The Candida albicans INT1 gene facilitates cecal colonization in endotoxin-treated mice

Systematic gene overexpression in Candida albicans identifies a regulator of early adaptation to the mammalian gut

Candida albicans is part of the human gastrointestinal (GI) microbiota. To better understand how C. albicans efficiently establishes GI colonisation, we competitively challenged growth of 572 signature-tagged strains (~10% genome coverage), each conditionally overexpressing a single gene, in the murine gut. We identified CRZ2, a transcription factor whose overexpression and deletion respectively increased and decreased early GI colonisation. Using clues from genome-wide expression and gene-set enrichment analyses, we found that the optimal activity of Crz2p occurs under hypoxia at 37°C, as evidenced by both phenotypic and transcriptomic analyses following CRZ2 genetic perturbation. Consistent with early colonisation of the GI tract, we show that CRZ2 overexpression confers resistance to acidic pH and bile salts, suggesting an adaptation to the upper sections of the gut. Genome-wide location analyses revealed that Crz2p directly modulates the expression of many mannosyltransferase- and cell-wall protein-encoding genes, suggesting a link with cell-wall function. We show that CRZ2 overexpression alters cell-wall phosphomannan abundance and increases sensitivity to tunicamycin, suggesting a role in protein glycosylation. Our study reflects the powerful use of gene overexpression as a complementary approach to gene deletion to identify relevant biological pathways involved in C. albicans interaction with the host environment.

Secondary bile acids inhibit Candida albicans growth and morphogenesis

Candida albicans is one of the most common causes of fungal infections in humans with a significant mortality rate. However, the factors involved in C. albicans gastrointestinal (GI) colonization remain unclear. We hypothesize that secondary bile acids have direct antifungal activity against C. albicans and may play a critical role in maintaining GI colonization resistance against C. albicans. In this study, we investigated the effect of secondary bile acids including lithocholic acid (LCA) and deoxycholic acid (DCA) on C. albicans growth and morphogenesis. Results indicate that LCA and DCA at in vivo cecal micelle concentrations inhibit C. albicans growth in vitro. Interestingly, LCA and DCA also significantly inhibited the germ tube, hyphae and biofilm formation in C. albicans. In addition, pre-treatment of C. albicans with LCA and DCA significantly reduced the percentage of C. albicans cells attached to a colon cancer cell line. Collectively, our results demonstrate that secondary bile acids play an important role in controlling the growth and morphological switching of C. albicans. Results from this study demonstrate that secondary bile acid possess direct antifungal activity against C. albicans, explaining a potential mechanism for gastrointestinal colonization resistance against C. albicans.

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

Background

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

Results

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

Conclusion

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

Electronic supplementary material

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

Novel Aggregation Properties of Candida albicans Secreted Aspartyl Proteinase Sap6 Mediate Virulence in Oral Candidiasis

Candida albicans, a commensal fungus of the oral microbiome, causes oral candidiasis in humans with localized or systemic immune deficiencies. Secreted aspartic proteinases (Saps) are a family of 10 related proteases and are virulence factors due to their proteolytic activity, as well as their roles in adherence and colonization of host tissues. We found that mice infected sublingually with C. albicans cells overexpressing Sap6 (SAP6 OE and a Δsap8 strain) had thicker fungal plaques and more severe oral infection, while infection with the Δsap6 strain was attenuated. These hypervirulent strains had highly aggregative colony structure in vitro and higher secreted proteinase activity; however, the levels of proteinase activity of C. albicans Saps did not uniformly match their abilities to damage cultured oral epithelial cells (SCC-15 cells). Hyphal induction in cells overexpressing Sap6 (SAP6 OE and Δsap8 cells) resulted in formation of large cell-cell aggregates. These aggregates could be produced in germinated wild-type cells by addition of native or heat-inactivated Sap6. Sap6 bound only to germinated cells and increased C. albicans adhesion to oral epithelial cells. The adhesion properties of Sap6 were lost upon deletion of its integrin-binding motif (RGD) and could be inhibited by addition of RGD peptide or anti-integrin antibodies. Thus, Sap6 (but not Sap5) has an alternative novel function in cell-cell aggregation, independent of its proteinase activity, to promote infection and virulence in oral candidiasis.

The iC3b receptor of Candida albicans and its roles in pathogenesis

On the basis of biochemical and immunologic studies, a receptor for iC3b with some activities reminiscent of the integrins CD11b and CD11c was defined on the cell wall of clinical and laboratory isolates of Candida albicans. The INT1 gene encodes a protein of 1659 amino acids; the Int1 protein participates in adhesion to epithelial cells in vitro and in vivo. Int1 is essential for hyphal morphogenesis and virulence in a murine model. Recent evidence points to the amino terminus of Int1 as the source of a peptide, Pep263, with superantigen-like activities.

Cleavage of E-cadherin: a mechanism for disruption of the intestinal epithelial barrier by Candida albicans

To investigate how intestinal epithelial cells respond to contact with Candida albicans, an organism able to invade the bloodstream via the gastrointestinal tract, we focused on the junction proteins occludin, E-cadherin, and desmoglein-2. The levels of these 3 junction proteins were reduced in lysates of human intestinal epithelial monolayers (Caco-2) after a 24-h inoculation with C. albicans, compared with lysates from Saccharomyces cerevisiae-inoculated monolayers. Treatment with pepstatin A did not change the effect of C. albicans on full-length occludin, desmoglein-2, and E-cadherin; however, pepstatin A enhanced the accumulation of a 35-kDa fragment derived from the intracellular portion of E-cadherin. This 35-kDa fragment also accumulated in the presence of gamma-secretase inhibitors. These observations suggest that enhancement of E-cadherin cleavage by C. albicans generates an intracellular E-cadherin fragment that can serve as a substrate for gamma-secretase. An 89-kDa extracellular fragment of E-cadherin was detected in supernatants of C. albicans-inoculated monolayers; this cleavage event was insensitive to both pepstatin A and gamma-secretase inhibitors. Transepithelial electrical resistance, a measure of monolayer integrity, decreased significantly and synchronously with increased generation of the 89-kDa extracellular E-cadherin fragment. Cleavage of E-cadherin may destabilize the homotypic interactions between adjacent epithelial cells and could contribute to loss of monolayer integrity. These experiments identify 2 E-cadherin cleavage events that are enhanced by contact with C. albicans: an intracellular cleavage event that generates a substrate for gamma-secretase and an extracellular cleavage event that is temporally associated with an increase in monolayer permeability.

Genetically regulated filamentation contributes to Candida albicans virulence during corneal infection

Candida albicans is a commensal fungus of the normal flora yet causes opportunistic infection following trauma or surgery and during immunosuppression. C. albicans virulence factors include morphogenesis into invasive filaments, adherence to host cells, and secretion of proteases. This study evaluated the role of fungal hyphal extension in experimental C. albicans keratitis using genetically altered yeast strains. Scarified corneas of adult BALB/c mice were topically inoculated with wild-type (SC5314) or 10 transposon-induced mutant strains of C. albicans and monitored for 4 days post inoculation (PI). In vitro growth kinetics and the yeast strains' ability to bud into pseudohyphae or hyphae were also compared. The wild-type human isolate had a high degree of virulence in the murine cornea, and four fungal strains deficient in genes regulating adherence or encoding membrane proteins did not significantly differ from the parental strain (P>0.3). Five yeast strains deficient in genes involved in filamentation resulted in fully or partially attenuated keratomycosis (P<0.0001). The overall growth kinetics of wild-type and mutant strains were similar in rich media (P>0.9), but mutants with deficient morphogenesis had reduced filamentation in vitro. Phenotypic switching from yeasts to filamentous forms facilitates the establishment and progression of experimental corneal disease by C. albicans.

Colonization and epithelial adhesion in the pathogenesis of neonatal candidiasis

Candida species are important nosocomial pathogens in the newborn population, particularly among the premature very-low-birth-weight infants in neonatal intensive care units. Candida colonization of the neonatal skin and gastrointestinal tract is an important first step in the pathogenesis of invasive disease. C albicans is the most commonly isolated species in colonized or infected infants. Over the past decade the incidence of both colonization and infection with other Candida species, particularly C parapsilosis, has risen dramatically. Colonization of the infant occurs early in life and is affected by a variety of common practices in neonatal intensive care. Microbial factors also augment colonization, including the ability of Candida to adhere to human epithelium. A better understanding of the complex interactions between host risk factors and virulence traits of colonizing yeast may allow the risk of systemic spread to be reduced in the population of premature infants.

Effect of lipopolysaccharide on virulence of intestinal candida albicans

BACKGROUND

Candida albicans is a polymorphic fungus that frequently causes systemic infection in postsurgical and trauma patients. Others have reported that Escherichia coli lipopolysaccharide (LPS) acts as a copathogen to enhance the virulence of parenteral C. albicans. Experiments were designed to clarify the effect of parenteral LPS on systemic candidiasis initiated via the oral route.

MATERIALS AND METHODS

Antibiotic-treated mice were orally inoculated with C. albicans CAF2 (wild-type) or mutant HLC54 (defective in filament formation), and were given 100 microg parenteral LPS 16 h before sacrifice. Separate groups of mice were additionally exposed to intermittent hypoxia prior to LPS. At sacrifice, cecal flora and microbial translocation to the mesenteric lymph nodes were quantified. C. albicans adherence to cultured HT-29 and Caco-2 enterocytes (pretreated with LPS, or calcium-free medium to expose the enterocyte lateral surface, or both) was quantified by enzyme-linked immunoabsorbent assay.

RESULTS

All mice had high numbers of cecal C. albicans, and LPS was associated with an additional increase in cecal concentrations of HLC54 but not CAF2. Translocation of HLC54, but not CAF2, appeared facilitated by hypoxia, but LPS did not facilitate translocation in any treatment group. Exposure of the lateral surface of cultured enterocytes had no effect on C. albicans adherence, although LPS consistently decreased adherence of both C. albicans strains.

CONCLUSIONS

In contrast to experiments where systemic candidiasis was initiated by the parenteral route, parenteral LPS did not act as a copathogen in mice with systemic candidiasis initiated by the oral route, and these results might be related to LPS-induced alterations in C. albicans adherence to host enterocytes.

Hypoxia and extraintestinal dissemination of Candida albicans yeast forms

Candida albicans is a pleomorphic fungus with budding yeast and filamentous forms, and is a frequent cause of complicating infections in patients who are postsurgical, in shock, and have trauma. Many cases of systemic candidiasis are thought to orginate from the intestine, but it is unclear if the filament or the yeast is the more invasive form. Because C. albicans is relatively noninvasive and because mesenteric ischemia is thought to facilitate extraintestinal microbial dissemination, wild-type C. albicans CAF2 and mutant HLC54 (defective in filament formation) were orally inoculated into antibiotic-treated mice that were housed exclusively in room air, or were intermittently exposed to 10% oxygen for 1-h intervals. Both strains of C. albicans colonized the cecum in similar numbers (approximately 10(6.7)/g). C. albicans translocation to the draining mesenteric lymph nodes was not detected in mice inoculated with CAF2 (normoxic or hypoxic) or in normoxic mice inoculated with HLC54, but was detected in 33% (P < 0.01) of hypoxic mice inoculated with HLC54. Using Caco-2 and HT-29 enterocytes cultivated on plastic dishes and pretreated for 48 h in 10% oxygen, adherence of C. albicans HLC54 was decreased compared with wild-type CAF2, and hypoxia had no noticeable effect on adherence of either CAF2 or HLC54. Using enterocytes cultivated on permeable 8-microm filters, transepithelial migration of C. albicans CAF2 and HLC54 appeared similar. Thus, C. albicans HLC54 (defective in filament formation) was more invasive in hypoxic mice compared with wild-type CAF2, and host factors (e.g., mesenteric ischemia) rather than an innate ability to interact with enterocytes might play a more important role in extraintestinal dissemination of C. albicans yeast forms.

Comparative virulence of Candida albicans yeast and filamentous forms in orally and intravenously inoculated mice

OBJECTIVE

Candida albicans, a dimorphic fungus that switches from yeast to filamentous forms, is a major cause of complicating systemic infection in intensive care patients. The aim of this study was to compare the pathogenic potential of C. albicans yeast and filamentous forms.

DESIGN

Separate groups of mice were inoculated either intravenously or orally with C. albicans CAF2 (wild type), HLC54 (yeast forms defective in filament formation), or BCa2-10 (constitutively filamentous). Mice were killed 1, 7, 14, and 21 days after intravenous C. albicans and kidneys and liver were quantitatively cultured; cohort groups were observed for mortality. Mice were pretreated with antibiotics for 3 days before oral inoculation with C. albicans, and killed 3 days later with dexamethasone administered for the latter 3 days; at sacrifice, the mesenteric lymph nodes and kidneys were cultured to monitor extraintestinal dissemination of C. albicans.

SETTING

University teaching hospital research laboratory.

SUBJECTS

Female, Swiss Webster, adult mice.

MEASUREMENTS AND MAIN RESULTS

In intravenously inoculated mice, mortality was highest with wild-type C. albicans CAF2 (92%), intermediate with HLC54 (56%), and not detected with constitutively filamentous BCa2-10 (0%); BCa2-10 was cleared from the kidney and liver, but CAF2 and HLC54 were recovered at approximately 10(5-7)/g kidney and 10(4-5)/g liver. There was only occasional mortality in orally inoculated mice and the numbers of cecal C. albicans CAF2 and HLC54 were similarly high (approximately 10(7)/g), whereas numbers of cecal BCa2-10 were at least 100-fold lower. Extraintestinal dissemination was greatest with HLC54, intermediate with CAF2, and undetectable with BCa2-10.

CONCLUSIONS

Of the three C. albicans strains studied, wild-type CAF2 was most virulent in intravenously inoculated mice and HLC54 (defective in filament formation) was most virulent in orally inoculated mice. The constitutively filamentous BCa2-10 was avirulent in both models, suggesting that filamentous forms by themselves might not be critically important for C. albicans virulence.

Cecal colonization and systemic spread of Candida albicans in mice treated with antibiotics and dexamethasone

Infections with Candida albicans have become a significant problem among very low birth weight infants in the neonatal intensive care unit. Risk factors are multiple and include administration of antibiotics and glucocorticoids, such as dexamethasone. Experiments were designed to study the combined effect of oral broad-spectrum antibiotics and parenteral dexamethasone on cecal colonization and extraintestinal dissemination of C. albicans in separate groups of mice that were orally inoculated with one of four C. albicans strains that were either wild-type INT1/INT1 or had one or more disruptions of the INT1 gene. Intestinal colonization was monitored by quantitative culture of the mouse cecum, and extraintestinal invasion was monitored by quantitative culture of the draining mesenteric lymph nodes and kidneys. At sacrifice, the average numbers of cecal C. albicans differed from 7.7 log(10)/g to 6.7 log(10)/g (p < 0.01) in mice orally inoculated with C. albicans containing two functional copies of INT1 and no functional copies of INT1, respectively. The incidence of extraintestinal dissemination to mesenteric lymph nodes and kidneys correspondingly varied from 57 to 13% (p < 0.01) and 83 to 4% (p < 0.01) in mice inoculated with these two C. albicans strains. Mice orally inoculated with C. albicans containing one functional copy of INT1 had intermediate levels of cecal colonization and extraintestinal dissemination. Thus, cecal colonization and extraintestinal dissemination of C. albicans was facilitated in antibiotic-treated mice given dexamethasone. In addition, the presence of two functional copies of the INT1 gene was associated with the greatest levels of cecal colonization and extraintestinal dissemination of C. albicans.

Gastrointestinal colonization by Candida albicans mutant strains in antibiotic-treated mice

Antibiotic-treated mice orally inoculated with one of three Candida albicans strains (including two mutant strains) or indigenous Candida pelliculosa showed levels of candidal gastrointestinal colonization that were strain specific. However, regardless of strain, the numbers of viable candida were intermediate to high in the stomach, were consistently lowest in the upper small intestine, and increased progressively down the intestinal tract.