Transferable or mobilisable antibiotic resistance in Shigella dysenteriae types 1, 2, 3, 4, 6 and 7 isolated in Ethiopia during 1974-85

A total of 199 Shigella dysenteriae isolates resistant to one or more antibiotics and belonging to types 1, 2, 3, 4, 6 and 7 was examined by one-step broth mating with Escherichia coli K12 and, if non-conjugative, additionally by triparental crosses with the conjugative plasmids X and delta. Of the S. dysenteriae type 1 (Shiga’s bacillus) strains, 96% harboured conjugative plasmids. During 1974-79, isolates of Shiga’s bacillus carried conjugative plasmids coding for ACSSuT (ampicillin, chloramphenicol, streptomycin, sulphonamide, tetracycline) resistance that transferred at low frequencies (less than 10(-4). After 1980, however, about 50% of isolates of Shiga’s bacillus with this resistance (R)-type carried conjugative plasmids that transferred at high frequencies (10 degrees-10(-2)) and that expressed the ACT determinant only. The introduction of a new clone of Shiga’s bacillus into Ethiopia in 1980 is suspected. Conjugative plasmids coding for SSuT resistance were detected in S. dysenteriae types 2, 3, and 4. Non-conjugative SSu determinants in S. dysenteriae type 3 were mobilised by conjugative plasmids X and delta. R-type CSSuT in strains of types 2 and 7, and R-type ACST in type-3 strains were neither transferable nor mobilisable and are probably determined chromosomally.