Molecular characterization of a conjugative R-lac plasmid in Salmonella typhi isolated from a patient with typhoid fever.

In a strain of Salmonella typhi isolated from a patient with post-operative typhoid fever, a 217-Kb conjugative plasmid has been detected which codes for lactose fermentation and drug resistance to sulphonamides (SA), chloramphenicol (CP), ampicillin (AP), kanamycin (KM) and trimethoprim (TP). The physical and genetic map was constructed. The lac region containing the lacI gene, the regulatory region and the lacZ and lacY genes was shown to be structurally identical to that of Escherichia coli K12. A transposon carrying genes for resistance to CP and AP, as well as four copies of IS1-like elements, were detected and localized on the plasmid. Their role in the emergence of such a large plasmid is discussed.