Heteroduplex analysis of Salmonella virulence plasmids and their prevalence in isolates of defined sources.

Strains of the Salmonella serovars S. typhimurium, S. enteritidis, S. dublin, and S. choleraesuis harbour large plasmids which are required for extraintestinal colonization after oral infection of mice. Electron microscopic heteroduplex analysis showed that these virulence plasmids share large regions of homology. Nine hundred and eighty-six isolates of different origins were analysed for the presence of these plasmids by using a cloned fragment of a S. choleraesuis virulence plasmid as a gene probe. Virulence plasmids were detected in nearly 100% of strains isolated from animal organs or human blood. Frequencies of detection ranged from 48 to 87% in strains of faecal, food or environmental origin. These results suggest that Salmonella virulence plasmids are required for systemic infections in humans and livestock.