Post-antibiotic suppression of bacterial growth

Persistent suppression of bacterial growth following exposure of both gram-positive and gram-negative bacteria to numerous antimicrobial agents was studied. The persis-tent, or postantibiotic, effect was quantitated by periodic counts of colony-forming units after removal of the drug by washing, dilution, or inactivation with penicillinase. Although a postantibiotic effect was observed with all drugs studied, there were marked differences among drugs in their postantibiotic effects on certain organisms. With gram-positive organisms, concentrations of fJ-Iactam antibiotics near the minimal inhibi-tory concentration produced persistent effects lasting 1-3 hr. With gram-negative or-ganisms much higher concentrations were required to elicit a postantibiotic effect. In-hibitors of protein and RNA synthesis produced the longest persistent suppression of growth, which was of comparable duration in gram-positive and gram-negative bacte-ria. Only a short persistent effect of gentamicin was observed with Staphylococcus aureus and Escherichia coli, but a postantibiotic effect lasting 1.6-2.6 hr was observed with Pseudomonas aeruginosa. The duration of the postantibiotic effect was related linearly to concentration of drug and duration of exposure up to a point of maxima