Use of molecular subtyping in surveillance for Salmonella enterica serotype typhimurium.

Background: Because Salmonella enterica serotype typhimurium is the most co mmon serotype isolated from persons with salmonellosis in the United States , it is difficult to detect unusual clusters or outbreaks. To determine whe ther molecular subtyping could be useful in public health surveillance for S. enterica serotype typhimurium, the Minnesota Department of Health initia ted the routine use of pulsed-field gel electrophoresis (PFGE) of isolates. Methods: Beginning in 1994, all S. enterica serotype typhimurium isolates s ubmitted by clinical laboratories to the Department of Health were subtyped by PFGE. A standard questionnaire was used to interview patients about pos sible sources of infection. Results: From 1994 through 1998, 998 cases of infection with S. enterica se rotype typhimurium were reported to the Minnesota Department of Health (4.4 cases per 100,000 person-years). PFGE was performed on 958 of the isolates (96 percent), and 174 different patterns were identified. Sixteen outbreak s with a common source were identified, accounting for 154 cases. PFGE subt yping made it possible to confirm 10 outbreaks that involved small numbers of cases in institutional settings. Of six larger, community-based outbreak s, four would probably not have been recognized without PFGE subtyping. The se four outbreaks accounted for 96 of the 154 culture-confirmed outbreak ca ses (62 percent). Fifty-six of 209 isolates tested for antimicrobial suscep tibility (27 percent) were resistant to at least five antimicrobial agents. The multidrug-resistant isolates identified had unique PFGE patterns. Conclusions: Routine molecular subtyping of S. enterica serotype typhimuriu m by PFGE can improve the detection of outbreaks and aid in the identificat ion of multidrug-resistant strains. Combining routine molecular subtyping w ith a method of rapid communication among public health authorities can imp rove surveillance for S. enterica serotype typhimurium infections. (N Engl J Med 2001;344:189-95.) Copyright (C) 2001 Massachusetts Medical Society.