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Molecular fingerprints of chromosomal genotypes in Salmonella typhimurium were generated by analysis of variation at the 16s vrn gene loci and the sites of the insertion sequence IS200. Genetic and reference strains of S. typhimuvium were compared with clinical phage type strains from cases of human salmonellosis. Three 1 6 s rrn profiles, one of which was predominant, were found. The copy number of the SaZmonelZa-specific insertion sequence IS200 varied from 6 to 12, and all insertions were chromosomal. Three of the insertion sites shared by all strains were serovar-specific for S. typhimurium. Thirteen distinct profiles of IS200 were detected, providing a high level of intraserovar strain discrimination. Profiles were generally more conserved among genetic and reference strains ; representatives of clinical phage type strains, which are recent human isolates, showed much greater diversity of IS200 profiles. Irrespective of their origin, strains could be assigned to IS200 profile groups, phylogenetically related lines identified by combinations of conserved insertion sites. Hybridization profiles of this mobile element are markers of intermediate and short-term evolution in S. typhimuvium. They provide a fingerprinting scheme for the purposes of genetics, and delineate a molecular typing scheme for the purposes of epidemiology.

SUMMARYThe routine methods used in the Enteric Reference Laboratory for the study of enterobacterial plasmids are described. The results of their application to plasmids of diverse origin, and their value for the categorization of those plasmids, are presented and discussed.

The susceptibilities to arsenic and cadmium together with the detection of plasmid DNA were evaluated for use as epidemiological markers for the subtyping of Listeria monocytogenes. Plasmid DNA was detected in 34% of 322 apparently unrelated isolates of L. monocytogenes. The resistance to cadmium and arsenic differentiated 565 apparently unrelated cultures into four groups, the smallest being 5% of cultures resistant to both agents, and the largest (53%) being sensitive to cadmium and resistant to arsenic. The resistance patterns to these agents and the presence of plasmid DNA varied markedly between the serotypes of the cultures. The detection of plasmid DNA was strongly associated with cadmium resistance in serogroup 1/2 cultures, but not within those of serogroup 4. Arsenic resistance was not associated with plasmid DNA. All methods were sufficiently stable to be useful for epidemiological investigations. The techniques described here offer simple methods which can be easily utilized in laboratories without a specialized expertise for this bacterium.