A combined approach using environmental yeasts and microbial indicators to assess aquatic pollution

Aquatic environments receiving anthropogenic inputs can act as reservoirs for opportunistic microorganisms and antifungal resistance, representing a potential environmental and public health concern. This study evaluated microbial contamination and antifungal susceptibility patterns in surface waters along a rural-urban gradient in a subtropical watershed using an integrated environmental monitoring approach. Physicochemical parameters and conventional microbial (Escherichia coli, Enterococcus spp., and heterotrophic bacteria) were quantified, and yeast abundance was determined using CHROMagar™ Candida. A total of 327 yeast isolates were evaluated for susceptibility to itraconazole (ITZ) and fluconazole (FCZ) using the CLSI M27-A4 broth microdilution method. Urban sites exhibited markedly higher microbial contamination compared with rural areas, with Enterococcus spp. reaching 36,000 colony-forming units (CFU)/100 ml and E. coli 19,863 most probable number (MPN)/100 ml. Yeast densities reached up to 351 CFU/100 ml in urban samples, exceeding concentrations typically reported for oligotrophic aquatic systems. Antifungal susceptibility testing revealed a high frequency of resistance among environmental isolates, ITZ (44.3%) compared with FCZ (24.46%). Statistical analyses indicated no significant spatial or seasonal differences in resistance patterns (p > 0.05), whereas resistance to ITZ was significantly higher than to FCZ (p < 0.001). The results indicate that anthropogenic pressure in urbanized watersheds may contribute to increased microbial contamination and the occurrence of antifungal-resistant yeasts in surface waters. This suggests a potential environmental pathway for the dissemination of resistance traits. The integration of conventional microbial indicators, environmental yeast monitoring, and antifungal susceptibility profiling provides a practical framework for environmental surveillance.