Spontaneous deletions of drug-resistance determinants from Salmonella typhimurium in Escherichia coli

Plasmids isolated from two different clinical isolates of Salmonella typhimurium, both resistant to the antibiotics ampicillin, tetracycline, streptomycin and chloramphenicol, were used to transform Escherichia coli. Segregation of antibiotic-resistance determinants occurred in both cases. Analysis of plasmids from one set of segregants by DNA-DNA hybridisation indicated that the segregation was due to precise deletions in the transforming plasmid.