Involvement of plasmids in determining bacteriophage sensitivity in Salmonella typhimurium: genetic and physical analysis of phagovar 204.

Strains resistant to the action of sulfa drugs and tetracycline were predominant among the antibiotic-resistant Salmonella typhimurium phagovar 204 isolated in Canada. Plasmid DNA was detected in cellular extracts of all strains examined. A number of these plasmids could be placed in specific incompatibility and size classes. Both resistance coding and cryptic plasmids were involved in determining phagovar 204. In one instance, phagovar 204 was derived from phagovar 36 in a two-step conjugation involving independent sulfadiazine and tetracycline resistance plasmids. In another, phagovar 204 was derived directly from phagovar 49 through the introduction of a single tetracycline-streptomycin R plasmid. The phagovar-determining plasmids ranged in size from 3.4 to 72 megadaltons.