Evaluation of isolated cases of salmonellosis by plasmid profile analysis: introduction and transmission of a bacterial clone by precooked roast beef.

In July-August 1981, the number of isolated cases of salmonellosis in New Jersey and Pennsylvania markedly increased. Concurrently, two outbreaks in these states caused by the same Salmonella serotypes were traced to a single brand of precooked roast beef. Plasmid profiles of outbreak and isolated strains were examined, and Salmonella newport isolates from the implicated meat and from cases in the two outbreaks were identified with a unique profile, present in 45% of reported strains from isolated cases in this area during the same period. Review of the food exposure histories in isolated cases demonstrated association between this plasmid profile and consumption of precooked roast beef (P = 0.003). Examination of S newport strains from other regions and intervals indicated that this strain was introduced into these two states sometime after March 1981. Thus, plasmid profile analysis was useful in studying the epidemiology of isolated cases of salmonellosis due to a sensitive common serotype.