Acquisition of antimicrobial resistance determinants by virulence plasmids specific for nontyphoid serovars of Salmonella enterica

Nontyphoid serovars of Salmonella enterica are one of the leading causes of bacterial food-borne disease. Most human infections are confined to the small intestine and associated with inflammatory diarrhoea. However, bacteria can also spread beyond the intestine causing focal or systemic infections, particularly in infants, the elderly and immuno-compromised hosts. Although usually not required, antimicrobial therapy is mandatory for the control of invasive infections and for people with recognized risk factors. A limited number of nontyphoid serovars carry virulence plasmids, which are actively evolving through loss and/or acquisition of genetic information. Of particular interest is the capture of genes conferring resistance to antimicrobials, which can result from cointegration with resistance plasmids but is more commonly mediated by mobile genetic elements, such as insertion sequences, transposons and integrons. Resistance plasmids carrying virulence genes of serovar specific plasmids, including the spv region, were also detected. This review will focus on hybrid virulence–resistance plasmids found in Typhimurium and its monophasic 4,5,12:i:- variant, Enteritidis and Choleraesuis, which rank among the most common and/or invasive nontyphoid serovars. The epidemiology of these plasmids, the mechanisms underlying their emergence and evolution, as well as the consequences for both the pathogen and its host, will be considered.