Involvement of a 70-kb plasmid of the epidemic Shigella dysenteriae type 1 (Dt66) strain in drug-resistance, lipopolysaccharide synthesis, and virulence.

The present work characterizes a 70 kb plasmid of the Indian epidemic isolate Shigella dysenteriae type 1 strain Dt66 in relation to virulence and drug-resistance characters. Curing of the plasmids of Dt66 strain by acriflavine (AF) yielded seven groups of AF-cured derivatives. One group of derivatives containing 5 plasmids (70-2.5 kb) as against 6 (120-2.5 kb) in the wild type showed resistance to chloramphenicol, pivmecillinam, streptomycin, and tetracycline (CmrPivrSmrTcr) but sensitivity for ampicillin and nalidixic acid (AmsNals). This derivative when used as donor in conjugation experiments with a plasmidless E. coli KL318 strain (AmsCmsPivsSmrTcsNalr) as recipient, transferred only its 70 kb plasmid into the recipient with concomitant transfer of the CmrPivr phenotype. Both the donor and the transconjugant exhibited a diffuse pattern of adherence and produced keratoconjunctivitis in guinea pigs. Transconjugants contained a 42 kDa lipopolysaccharide band which was absent in recipient. Moreover, both donor and transconjugants showed Congo red binding ability. The results suggest that the 70 kb plasmid of S. dysenteriae Dt66 strain encodes not only Cmr,Pivr character but also is associated with virulence-related characters such as Congo red binding, LPS-biosynthesis, HeLa cell adherence, and keratoconjunctivitis related to virulence.