EFFLUX-MEDIATED FLUOROQUINOLONE RESISTANCE IN STAPHYLOCOCCUS-AUREUS

Transport processes are used by all organisms to obtain essential nutr ients and to expel wastes and other potentially harmful substances fro m cells. Such processes are important means by which resistance to sel ected antimicrobial agents in bacteria is achieved. The recently descr ibed Staphylococcus aureus norA gene encodes a membrane-associated pro tein that mediates active efflux of fluoroquinolones from cells. SA-11 99B is a fluoroquinolone-resistant strain of S. aureus from which we c loned an allele of norA (norA1199). Similar to that of norA, the prote in product of norA1199 preferentially mediates efflux of hydrophilic f luoroquinolones in both S. aureus and an Escherichia coli host, a proc ess driven by the proton motive force. Determination of the nucleotide sequence of norA1199 revealed an encoded 388-amino-acid hydrophobic p olypeptide 95% homologous with the norA-encoded protein. Significant h omology with other proteins involved in transport processes also exist s, but especially with tetracycline efflux proteins and with the Bacil lus subtilis Bmr protein that mediates active efflux of structurally u nrelated compounds, including fluoroquinolones. In S. aureus, the norA 1199-encoded protein also appears to function as a multidrug efflux tr ansporter. Southern hybridization studies indicated that norA1199 (or an allele of it) is a naturally occurring S. aureus gene and that rela ted sequences are present in the S. epidermidis genome. The nucleotide sequence of the wild-type allele of norA1199, cloned from the fluoroq uinolone-susceptible parent strain of SA-1199B, did not differ from th at of norA1199 throughout the coding region. Northern (RNA) and Southe rn hybridization studies showed that increased transcription, and not gene amplification, of norA1199 is the basis for fluoroquinolone resis tance in SA-1199B.