Evaluation of Etest for determining in vitro susceptibility of yeast isolates to amphotericin B

We evaluated the performance of Etest using several different agar media fo r testing of amphotericin B against 660 clinical isolates of yeast species including Candida albicans, C. glabrata, C. tropicalis, C. parapsilosis, C. lusitaniae, C. krusei, Candida spp., Cryptococcus neoformans, and Saccharo myces cerevisiae. Two of the C. lusitaniae isolates represented strains wit h high-level amphotericin B resistance. All isolates were tested by NCCLS m icrodilution methods with XPMI 1640 medium and by Etest using RPMI agar wit h 2% glucose (RPG). A subset of 108 isolates was also tested by Etest using RPG, antibiotic medium 3 agar (AM3), Casitone agar (CAS), and Mueller-Hint on agar (MHA). The overall agreement between the NCCLS reference method and Etest using RPG was 98.3%. All of the Etest methods identified the two res istant strains (MICs, 4.0 to 16 mu g/mL), whereas the reference method fail ed to distinguish them from 18 other isolates with MICs of 2.0 mu g/mL. Amo ng the 20 isolates with reference MICs of 2.0 mu g/mL, 12 han MICs greater than or equal to 2.0 mu g/mL when tested by Etest with RPG (range 2.0 to 16 mu g/mL) compared ii with fight with AM3, two with GAS, and five with MHA. These data indicate that Etest identifies subpopulations of yeast isolates with high amphotericin B MICs. The greater sensitivity Of Etest for detect ion of amphotericin B resistance should be exploited in future surveillance studies. (C) 1998 Elsevier Science Inc.