| Opportunistic yeast infections are among the most prevalent fungal infections of humans,and most opportunistic yeast pathogens are common residents of the oral mucosa.As a result,understanding the oral microbiota has broad implications for the prevention and management of human infectious diseases causing by endogenous commensal yeasts.However,relatively little is known about the ecological and geographic patterns of genetic variation and drug susceptibility profiles of oral yeasts.The objective of this study was to investigate species,genotype,and antifungal susceptibility profiles of yeasts from the oral cavities of inpatients on Hainan Island and a global pattern based on published multilocus sequence data of Candida tropicalis.Candida tropicalis is among the most prevalent human pathogenic yeast species,especially prevalent in tropical and sub-tropical regions.Hainan is a tourist tropical island with abundant organic matter conducive for the growth of Candida tropicalis.We hypothesize that the favorable environmental conditions for Candida tropicalis in Hainan and frequent migration of people may allow the generation and maintenance of abundant genetic variation of this species within Hainan.Furthermore,Candida tropicalis is globally distributed.Over the several decades,a large number of studies have been published on the genetic diversity and molecular epidemiology of Candida tropicalis from different parts of the world.However,the global pattern of genetic variation remains largely unknown.We also conducted a meta-analysis of the published data to understand the patterns of strain and population relationships.The main results are as follows:1.Oral yeast species and drug resistance of InpatientsA total of 677 inpatient mucosal specimens were collected in this study.Among the 677 oral swabs,446 were found to contain yeasts,representing an oral yeast isolation rate of 65.9%.No difference in yeast isolation rates was found between oral samples from the two sexes P=0.685)but the prevalence of yeast colonization was significantly higher among those over 60 years old compared with their younger counterparts(P<0.05).459 strains of yeast belonging to 14 species were isolated from oral cavities of these 446 inpatients,the three most prevalent species were Candida albicans(231),Candida glabrata(99),and Candida tropicalis(66).Thirteen patients aged>60 years inpatients had more than two yeast species each.The in vitro antifungal drug susceptibility analyses found that 33.3%strains had different degrees of drug resistance,and more than half of these strains were resistant to two or more antifungal drugs.Overall,the resistance rates of the 459 yeast strains to amphotericin B,flucytosine,itraconazole,ketoconazole and fluconazole were 2.4%,10.0%,18.5%,10.7%and 19.2%respectively.2.Population genetic analyses of Candida tropicalis from HainanA total of 116 isolates were obtained from eight geographic regions located across the Hainan island.For each isolate,a total of 2677 bp from six gene loci were sequenced and 79(2.96%)polymorphic nucleotide sites were found in our sample,36 of these were new and had never been reported from other geographic regions.The combined analyses of sequence information from the six gene fragments identified a total of 94 diploid sequence types(DSTs).Among these 94 DSTs,only 14 have been reported previously and the remaining 80 DSTs were completely new to the database.Most of the genetic variation was found within individual strains and there was abundant evidence for gene flow among the seven geographic locations within Hainan.Mantel test identified no significant correlation between the diploid sequence types at the six loci and fluconazole susceptibility,non-synonymous mutation was found in the ERG11 gene of fluconazole-resistant strain vs.fluconazole-susceptible strain but with the same MLST genotype,consistent with multiple origins of fluconazole resistance in the Hainan population of Candida tropicalis.3.Genetic analyses of the global population of Candida tropicalisThe published multilocus sequence data at six loci for 876 isolates from 16 countries representing five continents were analyzed in this study.The results showed that 280 of the 2677(10.5%)analyzed nucleotides were polymorphic,resulting in a mean of 82(a range of 38 to 150)genotypes per locus and a total of 633 combined diploid sequence types(DSTs)based on the six loci.Among these,93 DSTs were shared by 336 strains,including 10 DSTs by strains from different continents.Analysis of Molecular Variance(AMOVA)results showed that 89%of the observed genetic variations were found within regional and national geographic populations while 11%was due to among-country separations.Pairwise geographic population analyses showed overall low but statistically significant genetic differentiation between most pairs of geographic populations,with the Singaporean and Indian populations being the most distinct from other populations.Mantel test showed no significant correlation between genetic distance and geographic distance among the geographic populations.Consistent with high genetic variation within and limited variations among geographic populations,results from STRUCTURE analyses showed that the 876 isolates could be grouped into 15 genetic clusters,with each cluster having a broad geographic distribution.Together,our results suggest frequent gene flows among certain regional,national,and continental populations of Candida tropicalis,resulting in abundant regional and national genetic diversitiesofthis important human fungal pathogen.In conclusion,the objective of this study was to:(i)investigate species and antifungal susceptibility profiles of yeasts from the oral cavities of inpatients on HainanIsland with special geographical and ecological environment,(ii)analyze the genetic diversity,population structure and the correlation between genotype and drug resistance of Candida tropicalis by molecular genetics method firstly,(iii)based on the published MLST data,analyze the global genetic variation model of Candida tropicalis.The study is helpful to better understand the prevalence of Candida tropicalis and the data will help develop effective prevention and control diseases caused by this pathogen. |
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