Minimal antibiotic concentrations of aminoglycosides and beta-lactam antibiotics for some gram-negative bacilli and gram-positive cocci.

The minimal antibiotic concentration (MAC) is the lowest concentration of an antibacterial agent that produces a decrease of 1 log in the number of organisms/ml as compared with a control culture in drug-free medium. Various gram-negative bacilli and gram-positive cocci were grown in the presence of amikacin, gentamicin, tobramycin, ampicillin, amoxicillin, oxacillin, carbenicillin, ticarcillin, and cefamandole at concentrations varying from eight times the minimal inhibitory concentration (MIC) to 1/128 of the MIC. Colony forming units (cfu) were counted, the MIC was determined, and the MIC:MAC ratio, which indicates the magnitude of the effective range, was calculated. The MIC:MAC ratio appears to be characteristic for a given species and antibiotic. There is no relation between the MICs and the MIC:MAC ratios. The highest ratios were given by Proteus mirabilis with aminoglycosides (MIC:MAC mean, 29.2 with tobramycin), and the lowest ratios were given with beta-lactam antibiotics by Pseudomonas aeruginosa and Streptococcus faecalis (MIC:MAC means, 2.1 with carbenicillin and cefamandole, respectively).