In investigating the epidemiological trends of Salmonella enterica serovar Goldcoast, we previously identified several closely related strains with different MICs to azithromycin and quinolones. Genome sequencing and comparison of two very similar multidrug-resistant (MDR) strains, R18.0877 and R18.1656, has led to the identification of an extra plasmid-borne ramA gene, ramAp, on the large IncHI2 plasmid carried by R18.0877. The ramAp gene is located in a 953-bp region on the plasmid, which is identical to that of the Klebsiella quasipneumoniae chromosomal ramA loci. A truncated ISEcp1 located at the adjacent upstream area of the putative regulatory region of ramAp may likely contribute to its mobilization and expression. Introducing the ramAp gene and the truncated ISEcp1 into Escherichia coli has resulted in elevated expression of efflux pump genes and elevated MICs to chloramphenicol, azithromycin, nalidixic acid, ciprofloxacin, sulfamethoxazole, trimethoprim, tetracycline, and tigecycline. The ramAp is an extra efflux pump activator gene that potentially could be transmitted with the IncHI2 plasmid among bacteria. It is plausible that, with high interspecific conservation, the plasmid-encoded regulator reduces drug susceptibility by activating existing efflux pump systems of the host and thus can be regarded as a new type of auxiliary antimicrobial resistance determinant. Sequences of similar plasmids were found worldwide. Its impact on the emergence of antimicrobial resistance among bacterial pathogens is worrisome.
Keywords: ISEcp1; Salmonella enterica serovar Goldcoast; antimicrobial drug resistance; efflux pump regulator; plasmid-borne; ramA.