Postantibiotic Suppression of Bacterial Growth

TLDR

Persistent suppression of bacterial growth after exposure to a variety of antimicrobial agents was investigated. The postantibiotic effect was quantified by repeatedly counting colony‑forming units after removing the drug by washing, dilution, or penicillinase inactivation. All tested antimicrobials produced a postantibiotic effect, but its magnitude varied: β‑lactams near MIC yielded 1–3 h suppression in gram‑positive bacteria, higher concentrations were needed for gram‑negatives, protein/RNA synthesis inhibitors gave the longest suppression in both groups, gentamicin had only a brief effect on *S.

Abstract

Persistent suppression of bacterial growth following exposure of both gram-positive and gram-negative bacteria to numerous antimicrobial agents was studied. The persistent, 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 β-Iactam antibiotics near the minimal inhibitory concentration produced persistent effects lasting 1–3 hr. With gram-negative organisms much higher concentrations were required to elicit a postantibiotic effect. Inhibitors of protein and RNA synthesis produced the longest persistent suppression of growth, which was of comparable duration in gram-positive and gram-negative bacteria. 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 maximal response. Persistent effects following exposure to antibiotics were also demonstrated in 90% human serum.