Plasmid characterization of Salmonella typhimurium transmitted from animals to humans

The transmission of pathogenic bacteria from animals to humans is widely studied because of its public health importance. In this study, we show the transmission of Salmonella typhimurium from cattle which had received no growth-promoting antibiotics to humans who had direct contact with the ill animals. On one cattle farm, the veterinarian attending the sick animals became ill, and two other individuals living on the farm later developed salmonellosis. The strains isolated from both humans and animals at one farm were identical as to antibiotic susceptibility and phage type, and they were specifically traced by the presence of a common 24-megadalton plasmid. Restriction enzyme digests of this plasmid from both human and animal strains were identical. At another farm, tetracycline-resistant S. typhimurium strains possessing a different profile (eight plasmids) were isolated from both animals and humans. The tetracycline-resistant clone was also isolated from animals at a third farm, but with animals and humans having no known contact with those of the other two farms.