Plasmid characterization of Shigella spp. isolated from children with shigellosis and asymptomatic excretors.

Strains of Shigella dysenteriae type 1 and Sh. flexneri isolated during 1981-82 from children with shigellosis and also from asymptomatic excretors were examined for drug resistance and for the presence of plasmids. Shigella strains isolated from dysenteric children showed multiple plasmid bands. Thirty-eight percent of the strains transferred the drug resistance factor(s), either partially or fully to a recipient Escherichia coli K12 strain. Plasmids in the molecular weight range from between 44-76 Mdal were correlated with drug resistance. It was found that multi-resistant clinical isolates generally harbour a single large transmissible plasmid. Strains isolated from asymptomatic excretors demonstrated plasmid patterns different from those isolated from children with shigellosis although the bands were relatively homogeneous within each group. Both the groups showed the presence of a 140 Mdal plasmid band conferring invasiveness and such strains gave a positive Sereny test. This study thus shows that Shigella strains from asymptomatic excretors also retain invasiveness.