Beta-lactamase induction and aminoglycoside susceptibility in Pseudomonas aeruginosa.

Recent clinical reports have described the emergence of cross-resistance to aminoglycosides during beta-lactam therapy. We therefore studied the effect of imipenem-mediated beta-lactamase induction on the susceptibilities of 92 clinical isolates of Pseudomonas aeruginosa to gentamicin, tobramycin, and amikacin. We performed a disc approximation test to look for truncation by imipenem of the inhibitory zone sizes of the aminoglycosides. We also compared aminoglycoside inhibitory zone sizes of strains grown in the presence and absence of subinhibitory concentrations of imipenem. Finally, we compared aminoglycoside MIC values determined with and without subinhibitory concentrations of imipenem. The results showed that simultaneous emergence of resistance to beta-lactams and aminoglycosides is very rare.