A New Antifungal Susceptibility Screening Test.



Proposed for use in the hospital laboratory is a simple, inexpensive antifungal susceptibility screening method which can produce results within 48 hours (h) after the isolation of pathogenic yeasts, cryptococci or filamentous fungi. A loopful of a just-turbid suspension of the isolate in water was inoculated deep into tubes containing semi-solid heart infusion agar with and without clinically relevant concentrations (conc) of pharmacy stock drug and incubated at 35°C for 48 h. Susceptibility in vitro (MIC) was defined as the lowest drug conc at which a prominent decrease in visible growth in the tube (no visible growth for amphotericin B (AB), and >= 75% or >= 50% growth reduction (GR) for fluconazole (FL)) compared to the growth control was observed. The ten candidate ATCC yeast quality control (QC) strains studied in accordance with the National Committee for Clinical Laboratory Standards M27-P guidelines by Pfaller et al in 1995 were each tested with this new screen method eight or more times to determine MICs using identical conc of drug. For AB, agreement with Pfaller's proposed QC ranges within 1 dilution (dil) was 99%; for FL at >= 75% GR agreement was 100% for seven strains. Three strains, ATCC 24433, 76615, and 750, showed a less sharp endpoint at >= 75% GR, but at >= 50% GR agreement was 100% within the proposed range. Furthermore, agreement within the proposed range for the suggested QC strains ATCC 22019 and 6258 was 100% for FL; for AB, agreement was 100% within 1 dil of the proposed range. Like Pfaller et al, we found the two strains ATCC 6258 (Candida krusei) and 90030 (Candida glabrata) to be relatively resistant to FL. Despite the limited number of resistant strains tested, our findings suggest that this screening method can adequately predict resistance to antifungal drugs. In addition, the susceptibility or resistance of 8 clinically significant filamentous fungi such as Aspergillus fumigatus to AB or FL was demonstrated by this screen within 48 h. Although results are preliminary, this antifungal susceptibility screen seems promising, cost-effective and worthy of concerted investigation.

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Full conference title: 

38th Interscience Conference on Antimicrobial Agents and Chemotherapy
    • ICAAC 38th