MOLECULAR CHARACTERIZATION OF MULTIRESISTANT STRAINS OF SALMONELLA-TYPHI FROM SOUTH-ASIA ISOLATED IN ONTARIO, CANADA

Two hundred and fourteen isolates of Salmonella typhi submitted to our laboratory between 1992 and 1996 were tested for susceptibility to 20 antimicrobial agents. Forty-eight of the 214 isolates (22.4%), recove red from individuals who had travelled in South Asia, were multiresist ant. Forty-four of the 48 isolates were resistant to ampicillin, chlor amphenicol, tetracycline, streptomycin, sulfamethoxazole, trimethoprim , cotrimoxazole, ticarcillin, and piperacillin; the other four isolate s were resistant to four to six agents. Forty-two of the multiresistan t isolates belonged to Vi phage type El, two isolates from the Punjab State belonged to phage type A, another from the Punjab State belonged to phage type E3, one isolate from Pakistan belonged to type M1, and one isolate from India belonged to type J1. Plasmids from 45 of 48 iso lates showed a temperature-sensitive mechanism of transfer to Escheric hia coli K-12 strains, characteristic of HI incompatibility group plas mids. The majority of plasmids had an estimated molecular weight of 12 0 MDa and encoded both citrate utilization and mercury resistance. Pla smids from three isolates had an estimated molecular weight of 112-115 MDa; one of these isolates encoded citrate utilization but not mercur y resistance. Analysis of isolates by pulsed-field gel electrophoresis after digestion with XbaI and SpeI indicated that the majority of mul tiresistant isolates shared a common restriction profile, while four i solates had unique patterns.