Preliminary evaluation of a semisolid agar antifungal susceptibility test for yeasts and molds

Abstract

This report presents a semisolid agar antifungal susceptibility (SAAS) method for the rapid susceptibility screening of yeasts and molds. The reproducibility and accuracy of the SAAS method were assessed by comparing the MICs of amphotericin B and fluconazole obtained for 10 candidate quality control (QC) American Type Culture Collection yeast strains in >/=15 replicates with those found by six independent laboratories using the National Committee for Clinical Laboratory Standards (NCCLS) M27-P broth macrodilution method (M. A. Pfaller et al., J. Clin. Microbiol. 33:1104-1107, 1995). Overall, 96% of MICs for both drugs fell within 1 log(2) dilution of the modal MIC for each strain. The MICs for amphotericin B showed 99% agreement with the NCCLS proposed QC ranges within 1 log(2) dilution. Likewise, the MICs for fluconazole at >/=75% growth reduction showed 99% agreement for seven strains. Three strains, Candida albicans ATCC 24333 and ATCC 76615 and Candida tropicalis ATCC 750, showed a less sharp fluconazole endpoint at >/=75% growth reduction, but at >50% growth reduction, the agreement was 98% within 1 log(2) dilution of the proposed range. The MIC agreement within the proposed range for the suggested QC strains Candida parapsilosis ATCC 22019 and Candida krusei ATCC 6258 was 100% for fluconazole and 100% within 1 log(2) dilution of the proposed range for amphotericin B. The SAAS method demonstrated the susceptibility or resistance of 25 clinical isolates of filamentous fungi such as Aspergillus fumigatus to amphotericin B, itraconazole, and fluconazole, usually within 48 h. Although the results are preliminary, this SAAS method is promising as a rapid and cost-effective screen and is worthy of concerted investigation.

FIG. 1

Growth of yeast (C. albicans) in screening test system after 48 h of incubation at 35°C. Tubes 1 and 2, side and facing views of 4+ growth (equal to that of the drug-free control); tube 3, 3+ (75% of growth control); tube 4, 2+ (50% of growth control); tube 5, 1+ (25% or less of growth control); tube 6, no growth.

FIG. 2

Growth controls of various filamentous fungi after 48 h of incubation at 35°C (72 h for Trichophyton). Tube 1, Aspergillus fumigatus; tube 2, Mucor sp.; tube 3, Fusarium sp.; tube 4, Trichophyton sp.; tube 5, uninoculated medium.

FIG. 3

Amphotericin B MICs obtained by screening test method () for the 10 ATCC yeast strains compared with the QC range of MICs obtained by the NCCLS M27-P broth macrodilution method () in the multilaboratory study by Pfaller et al. (11).

FIG. 4

Fluconazole MICs obtained by screening test method () for the 10 ATCC yeast strains compared with the QC range of MICs obtained by the NCCLS M27-P broth macrodilution method () in the multilaboratory study by Pfaller et al. (11).