Identification and characterization of a novel major facilitator superfamily (MFS) efflux pump conferring multidrug resistance in <i>Staphylococcus aureus</i> and <i>Staphylococcus epidermidis</i>

A novel major facilitator superfamily (MFS) efflux pump in <i>Staphylococcus</i>, designated Nms, was identified via topology prediction. The secondary structure indicated the presence of 12 transmembrane segments (TMSs) and characteristic motif A of MFS efflux pumps. Experimental verification of efflux activity was conducted using ethidium bromide accumulation and efflux assays and biofilm formation assays. Antimicrobial susceptibility testing and efflux pump inhibition confirmed that Nms effectively effluxed various antimicrobial agents to confer multidrug resistance. Comprehensive genomic analyses were used to assess the prevalence and possible origins of the <i>nms</i> gene. The results revealed that the <i>nms</i> gene was present in <i>Staphylococcus aureus</i> ST398/ST541 and <i>Staphylococcus epidermidis</i> ST570/ST1166 strains from global isolates. The transmission of <i>nms</i> was associated with the prevalence of <i>S. aureus</i> ST398-t571 in swine-derived samples from China. Phylogenetic analysis revealed that <i>nms-</i>positive strains formed a distinct clade separate from other <i>S. aureus</i> ST398 strains. Genetic analysis of the <i>nms</i> gene revealed a significant presence of plasmid-related mobile genetic elements, with extended nucleotide sequences containing circular intermediates exhibiting high homology with those found in an <i>S. aureus</i> plasmid. These findings suggested that the <i>nms</i> gene likely initially originated from plasmids and subsequently integrated into chromosomes. In conclusion, Nms is a novel MFS efflux pump that confers multidrug resistance to <i>S. aureus</i> and has been carried predominantly by ST398-t571 isolates in recent years. Ongoing surveillance is essential to elucidate the origin of <i>nms</i> in <i>S. aureus</i>, particularly MRSA ST398-t571, and to understand the transmission among humans, animals, and the environment.