ScienceDaily (May 26, 2006) — "Harmless" bacteria in the digestive tracts of dairy cows, may not be so harmless after all. They may be a reservoir for antibiotic resistance genes that can be transferred to more harmful, disease-causing bacteria, according to research presented at the 106th General Meeting of the American Society for Microbiology in Orlando, Florida.

"There is concern that veterinary therapeutic usage of antibiotics in animals is responsible for the emergence of drug-resistant Salmonella. For dairy farms recently surveyed in Georgia, there was no association found between antibiotic use and isolation of multi-drug resistant Salmonella. Where does the drug resistance in Salmonella originate?" says Susan Sanchez of the University of Georgia, who conducted the study with other researchers from the University of Georgia and the Universidad Complutense of Madrid.

Using DNA-based technology Sanchez and her colleagues discovered that a significant number of non-pathogenic, commensal bacteria isolated from cow manure contain the same antibiotic-resistance genes as those found in multi-drug resistant Salmonella strains found on the same farm. In addition, isolation of antibiotic-resistant Salmonella appears to be associated with whether or not commensal bacteria carry these resistance genes rather than antibiotic use on the farm.

"This work suggests that mobile DNA elements like plasmids are responsible for the rapid spread of drug resistance on farms. Ecology appears to play a more important role in the emergence of drug resistance in Salmonella than therapeutic antibiotic use," says Sanchez.

This research is supported by a grant from the National Institutes of Health.

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