Postantibiotic sub-MIC effects of vancomycin, roxithromycin, sparfloxacin, and amikacin.

The sub-MIC effects (SMEs) and the postantibiotic sub-MIC effects (PA SMEs) of vancomycin, roxithromycin, and sparfloxacin for Streptococcus pyogenes and Streptococcus pneumoniae and of amikacin for Escherichia coli and Pseudomonas aeruginosa were investigated. A postantibiotic effect was induced by exposing strains to 10x the MIC of the antibiotic for 2 h in vitro. After the induction, the exposed cultures were washed to eliminate the antibiotics. Unexposed controls were treated similarly. Thereafter, the exposed cultures (PA SME) and the controls (SME) were exposed to different subinhibitory concentrations (0.1, 0.2, and 0.3x the MIC) of the same drug and growth curves for a period of 24 h were compared. In general, the PA SMEs were much more pronounced than the SMEs. However, for amikacin and E. coli the SME of 0.2 and 0.3x the MIC also had an initial bactericidal effect. The longest PA SMEs were demonstrated for the combinations with the most pronounced killing during the induction and for the combinations which exhibited the longest PAEs.