Genetic evidence for a chromosomally integrated multiresistance plasmid in Salmonella dublin

Of 1099 isolates of Salmonella dublin during 1985-86, 11 (1%) were resistant to three or more antibiotics. Strain S4659/85, a multiresistant isolate, lacked autonomous R plasmids but showed incompatibility with incH2 plasmids and donated resistance determinants in matings. Transconjugants acquired incomplete R plasmids which integrated stably into a specific chromosomal site. These data provide an insight into the behaviour of R plasmids in S. dublin.