Comparative in vitro activity of ticarcillin, piperacillin, azlocillin and mezlocillin.

The in vitro activity of ticarcillin, piperacillin, azlocillin and mezlocillin was determined against 403 clinical isolates. At MIC50, piperacillin was 2 to 8 times more active than the other three compounds against Pseudomonas, Escherichia coli, Proteus, Citrobacter, Acinetobacter and Salmonella species. Against Klebsiella, Enterobacter, Haemophilus, Bacteroides spp. and non-beta-lactamase producing Staphylococcus aureus the activity of piperacillin was similar to one or more of the most effective agents. However, azlocillin and mezlocillin were more active than piperacillin against enterococci. Ticarcillin was the least active in vitro. Despite these significant differences at MIC50 amongst the four compounds, they became much less discernible at MIC90, obviously due to beta-lactamase producing strains under study. The spectrum of activity of piperacillin encompasses those of azlocillin and mezlociLlin together except for Gram-positive organisms.