Polymyxin Susceptibility in Pseudomonas aeruginosa Linked to the MexXY-OprM Multidrug Efflux System

The ribosome-targeting antimicrobial, spectinomycin (SPC), strongly induced themexXY genes of the MexXY-OprM multidrug efflux system inPseudomonas aeruginosa and increased susceptibility to the polycationic antimicrobials polymyxin B and polymyxin E, concomitant with a decrease in expression of the polymyxin resistance-promoting lipopolysaccharide (LPS) modification loci,arnBCADTEF and PA4773-74. Consistent with the SPC-promoted reduction inarn and PA4773-74 expression being linked tomexXY , expression of these LPS modification loci was moderated in a mutant constitutively expressingmexXY and enhanced in a mutant lacking the efflux genes. Still, the SPC-mediated increase in polymyxin susceptibility was retained in mutants lackingarnB and/or PA4773-74, an indication that their reduced expression in SPC-treated cells does not explain the enhanced polymyxin susceptibility. That the polymyxin susceptibility of a mutant strain lackingmexXY was unaffected by SPC exposure, however, was an indication that the unknown polymyxin resistance ‘mechanism’ is also influenced by the MexXY status of the cell. In agreement with SPC and MexXY influencing polymyxin susceptibility as a result of changes in the LPS target of these agents, SPC treatment yielded a decline in common polysaccharide antigen (CPA) synthesis in wild-typeP. aeruginosa but not in the ΔmexXY mutant. A mutant lacking CPA still showed the SPC-mediated decline in polymyxin MICs, however, indicating that the loss of CPA did not explain the SPC-mediated MexXY-dependent increase in polymyxin susceptibility. It is possible, therefore, that some additional change in LPS promoted by SPC-inducedmexXY expression impacted CPA synthesis or its incorporation into LPS and that this was responsible for the observed changes in polymyxin susceptibility.