Diversity, antifungal susceptibility, and biofilm formation of fish-associated yeasts from freshwater and marine fish.

This study investigated the diversity, antifungal susceptibility, and biofilm-forming capacity of yeasts isolated from freshwater and marine fish marketed for human consumption in Central Anatolia, Türkiye. A total of 109 yeast isolates recovered from 130 fish samples were identified by MALDI-TOF MS and assigned to seven genera and eleven species, with Candida, Rhodotorula, and Cryptococcus predominating. Candida zeylanoides was the most frequently recovered species and was widely distributed across fish species and habitats. Freshwater fish were predominantly colonized by Candida spp., whereas marine fish showed greater yeast diversity. Yeast isolation from muscle tissue was infrequent (7.7%) and restricted to freshwater fish, including Candida albicans and Candida tropicalis. Based on CLSI interpretive criteria, 62.5% of Candida isolates were resistant to fluconazole, whereas 9.4% and 28.1% were categorized as susceptible-dose dependent and susceptible, respectively. Among the antifungal agents evaluated, posaconazole exhibited the broadest inhibitory activity, itraconazole the lowest, while amphotericin B showed consistent activity across all isolates. Fluconazole susceptibility was significantly associated with tissue origin (p < 0.001), with resistance being most prevalent (96.6%) among skin-derived isolates. Biofilm formation was detected in 63.3% of isolates and differed significantly among yeast groups (p = 0.005). Strong biofilm-producing phenotypes were most frequently associated with C. zeylanoides and C. albicans, whereas all C. tropicalis isolates lacked detectable biofilm-forming capacity. These findings highlight the presence of diverse yeast communities in fish, including isolates with reduced antifungal susceptibility and biofilm-forming capacity, underscoring the need for continued surveillance within food safety and public health contexts.