SmeDEF multidrug efflux pump contributes to intrinsic multidrug resistance in Stenotrophomonas maltophilia

Stenotrophomonas maltophilia is an emerging nosocomial pathogen that displays high-level intrinsic resistance to a variety of structurally unrelated antimicrobial agents. Efflux mechanisms are known to contribute to acquired multidrug resistance in this organism, and indeed, one such multidrug efflux system, SmeDEF, was recently identified. Still, the importance of SmeDEF to intrinsic antibiotic resistance in S. maltophilia had not yet been determined. Reverse transcription-PCR confirmed expression of the smeDEF genes in wild-type S. maltophilia, and deletion of smeE or smeF in wild-type strains rendered the mutants hypersusceptible to several antimicrobials, suggesting that SmeDEF contributes to intrinsic antimicrobial resistance in this organism. Expression of smeDEF was also enhanced in an in vitro-selected multidrug-resistant mutant, although deletion of smeF but not of smeE in these mutants compromised antimicrobial resistance. Apparently, hyperexpressed SmeF is capable of functioning with additional multidrug efflux components to promote multidrug resistance in S. maltophilia.