Postantibiotic leukocyte enhancement: increased susceptibility of bacteria pretreated with antibiotics to activity of leukocytes.

Exposure of Escherichia coli and Staphylococcus aureus to high levels of certain antibiotics for brief periods of time increases the susceptibility of the organisms to the antimicrobial action of normal human leukocytes. The degree of sensitization varies with the antibiotic used for the pretreatment of the bacteria and is most pronounced in pretreatment of E. coli with chloramphenicol. Studies on the rates of phagocytosis and intracellular killing of E. coli indicate that the postantibiotic leukocyte enhancement effect operates through increased susceptibility of antibiotic-damaged bacteria to the intracellular killing mechanisms of leukocytes. These observations may explain the efficacy of antimicrobial dosage regimens in which drug levels are below inhibitory concentrations for part of the dosage interval.