INDUCIBLE NORA-MEDIATED MULTIDRUG-RESISTANCE IN STAPHYLOCOCCUS-AUREUS

The NorA protein of Staphylococcus aureus mediates the active efflux of hydrophilic fluoroquinolones from the cell, conferring low-level resistance upon the organism, This protein also is capable of transporting additional structurally diverse compounds, indicating that it has a broad substrate specificity, Increased transcription of the norA gene, leading to a greater quantity of the NorA protein within the cytoplasmic membrane, is felt to be the mechanism by which strains possessing such changes resist fluoroquinolones. S. aureus SA-1199 and its in vivo-selected derivative SA-1199B are fluoroquinolone-susceptible and -resistant isolates, respectively; SA-1199B resists hydrophilic fluoroquinolones via a NorA-mediated mechanism in a constitutive manner, SA-1199-3 is an in vitro-produced derivative of SA-1199 in which NorA-mediated multidrug resistance is expressed inducibly. Compared with organisms exposed to subinhibitory concentrations of a NorA substrate for the first time, preexposure of SA-1199-3 to such a compound followed by growth in the presence of that substrate results in the elimination of a 2- to 6-h period of organism killing that occurs prior to the onset of logarithmic growth, The uptake of radiolabeled fluoroquinolone is markedly reduced by preexposure of SA-1199-3 to NorA substrates; such prior exposure also results in a dramatic increase in RNA transcripts that hybridize with a norA probe, Preexposure of SA-1199 and SA-1199B to such substrates results in small increases or no increases in these transcripts, No sequence differences between SA-1199 and SA-1199-3 within the norA gene or flanking DNA were found, It appears likely that the regulation of nor A in SA-1199-3, which mag be effected by one or more genetic loci outside the norA region of the chromosome, differs from that of SA-1199 and SA-1199B.