In vitro activity of ceftazidime, cefotaxime and piperacillin against multi-resistant gram-negative bacteria tested with a modified agar dilution method.

The minimum inhibitory concentrations of ceftazidime, cefotaxime and piperacillin were determined for 870 multi-resistant gram-negative bacterial strains by means of a modified agar dilution technique using dishes with 25 compartments. This technique facilitates the reading of the MICs, reduces the risk of inoculation errors, and eliminates interference of interaction of bacterial strains. Ceftazidime and cefotaxime inhibited 90% of the Enterobacteriaceae strains at concentrations of 3.12 mg/l and 1.56 mg/l respectively. Ceftazidime and piperacillin inhibited 90% of the Pseudomonas aeruginosa strains at concentrations of 6.25 mg/l and 12.5 mg/l respectively. Mucoid Pseudomonas aeruginosa strains were more susceptible to the antibiotics tested, especially piperacillin, than non-mucoid strains.