[Comparative in vitro activity of 5 semi-synthetic penicillins against aerobic Gram-negative bacilli and enterococci (author's transl)].

The MICs and MBCs of mezlocillin, ticarcillin and piperacillin were determined by microdilution in liquid medium against 700 strains of Gram-negative bacilli and enterococci isolated from pathological materials and classified according to their sensitivity of ampicillin and carbenicillin. Strains sensitive to ampicillin and carbenicillin were usually sensitive to the other penicillins with weight for weight differences in activity. The MIC pf mezlocillin against 40% of ampicillin - and carbenicillin-resistant TEM-producing strains of E. coli ranged fron 32 to 64 micrograms/ml. In 50% of Klebsiella strains producing a broad-spectrum penicillinase which inhibited ampicillin and carbenicillin, the MIC of mezlocillin was equal or inferior to 8 micrograms/ml. Carbenicillin-resistant Enterobacter and Citrobacter strains were also resistant to the other penicillins under study. Mezlocillin and piperacillin showed some activity on a few strains of carbenicillin-resistant Serratia, Proteus and Acinetobacter spp. Piperacillin and, to a lesser degree, mezlocillin were active against those strains of Pseudomonas spp for which the MIC of carbenicillin was around 512 micrograms/ml. Mezlocillin and ampicillin were the most active of the antibiotics tested against Enterococci. The MBCs of all antibiotics in this study were strongly influenced by the size of the inoculum.