Decreased virulence of gentamicin-resistant strains of Pseudomonas aeruginosa in a rat model.

Previous studies have reported that GR strains of Pseudomonas aeruginosa were less likely to cause serious infections. In the present study, this clinical observation of decreased virulence of GR strains of P. aeruginosa was documented in Andriole's rat model. A significantly higher mortality in rats was noted with GS strains of P. aeruginosa (MIC less than 10 microgram/ml) compared to moderately resistant strains (MIC 12.5 to 312 microgram/ml) and highly resistant strains (MIC greater than 312 microgram/ml). Slower growth rates of GR strains of P. aeruginosa were observed in both the lag and the log phases as compared to GS strains. Average counts of in vivo quantitative blood cultures with the use of GR strains were lower at 30 min, 8 hr, and 24 hr compared to those with GS strains. The mortality in mice injected intraperitoneally with CFS's from GS strains (MIC 0.78 to 3.125 microgram/ml) was significantly higher at 8 and 24 hr than those injected with supernatants from GR strains (MIC 25 to 400 microgram/ml). Human neutrophils killed GR and GS strains of P. aeruginosa equally. Decreased virulence of GR strains of P. aeruginosa may be due in part to slower growth rates and to a decreased ability to produce heat-labile toxic components.