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Genetic and Biophysical Study of R Plasmids Conferring Sulfonamide Resistance in Shigella Strains Isolated in 1952 and 1956

School of Microbiology, University of Melbourne, Parkville, Victoria, 3052, Australia

ABSTRACT

The conjugative plasmids determining sulfonamide resistance in five Shigella strains, each isolated from a different patient, have been characterized. One S. flexneri 2a strain, isolated in 1952, harbored an fi⁺ plasmid of molecular weight 53 x 10⁶, which specified synthesis of F-like pili and bore determinants for sulfonamide resistance (Su) and bacteriocinogeny (Col). This plasmid was compatible with plasmids of groups F_I, F_II, I, and P. A second S. flexneri 2a strain isolated in 1952 harbored an fi^– plasmid of molecular weight 59 x 10⁶, bearing the Su determinant and compatible with all plasmids tested. This strain also harbored an fi⁺ group-F_II plasmid of molecular weight 42 x 10⁶, which bore the Col determinant and specified synthesis of F-like pili. Three S. dysenteriae 2 strains isolated in 1956 carried apparently identical fi^– plasmids of molecular weight 58 x 10⁶, which bore the Su determinant, could form transconjugants in Pseudomonas but not in Proteus, and were incompatible with the P-group plasmid RP4.