MexXY-OprM efflux pump is required for antagonism of aminoglycosides by divalent cations in Pseudomonas aeruginosa

Antagonism of aminoglycosides by divalent cations is well documented for Ps eudomonas aeruginosa and is regarded as one of the problems in aminoglycosi de therapy. It is generally considered that divalent cations interfere with uptake of aminoglycosides at both the outer and inner membranes. It has be en demonstrated recently that aminoglycosides can be removed from cells of P. aeruginosa by the three-component multidrug resistance efflux pump MexXY -OprM. We sought to investigate the interplay between efflux and uptake in resistance to aminoglycosides in P. aeruginosa. To do so, we studied the ef fects of the divalent cations Mg2+ and Ca2+ on susceptibility to aminoglyco sides in a wild-type strain of P. aeruginosa and in mutants either overexpr essing or lacking the MexXY-OprM efflux pump. MICs of gentamicin, streptomy cin, amikacin, apramycin, netilmicin, and arbekacin were determined in Muel ler-Hinton broth in the presence of cations added at concentrations that va ried from 0.125 to 8 mM. We found, unexpectedly, that while both Mg2+ and C a2+ antagonized aminoglycosides (up to a 64-fold decrease in susceptibility at 8 mM), antagonism was seen only in the strains of P. aeruginosa that co ntained the functional MexXY-OprM efflux pump. Our results indicate that in hibition of the MexXY-OprM efflux pump should abolish the antagonism of ami noglycosides by divalent cations, regardless of its precise mechanism. This may significantly increase the therapeutic index of aminoglycosides and im prove the clinical utility of this important class of antibiotics.