study of both aging and programed cell death (PCD) in the yeast models, with a focus on the biology of S. pombe and C. albicans. Particular emphasis is placed on the similarities and differences between the two types of aging, replicative aging, and chronological aging, and between the three types of cell death, intrinsic apoptosis, autophagic cell death, and regulated necrosis, found in these yeasts. The development of the additional microbial models for aging and PCD in the other yeasts may help further elucidate the mechanisms of longevity and cell death regulation in eukaryotes.

The evolutionary rewiring of ubiquitination targets has reprogrammed the regulation of carbon assimilation in the pathogenic yeast Candida albicans

Microbes must assimilate carbon to grow and colonize their niches. Transcript profiling has suggested that Candida albicans, a major pathogen of humans, regulates its carbon assimilation in an analogous fashion to the model yeast Saccharomyces cerevisiae, repressing metabolic pathways required for the use of alterative nonpreferred carbon sources when sugars are available. However, we show that there is significant dislocation between the proteome and transcriptome in C. albicans. Glucose triggers the degradation of the ICL1 and PCK1 transcripts in C. albicans, yet isocitrate lyase (Icl1) and phosphoenolpyruvate carboxykinase (Pck1) are stable and are retained. Indeed, numerous enzymes required for the assimilation of carboxylic and fatty acids are not degraded in response to glucose. However, when expressed in C. albicans, S. cerevisiae Icl1 (ScIcl1) is subjected to glucose-accelerated degradation, indicating that like S. cerevisiae, this pathogen has the molecular apparatus required to execute ubiquitin-dependent catabolite inactivation. C. albicans Icl1 (CaIcl1) lacks analogous ubiquitination sites and is stable under these conditions, but the addition of a ubiquitination site programs glucose-accelerated degradation of CaIcl1. Also, catabolite inactivation is slowed in C. albicans ubi4 cells. Ubiquitination sites are present in gluconeogenic and glyoxylate cycle enzymes from S. cerevisiae but absent from their C. albicans homologues. We conclude that evolutionary rewiring of ubiquitination targets has meant that following glucose exposure, C. albicans retains key metabolic functions, allowing it to continue to assimilate alternative carbon sources. This metabolic flexibility may be critical during infection, facilitating the rapid colonization of dynamic host niches containing complex arrays of nutrients.

Pathogenic microbes must assimilate a range of carbon sources to grow and colonize their hosts. Current views about carbon assimilation in the pathogenic yeast Candida albicans are strongly influenced by the Saccharomyces cerevisiae paradigm in which cells faced with choices of nutrients first use energetically favorable sugars, degrading enzymes required for the assimilation of less favorable alternative carbon sources. We show that this is not the case in C. albicans because there has been significant evolutionary rewiring of the molecular signals that promote enzyme degradation in response to glucose. As a result, this major pathogen of humans retains enzymes required for the utilization of physiologically relevant carbon sources such as lactic acid and fatty acids, allowing it to continue to use these host nutrients even when glucose is available. This phenomenon probably enhances efficient colonization of host niches where sugars are only transiently available.

Growth of Candida albicans cells on the physiologically relevant carbon source lactate affects their recognition and phagocytosis by immune cells

Candida albicans is a normal resident of the human gastrointestinal and urogenital tracts and also a prevalent fungal pathogen. During both commensalism and infection, it must match the immunological defenses of its host while adapting to environmental cues and the local nutrient status. C. albicans regularly colonizes glucose-poor niches, thereby depending upon alternative carbon sources for growth. However, most studies of host immune responses to C. albicans have been performed on fungal cells grown on glucose, and the extent to which alternative physiologically relevant carbon sources impact innate immune responses has not been studied. The fungal cell wall is decorated with multifarious pathogen-associated molecular patterns and is the main target for recognition by host innate immune cells. Cell wall architecture is both robust and dynamic, and it is dramatically influenced by growth conditions. We found that growth of C. albicans cells on lactate, a nonfermentative carbon source available in numerous anatomical niches, modulates their interactions with immune cells and the resultant cytokine profile. Notably, lactate-grown C. albicans stimulated interleukin-10 (IL-10) production while decreasing IL-17 levels, rendering these cells less visible to the immune system than were glucose-grown cells. This trend was observed in clinical C. albicans isolates from different host niches and from different epidemiological clades. In addition, lactate-grown C. albicans cells were taken up by macrophages less efficiently, but they were more efficient at killing and escaping these phagocytic cells. Our data indicate that carbon source has a major impact upon the C. albicans interaction with the innate immune system.

Metabolic activation of the HOG MAP kinase pathway by Snf1/AMPK regulates lipid signaling at the Golgi

Phosphatidylinositol-4-phosphate (PI(4)P) is an important regulator of Golgi function. Metabolic regulation of Golgi PI(4)P requires the lipid phosphatase Sac1 that translocates between endoplasmic reticulum (ER) and Golgi membranes. Localization of Sac1 responds to changes in glucose levels, yet the upstream signaling pathways that regulate Sac1 traffic are unknown. Here, we report that mitogen-activated protein kinase (MAPK) Hog1 transmits glucose signals to the Golgi and regulates localization of Sac1. We find that Hog1 is rapidly activated by both glucose starvation and glucose stimulation, which is independent of the well-characterized response to osmotic stress but requires the upstream element Ssk1 and is controlled by Snf1, the yeast homolog of AMP-activated kinase (AMPK). Elimination of either Hog1 or Snf1 slows glucose-induced translocation of Sac1 lipid phosphatase from the Golgi to the ER and thus delays PI(4)P accumulation at the Golgi. We conclude that a novel cross-talk between the HOG pathway and Snf1/AMPK is required for the metabolic control of lipid signaling at the Golgi.

Host carbon sources modulate cell wall architecture, drug resistance and virulence in a fungal pathogen

The survival of all microbes depends upon their ability to respond to environmental challenges. To establish infection, pathogens such as Candida albicans must mount effective stress responses to counter host defences while adapting to dynamic changes in nutrient status within host niches. Studies of C. albicans stress adaptation have generally been performed on glucose-grown cells, leaving the effects of alternative carbon sources upon stress resistance largely unexplored. We have shown that growth on alternative carbon sources, such as lactate, strongly influence the resistance of C. albicans to antifungal drugs, osmotic and cell wall stresses. Similar trends were observed in clinical isolates and other pathogenic Candida species. The increased stress resistance of C. albicans was not dependent on key stress (Hog1) and cell integrity (Mkc1) signalling pathways. Instead, increased stress resistance was promoted by major changes in the architecture and biophysical properties of the cell wall. Glucose- and lactate-grown cells displayed significant differences in cell wall mass, ultrastructure, elasticity and adhesion. Changes in carbon source also altered the virulence of C. albicans in models of systemic candidiasis and vaginitis, confirming the importance of alternative carbon sources within host niches during C. albicans infections.

Beyond homeostasis: a predictive-dynamic framework for understanding cellular behavior

Microbial regulatory strategies have long been understood in terms of the homeostatic framework, in which a response is interpreted as a restoring force counteracting the immediate intracellular consequences of a change in the environment. In this review, we summarize the breadth of recent discoveries of cellular behavior extending beyond the homeostatic framework. We argue that the nonrandom structure of native habitats makes environmental fluctuations inherently multidimensional. Beyond its utility for accurate perception of immediate events, the temporal regularity of this multidimensional correlation structure allows microbes to make predictions about the trajectory of their sensory environment. We describe recently discovered examples of such predictive behavior, their physiological benefits, and the underlying evolutionary forces shaping them. These observations compel us to go beyond homeostasis and consider a predictive-dynamic framework in which cellular behavior is orchestrated in response to the meaning of an environmental perturbation, not only its direct and immediate fitness consequences.

Stress response in Candida glabrata: pieces of a fragmented picture

Candida glabrata is closely related to yeast but obviously adapted to human commensalism. Communication with the environment is important to adjust allocation of resources between protection and proliferation in order to adapt to different situations in and outside of the host. Gene transcription regulated by environmental conditions is a major response strategy of simple fungal organisms. Differences to yeast include an extended repertoire of adhesive genes, and high drug, starvation and stress resistance. These properties largely do not originate from novel virulence genes but rather from adaptations of the transcriptional wiring. C. glabrata signaling pathways providing stress protection are adopted to meet conditions possibly encountered in a host-pathogen confrontation. The view on C. glabrata is getting clearer and points to a simple strategy combining resilience and a few adaptations.

Role of the HaHOG1 MAP kinase in response of the conifer root and butt rot pathogen (heterobasidion annosum) to osmotic and oxidative stress [corrected]

The basidiomycete Heterobasidion annosum (Fr.) Bref. s.l. is a filamentous white rot fungus, considered to be the most economically important pathogen of conifer trees. Despite the severity of the tree infection, very little is known about the molecular and biochemical aspects related to adaptation to abiotic stresses. In this study, the osmotic and oxidative tolerance as well as the role of the HaHOG1 Mitogen Activated Protein Kinase (MAPK) gene were investigated. The transcript levels of the yeast orthologues GPD1, HSP78, STL1, GRE2 and the ATPase pumps ENA1, PMR1, PMC1 known to have an important role in osmotolerance were also quantified under salt osmotic conditions. The HaHOG1 gene was used for a heterologous expression and functional study in the Saccharomyces cerevisiae Δhog1 strain. Moreover, the phosphorylation level of HaHog1p was studied under salt osmotic and oxidative stress. The result showed that H. annosum displayed a decreased growth when exposed to an increased concentration of osmotic and oxidative stressors. GPD1, HSP78, STL1 and GRE2 showed an induction already at 10 min after exposure to salt stress. Among the ATPase pumps studied, PMC1 was highly induced when the fungus was exposed to 0.2 M CaCl₂ for 60 min. The heterologous expression of the HaHOG1 sequence in yeast confirmed that the gene is able to restore the osmotolerance and oxidative tolerance in the S. cerevisiae hog1Δ mutant strain. The HaHog1p was strongly phosphorylated in the presence of NaCl, KCl, H₂O₂ but not in the presence of CaCl₂ and MgCl₂. The GFP-HaHog1p fusion protein accumulated in the nuclei of the S. cerevisiae hog1Δ cells when exposed to high osmotic conditions but not under oxidative stress. These results provide the first insights about the response of H. annosum to osmotic and oxidative stress and elucidate the role of the HaHOG1 gene in such conditions.

Posttranslational modifications of proteins in the pathobiology of medically relevant fungi

Posttranslational modifications of proteins drive a wide variety of cellular processes in eukaryotes, regulating cell growth and division as well as adaptive and developmental processes. With regard to the fungal kingdom, most information about posttranslational modifications has been generated through studies of the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, where, for example, the roles of protein phosphorylation, glycosylation, acetylation, ubiquitination, sumoylation, and neddylation have been dissected. More recently, information has begun to emerge for the medically important fungal pathogens Candida albicans, Aspergillus fumigatus, and Cryptococcus neoformans, highlighting the relevance of posttranslational modifications for virulence. We review the available literature on protein modifications in fungal pathogens, focusing in particular upon the reversible peptide modifications sumoylation, ubiquitination, and neddylation.

MAPKKK-independent regulation of the Hog1 stress-activated protein kinase in Candida albicans

The Hog1 stress-activated protein kinase regulates both stress responses and morphogenesis in Candida albicans and is essential for the virulence of this major human pathogen. Stress-induced Hog1 phosphorylation is regulated by the upstream MAPKK, Pbs2, which in turn is regulated by the MAPKKK, Ssk2. Here, we have investigated the role of phosphorylation of Hog1 and Pbs2 in Hog1-mediated processes in C. albicans. Mutation of the consensus regulatory phosphorylation sites of Hog1 (Thr-174/Tyr-176) and Pbs2 (Ser-355/Thr-359), to nonphosphorylatable residues, resulted in strains that phenocopied hog1Δ and pbs2Δ cells. Consistent with this, stress-induced phosphorylation of Hog1 was abolished in cells expressing nonphosphorylatable Pbs2 (Pbs2^AA). However, mutation of the consensus sites of Pbs2 to phosphomimetic residues (Pbs2^DD) failed to constitutively activate Hog1. Furthermore, Ssk2-independent stress-induced Hog1 activation was observed in Pbs2^DD cells. Collectively, these data reveal a previously uncharacterized MAPKKK-independent mechanism of Hog1 activation in response to stress. Although Pbs2^DD cells did not exhibit high basal levels of Hog1 phosphorylation, overexpression of an N-terminal truncated form of Ssk2 did result in constitutive Hog1 activation, which was further increased upon stress. Significantly, both Pbs2^AA and Pbs2^DD cells displayed impaired stress resistance and attenuated virulence in a mouse model of disease, whereas only Pbs2^AA cells exhibited the morphological defects associated with loss of Hog1 function. This indicates that Hog1 mediates C. albicans virulence by conferring stress resistance rather than regulating morphogenesis.

A mathematical model for adaptive prediction of environmental changes by microorganisms

Survival in natural habitats selects for microorganisms that are well-adapted to a wide range of conditions. Recent studies revealed that cells evolved innovative response strategies that extend beyond merely sensing a given stimulus and responding to it on encounter. A diversity of microorganisms, including Escherichia coli, Vibrio cholerae, and several yeast species, were shown to use a predictive regulation strategy that uses the appearance of one stimulus as a cue for the likely arrival of a subsequent one. A better understanding of such a predictive strategy requires elucidating the interplay between key biological and environmental forces. Here, we describe a mathematical framework to address this challenge. We base this framework on experimental systems featuring early preparation to either a stress or an exposure to improvement in the growth medium. Our model calculates the fitness advantage originating under each regulation strategy in a given habitat. We conclude that, although a predictive response strategy might by advantageous under some ecologies, its costs might exceed the benefit in others. The combined theoretical-experimental treatment presented here helps assess the potential of natural ecologies to support a predictive behavior.

Enzymatic dysfunction of mitochondrial complex I of the Candida albicans goa1 mutant is associated with increased reactive oxidants and cell death

We have previously shown that deletion of GOA1 (growth and oxidant adaptation) of Candida albicans results in a loss of mitochondrial membrane potential, ATP synthesis, increased sensitivity to oxidants and killing by human neutrophils, and avirulence in a systemic model of candidiasis. We established that translocation of Goa1p to mitochondria occurred during peroxide stress. In this report, we show that the goa1Δ (GOA31), compared to the wild type (WT) and a gene-reconstituted (GOA32) strain, exhibits sensitivity to inhibitors of the classical respiratory chain (CRC), including especially rotenone (complex I [CI]) and salicylhydroxamic acid (SHAM), an inhibitor of the alternative oxidase pathway (AOX), while potassium cyanide (KCN; CIV) causes a partial inhibition of respiration. In the presence of SHAM, however, GOA31 has an enhanced respiration, which we attribute to the parallel respiratory (PAR) pathway and alternative NADH dehydrogenases. Interestingly, deletion of GOA1 also results in a decrease in transcription of the alternative oxidase gene AOX1 in untreated cells as well as negligible AOX1 and AOX2 transcription in peroxide-treated cells. To explain the rotenone sensitivity, we measured enzyme activities of complexes I to IV (CI to CIV) and observed a major loss of CI activity in GOA31 but not in control strains. Enzymatic data of CI were supported by blue native polyacrylamide gel electrophoresis (BN-PAGE) experiments which demonstrated less CI protein and reduced enzyme activity. The consequence of a defective CI in GOA31 is an increase in reactive oxidant species (ROS), loss of chronological aging, and programmed cell death ([PCD] apoptosis) in vitro compared to control strains. The increase in PCD was indicated by an increase in caspase activity and DNA fragmentation in GOA31. Thus, GOA1 is required for a functional CI and partially for the AOX pathway; loss of GOA1 compromises cell survival. Further, the loss of chronological aging is new to studies of Candida species and may offer an insight into therapies to control these pathogens. Our observation of increased ROS production associated with a defective CI and PCD is reminiscent of mitochondrial studies of patients with some types of neurodegenerative diseases where CI and/or CIII dysfunctions lead to increased ROS and apoptosis.

Candida albicans--a pre-whole genome duplication yeast--is predominantly aerobic and a poor ethanol producer

Yeast species belonging to the lineage that underwent the whole genome duplication (WGD), and including Saccharomyces cerevisiae, can grow under anaerobiosis and accumulate ethanol in the presence of glucose and oxygen. The pre-WGD yeasts, which branched from the S. cerevisiae lineage just before the WGD event, including Kluyveromyces lactis, are more dependent on oxygen and do not accumulate large amounts of ethanol in the presence of excess oxygen. Yeasts that belong to the so-called 'lower branches' of the yeast phylogenetic tree and diverged from S. cerevisiae more than 200 million years ago have so far not been thoroughly investigated for their physiology and carbon metabolism. Here, we have studied several isolates of Candida albicans and Debaryomyces hansenii for their dependence on oxygen. Candida albicans grew very poorly at an oxygen concentration <1 p.p.m. and D. hansenii could not grow at all. In aerobic batch cultivations, C. albicans exhibited a predominantly aerobic metabolism, accumulating only small amounts of ethanol (0.01-0.09 g g(-1) glucose). Apparently, C. albicans and several other pre-WGD yeasts still exhibit the original traits of the yeast progenitor: poor accumulation of ethanol under aerobic conditions and strong dependence on the presence of oxygen.

Host cell invasion and virulence mediated by Candida albicans Ssa1

Candida albicans Ssa1 and Ssa2 are members of the HSP70 family of heat shock proteins that are expressed on the cell surface and function as receptors for antimicrobial peptides such as histatins. We investigated the role of Ssa1 and Ssa2 in mediating pathogenic host cell interactions and virulence. A C. albicans ssa1Δ/Δ mutant had attenuated virulence in murine models of disseminated and oropharyngeal candidiasis, whereas an ssa2Δ/Δ mutant did not. In vitro studies revealed that the ssa1Δ/Δ mutant caused markedly less damage to endothelial cells and oral epithelial cell lines. Also, the ssa1Δ/Δ mutant had defective binding to endothelial cell N-cadherin and epithelial cell E-cadherin, receptors that mediate host cell endocytosis of C. albicans. As a result, this mutant had impaired capacity to induce its own endocytosis by endothelial cells and oral epithelial cells. Latex beads coated with recombinant Ssa1 were avidly endocytosed by both endothelial cells and oral epithelial cells, demonstrating that Ssa1 is sufficient to induce host cell endocytosis. These results indicate that Ssa1 is a novel invasin that binds to host cell cadherins, induces host cell endocytosis, and is critical for C. albicans to cause maximal damage to host cells and induce disseminated and oropharyngeal disease.

Effects of calorie restriction on life span of microorganisms

Calorie restriction (CR) in microorganisms such as budding and fission yeasts has a robust and well-documented impact on longevity. In order to efficiently utilize the limited energy during CR, these organisms shift from primarily fermentative metabolism to mitochondrial respiration. Respiration activates certain conserved longevity factors such as sirtuins and is associated with widespread physiological changes that contribute to increased survival. However, the importance of respiration during CR-mediated longevity has remained controversial. The emergence of several novel metabolically distinct microbial models for longevity has enabled CR to be studied from new perspectives. The majority of CR and life span studies have been conducted in the primarily fermentative Crabtree-positive yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, but studies in primarily respiratory Crabtree-negative yeast and obligate aerobes can offer complementary insight into the more complex mammalian response to CR. Not only are microorganisms helping characterize a conserved cellular mechanism for CR-mediated longevity, but they can also directly impact mammalian metabolism as part of the natural gut flora. Here, we discuss the contributions of microorganisms to our knowledge of CR and longevity at the level of both the cell and the organism.

Stress signalling to fungal stress-activated protein kinase pathways

The ability of microorganisms to survive and thrive within hostile environments depends on rapid and robust stress responses. Stress-activated protein kinase (SAPK) pathways are important stress-signalling modules found in all eukaryotes, including eukaryotic microorganisms such as fungi. These pathways consist of a SAPK that is activated by phosphorylation through a kinase cascade, and once activated, the SAPK phosphorylates a range of cytoplasmic and nuclear target substrates, which determine the appropriate response. However, despite their conservation in fungi, mechanisms that have evolved to relay stress signals to the SAPK module in different fungi have diverged significantly. Here, we present an overview of the diverse strategies used in the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe, and the pathogenic fungus Candida albicans, to sense and transduce stress signals to their respective SAPKs.

Functional specialization and differential regulation of short-chain carboxylic acid transporters in the pathogen Candida albicans

The major fungal pathogen Candida albicans has the metabolic flexibility to assimilate a wide range of nutrients in its human host. Previous studies have suggested that C. albicans can encounter glucose-poor microenvironments during infection and that the ability to use alternative non-fermentable carbon sources contributes to its virulence. JEN1 encodes a monocarboxylate transporter in C. albicans and we show that its paralogue, JEN2, encodes a novel dicarboxylate plasma membrane transporter, subjected to glucose repression. A strain deleted in both genes lost the ability to transport lactic, malic and succinic acids by a mediated mechanism and it displayed a growth defect on these substrates. Although no significant morphogenetic or virulence defects were found in the double mutant strain, both JEN1 and JEN2 were strongly induced during infection. Jen1-GFP (green fluorescent protein) and Jen2-GFP were upregulated following the phagocytosis of C. albicans cells by neutrophils and macrophages, displaying similar behaviour to an Icl1-GFP fusion. In the murine model of systemic candidiasis approximately 20-25% of C. albicans cells infecting the kidney expressed Jen1-GFP and Jen2-GFP. Our data suggest that Jen1 and Jen2 are expressed in glucose-poor niches within the host, and that these short-chain carboxylic acid transporters may be important in the early stages of infection.