In vitro activity of mezlocillin and azlocillin compared with that of four other penicillins and two aminoglycosides

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Mezlocillin and azlocillin are both semisynthetic penicillins with activity against a broad spectrum of bacterial species, including Pseudomonas aeruginosa.1–10 In this report, we compare mezlocillin and azlocillin with ampicillin, carbenicillin, ticarcillin, and piperacillin. In addition, two aminoglycosides (gentamicin and amikacin) were also tested; they represent antimicrobics that are commonly used because of their broad spectrum of activity. The degree of cross-resistance to the six penicillins was also determined.

Materials and methods

Microdilution susceptibility tests were performed as described previously.11–14 Drug dilutions were prepared in cation-supplemented Mueller-Hinton broth and then dispensed into wells of microdilution trays. The wells were inoculated with 1 × 105 colony-forming units per milliliter and after 16 to 18 hours at 35 C, minimal inhibitory concentrations (MICs) were determined. The minimal lethal concentrations (MLCs) were determined by subculturing to blood agar plates, with the use of a disposable inoculum replicator that transfers approximately 5 μl from each well. The MLC was recorded as the lowest concentration that yielded no growth upon subculture. Bactericidal end points and inoculum density studies were performed in Kaiser Foundation Hospitals in Clackamas, Oregon. The remaining tests were performed at the Center for Disease Control, Atlanta, Georgia and at the University of California (Davis), Sacramento Medical Center, Sacramento, California. Tests with control strains confirmed that comparable results were obtained in all three institutions as previously documented.12–14

A total of 484 bacterial isolates were studied. Most of the strains were clinical isolates collected from six medical centers located in five separate . . .



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