Thirty-nine strains of Salmonella typhi, isolated in 1995 from four Districts in Pakistan,
Rawalpindi, Islamabad, Kharian and Jehlem, were catalogued and examined. Chromosomal
DNA from each isolate was digested with XbaI restriction endonuclease and subjected to
pulsed-field gel electrophoresis. Three clonal variants comprising of 17–19 DNA fragments
were identified. Antibiotic susceptibility testing identified that 37 of the S. typhi were resistant
to chloramphenicol, trimethoprim and ampicillin. These antibiotic resistance genes were found
to be located on one of four plasmids belonging to incompatibility group IncHI1 and ranging
in size from 150–175 Kb. The genes responsible for this resistance in each case were the
chloramphenicol acetyltransferase (CAT) type I, the dihydrofolate reductase (DHFR) type VII
and the β-lactamase TEM-1 respectively.