Location of plasmid-mediated citrate utilization determinant in R27 and incidence in other H incompatibility group plasmids

Molecular characterization of multiresistant strains ofSalmonella typhifrom 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%), recovered from individuals who had travelled in South Asia, were multiresistant. Forty-four of the 48 isolates were resistant to ampicillin, chloramphenicol, tetracycline, streptomycin, sulfamethoxazole, trimethoprim, cotrimoxazole, ticarcillin, and piperacillin; the other four isolates were resistant to four to six agents. Forty-two of the multiresistant isolates belonged to Vi phage type E1, 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 isolates showed a temperature-sensitive mechanism of transfer to Escherichia coli K-12 strains, characteristic of HI incompatibility group plasmids. The majority of plasmids had an estimated molecular weight of 120 MDa and encoded both citrate utilization and mercury resistance. Plasmids from three isolates had an estimated molecular weight of 112-115 MDa; one of these isolates encoded citrate utilization but not mercury resistance. Analysis of isolates by pulsed-field gel electrophoresis after digestion with XbaI and SpeI indicated that the majority of multiresistant isolates shared a common restriction profile, while four isolates had unique patterns.Key words: typing, multiresistant, Salmonella typhi.

Thermosensitive transfer of antimicrobial resistances and citrate utilization and cotransfer of hydrogen sulfide production from an Escherichia coli isolate

A hydrogen-sulfide producing, citrate-positive strain of Escherichia coli isolated from urinary tract infection was found to be resistant to chloramphenicol (Cm), tetracycline (Tc), streptomycin (Sm), trimethoprim (Tmp), sulfamethoxazole (Smx), and cotrimoxazole (Tmp/Smx). The strain contained 7 plasmids of molecular sizes 120, 35, 5.0, 3.2, 3.0, 2.6, and 2.4 megadaltons (Md), as detected by agarose gel electrophoresis of plasmid DNA. Thermosensitive transfer of Cm, Tc, and citrate utilization occurred conjugally to E. coli K-12 recipient strains at a frequency of approximately 10(-6) per donor cell after an 18 hour incubation. The transconjugants were also resistant to Sm and Smx and produced hydrogen-sulfide. Two plasmids of about 120 Md, pNH222, and 35 Md, pNH223, were detected in these transconjugants. Transformation or 1-hour conjugal transfer experiments at 26 degrees C with selection for Cm or Tc yielded only the 120 Md species. Plasmid pNH222 showed one way incompatibility with F plasmids, a characteristic shown previously to be typical of Inc HI1 plasmids. Transformation experiments with selection for Sm yielded transformants with one plasmid species, (pNH223), 35 Md in size, which carried the gene(s) for H2S production but not for raffinose fermentation. The fact that characteristics such as citrate utilization and hydrogen sulfide production, used in the identification of enteric bacteria, can be transmitted to E. coli by plasmids is of important taxonomic significance.