MexXY Efflux Pump is Required for Antagonism of Aminoglycosides by Divalent Cations in Pseudomonas aeruginosa
W. MAO, M. WARREN, A. LEE, A. MISTRY, O. LOMOVSKAYA
Microcide Pharmaceuticals, Inc., Mountain View, CA
Presentation Number: 1498
Antagonism of aminoglycosides (AG) by divalent cations is well documented for Pseudomonas aeruginosa and is regarded as one of the problems in AG therapy. It is generally considered that divalent cations interfere with uptake of AG at both the outer and inner membranes. It has been recently demonstrated that AGs can be effluxed out of cells of P. aeruginosa by the three-component multi-drug resistance efflux pump MexXY-OprM. We sought to investigate the interplay between efflux and uptake on resistance to AGs in P. aeruginosa. To do so, we studied the effects of the divalent cations Mg2+ and Ca2+ on susceptibility to AGs in a wild type strain of P. aeruginosa and in mutants overexpressing or lacking the MexXY-OprM efflux pump. MICs for gentamicin, streptomycin, amikacin, apramycin, nethilmicin and habekacin were determined in MHB in the presence of cations added at concentrations that varied from 0.125 mM to 8 mM. We found, unexpectedly, that both Mg2+ and Ca2+ antagonized AGs (up to 64-fold decrease in susceptibility at 8 mM), but only in the strains of P. aeruginosa that contained the functional MexXY-OprM efflux pump; almost no antagonism was seen at similar dication concentrations in mexX::Hg or oprM::Hg mutants. Our results indicate that decreased uptake contributes to the increased resistance to AGs only in the presence of active efflux. This implies that inhibition of the MexXY-OprM efflux pump should abolish the antagonism of aminoglycosides by divalent cations, regardless of its precise mechanism. This may significantly increase the therapeutic index of AGs and improve the clinical utility of this important class of antibiotics.
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