NorB, an Efflux Pump in Staphylococcus aureus Strain MW2, Contributes to Bacterial Fitness in Abscesses

While remaining a major problem in hospitals,Staphylococcus aureus is now spreading in communities. Strain MW2 (USA400 lineage) and other community methicillin-resistantS. aureus strains most commonly cause skin infections with abscess formation. Multidrug resistance (MDR) efflux pumps contribute to antimicrobial resistance but may also contribute to bacterial survival by removal of environmental toxins. InS. aureus , NorA, NorB, NorC, and Tet38 are chromosomally encoded efflux pumps whose overexpression can confer MDR to quinolones and other compounds (Nor pumps) or tetracyclines alone (Tet38), but the natural substrates of these pumps are not known. To determine the role of these efflux pumps in a natural environment in the absence of antibiotics, we used strain MW2 in a mouse subcutaneous abscess model and compared pump gene expression as determined by reverse transcription-PCR in the abscesses and in vitro.norB andtet38 were selectively upregulated in vivo more than 171- and 24-fold, respectively, whereasnorA andnorC were downregulated. These changes were associated with an increase in expression ofmgrA , which encodes a transcriptional regulator known to affect pump gene expression. In competition experiments using equal inocula of anorB ortet38 mutant and parent strain MW2, each mutant exhibited growth defects of about two- to threefold in vivo. In complementation experiments, a single-copy insertion ofnorB (but not a single-copy insertion oftet38 ) in theattB site withingeh restored the growth fitness of thenorB mutant in vivo. Our findings indicate that some MDR pumps, like NorB, can facilitate bacterial survival when they are overexpressed in a staphylococcal abscess and may contribute to the relative resistance of abscesses to antimicrobial therapy, thus linking bacterial fitness and resistance in vivo.