Multilocus sequence typing supports the hypothesis that cow- and human-associated Salmonella isolates represent distinct and overlapping populations

A collection of 179 human and 156 bovine clinicalSalmonellaisolates obtained from across New York state over the course of 1 year was characterized using serotyping and a multilocus sequence typing (MLST) scheme based on the sequencing of three genes (fimA,manB, andmdh). The 335 isolates were differentiated into 52 serotypes and 72 sequence types (STs). Analyses of bovine isolates collected on different farms over time indicated that specific subtypes can persist over time on a given farm; in particular, a number of farms showed evidence for the persistence of a specificSalmonella entericaserotype Newport sequence type. Serotypes and STs were not randomly distributed among human and bovine isolates, and selected serotypes and STs were associated exclusively with either human or bovine sources. A number of common STs were geographically widespread. For example, ST6, which includes isolates representing serotype Typhimurium as well as the emerging serotype 4,5,12:i:-, was found among human and bovine isolates in a number of counties in New York state. Phylogenetic analyses supported the possibility that serotype 4,5,12:i:- is closely related toSalmonellaserotype Typhimurium.Salmonellaserotype Newport was found to represent two distinct evolutionary lineages that differ in their frequencies among human and bovine isolates. A number ofSalmonellaisolates carried two copies ofmanB(33 isolates) or showed small deletion events infimA(nine isolates); these duplication and deletion events may provide mechanisms for the rapid diversification ofSalmonellasurface molecules. We conclude that the combined use of an economical three-gene MLST scheme and serotyping can provide considerable new insights into the evolution and transmission ofSalmonella.