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

A novel major facilitator superfamily (MFS) efflux pump in Staphylococcus, 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 nms gene. The results revealed that the nms gene was present in Staphylococcus aureus ST398/ST541 and Staphylococcus epidermidis ST570/ST1166 strains from global isolates. The transmission of nms was associated with the prevalence of S. aureus ST398-t571 in swine-derived samples from China. Phylogenetic analysis revealed that nms-positive strains formed a distinct clade separate from other S. aureus ST398 strains. Genetic analysis of the nms 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 S. aureus plasmid. These findings suggested that the nms 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 S. aureus and has been carried predominantly by ST398-t571 isolates in recent years. Ongoing surveillance is essential to elucidate the origin of nms in S. aureus, particularly MRSA ST398-t571, and to understand the transmission among humans, animals, and the environment.