Antifungal Breakpoints of Non-albicans Candida Clinical Isolates in Vitek-2 Compact

Sir,

The incidence and diversity of infections caused by non-albicans Candida species (NAC) have shown a substantial increase in recent years. Given the growing number of susceptible patients with compromised immunity as a consequence of HIV infection, cancer chemotherapy and transplantation associated immunosuppression, NAC has emerged as an eminent fungal pathogen worldwide. Pertaining to the diverse mechanisms of resistance to various antifungals agents among NAC, it has become imperative to identify the effective antifungals. A prospective study was conducted in Department of Microbiology in our tertiary care hospital from May to August, 2014. A total of 36 candida isolates (16 C. albicans, 11 C. tropicalis, 5 C. krusei, 2 C. glabrata and 2 C. parapsilosis) recovered from blood and urine were identified as per standard procedures. The NAC isolates (n=20) were selected for antifungal susceptibility testing (AFST) in Vitek-2 Compact using AST-YS07 card. MIC for fluconazole, voriconazole, caspofungin, micafungin and flucytosine were interpreted [Table/Fig-1] as per CLSI guideline [1]. Among the various AFST methods currently in use for yeasts, macrobroth dilution, microbroth dilution and disc diffusion tests are recommended by CLSI [2]. However, the utility of these tests are limited by the technically challenging procedure, difficulty of inoculums standardization and subjective variation in interpretation. Unlike bacterial pathogens, the slow growth rate of candida also precludes a short turnaround time. Early initiation of appropriate therapy is a critical factor determining the clinical outcome. The introduction of automated AFST has ensured prompt results in a standardized platform. However, there is a lack of published data of fungal resistance pattern determined by automated system in India. In this study, the mean analysis time for determining antifungal susceptibility by Vitek-2 Compact was 14.15 ± 2.49 hour. Vitek-2 utilizes a self-calibrating optic system comprising of light emitting diodes and photo detectors to continually monitor the growth of organisms inside the wells of the test cards, reducing the analysis time [3]. All five C. krusei isolates had fluconazole and flucytosine resistance with MIC > 8μg/dl. Two C. tropicalis strains showed fluconazole resistance and one had intermediate susceptibility to flucytosine. The median MICs for fluconazole, voriconazole, caspofungin, micafungin and flucytosine among C. tropicalis were 1, 0.12, 0.25, 0.06 and 18μg/dl respectively. In C. krusei the median MICs for these drugs were 16, 0.12, 0.25, 0.12 and 8μg/dl respectively. Our findings are in accordance with other studies. Tellapragada et al., found all NAC isolates except C. krusei were uniformly sensitive to fluconazole, while one C. parapsilosis and one C. tropicalis strain displayed voriconazole and caspofungin resistance respectively [4]. Echinocandins are novel synthetic lipopeptide antifungals which non-competitively inhibit the fungal β [1,3]-D-glucan synthase enzyme [5]. Echinocandin resistance among NAC species have been mainly reported in C. parapsilosis with FKS1 mutation [5]. In our study, caspofungin and micafungin were sensitive in all isolates, indicating their critical role in treatment of infections by azole resistant strains. However, further study with a larger sample size comparing Vitek-2 Compact results with other methods of MIC testing is required to confirm our findings. In conclusion, Vitek-2 Compact is essential in rapid determination of antifungal susceptibility of candida. However, the cost constraints may deter its incorporation in routine diagnostic protocol in resource poor settings.

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