Characterization of R-plasmids in environmental isolates ofsalmonella: Host range and stability

Four environmental isolates ofSalmonella, resistant to several drugs, were examined for plasmid carriage with four different plasmid DNA isolation procedures. The method of Birnboim and Doly gave the best results. Three of the strains possessed a single plasmid with molecular weights of 60 (kanamycin resistant), 44.5 (kanamcin resistant), and 23.4 Md (ampicillin and amoxicillin resistant); the other strain (resistant to tetracycline) harbored two plasmids of 69.8 and 2.2 Md. The 69.8 Md was the one responsible for resistance. All plasmids were fi^–, and the 44.5 Md Kc^r plasmid synthesized a sex pilus type F. Some properties related to the dissemination of R-plasmids, such as host range, transfer frequencies, and in vitro stability, were studied. Plasmids generally showed a wide host range and high stability in the transconjugants tested. It could be concluded that these plasmids may be widely disseminated in the environment studied.