Effect of curing R-plasmids in Salmonella species on their serum sensitivity

Thirteen salmonella strains of clinical isolates were tested for serum sensitivity, antibiotic resistance, colicin production and plasmid existence. Nine serum resistant strains were smooth and harboring plasmids of different sizes. Transfer of R-plasmids by conjugation from three strains to E. coli K12 conferred slight decrease in serum sensitivity. Curing the nine resistant strains with ethidium bromide or acriflavine affected their serum resistance, plasmid content and cell surface. Rough isolate of S. johanesberg became serum sensitive after curing the R-factors. The suggested mechanisms involved in the change in serum sensitivity after curing are discussed.