Measuring Minimum Inhibitory Concentrations in Mycobacteria

An agar dilution method for measuring minimum inhibitory concentrations (MICs) of Mycobacterium tuberculosis , based on the method of proportion, is described. Mycobacterium strains are grown on Middlebrook 7H10 (or 7H11) agar medium with twofold serially diluted drug concentrations in order to determine specific inhibitory values. The proportion of bacilli resistant to a given drug is determined by comparing the number of colony-forming units (CFU) on a drug-free control with those growing in the presence of drug within a specific concentration range. The MIC is defined as the lowest drug concentration that inhibits growth of more than 99% of a bacterial population of M. tuberculosis on solid Middlebrook medium within 21 days of incubation at 37�C. The proportion method, the absolute concentration method, and the resistant ratio method have traditionally been used as standard procedures for antimycobacterial drug-susceptibility testing (DST), and reference data are mainly based on these methods. DST concepts and alternative procedures that have been adopted for DST are also briefly discussed.