Background: Most of the shigellosis outbreaks in Korea have been caused by Shigella sonnei since late 1990’s. We analyzed 36 strains of S. sonnei isolated in South Korea from 1998 to 2001 by molecular epidemiologic tools to understand genetic relationship of the outbreaks.

Methods: The 36 strains of S. sonnei were tested for the presence of virulence genes (ial, ipaH, stx, set1A, set1B and sen) using polymerase chain reaction (PCR) method and for the production of Shiga-toxin using latex agglutination test. Seventeen representative strains were selected and their genetic relevance was analyzed by plasmid profile and pulsed-field gel electrophoresis(PFGE).

Results: By PCR, ipaH gene was detected in all 36 strains, set1B gene in 15 strains (41.7%), and sen gene in 16 strains (44.4%); all strains were negative for set1A gene. Although stx gene was positive in four strains by PCR method, the toxin was negative by latex agglutination test. The strains were differentiated into 11 groups by plasmid profile and 1 type with 3 subtypes (A-1, A-2 and A-3) by PFGE.

Conclusion: There was a wide range of diverse virulence genes present in the outbreak strains of S. sonnei. PFGE analysis indicated that all the strains tested were related with each other despite minor genotypic and phenotypic differences. A genetically identical clone of S. sonnei was estimated to be the cause of the outbreaks. (Korean J Clin Microbiol 2005;8(2):172-178)