Susceptibility testing of Aspergillus niger strains isolated from poultry to antifungal drugs - a comparative study of the disk diffusion, broth microdilution (M 38-A) and Etest® methods

The aim of this study was to determine the sensitivity of Aspergillus niger strains isolated from birds to available antifungal drugs using different in vitro assays - classical disk diffusion, Etest® and broth microdilution NCCLS/CLSI M 38-A. The study material consisted of about 2.000 swabs and samples from different species of birds. A. niger (n=10) was accounted for 6.81% of the total pool of strains isolated. Determinations were made for 13 antifungal drugs using the disk diffusion method. The A. niger exhibited high susceptibility to enilconazole, terbinafine, voriconazole, tioconazole and ketoconazole, low susceptibility to clotrimazole, miconazole and nystatin, and resistance to amphotericin B, itraconazole, pimaricin, fluconazole and 5-fluorocytosine. Minimum inhibitory concentration (MIC) was determined for 9 antifungal drugs using the micromethod of duplicate serial dilutions in a liquid medium. A. niger strains were most susceptible to enilconazole and voriconazole. MIC ranged from 0.0625 to 0.5 μg/ml for enilconazole, with MIC90-0.5 μg/ml and MIC50-0.125 μg/ml. The corresponding values for voriconazole were 0.25-1 μg/ml, 1 μg/ml and 0.5 μg/ml. MIC for amphotericin B and terbinafine ranged from 0.5 to 4 μg/ml, while the values for the remaining drugs were highly varied. MIC was measured by the gradient diffusion method using Etest® for 5 antifungal drugs: amphotericin B, fluconazole, itraconazole, ketoconazole and voriconazole. By far the highest susceptibility was obtained in the case of voriconazole, with MIC ranging from 0.0625 to 1 μg/ml. MIC for amphotericin B ranged from 0.25 to 4 μg/ml, for itraconazole and ketoconazole ranging from 0.5 to 16 μg/ml. Methods available for this purpose are not always applicable in field conditions. The present results indicate that the Etest® technique, due to its high percentage of agreement with the M 38-A microdilution method, should find application in medical and veterinary practice.