One predominant type of genetically closely related Shigella sonnei prevalent in four sequential outbreaks in school children.

Seventy-six Shigella sonnei isolates from four sequential outbreaks in school children were analyzed to determine their relatedness. Outbreak strains exhibited two major antibiograms, 9 plasmid profiles, 10 enterobacterial repetitive intergenic consensus sequence (ERIC)-PCR patterns, and 17 pulsed-field gel electrophoresis (PFGE) patterns. Of typing methods, ERIC-PCR types generally coincided with the PFGE types within these outbreak strains. However, ERIC-PCR analysis could not discriminate an epidemiologically unrelated strain from some outbreak strains. Further computer-assisted analysis for similarity of the PFGE patterns revealed that the main culprits of these four sequential outbreaks were strains of pulsotype C (88.2% of total outbreak isolates). The results indicate that PFGE can provide more explicit relatedness of outbreak strains than the other typing methods examined. In conclusion, based on PFGE analysis, one predominant pulsotype of multiple genetically related strains of S. sonnei was prevalent in these four sequential outbreaks.