In vitro comparison of mezlocillin and piperacillin plus tobramycin or gentamicin versus 100 gram-negative nosocomial bloodstream isolates

Abstract

We compared the in vitro activity of mezlocillin and piperacillin, alone and in combination with tobramycin or gentamicin, against clinical isolates of gram-negative bacilli from hospitalized patients with 100 distinct episodes of nosocomial bacteremia. The minimum inhibitory concentrations (MICs) necessary to inhibit 50% and 90% of isolates showed that piperacillin was most active against Pseudomonas aeruginosa. The MIC needed to inhibit 90% of isolates also showed that mezlocillin was more active against Enterobacter cloacae. Activities of the two acylaminopenicillins were comparable against the rest of the isolates. Combining the acylaminopenicillins with either gentamicin or tobramycin decreased the MICs fourfold or more for both combinations. Synergy occurred more frequently with mezlocillin-gentamicin (12%), followed by piperacillin-tobramycin (9%), mezlocillin-tobramycin (6%), and piperacillin-gentamicin (5%). Antagonism for Enterobacteriaceae isolates was observed most frequently with the combination of piperacillin plus tobramycin (20%), followed by mezlocillin plus tobramycin (17.6%), piperacillin plus gentamicin (12.9%), and mezlocillin plus gentamicin (8.2%). There are very few differences in the activities of mezlocillin and piperacillin combined with either gentamicin or tobramycin versus nosocomial gram-negative bloodstream isolates.