Molecular characterization of Salmonella Typhimurium highly successful outbreak strains.

Three large clusters of Salmonella Typhimurium infections in Denmark in 2008 and 2009 were defined by multilocus variable number of tandem repeat analysis (MLVA). One of these proved to be the hereto largest Danish cluster of salmonellosis with 1446 cases. Two smaller clusters with a total of 197 and 89 cases, respectively, were seen concurrently. These clusters shared epidemiological characteristics such as age distribution, geography, and time. To investigate the possible genetic relationship between the cluster strains, these were further characterized by phage typing, pulsed-field gel electrophoresis, and Optical Mapping. Although the MLVA method proved robust and well-performing in detecting and defining clusters, the employment of a second typing method detected an additional fourth cluster among the isolates. The cluster strains were stable throughout the almost 2-year period, even though we detected changes in three of five MLVA loci in a small fraction of isolates. These changes were mainly due to the gain or loss of single repeats. Optical Mapping of the large cluster strain indicated no increased content of virulence genes; however, Optical Mapping did reveal a large insert, a probable prophage, in the main cluster. This probable prophage may give the cluster strain a competitive advantage. The molecular methods employed suggested that the four clusters represented four distinct strains, although they seemed to be epidemiologically linked and shared genotypic characteristics.