Plasmid profiles of Shigella dysenteriae type 1 isolates from Ethiopia with special reference to R-plasmids

Plasmid profiles of 80 Shigella dysenteriae type 1 (Shiga’s bacillus) strains, collected between 1974 and 1985 in Ethiopia, were examined. Strains with the dominant antibiotic-resistance (R-) type–resistance to ampicillin (A), chloramphenicol (C), streptomycin (S), sulphadiazine (Su) and tetracycline (T)–showed two distinct plasmid profiles (PP). Six plasmid bands were demonstrated in “Ethiopian strains” with PP-1A isolated between 1974 and 1982. In mating experiments with these strains, Escherichia coli K12 recipients showed plasmids pYH10a (72 Mda, atypical Inc FIme, coding for ACSSuT resistance). Eight plasmid bands were demonstrated in strains with PP-2A. These strains were first isolated in 1980 and carried plasmid pYH11a (40 Mda, Inc X, coding for ACT resistance). Strains with PP-2A were identical with a “Zairian strain” described elsewhere. Strains with R-type ACT were “Zairian strains” lacking the 5.1- and 4.2-Mda plasmids. Those with R-type CSSuT were temporally clustered in 1978-1980 and carried plasmid pYH12 (58 Mda, Inc B, coding for the same R-type). A trimethoprim-resistant strain (Gimira strain) had a pattern of small plasmids similar to those of the “Zairian strain” and is probably a subclone of the latter. The fact that a limited number of plasmid profiles have remained constant over many years shows the limitation of plasmid profile analysis as an epidemiological tool. However, when the usual profile is known for a given area, identification of a distinctly different pattern becomes easy and epidemiologically useful.