Characterization of outbreak strains of shigella sonnei in the salt lake city area using plasmid dna analysis and pulsed-field gel electrophoresis

In 1994, the incidence of shigellosis in Utah was twice that reported for the United States as a whole. Greater than 40% of cases were due to Shigella sonnei, making it difficult to identify individual outbreaks and patterns of transmission. The majority of infections were in children less than 12 years of age; these children had attended 13 day-care centers. To better define S. sonnei transmission, epidemiologic data and isolates were collected for 90 cases during 8 months. Plasmid analysis and pulsed field gel electrophoresis were used to identify clusters. There were 11 plasmid patterns. Five patterns contained multiple isolates, while 6 patterns were unique. Pattern I included 52 isolates; 45 were associated with day-care centers. Pattern I infections occurred in 8 day-care centers, and the temporal clustering suggested spread between these centers. Pattern III was isolated from three additional day-care centers, suggesting a different group of related outbreaks. The 8 isolates of Pattern IV were associated with yet another day-care center. Pulsed-field gel electrophoresis (PFGE) of XbaI digested genomic DNA from representative strains from five plasmid patterns identified five PFGE patterns. Both plasmid analysis and PFGE can define otherwise undetected clusters and patterns of transmission of S. sonnei in communities. Real time use of either molecular subtyping method could improve the control of Shigella.