Studies on the development of chloramphenicol resistance in Salmonella typhimurium.

Chloramphenicol therapy of acute and chronic Salmonella typhimurium infection in mice did not lead to the development of chloramphenicol resistant mutants of this organism. However, chloramphenicol resistant organisms were readily produced in vitro. Transfer of chloramphenicol resistance from a donor strain of Escherichia coli X12 to a suitable recipient strain of S typhimurium 5235 occurred readily in the intestine of 15 out of 20 conventional mice, following oral administration of large doses of these strains supplemented by chloramphenicol therapy. When a similar system was used in untreated animals, only a small number of chloramphenicol resistant S typhimurium were isolated from three out of 18 mice. The virulence of chloramphenicol resistant S typhimurium produced in vitro and in vivo was similar to that of the sensitive parent strain.