Resistance to chloramphenicol and ampicillin of Salmonella typhimurium in Ontario, Canada.

Combined resistance to chloramphenicol and ampicillin in Salmonella typhimurium is appearing in Canada. Five cases of this type of infection have been noted. The isolates are resistant to streptomycin, sulfonamides, and tetracycline, in addition to ampicillin and and chloramphenicol; four isolates are also resistant to kanamycin. In each isolate the determinant for chloramphenicol resistance is linked to a conjugative plasmid. Four of the chloramphenicol plasmids appear to be related in resistance pattern, level of antibiotic resistance mediated, and temperature sensitivity of mating ability. The same four plasmids are associated in the primary isolates of Salmonella with three different kinds of genetic units mediating resistance to ampicillin. In only one of the five strains are the genes for chloramphenicol and ampicillin resistance linked in the same plasmid.