Range of antibacterial activity of antibiotics at subminimal inhibitory concentrations: the ratio of minimal inhibitory concentration to minimal antibiotic concentration.

The range of activity of subminimal inhibitory concentrations of some beta-lactam and aminoglycoside antibiotics against both gram-negative and gram-positive bacteria was determined and expressed as the ratio of the minimal inhibitory concentration (MIC) to the minimal antibiotic concentration (MAC). Ten strains of Staphylococcus aureus were tested with ampicillin, cefamandole, and cefoxitin. The mean MIC/MAC ratios were 9.4, 14.0, and 2.9, respectively. The mean MIC/MAC ratios for 10 strains of Escherichia coli that were tested with the same three antibiotics were 20.6, 25.2, and 15.1, respectively. The same strains of E. coli and S. aureus were tested with amikacin, gentamicin, and tobramycin. The respective mean MIC/MAC ratios were 13.1, 6.2, and 7.4 for E. coli and 3.7, 5.1, and 3.1 for S. aureus. Ten strains of Serratia marcescens were tested with amikacin, chloramphenicol, and these two drugs combined. The mean MIC/MAC ratios for amikacin and chloramphenicol were 6.8 and 2.1, respectively. For the combination, the mean MIC/MAC ratio was 9.2, a finding that indicates synergism. Determinations of MIC/MAC ratios could provide valuable information for the monitoring of antibacterial therapy, especially when drugs of significant toxicity are used.