Recovery Period and the Exposure of Bacteria to Subminimal Inhibitory Concentrations of Antibiotics*

Abstract

The minimal antibiotic concentration (MAC) refers to the lowest concentration of drug that results in a detectable effect on bacteria (e.g., inhibition of growth, change in morphology, and delay in recovery to normal growth in drug-free medium). Strains of Escherichia coli and of Staphylococcus aureus were subjected to a range of subminimal inhibitory concentrations of four drugs—ampicillin, gentamicin, rosaramicin, and tetracycline. Inhibition curves (percentage of normal growth vs. concentration of drug) were related to the period of recovery before resumption of normal growth, which was chosen to express an MAC value. In both E. coli and S. aureus, the longest delay in recovery of normal growth was observed with rosaramicin. Ampicillin resulted in a delay in recovery only with S. aureus.