The quorum-sensing molecule E,E-farnesol-its variable secretion and its impact on the growth and metabolism of Candida species

The role of serum albumin in Candida albicans filamentation, germ tube formation, and farnesol sequestration

Candida albicans is an opportunistic pathogen and colonizer of the human gut and mucosal membranes. C. albicans exhibits morphological plasticity, which is crucial for its fitness within the host and virulence. Morphogenesis in C. albicans is regulated, in part, by its production of farnesol, an autoregulatory molecule that inhibits filamentation. Morphogenesis is also regulated in response to external cues, such as serum, which stimulates hyphal formation by C. albicans. The precise mechanism by which serum stimulates hyphal formation is unknown. The most abundant serum protein is albumin. The binding affinity of albumin for nonpolar, fatty-acid-like molecules suggests that it may interact directly with exogenous farnesol and influence morphogenesis through sequestration of free farnesol. To test this hypothesis, we assessed whether albumin and albumin devoid of fatty acids (i) stimulated farnesol secretion and (ii) influenced the farnesol threshold required to inhibit filamentation. We found that albumin promoted farnesol secretion and filamentation, and the extent of its ability to do so was based on the presence or absence of bound fatty acids. We hypothesize that albumin not bound to fatty acids has the capacity to bind to farnesol and sequester it from C. albicans, encouraging filamentation.IMPORTANCEFor at least 50 years, researchers have wondered about the mechanisms by which serum stimulates germ tube formation (GTF) and hyphal growth in C. albicans. Here, we tested a model (Nickerson et al., Microbiol Mol Biol Rev 88:e00081-22, 2024, https://doi.org/10.1128/mmbr.00081-22) that serum promotes GTF and farnesol synthesis in part by extracting internal farnesol (Fi) from the cells toward the excess binding capacity of the albumins. The data presented here suggests that albumin not bound by fatty acids sequesters free farnesol thereby modulating filamentation and farnesol secretion by altering the equilibrium of internal vs external farnesol. We expect that the influence of secreted farnesol on cell morphology will differ during pathogenesis depending on location within the body, but sequestration of farnesol in the blood could mediate immune cell recruitment and promote hyphal formation.

The Candida albicans quorum-sensing molecule farnesol alters sphingolipid metabolism in human monocyte-derived dendritic cells

Candida albicans, an opportunistic fungal pathogen, produces the quorum-sensing molecule farnesol, which we have shown alters the transcriptional response and phenotype of human monocyte-derived dendritic cells (DCs), including their cytokine secretion and ability to prime T cells. This is partially dependent on the nuclear receptor peroxisome proliferator-activated receptor gamma (PPAR-γ), which has numerous ligands, including the sphingolipid metabolite sphingosine 1-phosphate. Sphingolipids are a vital component of membranes that affect membrane protein arrangement and phagocytosis of C. albicans by DCs. Thus, we quantified sphingolipid metabolites in monocytes differentiating into DCs by High-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). Farnesol increased the activity of serine palmitoyltransferase, leading to increased levels of 3-keto-dihydrosphingosine, dihydrosphingosine, and dihydrosphingosine 1-phosphate and inhibited dihydroceramide desaturase by inducing oxidative stress, leading to increased levels of dihydroceramide and dihydrosphingomyelin species and reduced ceramide levels. Accumulation of dihydroceramides can inhibit mitochondrial function; accordingly, farnesol reduced mitochondrial respiration. Dihydroceramide desaturase inhibition increases lipid droplet formation, which we observed in farnesol-treated cells, coupled with an increase in intracellular triacylglycerol species. Furthermore, inhibition of dihydroceramide desaturase with either farnesol or specific inhibitors impaired the ability of DCs to prime interferon-γ-producing T cells. The effect of farnesol on sphingolipid metabolism, triacylglycerol synthesis, and mitochondrial respiration was not dependent on PPAR-γ. In summary, our data reveal novel effects of farnesol on sphingolipid metabolism, neutral lipid synthesis, and mitochondrial function in DCs that affect their instruction of T cell cytokine secretion, indicating that C. albicans can manipulate host cell metabolism via farnesol secretion.IMPORTANCECandida albicans is a common commensal yeast, but it is also an opportunistic pathogen which is one of the leading causes of potentially lethal hospital-acquired infections. There is growing evidence that its overgrowth in the gut can influence diseases as diverse as alcohol-associated liver disease and COVID-19. Previously, we found that its quorum-sensing molecule, farnesol, alters the phenotype of dendritic cells differentiating from monocytes, impairing their ability to drive protective T cell responses. Here, we demonstrate that farnesol alters the metabolism of sphingolipids, important structural components of the membrane that also act as signaling molecules. In monocytes differentiating to dendritic cells, farnesol inhibited dihydroceramide desaturase, resulting in the accumulation of dihydroceramides and a reduction in ceramide levels. Farnesol impaired mitochondrial respiration, known to occur with an accumulation of dihydroceramides, and induced the accumulation of triacylglycerol and oil bodies. Inhibition of dihydroceramide desaturase resulted in the impaired ability of DCs to induce interferon-γ production by T cells. Thus, farnesol production by C. albicans could manipulate the function of dendritic cells by altering the sphingolipidome.

Physiological adventures in Candida albicans: farnesol and ubiquinones

SUMMARYFarnesol was first identified as a quorum-sensing molecule, which blocked the yeast to hyphal transition in Candida albicans, 22 years ago. However, its interactions with Candida biology are surprisingly complex. Exogenous (secreted or supplied) farnesol can also act as a virulence factor during pathogenesis and as a fungicidal agent triggering apoptosis in other competing fungi. Farnesol synthesis is turned off both during anaerobic growth and in opaque cells. Distinctly different cellular responses are observed as exogenous farnesol levels are increased from 0.1 to 100 µM. Reported changes include altered morphology, stress response, pathogenicity, antibiotic sensitivity/resistance, and even cell lysis. Throughout, there has been a dearth of mechanisms associated with these observations, in part due to the absence of accurate measurement of intracellular farnesol levels (Fi). This obstacle has recently been overcome, and the above phenomena can now be viewed in terms of changing Fi levels and the percentage of farnesol secreted. Critically, two aspects of isoprenoid metabolism present in higher organisms are absent in C. albicans and likely in other yeasts. These are pathways for farnesol salvage (converting farnesol to farnesyl pyrophosphate) and farnesylcysteine cleavage, a necessary step in the turnover of farnesylated proteins. Together, these developments suggest a unifying model, whereby high, threshold levels of Fi regulate which target proteins are farnesylated or the extent to which they are farnesylated. Thus, we suggest that the diversity of cellular responses to farnesol reflects the diversity of the proteins that are or are not farnesylated.

Deciphering the molecular components of the quorum sensing system in the fungus Ophiostoma piceae

IMPORTANCE

This manuscript presents a comprehensive study on the molecular mechanisms triggered by the quorum sensing (QS) molecule farnesol in the biotechnologically relevant fungus Ophiostoma piceae. We present for the first time, using a multiomics approach, an in-depth analysis of the QS response in a saprotroph fungus, detailing the molecular components involved in the response and their possible mechanisms of action. We think that these results are particularly relevant in the knowledge of the functioning of the QS in eukaryotes, as well as for the exploitation of these mechanisms.

Farnesol remodels the peritoneal cavity immune environment influencing Candida albicans pathogenesis during intra-abdominal infection

Candida albicans is a lifelong member of the mycobiome causing mucosal candidiasis and life-threatening, systemic, and intra-abdominal disease in immunocompromised and transplant patients. Despite the clinical importance of intra-abdominal candidiasis with mortality rates between 40% and 70%, the contribution of fungal virulence factors and host immune responses to disease has not been extensively studied. Secretion of the quorum-sensing molecule, farnesol, acts as a virulence factor for C. albicans during systemic infection, while inducing local, protective innate immune responses in oral models of infection. Previously, we reported that farnesol recruits macrophages to the peritoneal cavity in mice, suggesting a role for farnesol in innate immune responses. Here, we expand on our initial findings, showing that farnesol profoundly alters the peritoneal cavity microenvironment promoting innate inflammation. Intra-peritoneal injection of farnesol stimulates rapid local death of resident peritoneal cells followed by recruitment of neutrophils and inflammatory macrophages into the peritoneal cavity and peritoneal mesothelium associated with an early increase in chemokines followed by proinflammatory cytokines. These rapid inflammatory responses to farnesol significantly increase morbidity and mortality of mice with intra-abdominal candidiasis associated with increased formation of peritoneal adhesions, despite similar rates of fungal clearance from the peritoneal cavity and retro-peritoneal organs. C. albicans ddp3Δ/ddp3Δ knockout and reconstituted strains recapitulate these findings. This indicates that farnesol may be detrimental to the host during intra-abdominal infections. Importantly, our results highlight a need to understand how C. albicans virulence factors modulate the host immune response within the peritoneum, an exceedingly common site of Candida infection.

Synergistic Effect of Plant Compounds in Combination with Conventional Antimicrobials against Biofilm of Staphylococcus aureus, Pseudomonas aeruginosa, and Candida spp

Bacterial and fungal biofilm has increased antibiotic resistance and plays an essential role in many persistent diseases. Biofilm-associated chronic infections are difficult to treat and reduce the efficacy of medical devices. This global problem has prompted extensive research to find alternative strategies to fight microbial chronic infections. Plant bioactive metabolites with antibiofilm activity are known to be potential resources to alleviate this problem. The phytochemical screening of some medicinal plants showed different active groups, such as stilbenes, tannins, alkaloids, terpenes, polyphenolics, flavonoids, lignans, quinones, and coumarins. Synergistic effects can be observed in the interaction between plant compounds and conventional drugs. This review analyses and summarises the current knowledge on the synergistic effects of plant metabolites in combination with conventional antimicrobials against biofilms of Staphylococcus aureus, Pseudomonas aeruginosa, and Candida albicans. The synergism of conventional antimicrobials with plant compounds can modify and inhibit the mechanisms of acquired resistance, reduce undesirable effects, and obtain an appropriate therapeutic effect at lower doses. A deeper knowledge of these combinations and of their possible antibiofilm targets is needed to develop next-generation novel antimicrobials and/or improve current antimicrobials to fight drug-resistant infections attributed to biofilm.

Biofilm: The invisible culprit in catheter-induced candidemia

Candidemia is the most common form of invasive fungal infection associated with several risk factors, and one of them is the use of medical devices, to which microbial biofilms can attach. Candidemia related to the use of peripheral intravascular and central venous catheters (CVC) is referred to as Candida catheter-related bloodstream infection, with more than 90% being related to CVC usage. The infection is associated with a higher morbidity and mortality rate than nosocomial bacterial infections. Candida spp. can protect themselves from the host immune system and antifungal drugs because of the biofilm structure, which is potentiated by the extracellular matrix (ECM). Candida albicans and Candida parapsilosis are the most pathogenic species often found to form biofilms associated with catheter usage. Biofilm formation of C. albicans includes four mechanisms: attachment, morphogenesis, maturation and dispersion. The biofilms formed between C. albicans and non-albicans spp. differ in ECM structure and composition and are associated with the persistence of colonization to infection for various catheter materials and antifungal resistance. Efforts to combat Candida spp. biofilm formation on catheters are still challenging because not all patients, especially those who are critically ill, can be recommended for catheter removal; also to be considered are the characteristics of the biofilm itself, which readily colonizes the permanent medical devices used. The limited choice and increasing systemic antifungal resistance also make treating it more difficult. Hence, alternative strategies have been developed to manage Candida biofilm. Current options for prevention or therapy in combination with systemic antifungal medications include lock therapy, catheter coating, natural peptide products and photodynamic inactivation.

A clash of quorum sensing vs quorum sensing inhibitors: an overview and risk of resistance

Indiscriminate use of antibiotics to treat microbial pathogens has caused emergence of multiple drug resistant strains. Most infectious diseases are caused by microbes that are capable of intercommunication using signaling molecules, which is known as quorum sensing (QS). Such pathogens express their pathogenicity through various QS-regulated virulence factors. Interference of QS could lead to decisive results in controlling such pathogenicity. Hence, QS inhibition has become an attractive new approach for the development of novel drugs. Many quorum sensing inhibitors (QSIs) of diverse origins have been reported. It is imperative that more such anti-QS compounds be found and studied, as they have significant effect on microbial pathogenicity. This review attempts to give a brief account of QS mechanism, its inhibition and describes some compounds with anti-QS potential. Also discussed is the possibility of emergence of quorum sensing resistance.

Overview of Bioactive Fungal Secondary Metabolites: Cytotoxic and Antimicrobial Compounds

Microorganisms are known as important sources of natural compounds that have been studied and applied for different purposes in distinct areas. Specifically, in the pharmaceutical area, fungi have been explored mainly as sources of antibiotics, antiviral, anti-inflammatory, enzyme inhibitors, hypercholesteremic, antineoplastic/antitumor, immunomodulators, and immunosuppressants agents. However, historically, the high demand for new antimicrobial and antitumor agents has not been sufficiently attended by the drug discovery process, highlighting the relevance of intensifying studies to reach sustainable employment of the huge world biodiversity, including the microorganisms. Therefore, this review describes the main approaches and tools applied in the search for bioactive secondary metabolites, as well as presents several examples of compounds produced by different fungi species with proven pharmacological effects and additional examples of fungal cytotoxic and antimicrobial molecules. The review does not cover all fungal secondary metabolites already described; however, it presents some reports that can be useful at any phase of the drug discovery process, mainly for pharmaceutical applications.

Similarities and Differences among Species Closely Related to Candida albicans: C. tropicalis, C. dubliniensis, and C. auris

Although Candida species are widespread commensals of the microflora of healthy individuals, they are also among the most important human fungal pathogens that under certain conditions can cause diseases (candidiases) of varying severity ranging from mild superficial infections of the mucous membranes to life-threatening systemic infections. So far, the vast majority of research aimed at understanding the molecular basis of pathogenesis has been focused on the most common species—Candida albicans. Meanwhile, other closely related species belonging to the CTG clade, namely, Candida tropicalis and Candida dubliniensis, are becoming more important in clinical practice, as well as a relatively newly identified species, Candida auris. Despite the close relationship of these microorganisms, it seems that in the course of evolution, they have developed distinct biochemical, metabolic, and physiological adaptations, which they use to fit to commensal niches and achieve full virulence. Therefore, in this review, we describe the current knowledge on C. tropicalis, C. dubliniensis, and C. auris virulence factors, the formation of a mixed species biofilm and mutual communication, the environmental stress response and related changes in fungal cell metabolism, and the effect of pathogens on host defense response and susceptibility to antifungal agents used, highlighting differences with respect to C. albicans. Special attention is paid to common diagnostic problems resulting from similarities between these species and the emergence of drug resistance mechanisms. Understanding the different strategies to achieve virulence, used by important opportunistic pathogens of the genus Candida, is essential for proper diagnosis and treatment.

Quantitative assay for farnesol and the aromatic fusel alcohols from the fungus Candida albicans

Abstract

The dimorphic fungus Candida albicans is a commensal and opportunistic fungal pathogen of humans. It secretes at least four small lipophilic molecules, farnesol and three aromatic fusel alcohols. Farnesol has been identified as both a quorum sensing molecule (QSM) and a virulence factor. Our gas chromatography (GC)-based assay for these molecules exhibits high throughput, prevention of analyte loss by avoiding filtration and rotary evaporation, simultaneous cell lysis and analyte extraction by ethyl acetate, and the ability to compare whole cultures with their cell pellets and supernatants. Farnesol synthesis and secretion were separable phenomena and pellet:supernatant ratios for farnesol were high, up to 12:1. The assay was validated in terms of precision, specificity, ruggedness, accuracy, solution stability, detection limits (DL), quantitation limits (QL), and dynamic range. The DL for farnesol was 0.02 ng/µl (0.09 µM). Measurement quality was assessed by the relative error of the whole culture versus the sum of pellet and supernatant fractions (WPS). C. albicans strain SC5314 grown at 30 °C in complex and defined media (YPD and mRPMI) was assayed in biological triplicate 17 times over 3 days. Farnesol and the three aromatic fusel alcohols can be measured in the same assay. The levels of all four are greatly altered by the growth medium chosen. Significantly, the three fusel alcohols are synthesized during stationary phase, not during growth. They are secreted quickly without being retained in the cell pellet and may accumulate up to mM concentrations.

Key points

• Quantitative analysis of both intra- and extracellular farnesol, and aromatic fusel oils.

• High throughput, whole culture assay with simultaneous lysis and extraction.

• Farnesol secretion and synthesis are distinct and separate events.

The Antibiofilm Role of Biotics Family in Vaginal Fungal Infections

“Unity in strength” is a notion that can be exploited to characterize biofilms as they bestow microbes with protection to live freely, escalate their virulence, confer high resistance to therapeutic agents, and provide active grounds for the production of biofilms after dispersal. Naturally, fungal biofilms are inherently resistant to many conventional antifungals, possibly owing to virulence factors as their ammunitions that persistently express amid planktonic transition to matured biofilm state. These ammunitions include the ability to form polymicrobial biofilms, emergence of persister cells post-antifungal treatment and acquisition of resistance genes. One of the major disorders affecting vaginal health is vulvovaginal candidiasis (VVC) and its reoccurrence is termed recurrent VVC (RVVC). It is caused by the Candida species which include Candida albicans and Candida glabrata. The aforementioned Candida species, notably C. albicans is a biofilm producing pathogen and habitually forms part of the vaginal microbiota of healthy women. Latest research has implicated the role of fungal biofilms in VVC, particularly in the setting of treatment failure and RVVC. Consequently, a plethora of studies have advocated the utilization of probiotics in addressing these infections. Specifically, the excreted or released compounds of probiotics which are also known as postbiotics are being actively researched with vast potential to be used as therapeutic options for the treatment and prevention of VVC and RVVC. These potential sources of postbiotics are harnessed due to their proven antifungal and antibiofilm. Hence, this review discusses the role of Candida biofilm formation in VVC and RVVC. In addition, we discuss the application of pro-, pre-, post-, and synbiotics either individually or in combined regimen to counteract the abovementioned problems. A clear understanding of the role of biofilms in VVC and RVVC will provide proper footing for further research in devising novel remedies for prevention and treatment of vaginal fungal infections.

Fungal Quorum-Sensing Molecules: A Review of Their Antifungal Effect against Candida Biofilms

The number of effective therapeutic strategies against biofilms is limited; development of novel therapies is urgently needed to treat a variety of biofilm-associated infections. Quorum sensing is a special form of microbial cell-to-cell communication that is responsible for the release of numerous extracellular molecules, whose concentration is proportional with cell density. Candida-secreted quorum-sensing molecules (i.e., farnesol and tyrosol) have a pivotal role in morphogenesis, biofilm formation, and virulence. Farnesol can mediate the hyphae-to-yeast transition, while tyrosol has the opposite effect of inducing transition from the yeast to hyphal form. A number of questions regarding Candida quorum sensing remain to be addressed; nevertheless, the literature shows that farnesol and tyrosol possess remarkable antifungal and anti-biofilm effect at supraphysiological concentration. Furthermore, previous in vitro and in vivo data suggest that they may have a potent adjuvant effect in combination with certain traditional antifungal agents. This review discusses the most promising farnesol- and tyrosol-based in vitro and in vivo results, which may be a foundation for future development of novel therapeutic strategies to combat Candida biofilms.

In vitro and in vivo Effect of Exogenous Farnesol Exposure Against Candida auris

The spreading of multidrug-resistant Candida auris is considered as an emerging global health threat. The number of effective therapeutic regimens is strongly limited; therefore, development of novel strategies is needed. Farnesol is a quorum-sensing molecule with a potential antifungal and/or adjuvant effect; it may be a promising candidate in alternative treatment against Candida species including C. auris. To examine the effect of farnesol on C. auris, we performed experiments focusing on growth, biofilm production ability, production of enzymes related to oxidative stress, triazole susceptibility and virulence. Concentrations ranging from 100 to 300 μM farnesol caused a significant growth inhibition against C. auris planktonic cells for 24 h (p < 0.01-0.05). Farnesol treatment showed a concentration dependent inhibition in terms of biofilm forming ability of C. auris; however, it did not inhibit significantly the biofilm development at 24 h. Nevertheless, the metabolic activity of adhered farnesol pre-exposed cells (75 μM) was significantly diminished at 24 h depending on farnesol treatment during biofilm formation (p < 0.001-0.05). Moreover, 300 μM farnesol exerted a marked decrease in metabolic activity against one-day-old biofilms between 2 and 24 h (p < 0.001). Farnesol increased the production of reactive species remarkably, as revealed by 2',7'-dichlorofluorescein (DCF) assay {3.96 ± 0.89 [nmol DCF (OD₆₄₀)^-1] and 23.54 ± 4.51 [nmol DCF (OD₆₄₀)^-1] for untreated cells and farnesol exposed cells, respectively; p < 0.001}. This was in line with increased superoxide dismutase level {85.69 ± 5.42 [munit (mg protein)^-1] and 170.11 ± 17.37 [munit (mg protein)^-1] for untreated cells and farnesol exposed cells, respectively; p < 0.001}, but the catalase level remained statistically comparable between treated and untreated cells (p > 0.05). Concerning virulence-related enzymes, exposure to 75 μM farnesol did not influence phospholipase or aspartic proteinase activity (p > 0.05). The interaction between fluconazole, itraconazole, voriconazole, posaconazole, isavuconazole and farnesol showed clear synergism (FICI ranges from 0.038 to 0.375) against one-day-old biofilms. Regarding in vivo experiments, daily 75 μM farnesol treatment decreased the fungal burden in an immunocompromised murine model of disseminated candidiasis, especially in case of inocula pre-exposed to farnesol (p < 0.01). In summary, farnesol shows a promising therapeutic or adjuvant potential in traditional or alternative therapies such as catheter lock therapy.

Farnesol, a Quorum-Sensing Molecule of Candida Albicans Triggers the Release of Neutrophil Extracellular Traps

The efficient growth of pathogenic bacteria and fungi in the host organism is possible due to the formation of microbial biofilms that cover the host tissues. Biofilms provide optimal local environmental conditions for fungal cell growth and increased their protection against the immune system. A common biofilm-forming fungus—Candida albicans—uses the quorum sensing (QS) mechanism in the cell-to-cell communication, which determines the biofilm development and, in consequence, host colonization. In the presented work, we focused on the ability of neutrophils—the main cells of the host’s immune system to recognize quorum sensing molecules (QSMs) produced by C. albicans, especially farnesol (FOH), farnesoic acid (FA), and tyrosol (TR), with emphasis on the neutrophil extracellular traps (NETs) formation in a process called netosis. Our results showed for the first time that only farnesol but not farnesolic acid or tyrosol is capable of activating the NET production. By using selective inhibitors of the NET signaling pathway and analyzing the activity of selected enzymes such as Protein Kinase C (PKC), ERK1/2, and NADPH oxidase, we showed that the Mac−1 and TLR2 receptors are responsible for FOH recognizing and activating the reactive oxygen species (ROS)-dependent netosis pathway.

Filamentous Non-albicans Candida Species Adhere to Candida albicans and Benefit From Dual Biofilm Growth

Non-albicans Candida species (NACS) are often isolated along with Candida albicans in cases of oropharyngeal candidiasis. C. albicans readily forms biofilms in conjunction with other oral microbiota including both bacteria and yeast. Adhesion between species is important to the establishment of these mixed biofilms, but interactions between C. albicans and many NACS are not well-characterized. We adapted a real-time flow biofilm model to study adhesion interactions and biofilm establishment in C. albicans and NACS in mono- and co-culture. Out of five NACS studied, only the filamenting species C. tropicalis and C. dubliniensis were capable of adhesion with C. albicans, while C. parapsilosis, C. lusitaniae, and C. krusei were not. Over the early phase (0-4 h) of biofilm development, both mono- and co-culture followed similar kinetics of attachment and detachment events, indicating that initial biofilm formation is not influenced by inter-species interactions. However, the NACS showed a preference for inter-species cell-cell interactions with C. albicans, and at later time points (5-11 h) we found that dual-species interactions impacted biofilm surface coverage. Dual-species biofilms of C. tropicalis and C. albicans grew more slowly than C. albicans alone, but achieved higher surface coverage than C. tropicalis alone. Biofilms of C. dubliniensis with C. albicans increased surface coverage more rapidly than either species alone. We conclude that dual culture biofilm of C. albicans with C. tropicalis or C. dubliniensis offers a growth advantage for both NACS. Furthermore, the growth and maintenance, but not initial establishment, of dual-species biofilms is likely facilitated by interspecies cell-cell adherence.

Effect of quorum sensing molecules and natamycin on biofilms of Candida tropicalis and other yeasts isolated from industrial juice filtration membranes

AIMS

Cells limit the cell number of dense biofilms by releasing self-inhibitory molecules. Here, we aim to assess the effectiveness of yeast quorum sensing (QS) molecules and the antifungal agent natamycin against yeast biofilms of strains commonly isolated from fruit juice ultrafiltration membranes.

METHODS AND RESULTS

Yeast QS molecules, such as tyrosol, 2-phenylethanol and farnesol, were detected by solvent extraction and HS-SPME GC-MS in Candida tropicalis cultures. The effect of QS molecules on mono- and multispecies biofilms formed by Rhodotorula mucilaginosa, C. tropicalis, Candida krusei and Candida kefyr was evaluated by plate count and epifluorescence microscopy. Farnesol caused a decrease in cell number and disrupted mono- and multispecies yeast biofilms during adhesion (0·6 mmol l^-1 ). 2-phenyl ethanol 1·2 mmol l^-1 stimulated biofilm density and increased cell number in both mono- and multispecies biofilms, while tyrosol did not show effects when tested against C. tropicalis biofilms (0·05-1·2 mmol l^-1 ). Natamycin caused a strong decrease in cell number and disruption of biofilm structure in C. tropicalis biofilms at high concentrations (0·3-1·2 mmol l^-1 ). The combination of farnesol 0·6 mmol l^-1 and natamycin at 0·01 mmol l^-1 , the maximum concentration of natamycin accepted for direct addition into fruit juices, effectively reduced cell counts and disrupted the structure of C. tropicalis biofilms.

CONCLUSION

Farnesol 0·6 mmol l^-1 significantly increased the inhibition exerted by natamycin 0·01 mmol l^-1 (~5 ppm) reducing biofilm development from juice on stainless steel surfaces.

SIGNIFICANCE AND IMPACT OF THE STUDY

These results support the use of QS molecules as biofilm inhibitors in beverages and would certainly inspire the design of novel preservative and cleaning products for the food industry based on combinatory approaches.

Growth Regulation in Amphibian Pathogenic Chytrid Fungi by the Quorum Sensing Metabolite Tryptophol

Amphibians face many threats leading to declines and extinctions, but the chytrid fungal skin pathogens Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal) have been identified as the causative factors leading to one of the greatest disease-driven losses of amphibian biodiversity worldwide. Infection may lead to different clinical outcomes, and lethal infections are commonly associated with unrestricted, exponential fungal growth in the amphibian epidermis. Mechanisms underpinning Bd and Bsal growth in the amphibian host are poorly understood. Here, we describe a quorum sensing mechanism that allows cell-to-cell communication by Bd and Bsal in order to regulate fungal densities and infection strategies. Addition of chytrid culture supernatant to chytrid cultures resulted in a concentration-dependent growth reduction and using dialysis, small metabolites were shown to be the causative factor. U-HPLC-MS/MS and in vitro growth tests identified the aromatic alcohol tryptophol as a key metabolite in regulating fungal growth. We determined tryptophol kinetics in both Bd and Bsal and confirmed the autostimulatory mode of action of this quorum sensing metabolite. Finally, we linked expression of genes that might be involved in tryptophol production, with in vitro and in vivo chytrid growth. Our results show that Bd and Bsal fungi use tryptophol to act as multicellular entities in order to regulate their growth.

Metabolic Profiling of Candida auris, a Newly-Emerging Multi-Drug Resistant Candida Species, by GC-MS

Candida auris, a newly-emerging Candida species, is a serious global health threat due to its multi-drug resistant pattern, difficulty to diagnose, and the high mortality associated with its invasive and bloodstream infections. Unlike C. albicans, and C. dubliniensis which can form true hyphae, C. auris grows as yeast or pseudohyphae and is capable of developing biofilms. The reasons for the inability of C. auris to form true hyphae are currently unknown. Metabolites secreted by microorganisms, including Candida, are known as important factors in controlling morphogenesis and pathogenesis. Metabolic profiling of C. auris and C. albicans cultures was performed using gas chromatography–mass spectrometry (GC–MS). Compared to C. albicans, C. auris secreted several hyphae-inhibiting metabolites, including phenylethyl, benzyl and isoamyl alcohols. Furthermore, a biofilm-forming metabolite—tyrosol—was identified. On the other hand, several other biomarkers identified from C. auris but not from C. albicans cultures may be produced by the organism to overcome the host immune system or control fungal adaptations, and hence ease its invasion and infections. The results from this study are considered as the first identification of C. auris metabolic activities as a step forward to understand its virulence mechanisms.

Farnesol inhibits planktonic cells and antifungal-tolerant biofilms of Trichosporon asahii and Trichosporon inkin

Trichosporon species have been considered important agents of opportunistic systemic infections, mainly among immunocompromised patients. Infections by Trichosporon spp. are generally associated with biofilm formation in invasive medical devices. These communities are resistant to therapeutic antifungals, and therefore the search for anti-biofilm molecules is necessary. This study evaluated the inhibitory effect of farnesol against planktonic and sessile cells of clinical Trichosporon asahii (n = 3) andTrichosporon inkin (n = 7) strains. Biofilms were evaluated during adhesion, development stages and after maturation for metabolic activity, biomass and protease activity, as well as regarding morphology and ultrastructure by optical microscopy, confocal laser scanning microscopy, and scanning electron microscopy. Farnesol inhibited Trichosporon planktonic growth by 80% at concentrations ranging from 600 to 1200 μM for T. asahii and from 75 to 600 μM for T. inkin. Farnesol was able to reduce cell adhesion by 80% at 300 μM for T. asahii and T. inkin at 600 μM, while biofilm development of both species was inhibited by 80% at concentration of 150 μM, altering their structure. After biofilm maturation, farnesol decreased T. asahii biofilm formation by 50% at 600 μM concentration and T. inkin formation at 300 μM. Farnesol inhibited gradual filamentation in a concentration range between 600 and 1200 μM. Farnesol caused reduction of filament structures of Trichosporon spp. at every stage of biofilm development analyzed. These data show the potential of farnesol as an anti-biofilm molecule.

Role of quorum sensing and chemical communication in fungal biotechnology and pathogenesis

Microbial cells do not live in isolation in their environment, but rather they communicate with each other using chemical signals. This sophisticated mode of cell-to-cell signalling, known as quorum sensing, was first discovered in bacteria, and coordinates the behaviour of microbial population behaviour in a cell-density-dependent manner. More recently, these mechanisms have been described in eukaryotes, particularly in fungi, where they regulate processes such as pathogenesis, morphological differentiation, secondary metabolite production and biofilm formation. In this manuscript, we review the information available to date on these processes in yeast, dimorphic fungi and filamentous fungi. We analyse the diverse chemical 'languages' used by different groups of fungi, their possible cross-talk and interkingdom interactions with other organisms. We discuss the existence of these mechanisms in multicellular organisms, the ecophysiological role of QS in fungal colonisation and the potential applications of these mechanisms in biotechnology and pathogenesis.

Candida albicans Zn Cluster Transcription Factors Tac1 and Znc1 Are Activated by Farnesol To Upregulate a Transcriptional Program Including the Multidrug Efflux Pump CDR1

Farnesol, a quorum-sensing molecule, inhibits Candida albicans hyphal formation, affects its biofilm formation and dispersal, and impacts its stress response. Several aspects of farnesol's mechanism of action remain incompletely uncharacterized. Among these are a thorough accounting of the cellular receptors and transporters for farnesol. This work suggests these processes are linked through the Zn cluster transcription factors Tac1 and Znc1 and their induction of the multidrug efflux pump Cdr1. Specifically, we have demonstrated that Tac1 and Znc1 are functionally activated by farnesol through a mechanism that mimics other means of hyperactivation of Zn cluster transcription factors. This is consistent with our observation that many genes acutely induced by farnesol are dependent on TAC1, ZNC1, or both. A related molecule, 1-dodecanol, invokes a similar TAC1-ZNC1 response, while several other proposed C. albicans quorum-sensing molecules do not. Tac1 and Znc1 both bind to and upregulate the CDR1 promoter in response to farnesol. Differences in inducer and DNA binding specificity lead to Tac1 and Znc1 having overlapping, but nonidentical, regulons. Induction of genes by farnesol via Tac1 and Znc1 was inversely related to the level of CDR1 present in the cell, suggesting a model in which induction of CDR1 by Tac1 and Znc1 leads to an increase in farnesol efflux. Consistent with this premise, our results show that CDR1 expression, and its regulation by TAC1 and ZNC1, facilitates growth in the presence of high farnesol concentrations in C. albicans and in certain strains of its close relative, C. dubliniensis.

G1 and S phase arrest in Candida albicans induces filamentous growth via distinct mechanisms

Candida albicans is an opportunistic fungal pathogen. In immunocompromised individuals, it can cause bloodstream infections with high mortality rates. The ability to switch between yeast and hyphal morphologies is a critical virulence factor of C. albicans. In response to diverse environmental cues, several signaling pathways are activated resulting in filamentous growth. Interestingly, cell cycle arrest can also trigger filamentous growth although the pathways involved are not well-understood. Here, we demonstrate that the cAMP-PKA pathway is involved in the filamentous growth caused by G1 arrest due to the depletion of the G1 cyclin Cln3 and S phase arrest due to hydroxyurea treatment. The downstream mechanisms involved in filamentation are different between the two cell cycle arrest phenomena. Cln3-depleted cells require HGC1 and UME6 for filamentous growth, but hydroxyurea-induced filamentation does not. Also, the hyphal repressor Nrg1 is not involved in the suppression of Cln3-depletion and hydroxyurea-induced filamentous growth. The findings highlight the complexity of the signaling networks that control filamentous growth in which different mechanisms downstream of the cAMP-PKA pathway are activated based on the nature of the inducing signals.

Quorum sensing: A less known mode of communication among fungi

Quorum sensing (QS), a density-dependent signaling mechanism of microbial cells, involves an exchange and sense of low molecular weight signaling compounds called autoinducers. With the increase in population density, the autoinducers accumulate in the extracellular environment and once their concentration reaches a threshold, many genes are either expressed or repressed. This cell density-dependent signaling mechanism enables single cells to behave as multicellular organisms and regulates different microbial behaviors like morphogenesis, pathogenesis, competence, biofilm formation, bioluminescence, etc guided by environmental cues. Initially, QS was regarded to be a specialized system of certain bacteria. The discovery of filamentation control in pathogenic polymorphic fungus Candida albicans by farnesol revealed the phenomenon of QS in fungi as well. Pathogenic microorganisms primarily regulate the expression of virulence genes using QS systems. The indirect role of QS in the emergence of multiple drug resistance (MDR) in microbial pathogens necessitates the finding of alternative antimicrobial therapies that target QS and inhibit the same. A related phenomenon of quorum sensing inhibition (QSI) performed by small inhibitor molecules called quorum sensing inhibitors (QSIs) has an ability for efficient reduction of gene expression regulated by quorum sensing. In the present review, recent advancements in the study of different fungal quorum sensing molecules (QSMs) and quorum sensing inhibitors (QSIs) of fungal origin along with their mechanism of action and/or role/s are discussed.

Candida Biofilms: Threats, Challenges, and Promising Strategies

Candida species are fungal pathogens known for their ability to cause superficial and systemic infections in the human host. These pathogens are able to persist inside the host due to the development of pathogenicity and multidrug resistance traits, often leading to the failure of therapeutic strategies. One specific feature of Candida species pathogenicity is their ability to form biofilms, which protects them from external factors such as host immune system defenses and antifungal drugs. This review focuses on the current threats and challenges when dealing with biofilms formed by Candida albicans, Candida glabrata, Candida tropicalis, and Candida parapsilosis, highlighting the differences between the four species. Biofilm characteristics depend on the ability of each species to produce extracellular polymeric substances (EPS) and display dimorphic growth, but also on the biofilm substratum, carbon source availability and other factors. Additionally, the transcriptional control over processes like adhesion, biofilm formation, filamentation, and EPS production displays great complexity and diversity within pathogenic yeasts of the Candida genus. These differences not only have implications in the persistence of colonization and infections but also on antifungal resistance typically found in Candida biofilm cells, potentiated by EPS, that functions as a barrier to drug diffusion, and by the overexpression of drug resistance transporters. The ability to interact with different species in in vivo Candida biofilms is also a key factor to consider when dealing with this problem. Despite many challenges, the most promising strategies that are currently available or under development to limit biofilm formation or to eradicate mature biofilms are discussed.

Candida tropicalis Isolates from Mexican Republic Exhibit High Susceptibility to Bleomycin and Variable Susceptibility to Hydrogen Peroxide

Candida sp. are found as part of the commensal flora in humans but can cause invasive candidiasis in patients with severe underlying disease, especially cancer patients. These patients are frequently subjected to nonsurgical anticancer treatments such as ionizing radiation and anticancer drugs, which kill proliferating human cells by damaging DNA but also affect the microbiota of the patient. C. tropicalis, an emerging fungal pathogen, is associated with high mortality rates of cancer patients especially in tropical regions. In this study, we have investigated the in vitro susceptibility of 38 C. tropicalis clinical isolates from several Mexican hospitals to chronic treatments with several DNA damaging agents, including oxidizing compounds and anticancer drugs. C. tropicalis isolates displayed a high variability in their susceptibility to hydrogen peroxide (H₂O₂) while showing a high susceptibility to bleomycin (BLM), an anticancer drug that causes double-strand breaks in DNA. This contrasted with the moderate-to-high resistance exhibited by several C. albicans laboratory strains. At least for the C. tropicalis reference strain MYA3404, this susceptibility was hardly modified by the presence of serum. Our results open the possibility of using susceptibility to BLM to differentiate between C. tropicalis and C. albicans; however, analysis of a larger number of isolates is required. The use of BLM for prevention of C. tropicalis infections in neutropenic patients with cancer should be also evaluated. Finally, the variable susceptibility to H₂O₂ might be due to allelic variation of the histone acetyl-transferase complex which modulates the induction kinetics of H₂O₂-induced genes in C. tropicalis.

An Update on Candida tropicalis Based on Basic and Clinical Approaches

Candida tropicalis has emerged as one of the most important Candida species. It has been widely considered the second most virulent Candida species, only preceded by C. albicans. Besides, this species has been recognized as a very strong biofilm producer, surpassing C. albicans in most of the studies. In addition, it produces a wide range of other virulence factors, including: adhesion to buccal epithelial and endothelial cells; the secretion of lytic enzymes, such as proteinases, phospholipases, and hemolysins, bud-to-hyphae transition (also called morphogenesis) and the phenomenon called phenotypic switching. This is a species very closely related to C. albicans and has been easily identified with both phenotypic and molecular methods. In addition, no cryptic sibling species were yet described in the literature, what is contradictory to some other medically important Candida species. C. tropicalis is a clinically relevant species and may be the second or third etiological agent of candidemia, specifically in Latin American countries and Asia. Antifungal resistance to the azoles, polyenes, and echinocandins has already been described. Apart from all these characteristics, C. tropicalis has been considered an osmotolerant microorganism and this ability to survive to high salt concentration may be important for fungal persistence in saline environments. This physiological characteristic makes this species suitable for use in biotechnology processes. Here we describe an update of C. tropicalis, focusing on all these previously mentioned subjects.

The Effect of Novel Heterocyclic Compounds on Cryptococcal Biofilm

Biofilm formation by microorganisms depends on their communication by quorum sensing, which is mediated by small diffusible signaling molecules that accumulate in the extracellular environment. During human infection, the pathogenic yeast Cryptococcus neoformans can form biofilm on medical devices, which protects the organism and increases its resistance to antifungal agents. The aim of this study was to test two novel heterocyclic compounds, S-8 (thiazolidinedione derivative, TZD) and NA-8 (succinimide derivative, SI), for their anti-biofilm activity against strains of Cryptococcus neoformans and Cryptococcus gattii. Biofilms were formed in a defined medium in 96-well polystyrene plates and 8-well micro-slides. The effect of sub-inhibitory concentrations of S-8 and NA-8 on biofilm formation was measured after 48 h by a metabolic reduction assay and by confocal laser microscopy analysis using fluorescent staining. The formation and development of cryptococcal biofilms was inhibited significantly by these compounds in concentrations below the minimum inhibitory concentration (MIC) values. These compounds may have a potential role in preventing fungal biofilm development on indwelling medical devices or even as a therapeutic measure after the establishment of biofilm.

A functional link between hyphal maintenance and quorum sensing in Candida albicans

Morphogenesis in Candida albicans requires hyphal initiation and maintenance, and both processes are regulated by the fungal quorum sensing molecule (QSM) farnesol. We show that deletion of C. albicans EED1, which is crucial for hyphal extension and maintenance, led to a dramatically increased sensitivity to farnesol, and thus identified the first mutant hypersensitive to farnesol. Furthermore, farnesol decreased the transient filamentation of an eed1Δ strain without inducing cell death, indicating that two separate mechanisms mediate quorum sensing and cell lysis by farnesol. To analyze the cause of farnesol hypersensitivity we constructed either hyperactive or deletion mutants of factors involved in farnesol signaling, by introducing the hyperactive RAS1^G13V or pADH1-CYR1^CAT allele, or deleting CZF1 or NRG1 respectively. Neither of the constructs nor the exogenous addition of dB-cAMP was able to rescue the farnesol hypersensitivity, highlighting that farnesol mediates its effects not only via the cAMP pathway. Interestingly, the eed1Δ strain also displayed increased farnesol production. When eed1Δ was grown under continuous medium flow conditions, to remove accumulating QSMs from the supernatant, maintenance of eed1Δ filamentation, although not restored, was significantly prolonged, indicating a link between farnesol sensitivity, production, and the hyphal maintenance-defect in the eed1Δ mutant strain.

Activity of tyrosol against single and mixed-species oral biofilms

AIM

This study aimed to evaluate the effect of tyrosol on the formation of single and mixed biofilms of Candida albicans ATCC 10231, Candida glabrata ATCC 90030 and Streptococcus mutans ATCC 25175 formed on acrylic resin (AR) and hydroxyapatite (HA) surfaces.

METHODS AND RESULTS

Single and mixed biofilms were formed on AR and HA in the presence of tyrosol at 50, 100 and 200 mmol l(-1), during 48 h. Next, antimicrobial activity was assessed through metabolic activity (XTT reduction assay) and the number of colony-forming units (CFUs). Scanning electron microscopy observations were performed in order to analyse biofilm structure. Tyrosol, mainly at 200 mmol l(-1), significantly decreased the metabolic activity and number of CFUs for all single and mixed-species biofilms formed on both surfaces. SEM images suggested cell damage caused by tyrosol.

CONCLUSION

Tyrosol showed inhibitory effects against biofilms formed by important oral pathogens.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first study showing the antibiofilm effect of tyrosol on Candida species and Strep. mutans in single and mixed cultures. These results may be useful in the development of topical therapies focused on preventing biofilm-associated oral diseases, such as denture stomatitis and dental caries.

Comparison of E,E-Farnesol Secretion and the Clinical Characteristics of Candida albicans Bloodstream Isolates from Different Multilocus Sequence Typing Clades

Using multilocus sequence typing (MLST), Candida albicans can be subdivided into 18 different clades. Farnesol, a quorum-sensing molecule secreted by C. albicans, is thought to play an important role in the development of C. albicans biofilms and is also a virulence factor. This study evaluated whether C. albicans bloodstream infection (BSI) strains belonging to different MLST clades secrete different levels of E,E-farnesol (FOH) and whether they have different clinical characteristics. In total, 149 C. albicans BSI isolates from ten Korean hospitals belonging to clades 18 (n = 28), 4 (n = 23), 1 (n = 22), 12 (n = 17), and other clades (n = 59) were assessed. For each isolate, the FOH level in 24-hour biofilms was determined in filtered (0.45 μm) culture supernatant using high-performance liquid chromatography. Marked differences in FOH secretion from biofilms (0.10–6.99 μM) were observed among the 149 BSI isolates. Clade 18 isolates secreted significantly more FOH than did non-clade 18 isolates (mean ± SEM; 2.66 ± 0.22 vs. 1.69 ± 0.10 μM; P < 0.001). Patients with isolates belonging to clade 18 had a lower mean severity of illness than other patients, as measured using the “acute physiology and chronic health evaluation” (APACHE) III score (14.4 ± 1.1 vs. 18.0 ± 0.7; P < 0.05). This study provides evidence that C. albicans BSI isolates belonging to the most prevalent MLST clade (clade 18) in Korea are characterized by increased levels of FOH secretion and less severe illness.

Natural Sources as Innovative Solutions Against Fungal Biofilms

Fungal cells are capable of adhering to biotic and abiotic surfaces and form biofilms containing one or more microbial species that are microbial reservoirs. These biofilms may cause chronic and acute infections. Fungal biofilms related to medical devices are particularly responsible for serious infections such as candidemia. Nowadays, only a few therapeutic agents have demonstrated activities against fungal biofilms in vitro and/or in vivo. So the discovery of new anti-biofilm molecules is definitely needed. In this context, biodiversity is a large source of original active compounds including some that have already proven effective in therapies such as antimicrobial compounds (antibacterial or antifungal agents). Bioactive metabolites from natural sources, useful for developing new anti-biofilm drugs, are of interest. In this chapter, the role of molecules isolated from plants, lichens, algae, microorganisms, or from animal or human origin in inhibition and/or dispersion of fungal biofilms (especially Candida and Aspergillus biofilms) is discussed. Some essential oils, phenolic compounds, saponins, peptides and proteins and alkaloids could be of particular interest in fighting fungal biofilms.

Candida Biofilms: Development, Architecture, and Resistance

Intravascular device-related infections are often associated with biofilms (microbial communities encased within a polysaccharide-rich extracellular matrix) formed by pathogens on the surfaces of these devices. Candida species are the most common fungi isolated from catheter-, denture-, and voice prosthesis-associated infections and also are commonly isolated from contact lens-related infections (e.g., fungal keratitis). These biofilms exhibit decreased susceptibility to most antimicrobial agents, which contributes to the persistence of infection. Recent technological advances have facilitated the development of novel approaches to investigate the formation of biofilms and identify specific markers for biofilms. These studies have provided extensive knowledge of the effect of different variables, including growth time, nutrients, and physiological conditions, on biofilm formation, morphology, and architecture. In this article, we will focus on fungal biofilms (mainly Candida biofilms) and provide an update on the development, architecture, and resistance mechanisms of biofilms.

The fungal quorum-sensing molecule farnesol activates innate immune cells but suppresses cellular adaptive immunity

Farnesol, produced by the polymorphic fungus Candida albicans, is the first quorum-sensing molecule discovered in eukaryotes. Its main function is control of C. albicans filamentation, a process closely linked to pathogenesis. In this study, we analyzed the effects of farnesol on innate immune cells known to be important for fungal clearance and protective immunity. Farnesol enhanced the expression of activation markers on monocytes (CD86 and HLA-DR) and neutrophils (CD66b and CD11b) and promoted oxidative burst and the release of proinflammatory cytokines (tumor necrosis factor alpha [TNF-α] and macrophage inflammatory protein 1 alpha [MIP-1α]). However, this activation did not result in enhanced fungal uptake or killing. Furthermore, the differentiation of monocytes to immature dendritic cells (iDC) was significantly affected by farnesol. Several markers important for maturation and antigen presentation like CD1a, CD83, CD86, and CD80 were significantly reduced in the presence of farnesol. Furthermore, farnesol modulated migrational behavior and cytokine release and impaired the ability of DC to induce T cell proliferation. Of major importance was the absence of interleukin 12 (IL-12) induction in iDC generated in the presence of farnesol. Transcriptome analyses revealed a farnesol-induced shift in effector molecule expression and a down-regulation of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor during monocytes to iDC differentiation. Taken together, our data unveil the ability of farnesol to act as a virulence factor of C. albicans by influencing innate immune cells to promote inflammation and mitigating the Th1 response, which is essential for fungal clearance.

Farnesol is a quorum-sensing molecule which controls morphological plasticity of the pathogenic yeast Candida albicans. As such, it is a major mediator of intraspecies communication. Here, we investigated the impact of farnesol on human innate immune cells known to be important for fungal clearance and protective immunity. We show that farnesol is able to enhance inflammation by inducing activation of neutrophils and monocytes. At the same time, farnesol impairs differentiation of monocytes into immature dendritic cells (iDC) by modulating surface phenotype, cytokine release and migrational behavior. Consequently, iDC generated in the presence of farnesol are unable to induce proper T cell responses and fail to secrete Th1 promoting interleukin 12 (IL-12). As farnesol induced down-regulation of the granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor, desensitization to GM-CSF could potentially explain transcriptional reprofiling of iDC effector molecules. Taken together, our data show that farnesol can also mediate Candida-host communication and is able to act as a virulence factor.

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.

Therapeutic potential of thiazolidinedione-8 as an antibiofilm agent against Candida albicans

Candida albicans is known as a commensal microorganism but it is also the most common fungal pathogen in humans, causing both mucosal and systemic infections. Biofilm-associated C. albicans infections present clinically important features due to their high levels of resistance to traditional antifungal agents. Quorum sensing is closely associated with biofilm formation and increasing fungal pathogenicity. We investigated the ability of the novel bacterial quorum sensing quencher thiazolidinedione-8 (S-8) to inhibit the formation of, and eradication of mature C. albicans biofilms. In addition, the capability of S-8 to alter fungal adhesion to mammalian cells was checked. S-8 exhibited specific antibiofilm and antiadhesion activities against C. albicans, at four- to eightfold lower concentrations than the minimum inhibitory concentration (MIC). Using fluorescence microscopy, we observed that S-8 dose-dependently reduces C. albicans–GFP binding to RAW macrophages. S-8 at sub-MICs also interfered with fungal morphogenesis by inhibiting the yeast-to-hyphal form transition. In addition, the tested agent strongly affected fungal cell wall characteristics by modulating its hydrophobicity. We evaluated the molecular mode of S-8 antibiofilm and antiadhesion activities using real-time RT-PCR. The expression levels of genes associated with biofilm formation, adhesion and filamentation, HWP1, ALS3 and EAP1, respectively, were dose-dependently downregulated by S-8. Transcript levels of UME6, responsible for long-term hyphal maintenance, were also significantly decreased by the tested agent. Both signaling pathways of hyphal formation-cAMP-PKA and MAPK-were interrupted by S-8. Their upstream general regulator RAS1 was markedly suppressed by S-8. In addition, the expression levels of MAPK cascade components CST20, HST7 and CPH1 were downregulated by S-8. Finally, transcriptional repressors of filament formation, TUP1 and NRG1, were dramatically upregulated by our compound. Our results indicate that S-8 holds a novel antibiofilm therapeutic mean in the treatment and prevention of biofilm-associated C. albicans infections.

Mechanisms of Candida biofilm drug resistance

Candida commonly adheres to implanted medical devices, growing as a resilient biofilm capable of withstanding extraordinarily high antifungal concentrations. As currently available antifungals have minimal activity against biofilms, new drugs to treat these recalcitrant infections are urgently needed. Recent investigations have begun to shed light on the mechanisms behind the profound resistance associated with the biofilm mode of growth. This resistance appears to be multifactorial, involving both mechanisms similar to conventional, planktonic antifungal resistance, such as increased efflux pump activity, as well as mechanisms specific to the biofilm lifestyle. A unique biofilm property is the production of an extracellular matrix. Two components of this material, β-glucan and extracellular DNA, promote biofilm resistance to multiple antifungals. Biofilm formation also engages several stress response pathways that impair the activity of azole drugs. Resistance within a biofilm is often heterogeneous, with the development of a subpopulation of resistant persister cells. In this article we review the molecular mechanisms underlying Candida biofilm antifungal resistance and their relative contributions during various growth phases.

Anti-Candida albicans biofilm effect of novel heterocyclic compounds

OBJECTIVES

The aims of this study were to develop new anti-biofilm drugs, examine their activity against Candida albicans biofilm and investigate their structure-activity relationship and mechanism of action.

METHODS

A series of thiazolidinedione and succinimide derivatives were synthesized and their ability to inhibit C. albicans biofilm formation and destroy pre-formed biofilm was tested. The biofilms' structure, metabolic activity and viability were determined by XTT assay and propidium iodide and SYTO 9 live/dead stains combined with confocal microscopic analysis. The effect of the most active compounds on cell morphology, sterol distribution and cell wall morphology and composition was then determined by specific fluorescent stains and transmission electron microscopy.

RESULTS

Most of the compounds were active at sub-MICs. Elongation of the aliphatic side chain resulted in reduced anti-biofilm activity and the sulphur atom contributed to biofilm killing, indicating a structure-activity relationship. The compounds differed in their effects on biofilm viability, yeast-to-hyphal form transition, hyphal morphology, cell wall morphology and composition, and sterol distribution. The most effective anti-biofilm compounds were the thiazolidinedione S8H and the succinimide NA8.

CONCLUSIONS

We developed novel anti-biofilm agents that both inhibited and destroyed C. albicans biofilm. With some further development, these agents might be suitable for therapeutic purposes.

Determination of biofilm production by Candida tropicalis isolated from hospitalized patients and its relation to cellular surface hydrophobicity, plastic adherence and filamentation ability

Candida tropicalis is an emerging virulent species. The aim of this study is to determine the biofilm-forming ability of 29 strains of C. tropicalis isolated from inpatients, and to examine its relation with other virulence factors such as cellular surface hydrophobicity (CSH), immediate (15 min, IA) and late (24 h, LA) plastic adherence and filamentation ability. The study was performed in parallel using two incubation temperatures - 37 and 22 °C - to determine the effect of growth temperature variations on these pathogenic attributes of C. tropicalis. Biofilm formation (BF) was measured by optical density (OD) and by XTT reduction (XTT); Slime index (SI), which includes growth as a correction factor in BF, was calculated in both methods. All strains were hydrophobic and adherent - at 15 min and 24 h - at both temperatures, with higher values for 22 °C; the adhered basal yeast layer appears to be necessary to achieve subsequent development of biofilm. Filamentation ability varied from 76.2% of strains at 37 °C to 26.6% at 22 °C. All C. tropicalis strains were biofilm producers, with similar results obtained using OD determination and XTT measurement to evaluation methods; SI is useful when good growth is not presented. BF at 37 °C was similar at 24 h and 96 h incubation; conversely, at 22 °C, the highest number of biofilm-producing strains was detected at 96 h. CSH is an important pathogenic factor which is involved in adherence, is influenced by the filamentation of yeast, and plays a critical role in BF.

Pseudomonas aeruginosa lipopolysaccharide inhibits Candida albicans hyphae formation and alters gene expression during biofilm development

Elucidation of bacterial and fungal interactions in multispecies biofilms will have major impacts on understanding the pathophysiology of infections. The objectives of this study were to (i) evaluate the effect of Pseudomonas aeruginosa lipopolysaccharide (LPS) on Candida albicans hyphal development and transcriptional regulation, (ii) investigate protein expression during biofilm formation, and (iii) propose likely molecular mechanisms for these interactions. The effect of LPS on C. albicans biofilms was assessed by XTT-reduction and growth curve assays, light microscopy, scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM). Changes in candidal hypha-specific genes (HSGs) and transcription factor EFG1 expression were assessed by real-time polymerase chain reaction and two-dimensional gel electrophoresis, respectively. Proteome changes were examined by mass spectrometry. Both metabolic activities and growth rates of LPS-treated C. albicans biofilms were significantly lower (P < 0.05). There were higher proportions of budding yeasts in test biofilms compared with the controls. SEM and CLSM further confirmed these data. Significantly upregulated HSGs (at 48 h) and EFG1 (up to 48 h) were noted in the test biofilms (P < 0.05) but cAMP levels remained unaffected. Proteomic analysis showed suppression of candidal septicolysin-like protein, potential reductase-flavodoxin fragment, serine hydroxymethyltransferase, hypothetical proteins Cao19.10301(ATP7), CaO19.4716(GDH1), CaO19.11135(PGK1), CaO19.9877(HNT1) by P. aeruginosa LPS. Our data imply that bacterial LPS inhibit C. albicans biofilm formation and hyphal development. The P. aeruginosa LPS likely target glycolysis-associated mechanisms during candidal filamentation.

Candida species: new insights into biofilm formation

Biofilms of Candida albicans, Candida parapsilosis, Candida glabrata and Candida tropicalis are associated with high indices of hospital morbidity and mortality. Major factors involved in the formation and growth of Candida biofilms are the chemical composition of the medical implant and the cell wall adhesins responsible for mediating Candida-Candida, Candida-human host cell and Candida-medical device adhesion. Strategies for elucidating the mechanisms that regulate the formation of Candida biofilms combine tools from biology, chemistry, nanoscience, material science and physics. This review proposes the use of new technologies, such as synchrotron radiation, to study the mechanisms of biofilm formation. In the future, this information is expected to facilitate the design of new materials and antifungal compounds that can eradicate nosocomial Candida infections due to biofilm formation on medical implants. This will reduce dissemination of candidiasis and hopefully improve the quality of life of patients.