Sequential acquisition of R-plasmids in vivo by Salmonella typhimurium.

Salmonella typhimurium, resistant only to trimethoprim and sulphamethoxazole, was isolated from the faeces and blood of a chronic alcoholic patient in acute renal failure. The isolates harboured an 18 Md non-conjugative plasmid. He was dialysed peritoneally and treated with ampicillin; four days later there was no clinical improvement and his peritoneal dialysis fluid (PDF) had become infected. Salm. typhimurium was isolated from faeces and PDF. Both isolates were additionally resistant to ampicillin and contained two plasmids (55 Md and 18 Md). Therapy was changed to chloramphenicol and gentamicin was added to the PDF. Two weeks later Salm. typhimurium was again isolated from PDF and faeces. The PDF isolate was unchanged but 4% of the colonies isolated from this faecal specimen were resistant to chloramphenicol and had acquired an additional 62 Md plasmid. From all PDF and faecal specimens two different strains of Escherichia coli and one strain of Klebsiella pneumoniae were isolated which contained plasmids indistinguishable, on the basis of molecular weight and transferable resistance markers, from those acquired by Salm. typhimurium. The transferability of these plasmids in vitro to E. coli K12 and to the patient's initial Salm. typhimurium was studied and the results discussed.