Method for the determination of minimum inhibitory concentration (MIC) by broth dilution of fermentative yeasts

Abstract

Dilution methods are used to determine the mini-mum inhibitory concentrations (MICs) of antimi-crobial agents and are the reference methods for antimicrobial susceptibility testing. In dilution tests, microorganisms are tested for their ability to produce visible growth in microtitration plate wells of broth (broth microdilution) containing serial dilutions of the antimicrobial agents. The MIC is de®ned as the lowest concentration of an antimicrobial agent that inhibits the growth of a microorganism. The method described in this document is intended for testing yeasts that cause clinically signi®cant infections (primarily Candida species). This standard encompasses only those yeasts that are able to ferment glucose. Thus, testing the susceptibility of nonfermentative yeasts such as Cryptococcus neoformans var. neoformans cannot be determined by the current procedure nor is the methodology suitable for the yeast forms of dimorphic fungi. P U R P O S E The standard method described herein is intended to provide a valid method for testing the suscept-ibility to antifungal agents of yeasts that ferment glucose. The method is intended primarily to facilitate an acceptable degree of conformity, i.e. agreement within speci®ed ranges, between laboratories in measuring the susceptibility of yeasts to antifungal agents. The method is designed to be easy to perform, rapid, economic, and to be suitable for reading with microtitration plate readers, which allows direct transfer, storage and manipulation of data with a computer. The method is also intended to produce concordant results with the American National Committee for Clinical and Laboratory Standards (NCCLS) docu-ment on antifungal susceptibility testing of yeasts, Approved Standard M27-A2 [1].