Cow Manure May Lead to More Antibiotic Resistant Bacteria on Food

It turns out that cow manure may be more dangerous than anyone expected (and not just because of the smell). Manure can contain a surprising number of antibiotic resistance genes from the cows' gut bacteria. The findings hint that it's possible that cow manure is a potential source of new types of antibiotic resistance genes that can transfer to bacteria in soils where the manure is used as fertilizer.

Farmers use raw or composted cow manure on some vegetable crops as fertilizer. This means that it's possible that residual manure bacteria could cling to these crops and then move to grocery stores and the human ecosystem. Already, thousands of antibiotic resistance (AR) genes have been identified. While the majority of these don't pose a problem when found in harmless bacteria, there's the possibility that they could appear in certain types of pathogenic bacteria that cause food-borne illnesses.

"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," said Fabienne Wichmann, one of the researchers, in a news release.

The scientists examined which AR genes were present in cow manure. In the end, they found 80 unique and functional AR genes. These genes were actually able to make a laboratory strain of E. coli bacteria resistant to one of four types of antibiotics. The researchers 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," said Jo Handelsman, one of the researchers, in a news release. "But also, these are evolutionarily distant from the genes we already have in the genetic databases, which largely represent AR genes we see in the clinic."

This actually may be good news. It could potentially mean that AR genes from cow gut bacteria aren't currently causing problems for human patients. That said, this is just the first in a sequence of studies to find out whether these genes have the potential to move into the human ecosystem.

"We're hoping this study will open up a larger field of surveillance, to start looking at new types of resistance before they show up in the clinic," said Handelsman in a news release.

The findings are published in the journal mBio.