Antifungal Susceptibility of Rare Yeasts Causing Vulvovaginal Candidiasis in the Acidic Vaginal Environment.

Objective: To investigate the in vitro potency of 13 antifungals against emerging rare yeast species causing vulvovaginal candidiasis (VVC) under the influence of acidic pH. Methods: Antifungal susceptibility was determined according to CLSI M27/M44-ed3 with minor modifications. In total, 42 isolates of rare yeast were isolated from the vaginal samples based on molecular identification, including Results: The antifungal agents most affected by a lower pH were terconazole (3 two-fold dilutions higher, MIC >0.5-8 mg/L) for all yeast isolates, followed by amphotericin B (2 two-fold dilutions higher, MIC >2 mg/L) for 93% of isolates, nystatin (2 two-fold dilutions higher, MIC >4-8 mg/L) for 95% of isolates, and micafungin (2 two-fold dilutions higher, MIC >0.125-1 mg/L) for 50% of isolates, compared with those at pH 7.0, indicating a high percentage of low activity against the yeast species. Fluconazole, voriconazole, miconazole, and clotrimazole were the least affected, while anidulafungin, caspofungin, and 5-flucytosine were not affected significantly by pH, showing activity against most rare yeast species at both pH levels. Conclusion: The noted shift toward rare yeasts could present a challenge to the clinical treatment of VVC. Considering the influence of acidic pH, our results reveal apparently drug-specific and genus-specific trends in antifungal susceptibility. The variability of rare yeasts with regard to antifungal susceptibility at acidic pH warrants further research on this topic.