BETA-LACTAMASE INHIBITORS ARE SUBSTRATES FOR THE MULTIDRUG EFFLUX PUMPS OF PSEUDOMONAS-AERUGINOSA

The MexAB-OprM multidrug efflux system exports a number of antimicrobi al compounds, including beta-lactams, In an attempt to define more ful ly the range of antimicrobial compounds exported by this system, and, in particular, to determine whether beta-lactamase inhibitors were als o accommodated by the MexAB-OprM pump, the influence of pump status (i ts presence or absence) on the intrinsic antibacterial activities of t hese compounds and on their abilities to enhance beta-lactam susceptib ility in intact cells was assessed. MIC determinations clearly demonst rated that all three compounds tested, clavulanate, cloxacillin, and B RL42715, were accommodated by the pump. Moreover, by using beta-lactam s which were readily hydrolyzed by the Pseudomonas aeruginosa class C chromosomal beta-lactamase, it was demonstrated that elimination of th e mexAB-oprM-encoded efflux system greatly enhanced the abilities of c loxacillin and BRL42715 (but not clavulanate) to increase beta-lactam susceptibility, With beta-lactams which were poorly hydrolyzed, howeve r, the inhibitors failed to enhance beta-lactam susceptibility in MexA B-OprM(+) strains, although BRL42715 did enhance beta-lactam susceptib ility in MexAB-OprM(-) strains, suggesting that even with poorly hydro lyzed beta-lactams this inhibitor was effective when it was not subjec ted to efflux. MexEF-OprN-overexpressing strains, but not MexCD-OprJ-o verexpressing strains, also facilitated resistance to beta-lactamase i nhibitors, indicating that these compounds are also substrates for the MexEF-OprN pump. These data indicate that an ability to inactivate Me xAB-OprM (and like efflux systems in other bacteria) will markedly enh ance the efficacies of beta lactam-beta-lactamase inhibitor combinatio ns in treating bacterial infections.