Background: We observed a stepwise evolution of resistance to aminoglycosides in Pseudomonas aeruginosa by tracking the development of multidrug resistance in a series of clinical isolates from a patient with a serious burn injury.
Methods and Results: Fifteen isolates sequentially recovered from burn wounds fell into 4 groups (group 1 [G1], G2, G3, and G4) in the order of appearance in the evolution of resistance as assessed by antibiotic susceptibility profiles. In contrast to G1, which was sensitive to aminoglycosides, there was a stepwise evolution in resistance to aminoglycosides: gentamicin alone (G2), gentamicin and tobramycin (G3), and finally, amikacin (a panaminoglycoside/multidrug resistance; G4). All but the G1 strains were positive for metallo-β-lactamase and resistant to carbapenems and fluoroquinolones. When the isolates were screened for the expression of Mex efflux pumps by reverse transcriptase polymerase chain reaction, all 4 groups showed expression of the mexA but not mexC genes. The expression of mexE messenger RNA was observed in G2, G3, and G4, and that of mexX was observed in G4. The first isolates recovered for each of the 4 groups were further characterized as their representatives (G1-1, G2-1, G3-1, and G4-1) for the expression levels of Mex efflux pumps by a Western blotting analysis. In concomitance with the evolution of drug resistance, MexE protein was found to be differentially expressed. It was expressed in G2-1, up in G3-1 and back down in G4-1. The MexX protein was expressed only in G4-1 and may play a role in establishing amikacin resistance.
Conclusions: These results suggest that a series of clinical isolates of a metallo-β-lactamase-producing P. aeruginosa has acquired panaminoglycoside-resistant profiles in a stepwise manner via differential coexpression of the Mex efflux pumps.