Effect of multidrug resistance-conferring mutations on the fitness and virulence of Pseudomonas aeruginosa

Background: Multidrug resistance has become a quandary in the treatment of bacterial infections. The effect of resistance mutations and the fitness cost on the pathogenicity of Pseudomonas aeruginosa is not well established. The objective of this study was to examine the impact of multidrug resistance on the fitness and virulence of P. aeruginosa. Methods: Fourteen P. aeruginosa strains with various resistance mechanisms were used. In vitro growth of these isolates was investigated in full-strength and 0.25-strength Mueller-Hinton broth (MHB). Exponential growth rates were estimated from serial bacterial burden over 24 h. In vitro growth of two multidrug-resistant strains (PAO1 Delta mexR Delta oprD and PA9019) was studied when each was grown in co-culture with wild-type strain PAO1. In vivo growth was compared between PAO1 and PAO1 Delta mexR Delta opD using a murine pneumonia model; virulence over 10 days was studied in six isolates. Results: Significant reduction in growth rate was observed in selected mutants (P<0.01). PAO1 out-competed PAO1 Delta mexR Delta oprD and PA9019 in vitro, and in vivo growth of PAO1 was faster than PAO1 Delta mexR Delta oprD. Compared with PAO1, PAO1 Delta mexR and PAO1 Delta oprD showed a slight reduction in mortality rate; significantly lower mortality was seen in PAO1 Delta mexR Delta oprD (P<0.01). However, virulence of PA9019 was not significantly different from that of PAO1. Conclusions: Specific resistance mutations were associated with fitness cost in P. aeruginosa, and accumulation of such mutations was associated with a reduction in virulence. However, it was difficult to predict the impact in clinical isolates. Knowledge of multidrug resistance mechanisms and compensatory mutations would likely be helpful.