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A large outbreak of Shigella sonnei gastrointestinal infections occurred at Cheju Island in Korea from May to August 2000. We selected 54 strains which were isolated from the primary treatment failure cases in the outbreak, and characterized the resistance-determining region of the R-plasmid. The 54 strains showed same antimicrobial resistance patterns; resistance against ampicillin, streptomycin, tetracycline, and trimethoprim-sulfamethoxazole. The resistance to ampicillin, streptomycin, and tetracycline were mediated by a conjugable plasmid of about 80 kb size, but the trimethoprim- sulfamethoxazole resistance was not transferred by this plasmid. The R-determining region of the plasmid was cloned and characterized. The 8,384 bp sequences contained resistance genes in the following order:strA, strB, tetR, tetA, and sul1. Fifty four isolates harbored the same sized plasmid and showed same ribotyping patterns, which suggested the clonal spread of S. sonnei in the outbreak.
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