Determination of minimum inhibitory concentrations

Minimum inhibitory concentrations (MICs) are defined as the lowest concentr ation of an antimicrobial that will inhibit the visible growth of a microor ganism after overnight incubation, and minimum bactericidal concentrations (MBCs) as the lowest concentration of antimicrobial that will prevent the g rowth of an organism after subculture on to antibiotic-free media. MiCs are used by diagnostic laboratories mainly to confirm resistance, but most oft en as a research tool to determine the in vitro activity of new antimicrobi als, and data from such studies have been used to determine MIC breakpoints . MBC determinations are undertaken less frequently and their major use has been reserved for isolates from the blood of patients with endocarditis. S tandardized methods for determining MiCs and MBCs are described in this pap er. Like all standardized procedures, the method must be adhered to and may not be adapted by the user. The method gives information on the storage of standard antibiotic powder, preparation of stock antibiotic solutions, med ia, preparation of inocula, incubation conditions, and reading and interpre tation of results. Tables giving expected MIC ranges for control NCTC and A TCC strains are also supplied.