In a path-breaking research, scientists in the US have found that cow manure is a potential source of new types of antibiotic resistance genes that transfer to bacteria in the soils where food is grown.
"Manure from dairy cows, which is commonly used as a farm soil fertiliser, contains a surprising number of newly identified antibiotic resistance genes from the cows' gut bacteria," said lead author Fabienne Wichmann, former post-doctoral researcher at Yale University in New Haven, Connecticut.
Thousands of antibiotic resistance (AR) genes have already been identified, but the vast majority of them do not pose a problem when found in harmless bacteria.
The real worry is when these genes appear in the types of pathogenic bacteria that cause food-borne illnesses or hospital infections.
"Since there is a connection between AR genes found in environmental bacteria and bacteria in hospitals, we wanted to know what kind of bacteria are released into the environment via this route of manure fertilisation," Wichmann explained.
Farmers use raw or composted cow manure on some vegetable crops, which could lead to a scenario where residual manure bacteria might cling to produce and they or their genes might move to the human ecosystem.
"Is this a route for movement of these genes from the barn to the table?" asked Jo Handelsman, a microbiologist at Yale.
To understand this, Handelsman's team used a powerful screening-plus-sequencing approach to identify 80 unique and functional AR genes.
The genes made a laboratory strain of Escherichia coli bacteria resistant to one of four types of antibiotics - beta-lactams (like penicillin), aminoglycosides (like kanamycin), tetracycline or chloramphenicol.
The team also found an entire new family of AR genes that confer resistance to chloramphenicol antibiotics which are commonly used to treat respiratory infections in livestock.
"The diversity of genes we found is remarkable in itself considering the small set of five manure samples," Handelsman noted.
That might signal good news that AR genes from cow gut bacteria are not currently causing problems for human patients.
But, Wichmann points out, another possibility is that "cow manure harbours an unprecedented reservoir of AR genes" that could be next to move into humans.
The study was reported in mBio, the online open-access journal of the American Society for Microbiology.
