A replicator method for the combined determination of minimum inhibitory concentration and minimum bactericidal concentration

Minimal bactericidal concentrations (MBCs) of nine antimicrobial agents were determined for clinical isolates by a replica plating method. Membranes were placed on the antibiotic-containing plates and the organisms replicated onto the membranes. After 18 h incubation the minimal inhibitory concentrations (MICs) were determined, the membranes were transferred to antibiotic-free plates and incubated a further 18 h and the MBCs determined. MICs and MBCs were also determined in broth. The reproducibility of the 'membrane' method and the agreement of these results for MIC and MBC with the agar and/or broth methods was satisfactory for most antibiotics, within one two-fold dilution. With sulphamethoxazole, trimethoprim and co-trimoxazole the results were less satisfactory, especially with Gram-negative rods, but agreement within two dilutions could be achieved.