IN-VITRO ISOLATION AND ANTIFUNGAL SUSCEPTIBILITY OF AMPHOTERICIN B-RESISTANT MUTANTS OF ASPERGILLUS-FUMIGATUS

Aspergillus fumigatus mutants resistant to amphotericin B were selecte d in the laboratory following UV irradiation. A total of 18 colonies ( frequency 1.8 x 10(-7)) that grew in the presence of amphotericin B (8 mg/L and 16 mg/L) on peptone yeast extract glucose agar were tested f or their susceptibility to amphotericin B, nystatin, azoles, and the e chinocandin L-743872. Ten of the 18 isolates showed an eight-fold rise in amphotericin B MIC (4 mg/L) compared with the susceptible parent w hereas the remaining isolates showed a 16 to 32-fold rise in amphoteri cin B MIC (8-16 mg/L). Subculturing of three representatives from each of the groups that had MIC values of 4 mg/L and 8-16 mg/L for six cyc les revealed that the resistance trait was stably expressed. All ampho tericin B-resistant isolates showed a significant level of cross-resis tance to nystatin but not to azoles and L-743872. Kill-curve studies w ith amphotericin B revealed that the killing of the resistant isolates was significantly less than that of the susceptible parent strain. Th ese results show that amphotericin B-resistant mutants of A. fumigatus can be isolated in the laboratory following a single-step UV-induced mutagenesis and suggest that similar mechanisms could operate in natur e for the emergence of resistance in clinical isolates.