Phosphatase Activity Among Candida Species and Other Yeasts Isolated from Clinical Material

A group of 277 yeasts isolated from burned children and 14 reference strains were tested for phosphatase activity by using phenolphthalein phosphate substrates. Phosphatase activity was widely distributed among various species and strains representing seven genera. Candida albicans, which was the most common yeast isolated from clinical material, was notably absent in producing the enzyme, whereas Candida tropicalis was the most consistent, strong, and rapidly active phosphatase-producing organism. The characteristic enzyme activity of a selected isolate of C. tropicalis was demonstrated in the presence of concentrations of inorganic phosphate which inhibited enzyme activity of other species. The greater enzyme activity of C. tropicalis was not related to more rapid or greater cell growth or decrease in the pH of culture media. Extracellular constitutive heat-labile acid phosphatase was found in broth filtrates of C. tropicalis, C. krusei, and a strain of Staphylococcus aureus.     

Induction of secretory acid proteinase in Candida albicans.

Candida albicans and some other pathogenic Candida species, when grown in a medium containing a protein as a sole source of nitrogen, secrete an acid proteinase. Culture supernatants were assayed for proteinase activity, and were also analysed by Western blotting with antibodies raised and affinity-purified against proteinase of C. albicans. Proteinases secreted by C. tropicalis and C. parapsilosis were antigenically related to that of C. albicans, but had different molecular masses. The proteinases secreted by C. lipolytica, C. rugosa and C. lusitaniae were not antigenically related. The kinetics of proteinase secretion by C. albicans were monitored by activity and by Western blotting. With BSA as the nitrogen source, proteinase secretion increased exponentially until about 16 h. Culture supernatants of BSA-grown cultures accumulated proteinase to about a 1000-fold higher level than those of ammonium-sulphate-grown cultures. In vivo labelling experiments showed that proteinase was not detectably accumulated in the cells, but was secreted immediately after synthesis. Immunoprecipitation of in vitro translated poly(A)-containing RNA identified a putative pre-protein of about 54 kDa. As well as BSA, other proteins (haemoglobin, ovalbumin, histone), peptone and tryptone, when used as nitrogen sources, could induce proteinase, but to different levels. When Casamino acids or an amino acid mixture (equivalent to the composition of BSA) was used as nitrogen source, no induction was observed. Ammonium sulphate, or any other ammonium salt, repressed secretion of proteinase.     

Purification and characterization of secretory proteinase of Candida albicans.

Proteolytic activity of medically important yeasts was tested in both YCB-BSA agar and medium. All of 134 strains of Candida albicans, 13 of 18 strains of Candida tropicalis and 11 of 18 strains of Candida parapsilosis had this activity, while none of 52 Candida glabrata strains or of 11 Cryptococcus neoformans strains tested had proteolytic activity. Strains of C. albicans fell into five groups based on the level and time-course of in vitro proteinase productivity. Five strains randomly selected from each group were tested for pathogenicity in mice. The strain possessing the strongest pathogenicity was used to purify proteinase. The molecular weight of the proteinase was approximately 44,000 daltons and its isoelectric point was pH 4.2. Optimal pH of the proteinase was 3.2 and the enzyme was stable below pH 7.0 and lost its activity above pH 8.0 at 37 C in a 60-min incubation. The 23 amino acid sequence of the proteinase N-terminus was determined.     

Distribution of Candida species in different clinical sources in Delhi, India, and proteinase and phospholipase activity of Candida albicans isolates

Eighty-five isolates of Candida recovered from three hundred and fifty diverse clinical sources, viz. respiratory tract (sputum, bronchial washing,bronchoalveolar lavage, tracheal aspirate), blood, urine, high vaginal swab, skin and plastic devices, were studied in detail for their morphological and biochemical characters. Seven species of Candida were identified, viz., C. albicans (45.8%), C. tropicalis (24.7%), C. parapsilosis (10.5%), C. krusei (7.0%), C. kefyr (7.0%), C. guilliermondii (3.5%), and C. glabrata (1.1%). C. albicans was the predominant species isolated from all clinical specimens, except blood from which C. krusei was most frequently (38.4%) recovered. Out of 39 isolates of C. albicans, 26 (66.6%) and 19 (48.7%) exhibited strong proteinase and phospholipase activity respectively. There was a higher prevalence of proteinase producing strains amongst the vaginal and skin isolates than that in urinary and respiratory isolates. Also a greater number of phospholipase producing strains was observed in the vaginal and urinary isolates than that in the respiratory and skin isolates.     

Comparison of secretory acid proteinases from Candida tropicalis, C. parapsilosis and C. albicans.

Acid proteinases secreted by Candida tropicalis and C. parapsilosis were newly isolated. Their physico-chemical and enzymatic properties of molecular weight, pH stability, isoelectric points, specific activity, and N-terminal amino acid sequences were determined and compared with those of a C. albicans acid proteinase. The two acid proteinases secreted by C. parapsilosis were found to be new enzymes in their molecular weights. The acid proteinases from C. tropicalis and C. parapsilosis showed lower activity at neutral pH, less resistance to neutral and alkaline pH than that from C. albicans, and a half or a third of the specific activity of the C. albicans enzyme. These differences seemed to be associated with the difference of pathogenesis between Candida species. Of the 31 N-terminal amino acids, the enzymes of these three Candida species revealed 12 homologous amino acids.     

口腔黏膜真菌鉴定方法的比较

比较常见用于黏膜真菌菌种鉴别的多种方法,探寻最佳的鉴别方法。采集230例普通人群口腔黏膜样本,分别用玉米吐温-80培养观察厚膜孢子法、糖发酵生化反应法、CHROMagar假丝酵母菌显色培养基法、ITS基因的PCR-RFLP(聚合酶链反应-限制性片段长度多态性)法、ITS测序菌种鉴定法,鉴别真菌各菌株。结果显示:有56例菌株至少通过1种方法检出真菌;玉米吐温-80分离培养假丝酵母菌37株;50例菌株ITS基因测序共鉴定出8个菌种,白假丝酵母菌(C.albicans)29株,近平滑假丝酵母菌(C.parapsilosis)10株,热带假丝酵母菌(C.tropicalis)5株,Candida metapsilosis 1株,Lodderomyces elongisporus 1株,克柔假丝酵母菌(Candida krusei)1株,乙醇假丝酵母菌(C.ethanolica)1株,季也蒙毕赤酵母菌(Pichia guilliermondii)2株;CHROMagar假丝酵母菌显色培养基法鉴定出3种菌株,分别是白假丝酵母菌、热带假丝酵母菌、近平滑假丝酵母菌;PCR-RFLP法检出5种菌株,分别是白假丝酵母菌、热带假丝酵母菌、近平滑假丝酵母菌、季也蒙毕赤酵母菌、克柔假丝酵母菌,与基因的测序鉴定一致率为91%;糖发酵生化反应法阳性标本占被检出真菌例数的46.4%(26/56)。结果表明:ITS基因的测序法可以准确鉴定真菌各个菌种;PCR-RFLP法能鉴定常见的菌种,但操作繁琐;CHROMagar假丝酵母菌显色培养基法能快速准确鉴别3种常见假丝酵母菌菌种;玉米吐温-80可以准确培养鉴别白假丝酵母菌;糖发酵生化反应法,缺乏足够的敏感度和特异性,难以准确鉴别各个菌种。     

A comparison of secretory proteinases from different strains of Candida albicans

Randomly selected strains of Candida albicans were grown with bovine serum albumin (BSA) as a single nitrogen source. From all strains tested, culture supernatant contained carboxyl proteinase (E.C.3.4.23) as has been shown that with hemoglobin as a substrate and by specific inhibition with pepstatin-A. According to the separation pattern of BSA fragments, secretory proteinases from C. albicans belong to at least three groups. We have purified the partially proteolytic enzyme of strain 113 and have compared its properties with those of the totally proteolytic enzyme of strain CBS 2730. Both enzymes have virtually identical molecular weight (ca. 44,000) and cross-react immunologically; they differ in pH optimum, isoelectric point, substrate specificity, and resistance against alkali. IgG1, which is the prevalent immunoglobulin of human serum, was not cleaved by enzyme 113. Immunoglobulins A1, A2 and secretory component were cleaved by both enzymes, which points to a role of the secretory proteinases in the persistence of yeasts on mucous membranes. Differences in the course of alkaline denaturation indicate that only a fraction of strain-specific proteinases is capable to convey long-range effects in the host.     

Detection of specificity of a new antigen in Candida tropicalis and its evaluation by taxonomic DNA analyses

Monospecific factor serum for identifying Candida tropicalis was obtained either from rabbit antiserum to heated cells of C. tropicalis M 1519 (S 96) or from antiserum to C. tropicalis IFO 1400, by adsorption with heated cells of Candida albicans serotype A, or C. albicans (A) and Candida krusei, respectively. We designated this adsorbed serum factor t serum. The monospecific factor serum reacted with 31 out of 32 strains of C. tropicalis, only when tested on heat-treated cell antigens, whereas it did not react with any of 72 strains of the six other medically important species of Candida. The morphological and physiological characteristics of the one strain of C. tropicalis that did not react with the factor t serum, designated the t- -strain, were shown to be similar to those of the type strain of C. tropicalis by most of the methods employed for identifying Candida. Therefore, cell wall mannan from the t- -strain was compared with that from several typical strains of C. tropicalis for its specificity by the precipitation reaction and also for its 1H-nuclear magnetic resonance spectrum. The results showed that these mannans are similar to each other serologically and physicochemically, suggesting that the new antigen t is not mannan. Taxonomic characterization of the t-- and several typical strains of C. tropicalis was carried out by determining the mol% G+C of their DNA and also their DNA homology. Although the mol% G+C values of four typical strains of C. tropicalis were fairly similar (35.2 to 36.2 mol% by the Tm method and 35.5 to 36.4 mol% by the HPLC method), the t- -strain had a G+C content of 44.1 (Tm) and 43.3 (HPLC) mol%. Furthermore, the DNAs of the t- -strain and the type strain of C. tropicalis showed only 18.2% relatedness. These results suggest that the antigen corresponding to serum factor t exists only in the cell wall of C. tropicalis strains, not in those of the other medically important Candida, and that the t- -strain should not be classified as C. tropicalis. In conclusion, the taxonomic value and usefulness of factor t serum is primarily for differentiating C. tropicalis from C. albicans serotype A serologically.     

Effect of the incubation conditions on the production of patulin by Penicillium griseofulvum isolated from wheat

A total of 290 Candida isolates from patients were investigated for in vitro proteinase production. Overall, sixty percent of these strains were found to be proteinase producers. Of the C. albicans strains, 81.4% of the significant isolates in contrast to 19.7% of nonsignificant isolates were proteinase producers, the difference being statistically significant (P<0.001). Amongst the different Candida species, the proteinase production was found not only in Candida albicans, but also in C. tropicalis, C. parapsilosis and C. glabrata. Thus this in vitro method of demonstration of proteinase may be a good adjunct to smear and culture examination in identifying pathogenic Candida species from anatomical sites where they can also be present as commensals.     

血培养分离出212株念珠菌的菌种分布及耐药性分析

目的分析血培养分离出的念珠菌的菌种分布及耐药分析,为临床念珠菌血症的诊治提供理论依据。方法回顾性分析2017年1月—2020年12月期间,本院血培养分离出的念珠菌的菌种分布、药敏试验结果及患者血培养分离出念珠菌前后96 h内的G试验结果。结果血培养中分离出念珠菌314例,阳性率2.1%,其中非重复分离株212例。检出率最高的是近平滑念珠菌(72株,34.0%),其次是白念珠菌(55株,25.9%)和光滑念珠菌(28株,13.2%)。念珠菌检出率最高的科室为ICU(62株,29.2%),其次是新生儿科(39株,18.4%)和血液科(20株,9.4%)。检出的212株念珠菌除一株近平滑念珠菌为两性霉素B的非野生株,其余均为两性霉素B的野生株。白念珠菌对唑类药物的敏感率超过90%。但光滑念珠菌和热带念珠菌对唑类药物的敏感性较低。血培养分离出念珠菌的前后96 h内,G试验的阳性率为73.7%。结论本院念珠菌检出率前3位分别是近平滑念珠菌、白念珠菌和光滑念珠菌。光滑念珠菌、热带念珠菌对唑类药物敏感性比较低,在经验用药时需要合理选择抗真菌药物。G试验对念珠菌血症有较高的价值,需要对结果进行动态监测。     

Evaluation of a new chromogenic medium (Candida ID) for the isolation and presumptive identification of Candida albicans and other medically important yeasts

Candidiasis is a frequent human infection caused mainly by Candida albicans. However, other species are emerging as important pathogens, as Candida glabrata, Candida parapsilosis, Candida tropicalis, Candida krusei or Candida guilliermondii. Rapid identification of clinical isolates could facilitate diagnosis and treatment. Candida ID (bioMerieux, Spain) is a new medium for the isolation and presumptive identification of yeasts: C. albicans grows as blue colonies, and C. tropicalis, C. guilliermondii, Candida kefyr and Candida lusitaniae as pink ones. The utility of Candida ID was evaluated with more than 700 clinical isolates and type culture collection strains from different genera including Candida, Cryptococcus, Saccharomyces, and Rhodotorula. Presumptive identification was confirmed by germ tube test, microscopic morphology and chlamydoconidia production on corn meal agar and carbohydrate assimilation on API-ATB ID 32C or Vitek (bioMerieux). Growth on Candida ID was rapid (18-24 h) for most of the yeast strains tested. Sensitivity and specificity of identification of C. albicans was significantly high (>98%), since a very low number of isolates were found to be false negative or false positive. A better result was obtained for species growing as pink colonies (>99.5%). Detection of different species of medical important yeasts was easy with Candida ID, as perfectly distinct colors and textures of colonies were observed on this medium. Candida ID allowed the discrimination between C. glabrata (creamy and smooth) and C. krusei (rough and white) colonies. Other species showed different colony textures and colours, white being the predominant colour. Candida ID was very useful for the presumptive identification C. albicans isolates.     

Proteinase production and pathogenicity of Candida albicans. II. Virulence for mice of C. albicans strains of different proteinase activity

We have studied the virulence for mice of Candida albicans strains with different proteinase activity in the culture. The mortality rates for mice infected with the type Ia strains, which secrete proteinase whose activity porsisted for a week in vitro, were higher than those infected with the type Ib strains, which secrete proteinase whose activity declined at 2 or 3 days in vitro and the type II proteinase-deficient strains. This was substantiated by the number of colony-forming units (CFU) recovered from kidneys of mice infected with C. albicans. In the kidney tissues of mice infected with the type Ia strains, extensive invasion by fungal cells and the secretion of proteinase were histologically demonstrated, while in those infected with the type Ib and II strains fungal cells were rarely found. However, the mice infected with the type Ib strain NUM 978 were an exception; the recovery of CFU from the kidney was high, but the animals survived longer. Histologically, Candida cells were not colonized but interspersed in the tissue. Type II strain NUM 584 was found to be moderately virulent when infected at a high dose. These observations indicate that the proteinase plays a role in type Ia strains but that other factors are involved in the type Ib or II strains for the establishment of pathogenicity of C. albicans.     

Characterization of agglutinin-like sequence genes from non-albicans Candida and phylogenetic analysis of the ALS family

The ALS (agglutinin-like sequence) gene family of Candida albicans encodes cell-surface glycoproteins implicated in adhesion of the organism to host surfaces. Southern blot analysis with ALS-specific probes suggested the presence of ALS gene families in C. dubliniensis and C. tropicalis; three partial ALS genes were isolated from each organism. Northern blot analysis demonstrated that mechanisms governing expression of ALS genes in C. albicans and C. dubliniensis are different. Western blots with an anti-Als serum showed that cross-reactive proteins are linked by beta 1,6-glucan in the cell wall of each non-albicans Candida, suggesting similar cell wall architecture and conserved processing of Als proteins in these organisms. Although an ALS family is present in each organism, phylogenetic analysis of the C. albicans, C. dubliniensis, and C. tropicalis ALS genes indicated that, within each species, sequence diversification is extensive and unique ALS sequences have arisen. Phylogenetic analysis of the ALS and SAP (secreted aspartyl proteinase) families show that the ALS family is younger than the SAP family. ALS genes in C. albicans, C. dubliniensis, and C. tropicalis tend to be located on chromosomes that also encode genes from the SAP family, yet the two families have unexpectedly different evolutionary histories. Homologous recombination between the tandem repeat sequences present in ALS genes could explain the different histories for co-localized genes in a predominantly clonal organism like C. albicans.     

Diabetes mellitus and candidiases

Patients in various clinical states of diabetes mellitus (according to the recommendation of the American Diabetes Association) as a primary diagnosis were examined for fungal infections by Candida species. Candida spp. were detected in urine, in the material taken from the mouth cavity, nails, skin lesions, ears and eyes, by cultivation on the Sabouraud agar, CHROMagar Candida, and by saccharide assimilation. In the group of diabetics with symptoms of oral candidiasis and denture stomatitis C. albicans was identified in 8 cases, C. tropicalis in 3, C. parapsilosis in 2; 1 strain of C. guilliermondii was also isolated. In patients with urinary tract infections the presence of C. albicans was shown in 12 cases; C. parapsilosis was detected in 6 cases and two strains of each C. tropicalis and C. krusei were also isolated. In patients with leg ulcers C. albicans (25 cases), C. parapsilosis (5), C. tropicalis (3) and one strain of each C. krusei and C. robusta were isolated. Otomycosis was associated with one strain of C. albicans, C. parapsilosis, C. tropicalis and C. guilliermondii. C. albicans was most frequently associated with onychomycosis, paronychia and endophthalmitis; C. parapsilosis was the second most rated yeast.     

Proteinase production and pathogenicity of Candida albicans. I. Invasion into chorioallantoic membrane by C. albicans strains of different proteinase activity

In order to investigate a role of proteinase in the pathogenesis of Candida infections, invasion of C. albicans strains of different proteinase activity into the chorioallantoic membrane (CAM) of developing chicks was studied. Eight strains were used after examining the inducible proteinase activity in the culture containing bovine serum albumin as the sole source of nitrogen. Six were proteinase-producing strains (type I) and two were proteinase-deficient ones (type II). Type I strains were subdivided into type Ia strains in which the proteinase activity persisted for a week in the in vitro culture and type Ib ones in which the enzyme activity was lost by the 7th day after inoculation. By inoculation onto CAM, the type I strains could invade the tissue in which secreted proteinase was detected on the periphery of the invading Candida cells by immunohistochemical method. At an early stage of the infection, proteinase secretion was detected on the surface of the yeast cells before their entry into the tissue. The type II strains remained on the surface of the CAM and did not invade the tissue where the secretion of the enzyme was not detected. The mortality rate of the chick embryo was not correlated with the degree of proteinase production of these strains. Two type Ib strains invaded the CAM tissue and elicited some tissue reactions by the host, yielding a low mortality rate of the chick embryos. These results suggested that the secretion of proteinase was an important factor for the invasion of CAM but other factors were also involved for the pathogenicity of C. albicans.     

米诺环素对临床几种常见假丝酵母菌体外药敏的初步探究

探究米诺环素在体外对常见酵母菌的药敏特点。采用Rosco纸片扩散法对168株常见的几种假丝酵母菌进行米诺环素的药敏试验。米诺环素有抑菌环的比率:白假丝酵母菌41%,光滑假丝酵母菌1.2%,热带假丝酵母菌和克柔假丝酵母菌没有抑菌环。在常见几种酵母菌中,米诺环素几乎只对白假丝酵母菌在体外有抗菌活性。     

Evaluation of biochemical and serological methods to identify and clustering yeast cells of oral Candida species by CHROMagar test, SDS-PAGE and ELISA.

The purpose of this work was to evaluate biochemical and serological methods to characterize and identify Candida species from the oral cavity. The strains used were five Candida species previously identified: C. albicans, C. guilliermondii, C. parapsilosis, C. krusei, C. tropicalis, and Kluyveromyces marxianus, as a negative control. The analyses were conducted through the SDS-PAGE associated with statistical analysis using software, chromogenic medium, and CHROMagar Candida (CA), as a differential medium for the isolation and presumptive identification of clinically important yeasts and an enzyme-linked immunoabsorbent assay (ELISA), using antisera produced against antigens from two C. albicans strains. This method enabled the screening of the three Candida species: C. albicans, C. tropicalis, and C. krusei, with 100% of specificity. The ELISA using purified immunoglobulin G showed a high level of cross-reaction against protein extracts of Candida species. The SDS-PAGE method allowed the clustering of species-specific isolates using the Simple Matching coefficient, S(SM) = 1.0. The protein profile analysis by SDS-PAGE increases what is known about the taxonomic relationships among oral yeasts. This methodology showed good reproducibility and allows collection of useful information for numerical analysis on information relevant to clinical application, and epidemiological and systematical studies.     

New chromogenic agar medium for the identification of Candida spp

A new chromogenic agar medium (Candida diagnostic agar [CDA]) for differentiation of Candida spp. is described. This medium is based on Sabouraud dextrose agar (Oxoid CM41) and contains (per liter) 40.0 g of glucose, 10.0 g of mycological peptone, and 15.0 g of agar along with a novel chromogenic glucosaminidase substrate, ammonium 4-(2-[4-(2-acetamido-2-deoxy-beta-D-glucopyranosyloxy)-3-methoxyphenyl]-vinyl)-1-(propan-3-yl-oate)-quinolium bromide (0.32 g liter(-1)). The glucosaminidase substrate in CDA was hydrolyzed by Candida albicans and Candida dubliniensis, yielding white colonies with deep-red spots on a yellow transparent background after 24 to 48 h of incubation at 37 degrees C. Colonies of Candida tropicalis and Candida kefyr were uniformly pink, and colonies of other Candida spp., including Candida glabrata and Candida parapsilosis, were white. CDA was evaluated by using 115 test strains of Candida spp. and other clinically important yeasts and was compared with two commercially available chromogenic agars (Candida ID agar [bioMerieux] and CHROMagar Candida [CHROMagar Company Ltd.]). On all three agars, colonies of C. albicans were not distinguished from colonies of C. dubliniensis. However, for the group containing C. albicans plus C. dubliniensis, both the sensitivity and the specificity of detection when CDA was used were 100%, compared with values of 97.6 and 100%, respectively, with CHROMagar Candida and 100 and 96.8%, respectively, with Candida ID agar. In addition, for the group containing C. tropicalis plus C. kefyr, the sensitivity and specificity of detection when CDA was used were also 100%, compared with 72.7 and 98.1%, respectively, with CHROMagar Candida. Candida ID agar did not differentiate C. tropicalis and C. kefyr strains but did differentiate members of a broader group (C. tropicalis, C. kefyr, Candida lusitaniae plus Candida guilliermondii); the sensitivity and specificity of detection for members of this group were 94.7 and 93.8%, respectively. In addition to the increased sensitivity and/or specificity of Candida detection when CDA was used, differentiation of colony types on CDA (red spotted, pink, or no color) was unambiguous and did not require precise assessment of colony color.     

Characterization of Candida antigens by crossed-immunoaffinoelectrophoresis

The antigens of three Candida albicans strains (3153 A, 3156 B and CBS 1905) and one C. tropicalis strain were studied by means of crossed-immunoaffinoelectrophoresis with the corresponding polyvalent antisera. Most antigens (from 63.8% to 77.7% depending on the strain) were bound to concanavalin A-sepharose and about 20% to blue cibacron-sepharose for all the strains tested. Free concanavalin A, wheat germ lectin-sepharose and Helix pomatia lectin-sepharose revealed differences between C. albicans 3153 A and C. albicans CBS 1905 on the one hand and C. albicans 3156 B and C. tropicalis on the other, since affinity percentages were from 4.2 to 10.2 and from 14.2 to 20.0 respectively. Among 10 previously described species-specific antigens of C. albicans, 4 were never bound and 5 were bound to concanavalin A-sepharose which was considered an unsuitable agent for antigen purification since it retained 77% of C. albicans antigens. One important species-specific antigen was bound to blue cibacron sepharose and the corresponding purification could be undertaken. Similar results were found for 12 species-specific antigens of C. tropicalis. Blue cibacron-sepharose as well as wheat germ lectin or Helix pomatia lectin-sepharose were found suitable agents for purification of some of them.     

N-acetylglucosamine induces white-to-opaque switching and mating in Candida tropicalis, providing new insights into adaptation and fungal sexual evolution

Pathogenic fungi are capable of switching between different phenotypes, each of which has a different biological advantage. In the most prevalent human fungal pathogen, Candida albicans, phenotypic transitions not only improve its adaptation to a continuously changing host microenvironment but also regulate sexual mating. In this report, we show that Candida tropicalis, another important human opportunistic pathogen, undergoes reversible and heritable phenotypic switching, referred to as the "white-opaque" transition. Here we show that N-acetylglucosamine (GlcNAc), an inducer of white-to-opaque switching in C. albicans, promotes opaque-cell formation and mating and also inhibits filamentation in a number of natural C. tropicalis strains. Our results suggest that host chemical signals may facilitate this phenotypic switching and mating of C. tropicalis, which had been previously thought to reproduce asexually. Overexpression of the C. tropicalis WOR1 gene in C. albicans induces opaque-cell formation. Additionally, an intermediate phase between white and opaque was observed in C. tropicalis, indicating that the switching could be tristable.