NDLI: The Trailing End Point Phenotype in Antifungal Susceptibility Testing Is pH Dependent

Content Provider	PubMed Central
Author	Marr, Kieren A. Rustad, Tige R. Rex, John H. White, Theodore C.
Copyright Year	1999
Abstract	The interpretation of end points in azole antifungal drug susceptibility testing is problematic, in part due to incomplete growth inhibition of Candida species. Such trailing growth can cause the MICs of fluconazole for some isolates to be low (<1 μg/ml) after 24 h of growth but much higher (>64 μg/ml) after 48 h. Isolates having this type of growth have been described as having a low-high phenotype. Although these isolates would be considered resistant by current National Committee of Clinical Laboratory Standards definitions, growing evidence suggests that they are susceptible in vivo. To further characterize these isolates in vitro, microdilution susceptibility testing comparing the complex defined medium RPMI 1640 to a defined minimal medium (yeast nitrogen broth) was performed. Isolates having trailing growth in MOPS (morpholinepropanesulfonic acid)-buffered RPMI 1640 (pH 7.0) were found to have clear end points in the minimal medium at its native pH of 4.5. The pH of the medium influenced the low-high phenotype, as these same isolates trailed in minimal medium adjusted to a pH of ≥6.0 but did not trail in RPMI 1640 adjusted to a pH of ≤5.0. This pH effect was independent of the medium buffering capacity, as trailing was decreased in both minimal medium and RPMI 1640 (pH 4.5) buffered in citrate. Adjustment in the pH of MOPS-buffered RPMI 1640 reduced trailing in multiple strains of Candida albicans without affecting the MICs for isolates having known susceptible (low-low) and resistant (high-high) phenotypes. Adjustment of the medium pH could be considered to eliminate trailing in azole drug susceptibility testing.
Starting Page	1383
File Format	PDF
ISSN	10986596
e-ISSN	10986596
Journal	Antimicrobial Agents and Chemotherapy
Issue Number	6
Volume Number	43
Language	English
Publisher	American Society for Microbiology
Publisher Date	1999-06-01
Access Restriction	Open
Rights Holder	American Society for Microbiology
Subject Keyword	Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Infectious Diseases Pharmacology Pharmacology (medical)

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