Statistical comparison of the antibacterial activities of broad-spectrum penicillins against gram-negative bacilli

Minimal inhibitory concentrations (MICs) or two alpha-carboxypenicillins (carbenicillin and ticarcillin) and three acylaminopenicillins (azlocillin, mezlocillin, and piperacillin) for 300 aerobic and facultative gram-negative bacilli were determined by a microdilution method and compared by parametric statistical tests. Within each group of penicillins, MICs were highly interrelated; MICs of one antibiotic were readily predictable based on knowledge of MICs of another antibiotic. Ticarcillin was consistently more active than carbenicillin by approximately one dilution step, but the relative activities of the acylaminopenicillins varied by bacterial species. The acylaminopenicillins were generally more active than the alpha-carboxypenicillins, particularly against alpha-carboxypenicillin-resistant organisms. There were exceptions, however, and antibiotic MICs in one group were not readily predictable on the basis of the knowledge of antibiotic MICs in the other group. The enhanced antibacterial potencies and spectra of the acylaminopenicillins against gram-negative bacilli make these antibiotics potentially useful therapeutic agents. It is not necessary for clinical laboratories to routinely perform susceptibility tests with all five antibiotics.