Allele-based analysis revealed the critical functions of region 277-297 in the NorA efflux pump of Staphylococcus aureus.

OBJECTIVES

The NorA efflux pump in Staphylococcus aureus mediates resistance to many fluoroquinolone (FQ) antibiotics. Three norA alleles with high sequence similarity are found in various S. aureus strains exhibiting different FQ resistance profiles. This study aimed to elucidate the underlying molecular basis for the varying efflux activity of these three allelic variations.

METHODS

The norA genotypes of 20 S. aureus isolates were analysed. Multiple alignments and conservative analyses were conducted to explore the evolutionary variations. After heterologous expression in Escherichia coli, seven mutants were constructed for MIC tests, efflux activity and conformational change measurements.

RESULTS

Three NorA alleles were identified that displayed different FQ MICs and varying efflux activity for ethidium bromide, with the NorAII protein showing the strongest activity. A total of 29 single amino acid polymorphisms were identified by conservative analysis within three allelic peptides, with seven sites densely distributed in the 277-297 region. Mutations of these seven residues in NorAII all significantly impaired drug resistance and efflux activity, and three key mutants showed conformational changes in fluorescence resonance energy transfer (FRET) analysis.

CONCLUSIONS

Evolutionary variations of the 277-297 region could be a major explanation for the functional difference of three norA alleles and serve as a potential target for the development of novel NorA inhibitors.