Antimicrobial resistance pattern and plasmid profile of Salmonella typhi isolated from an outbreak in Tehran province.

The antimicrobial resistance patterns and plasmid profiles of Salmonella typhi isolates from sporadic cases (n = 33) and an outbreak (n = 48) were compared. Of 28 sporadic drug-resistant isolates, 24 (85.7%) were multiply resistant. The predominant antibiotic resistance pattern was TeCmSmSxTAp, which was also the most common pattern of the outbreak isolates. 13 drug-resistant strains isolated before the outbreak (46.4%) were able to transfer the whole resistance pattern or part of it to Escherichia coli K 12 by conjugation. Although 20 of the sporadic strains contained plasmid DNA, transferable R plasmids were only detected in 13 (65%) of them. Among the outbreak strains, the rate of R plasmid transfer was 92.3%, with only the TeCmSmSxTAp pattern transferred. Plasmid profiling and Hind III endonuclease digestion of plasmid DNA identified a 91.2 megadaltons (Mda) plasmid that was recovered from most of the outbreak isolates and from 4 strains collected before the outbreak. This plasmid coded for TeCmSmSxtAp and transferred the pattern of resistance in toto. The results indicate multidrug-resistant S. typhi as a potential cause of infection in the region.