Postantibiotic effects of norfloxacin and netilmicin and their influence on the biological properties of Salmonella strains.

The postantibiotic effect (PAE) and the postantibiotic sub-MIC effect (PA SME) of norfloxacin and netilmicin on two clinical strains--Salmonella typhimurium and Salmonella enteritidis was investigated. After both PAE and PA SME of antibiotics were studied, we determined their effect on the induction of a prophage in the lysogenic S. typhimurium and on Congo red binding by both serovars, as an indicator of invasive ability in vitro. The PAE was induced by 2.MIC and 4.MIC of norfloxacin and netilmicin for 0.5 h. Norfloxacin induced a longer lasting PAE on both Salmonella serovars as compared to netilmicin. Supra-subinhibitory concentrations (PA SMEs) delayed regrowth of tested strains. The PA SMEs of norfloxacin as well as of netilmicin (except 2.MIC + 0.1.MIC concentration) did not allow regrowth of S. enteritidis. The prophage-inductive ability of norfloxacin was more expressive after PA SMEs than PAE. The PA SMEs of netilmicin caused loss of Congo red binding by S. typhimurium cells and decreased this binding by S. enteritidis cells.