[In vitro effect of piperacillin on aerobic bacteria. Variations according to the phenotypes of resistance to beta-lactam antibiotics].

In a first study the MICs of piperacillin (PIP), mezlocillin (MEZ), azlocillin (AZL), carbenicillin (CARB) and ticarcillin (TIC) against 563 strains of aerobic bacteria were compared. The MIC50 of PIP against E. coli and P. mirabilis was 0.5 to 1 mg/l, that is equal to those of MEZ and TIC but 2 or 3 times lower than those of AZL and CAR. The MIC50 of PIP and MEZ against Klebsiella, a species that is naturally resistant to TIC and CAR, was 4 mg/l. Enterobacter, Serratia and Citrobacter showed two populations of strains: the first was sensitive to the five penicillins tested (MIC50 of PIP: 1 to 2 mg/l); the second was resistant to all of them. PIP and AZL were more active (MIC50 8 mg/l) against Ps. aeruginosa than MEZ, TIC and CAR. All five penicillins had limited activity against Acinetobacter (MIC50 less than 16 mg/l). PIP, MEZ and AZL were more active than TIC and CAR against enterococci. In a second study, the activity of PIP was evaluated by standard sensitivity tests on 4993 strains of enterobacteria, Ps. aeruginosa and Acinetobacter classified according to their phenotype of resistance to beta-lactam antibiotics. PIP was regularly active against enterobacteria and Ps. aeruginosa strains devoid of acquired resistance. The activity of PIP was significantly reduced against enterobacteria strains with a phenotype suggesting induced penicillinase production. However, the inhibition zone diameters, although reduced, remained within what is now considered the sensitivity range (notably with E. coli and Proteus spp), which raises the problem of in vitro tests interpretation. PIP was active against E. coli strains with a "cephalosporinase" phenotype, but inactive against cefotaxime resistant strains of: Enterobacter, Citrobacter and Serratia. The activity of PIP on CAR-resistant Ps. aeruginosa strains was significantly reduced, but the inhibition zone diameter in one quarter of them was still within limits of sensitivity.