In vitro activity of piperacillin compared with that of ampicillin, ticarcillin, azlocillin, and mezlocillin.

The antibacterial activity of ampicillin, ticarcillin, azlocillin, mezlocillin and piperacillin was investigated in 1112 fresh clinical isolates using the microdilution technic. Ampicillin was the most active agent against enterococci and ticarcillin against Staphylococcus aureus, 91% of the latter being inhibited by 32 mg/l. 95% of strains of Pseudomonas aeruginosa were susceptible to piperacillin and 90% to ticarcillin and azlocillin. Piperacillin was still active against half of the azlocillin-resistant strains of Pseudomonas aeruginosa. Against the Enterobacteriaceae, mezlocillin and piperacillin were equally active: 31% of these strains were resistant to both drugs. Amongst the various strains the percentage of resistance was highest in Klebsiella (60%). Analysis of resistance patterns of Enterobacteriaceae showed almost completely parallel resistance between these two drugs the difference being less than 3%. Piperacillin, however, had the broadest spectrum of all the penicillins tested.