Altered susceptibility of rare yeast species causing vulvovaginal candidiasis at acidic vaginal environment

This study evaluated potency of antifungals against the rare emerging yeast causing VVC under the influence of acid pH (4.5). The antifungal susceptibility was evaluated by CLSI-M27A4 for 25 isolates represented 11 rare yeast species recovered from VVC patients. Among 11 yeast species (25 isolates), Clavispora lusitaniae (n=9) was the most common species, followed by Cyberlindnera fabianii (n=4) , Kodamaea ohmeri, Candida metapsilosis and Candida bracarensis (n=2, respectively) , Pichia norvegensis, Kluyveromyces marxianus, Diutina rugosa, Tricholoma matsutake, Candida nivariensis and Candida africana (n=1, respectively). The antifungal agents most affected by a lower pH were terconazole (4-fold higher, MIC >0.5-8mg/L) to all 11 yeast species; followed by amphotericin B (3-fold higher, MIC >2mg/L) to 9 species except K. ohmeri and D. rugose ; nystatin (2-fold higher, MIC >2 - 4 mg/L) to 9 species except C. nivariensis and C . africana ; micafungin (2-fold higher, MIC >0.125 - 1 mg/L) to 8 species except C. metapsilosis, D. rugosa and Tricholoma matsutake by acid pH, respectively. 5-flucytosine was not affected by pH to all tested yeast species, and maintained activity at low PH.

The rare yeast species tested shared high MICs of terconazole, amphotericin B, nystatin at pH4.5 compared with those at pH7.0. The in vitro antifungal susceptibility at pH 7.0 did not consistently predict the activity of antifungals against those rare yeast species at vaginal acidic pH. Further investigations assessing the clinical utility of antifungal susceptibility testing should take these findings into consideration.