Plasmid profiles of antibiotic-resistant Shigella dysenteriae types 2, 3, 4, 6 and 7 isolated in Ethiopia during 1976-85.

Plasmid profile analysis by agarose gel electrophoresis was carried out on 37 drug-resistant strains of Shigella dysenteriae types 2, 3, 4, 6 and 7. These strains were collected between 1976 and 1985 in Addis Ababa, Ethiopia. The plasmid profile of S. dysenteriae type 2 strains with R-type CSSuT did not show middle-sized plasmids likely to code for CSSuT resistance. All strains contained a large plasmid of about 120 megadaltons (MDa), and a cryptic plasmid of about 2.2 MDa. The plasmid profiles of S. dysenteriae type 3 with R-types ACSSuT, SSuT and SSu showed a 4.2 MDa SSu-determinant, which was demonstrated in Escherichia coli K12 recipients resulting from triparental crosses. The ACT determinant in S. dysenteriae type 3 with R-type ACSSuT is probably chromosomally mediated. Cryptic plasmids of about 3.0 and 2.2 MDa were found in all S. dysenteriae type 3 isolates. The 4.2 MDa plasmid featured prominently in the plasmid profiles of S. dysenteriae types 4, 6 and 7 with R-types SSuT and SSu. However, this plasmid was not mobilizable by triparental crosses. There was a relative paucity of transferable plasmids in non-Shiga bacillus isolates. However, incompatibility group N plasmids, coding for tetracycline resistance, were detected.