Study finds antibiotic-resistance genes in cattle manure

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Researchers from Yale and the University of Connecticut report detecting genes relating to bacterial resistance to antibiotics in the manure of dairy cows.

The findings create some concern that introduction of these genes into the environment as manure is used as fertilizer could lead to the emergence of antibiotic-resistant pathogens in humans or animals. The research results are published in the April 22 issue of mBio the online open-access journal of the American Society for Microbiology (ASM).

The researchers used a new combination of methods to examine the resistome of dairy cow manure, and constructed metagenomic libraries with DNA extracted from manure screened for resistance to beta-lactams, phenicols, aminoglycosides, and tetracyclines. Functional screening identified 80 different antibiotic resistance genes whose protein sequences were on average 50 to 60 percent identical to sequences deposited in GenBank. The resistance genes were frequently found in clusters and originated from a taxonomically diverse set of species, suggesting that some microorganisms in manure harbor multiple resistance genes.

“The diversity of genes we found is remarkable in itself considering the small set of five manure samples,” says Jo Handelsman, PhD, senior study author and microbiologist at Yale. “But also, these are evolutionarily distant from the genes we already have in the genetic databases, which largely represent (antibiotic resistant) genes we see in the clinic.”

According to a release from ASM, that might signal good news that antibiotic-resistant genes from cow gut bacteria are not currently causing problems for human patients. But, Wichmann points out, another possibility is that “cow manure harbors an unprecedented reservoir of AR genes” that could be next to move into humans.

Potentially, manure bacteria that contain antibiotic-resistant genes could colonize humans, or the genes from benign bacteria could move to human pathogens through a process called horizontal gene transfer. Gene transfer enables genes to jump between microorganisms that are not related, and it occurs in most environments that host bacteria, according to the ASM release.

The researchers hope the study will open up a larger field of surveillance to find and identify new types of resistance before they affect humans.

The mBio article is available online. 

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shaun evertson    
Nebraska  |  April, 25, 2014 at 08:45 AM

Common sense questions. Since we've only been looking for ab resistant genes for a very short while, how do we know they haven't always been present? The authors state that "the increasing prevalence of antibiotic resistance among bacteria is one of the most intractable challenges in 21st-century public health." How do they define "increasing?" Are real rates of ab resistance increasing? Who has compared rates of ab resistance in early ab's to the rates of resistance to today's ab's? We do know that resistance is a natural evolutionary process. This story and the tone of the paper suggest that ab resistance today is somehow extra-natural. Where are the actual, objective data which prove this?

Karen S    
Missouri  |  April, 25, 2014 at 10:16 AM

Good point, Shaun. Thank you - hope you get a response as I'm curious as well.

John maday    
Colorado  |  April, 25, 2014 at 10:38 AM

Yes, that’s a good point. Bacteria have been trying to stay ahead of the naturally occurring substances that kill them for millions of years. It seems I’ve heard of some research intended to identify some “baseline” levels for natural occurrence of AB resistance, but it’s an area where we probably have a lot yet to learn.

teddie Rahube    
London, Canada  |  April, 25, 2014 at 03:47 PM

Antibiotic resistance genes have alsways been there in the environment long before antibiotics were used, the issue of 'increasing' and prevalence is based on quantification over-time, and the current data shows increase in both diversity and magnitude in various samples linked with antibiotic use. There is clear evidence and lots of data outlining the impact anthropogenic activities (which involve intesive antibiotic use in humans and animals) as a major factor accelerating this evoultionary process

shaun evertson    
Nebraska  |  April, 28, 2014 at 12:48 PM

I'm just not seeing any clear evidence or data. I'm seeing a lot of supposition regarding anthropogenisis but no studies, data or evidence supporting those ideas. There would clearly be an increase in mutagenisis concurrent with the post-1945 increase in antibiotic use, however, the fact that there has been no epidemic of resistant disease in more than 70 years suggests that such an epidemic is unlikely to occur. As Scott Hurd (RIP) said, "if there was going to be an epidemic of resistance related to antibiotic use in agriculture it would have occurred by now. The fact that it has not means that antibiotic use in animals is not a major risk to human health." Furthermore, context is important in this discussion. In the 44 year that have passed since the first Earth Day, the human population has more than doubled, life span has increased by more than 20 years, famine and starvation has decreased, poverty has decreased, etc. Those who are pushing antrhopogenic-driven disaster scenarios which ignore reality are dishonest brokers.

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