Determination of the range of antibacterial activity by use of viable counts

The activity of three aminoglycosides and six beta-lactam antibiotics on strains of Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Staphylococcus aureus, and enterococci was studied. The minimal inhibitory concentrations (MICs), the minimal bactericidal concentrations (MBCs), and the minimal antibiotic concentrations (MACs) were determined after 5 h of incubation in broth cultures by colony-forming-unit counts. The MICs were also determined by agar dilution after 24 h of incubation. The MICs on agar after 24 h of incubation were higher than those in broth after 5 h of incubation. The differences ranged from 1.1- to 14.2-fold, but in most cases were only three- to fivefold (P less than 0.05 to less than 0.001). The MBCs at 5 and 24 h were comparable in 71% of tests. For current practice, the MBC of enterococci can be determined after 5 h of incubation with antibiotics. The aminoglycosides showed MBCs which were closer to the MICs than were those of the beta-lactam antibiotics, which required a higher multiple of the MIC to show a bactericidal effect. The MBCs of oxacillin and cefamandole for S. aureus after 5 h of incubation were greater than 128 times the respective MICs. The MACs ranged from 1/1.5 to 1/7 of the 5-h MICs. The three endpoints, MIC, MBC, and MAC, indicate the antibacterial range of an antibiotic in terms of inhibition of growth and bacterial survival. The data suggest that the antibacterial range of an antibiotic is similar for most strains of a given species and is, to some extent, a characteristic of similar antibiotics.