Piperacillin (T-1220), a semisynthetic derivative of aminobenzylpenicillin, was more active than either carbenicillin or ticarcillin against Pseudomonas aeruginosa; over 60% of isolates were inhibited at a concentration of 6.3 ug/ml. Piperacillin was bactericidal for 84% of Pseudomonas strains at 100 ,ug/ml, carbenicillin killed 60%, and ticarcillin killed 68% at that concentration. Piperacilhin was also more active than the other penicillins against isolates of Escherichia coli, Enterobacter, and Proteus mirabilis. The combination of piperacillin and tobramycin, demonstrating synergistic inhibition of 87% of strains of P. aeruginosa, was the most active of the penicillin-aminoglycoside combinations tested for synergism.Piperacillin sodium (T-1220) is a new semisynthetic penicillin with a broad spectrum of activity against gram-negative bacteria. In recent studies, many isolates of Pseudomonas aeruginosa and members of the Enterobacteriaceae were noted to be more susceptible to piperacillin than to carbenicillin (6,14,15). In the current study, the in vitro activity of piperacillin was compared with that of carbenicillin and ticarcillin against P. aeruginosa, and with that of carbenicillin, ticarcillin, ampicillin, cephalothin, and cefamandole against various Enterobacteriaceae. In addition, we compared combinations of piperacillin or carbenicillin with gentamicin or tobramycin for synergistic activity against P. aeruginosa. MATERIALS AND METHODSBacterial isolates. Fifty isolates each of Escherichia coli, Klebsiellapneumoniae, Proteus mirabilis, and P. aeruginosa, 37 of Enterobacter spp., 28 indolepositive Proteus, 13 Providencia stuartii, and 5 Serratia marcescens constituted the 283 bacterial isolates obtained from the diagnostic microbiology laboratory of the Royal Victoria Hospital. The identity of all isolates was confirmed by standard methods (3, 9). Not more than one isolate of a given species per patient was included.Culture medium. Mueller-Hinton broth was supplemented with calcium chloride and magnesium chloride (12). The final calcium concentration was 5.4 to t Present address: Division of Virology, New Addenbrooke's Hospital, Cambridge, England.6.0 mg/dl, and the magnesium concentration was 2.2 to 2.5 mg/dl in the various batches used.Determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). A microtitration broth dilution technique similar to that described by Harwick et al. was employed (8). Stock solutions of piperacillin, carbenicillin, and ticarcillin were prepared in Mueller-Hinton broth at a concentration of 533 ,g/ml. After appropriate dilutions in Mueller-Hinton broth and addition of the bacterial inoculum, the antibiotic concentrations ranged from 400 to 0.8 ,ug/ml, in a final volume of 200 ,ul. For ampicillin, cephalothin, and cefamandole, stock solutions were prepared at a concentration of 266 ,ug/ml, yielding antibiotic concentrations of 200 to 0.4 'Ug/ml.
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