The present study verifies the usefulness of PFGE in characterizing and comparing strains of S. sonnei. Minor variations among S. sonnei isolates could be detected by PFGE.

Plasmid profiling was used to characterize 219 strains of Shigella species isolated from sporadic cases of shigellosis in Malaysia during the period 1994-2000. Heterogeneous plasmid patterns were observed in all Shigella spp. There was a correlation between plasmid patterns and serotypes of S. flexneri, S. dysenteriae and S. sonnei. Five common small plasmids (>20.0 kb) were observed in S. flexneri 1b and 2a, whereas six common small plasmids were found in serotype 3a. Some of these plasmids appeared to maintain their existence stably in each individual serotype. Plasmids of size 11.40 and 4.20 kb were present only in S. flexneri 2a isolates, whereas the 4.40 kb plasmid was unique for serotype 3a. Large (>150 kb) or mid-range plasmid (20.0-150 kb) was not observed from any S. flexneri 1b isolates. Eighty-nine percent of S. flexneri of various serotypes harboured the plasmid of 3.20 kb. All S. dysenteriae type 2 isolates harboured the 9.00 kb plasmid, while four common small plasmids were found in S. sonnei isolates. The 2.10 kb plasmid was only seen in S. sonnei. Streptomycin resistance in S. dysenteriae type 2 and multi-drug resistance in S. sonnei may be associated with the 9.00 and 14.8 kb plasmids, respectively. Plasmid profiling provided a further discrimination beyond serotyping and a useful alternative genotypic marker for differentiation of Shigella species. To the best of our knowledge, this is the first report on the plasmid prevalence of the Malaysian Shigella species.