Identification of contemporary plasmid virulence genes in ancestral isolates of Salmonella enteritidis and Salmonella typhimurium

Six epidemiologically distinct ancestral strains of Salmonella enteritidis and 5 of S. typhimurium from the pre-antibiotic era were examined for plasmid content, and for presence of plasmid genes implicated in mouse-virulence. Five sizes of plasmid were detected in S. enteritidis varying from 1 to 60 MDa. Two sizes of plasmid were found in S. typhimurium, 28 and 60 MDa. Plasmids of the same size were not common to both serovars. The HindIII restriction patterns of 3 of the ancestral S. enteritidis plasmids were identical to the modern 38 MDa plasmid, while all contained identical bands of 3.5, 2.7 and 1.9 kb. All the 60-MDa S. typhimurium plasmids, ancestral and contemporary, had an identical restriction pattern. Three different sized S. enteritidis plasmids and one size S. typhimurium plasmid contained a 3.5-kb DNA fragment carrying the virulence locus VirA. The VirB virulence locus was located on a 2.7-kb DNA fragment in S. enteritidis and on a 2.5-kb fragment in S. typhimurium. Both loci were precisely conserved between the ancestral strains and the modern representatives of both serovars.