Genome variation and molecular epidemiology of Salmonella Typhimurium pathovariants

Salmonella enterica serovar Typhimurium (S Typhimurium) is one of approximately 2500 distinct serovars of the genus Salmonella, but is exceptional in its wide distribution in the environment, livestock, and wild animals. S Typhimurium causes a large proportion of non-typhoid Salmonella (NTS) infections, accounting for a quarter of infections, second only to SEnteritidis in incidence. S Typhimurium was once considered the archetypal broad host range Salmonella serovar due to its wide distribution in livestock and wild animals and much of what we know of the interaction of Salmonella with the host comes from research using a small number of laboratory strains of the serovar (LT2, SL1344, ATCC14028). But it has become clear that these strains do not reflect the genotypic or phenotypic diversity of S Typhimurium. Here we review the epidemiological record of S Typhimurium and studies of the host-pathogen interactions of diverse strains of S Typhimurium. We present the concept of distinct pathovariants of S Typhimurium that exhibit diversity of host range, distribution in the environment, pathogenicity and risk to food safety. We review recent evidence from whole genome sequencing that has revealed the extent of genomic diversity of S Typhimurium pathovariants, the genomic basis of differences in the level of risk to human and animal health, and the molecular epidemiology of prominent strains. An improved understanding of the impact of genome variation of bacterial pathogens on pathogen-host and pathogen-environment interactions has the potential to improve quantitative risk assessment and reveal how new pathogens evolve.