Pharmacodynamic parameters of gentamicin and their effect on biological properties of gram-negative bacteria.

The in vitro postantibiotic effect (PAE) and the postantibiotic effect of subinhibitory concentrations (PA SME) of gentamicin were investigated on clinical isolates of Salmonella typhimurium, Salmonella enteritidis and Pseudomonas aeruginosa. The PAE was induced by 2 x MIC and 4 x MIC of gentamicin for 0.5 h. The PA SME were studied by the addition of 0.1, 0.2 and 0.3 x MIC during the postantibiotic phase of the bacteria. The S. enteritidis strain did no regrow after affecting of supra-subinhibitory concentrations for 24 h with exception of the concentration 2 x MIC + 0.1 x MIC. The PAEs against P. aeruginosa were nearly identical for all the suprainhibitory concentrations tested (4.4-4.6 h) and no regrowth after PA SME was observed. The studied pharmacodynamic parameters decreased the surface hydrophobicity of Salmonella sp., mainly of S. enteritidis, evaluated by the ability to bind Congo red and by the aggregation in solutions of ammonium sulphate (SAT). Gentamicin in suprainhibitory concentrations expressively decreased the phospholipase C production--an important virulence factor of P. aeruginosa.