Identification and Characterization of a Novel Major Facilitator Superfamily Efflux Pump, SA09310, Mediating Tetracycline Resistance in Staphylococcus aureus

Drug efflux systems have recently been recognized as an important mechanism of multidrug resistance in bacteria. Here, we described the identification and characterization of a novel chromosomally encoded multidrug efflux pump (SA09310) in Staphylococcus aureus. SA09310 is a 43-kDa protein with 12 transmembrane helices. The conserved amino acid sequence motifs of the major facilitator superfamily (MFS) were identified in the protein SA09310, which indicated that SA09310 belonged to the MFS transporters. Expression of the sa09310 gene was induced by different types of antibiotics, including aminoglycoside, tetracycline, macrolides, and chloramphenicol. An sa09310 gene knockout mutant (Δsa09310) was constructed, and its susceptibility to 30 different antibiotics was evaluated. The Δsa09310 mutant exhibited increased sensitivity to tetracycline and doxycycline, with 64-fold- and 8-fold-decreased MICs, respectively. The mechanism of SA09310 mediation of tetracycline resistance was demonstrated by its ability to extrude intracellular tetracycline from within the cells into the environment. The efflux activity of SA09310 was further confirmed by ethidium bromide (EtBr) accumulation and efflux assays. In addition, the efflux activity of SA09310 was observed to be blocked by the known efflux pump inhibitor carbonyl cyanide chlorophenylhydrazone (CCCP), which provided direct evidence that suggested the H(+)-dependent activity of the SA09310 efflux pump. The conservation of SA09310 homologs in Staphylococcus indicated the universal function of these SA09310-like protein clusters. In conclusion, the function-unknown protein SA09310 has been identified and characterized as a tetracycline efflux pump mediating tetracycline resistance in S. aureus.