Antibiotic Resistant Bacteria Found In Cow Manure Used For Fertilization; Could Lead To More Infections

By Shweta Iyer on April 22, 2014 12:36 PM EDT

cow
Researchers have found antibiotic-resistant bacteria in cow manure used for agricultural fertilization. (Photo: b3d, CC BY 2.0)

Cow manure is known to be rich in minerals, and is widely used as an agricultural fertilizer. But a recent study has found that many samples of manure hold bacteria with antibiotic-resistant genes, which originate in the bovine gastrointestinal system. The authors speculate the resistance may transfer from the manure into the soil being fertilized for agriculture, which may have implications for human health.

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Although antibiotic-resistant (AR) genes are quite widespread, with thousands of harmless bacteria having them, the problem comes when AR genes become present in pathogenic bacteria responsible for foodborne illness, among other infections that require hospitalization. "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 (manure fertilization),"  said Fabienne Wichmann, lead author of the study and former postdoctoral researcher at Yale University, in a press release.

Bacteria with AR genes may enter the food chain when farmers use raw or composted cow dung to fertilize plants, and a certain amount of residual bacteria remain in the yield, Yale microbiologist Jo Handelsman said. Eventually, this can transfer to humans.

To see how much bacteria cow manure had, the team screened and sequenced the manure, finding 80 unique and functional AR genes. The genes, when used on a laboratory strain of Escherichia coli (E. coli) bacteria, made it resistant to one of four types of antibiotics, which included beta-lactams (penicillin), aminoglycosides (kanamycin), tetracycline, and chloramphenicol.

Most of the 80 AR genes discovered were new, yet about 75 percent of them had sequences that were only a bit related to AR genes already discovered. 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. 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," Handelsman said in the release.

This means that the AR genes found are not yet causing diseases in humans. But that doesn't mean they won't eventually move in, Wichmann said. But there's still a lot of work to be done, as "this is just the first in a sequence of studies," Handelsman said. They still need to determine whether the genes will move from the barn to the dinner table.

If they don't enter through that route, they could also be acquired through bacteria that already colonize humans. There are already instances in which bacteria have transferred from farm animals to their human handlers. Gene transfer enables genes to jump between microorganisms that are not related, and it occurs in most environments that host bacteria.

If pathogenic manure bacteria acquire antibiotic resistance then there could be several complications in treating diseases. Alternatively, benign bacteria in manure might transfer resistant genes to pathogens at any point along the path - in manure, soil, food, or humans.

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

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