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Cow manure is an important fertilizer, but could it give us more than we bargained for — like deadly bacteria resistant to antibiotics? A new study reveals 80 previously unknown antibiotic resistance genes.

Could the cow manure that plays such an essential role in fertilizing many crops uses for human consumption pose a threat to humans? Researchers from Yale University, taking a closer look at Connecticut dairy cows' manure, found 80 different antibiotic resistance genes in their gut bacteria.

Could bacteria resistant to antibiotics make their way to human populations via crops that are fertilized by cow dung? That is exactly what the research team wanted to find out. Fabienne Wichmann, lead study author and former postdoctoral researcher at Yale, said:

"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 fertilization."

'Remarkable Diversity' Of Genes

Wichmann's team used a "powerful screening-plus-sequencing approach" to find out just what is lurking in the cow dung used to fertilize our crops. Eighty different AR genes were found to be present in the samples the team sequenced. Three quarters of these genes were previously completely unknown, and further investigation revealed that they were only distantly related to those antibiotic resistance (AR) genes that had already been identified before.

This means that AR genes are rapidly proliferating. While the good news is that most of the genes the research team came across aren't dangerous to humans because the bacteria they are present in don't pose a threat, the bad news is that that could soon change.

When the researchers applied the genes to lab strains of the E Coli bacteria, the bacteria became resistant to several commonly used antibiotics, including penicillin and tetracycline. 

What's more, the stool samples the Yale team used to make their conclusions came from only four cows on a single farm. Senior study author and microbiologist at Yale Jo Handelsman put it like this: "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."

Are These AR Genes A Threat To Humans?

The three quarters of AR genes that were newly identified might be only distantly related to those AR genes that are currently known to occur in humans, but that doesn't necessarily mean they can't pose a threat to humans in the future. The antibiotic resistance genes have the potential to make humans very ill without much hope of effective treatment through two separate mechanisms. 

The first is direct. Farmers, who are in close contact with cows on a daily basis, could potentially be colonized by some of the bacteria which were found to contain AR genes — and some of those bacteria may indeed prove to be harmful to people. The second is less direct. We already know that genes can transfer from one micro-organism to another unrelated one.

Genes could be transferred from one bacterium to another in manure, soil, food or even within the human body itself. If this process, known as "horizontal gene transfer", takes place, bacteria that are already known to make humans ill could take possession of these newly found AR genes. This would mean that we could soon run out of treatment options for known pathogens. 

Is this something we need to start panicking about? Handelsman says that we need to conduct further research to find out whether these genes could really move from the barn to the clinic. 

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