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

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 × 10 6 , 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 α , and P. A second S. flexneri 2a strain isolated in 1952 harbored an fi − plasmid of molecular weight 59 × 10 6 , bearing the Su determinant and compatible with all plasmids tested. This strain also harbored an fi + group-F II plasmid of molecular weight 42 × 10 6 , 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 × 10 6 , which bore the Su determinant, could form transconjugants in Pseudomonas but not in Proteus , and were incompatible with the P-group plasmid RP4.