In vitro susceptibility testing of yeasts to nystatin – low minimum inhibitory concentrations suggest no indication of in vitro resistance of Candida albicans, Candida species or non-Candida yeast species to nystatin

In total, 14 yeast strains originating from patients with dermatomycoses, and 7 control strains (isolates from strain collections and collaborative ring trials) were investigated regarding their in vitro susceptibility to the polyene antifungal agent nystatin. Testing was performed using a broth microdilution assay based on the standardized method of susceptibility testing of yeasts per EUCAST (The European Committee on Antimicrobial Susceptibility Testing). Minimum inhibitory concentrations (MIC) for nystatin were measured. The reading of the MIC values was performed by both visual examination, and spectrophotometric measuring after 24 and 48 hours’ incubation time at 36°C. The visual read-out of growth inhibition revealed MICs for nystatin in a range from 3.7 to 7.4 IU/mL (0.625 to 1.25 μg/mL) for all Candida species tested. One of the Candida (C.) albicans strains, and both strains of C. glabrata and C. tropicalis, showed low MIC values of 3.7 IU/mL (0.625 μg/mL). Geotrichum candidum and Trichosporon mucoides were also inhibited by nystatin. The control strains (C. albicans, C. glabrata, C. parapsilosis, C. krusei and C. tropicalis) confirmed the values which were found for the wild strains. The spectrophotometric measuring of the turbidimetry revealed slightly lower MIC values for Candida species. Spectrophotometric measurment of Geotrichum candidum and Trichosporon mucoides was unsuccessful or not possible; however, visual reading of the results was carried out effectively. Nystatin showed very good in vitro activity against these non-Candida yeast species. In conclusion, very good in vitro activity of nystatin against all tested yeast strains could be detected. The in vitro efficacy was independent of the origin of the strains, as both the wild strains isolated from patients in this study, and the control strains originating from strain collections, were inhibited. Correspondence to: Dr. Pietro Nenoff, Dermatology, Laboratory Medicine, Allergy & Andrology Specialist; Main Focus: Tropical and Travel Dermatology (DDA), Mölbiser Hauptstraße 8, 04571 Rötha/OT Mölbis, Germany; Tel: +4934347-50 323; Fax +49-34347-50 123; E-mail: pietro.nenoff@gmx.de Received: October 31, 2016; Accepted: November 17, 2016; Published: November 22, 2016 Introduction The common use of azole antifungal agents, in particular oral fluconazole, but also topical azoles, for the treatment of cutaneous and oral candidiasis, and possibly due to the use of azole fungicides in agricultural crop protection, has led to the emergence of azole and echinocandine resistance of clinical isolates of Candida (C.) albicans and other Candida species [1]. An alternative therapeutic approach is the use of topical polyene antifungals such as amphotericin B or nystatin. Only limited current data exists regarding Candida species which are resistant to polyene antifungals. Recently, a total of 201 clinical C. albicans isolates from Turkey investigated by Etest were found to be susceptible to amphotericin B [2]. A similar situation can be found for nystatin. However, in vitro susceptibility testing studies have shown a low percentage of Candida species isolated from HIV positive patients to be resistant to nystatin [3]. In this study, a total of 14 currently isolated yeast strains from dermatomycosis patients, and seven reference strains (controls), including isolates from the DSMZ strain collection (German Collection of Microorganisms and Cell Cultures, Braunschweig, Germany) and inter-laboratory ring tests of INSTAND e.V. (the German Society for Promoting Quality Assurance in Medical Laboratories, Düsseldorf, Germany), were tested for their in vitro susceptibility to the polyene antifungal agent nystatin. A broth microdilution assay which corresponded to the method of the European Committee on Antimicrobial Susceptibility Testing (EUCAST), and allowed for determination of the minimal inhibitory concentration (MIC) of nystatin, was used for this purpose. Materials and methods