Comparative evaluation of molecular typing of strains from a national epidemic due to Salmonella brandenburg by rRNA gene and IS200 probes and pulsed-field gel electrophoresis

Abstract

In Switzerland in 1992 there was a prolonged series of outbreaks of human salmonellosis caused by a previously rare serotype, Salmonella brandenburg. In order to examine the genotypic basis of the epidemic, molecular typing was applied to representative strains of this serovar isolated between 1983 and 1992. These included sporadic human isolates up to 1985, isolates from unrelated geographical areas, and Swiss isolates from humans, animals, and meat products isolated after 1991. Plasmid profiling was not found to be applicable to S. brandenburg, but chromosomal typing was accomplished by analyzing restriction fragment length polymorphisms with DNA probes for three marker loci; the 16S and 23S rRNA genes and sites of insertion of the mobile DNA element IS200. The macrorestriction profiles of the whole genome were examined by pulsed-field gel electrophoresis, which proved to be the most discriminatory of the typing methods. The study demonstrated the comparative value and complementary relationship between these typing methods for epidemiological purposes. All approaches concurred in identifying the 1992 isolates as a single genotypic clone, which was present in multiple (food) vehicles of infection. They were distinct from sporadic isolates of this serovar and from strains of S. brandenburg isolated in other countries.