Genetic diversity of the Staphylococcus aureus efflux pump gene norA

Efflux pumps are capable of extruding multiple antimicrobial compounds from the cell, contributing to the development of resistance phenotypes. In Staphylococcus aureus one of the most well studied efflux pumps is NorA, which has been related to reduced susceptibility to fluoroquinolones and biocides. Earlier studies described two alleles of NorA encoding gene, norAI and norAII. However, the relevance of each norA allele in the clinical setting is scarcely documented. In our study, the promoter region and structural norA gene sequences were analyzed in representatives of a collection of clinical S. aureus isolates and in the reference strain ATCC25923, in order to ascertain the occurrence of each norA allele and establish a correlation between the norA alleles and susceptibility phenotypes to different antimicrobial compounds. Among the isolates carrying norAI, three types of alterations were found, namely an insertion downstream of the -10 consensus promoter sequence and the mutations Gly147Ser and Gly291Asp. The norAII allele showed a higher number of mutations when compared to the norAII allele described earlier, of which only one resulted in an aminoacid substitution, Asn200Asp. RT-qPCR assays and MIC determinations showed no direct correlation between norA alleles and the strain susceptibility profile towards fluoroquinolones and biocides. Further characterization of norA diversity should clarify the importance of the different NorA variants and their interplay with the remaining S. aureus efflux pumps.