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Gina Solomon’s Blog

The Spread of Antibiotic Resistant Bacteria from Chickens to Farmers

Gina Solomon

Posted May 31, 2011

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During the past month, I have been privileged to have a talented young physician, Dr. Julia MacIsaac, working with me at NRDC. Dr. MacIsaac is a specialist in internal medicine who is pursuing subspecialty training at UCSF in occupational and environmental medicine. This month, she has been digging deep into the medical literature about antibiotic use in agriculture and the problem of antibiotic resistance. Here is a blog she wrote about this important topic:

I have been researching the human health implications of feeding antibiotics to healthy livestock – a common practice in U.S. agribusiness. As a physician, this issue is important to me because antibiotic-resistant bacteria make my patients sicker and make the drugs that I rely on less effective. In my research, I came across a particularly compelling study – simple, yet elegant – that captures the essence of the problem.

Dr. Stuart Levy, a physician at Tufts University, tested the bacteria that live in the intestines of farmers and farm communities. As background, it’s important to know that each one of us have millions of bacteria that live in our intestines that are necessary for digestion – they are the healthy bacteria, or so-called bacteria ‘flora’.  But, just like bacteria that cause disease, these bacteria can become resistant to antibiotics.  Then they can spread the genes that make them resistant to other bacteria that cause disease.

What Dr. Levy and his colleagues did was this: they studied a farm that had not yet incorporated antibiotics into the feed of their chickens – in fact, no feed with antibiotics had been used in the area for at least 7 years prior to the study.  They took fecal (stool) samples and studied the bacteria in the stool.  They recorded changes over time in intestinal bacteria from the chickens, farm dwellers, and their neighbors.  They checked how many of these bacteria were antibiotic resistant, and found that very few were resistant at the beginning of the study.

Then the farm introduced a chicken feed that had an antibiotic in it.  It was a tetracycline antibiotic, a drug commonly used in livestock for growth promotion and a drug also used commonly in medicine to treat a variety of common human ailments.

What the group found was this:  Within one week of introducing the tetracycline supplemented feed, the chickens’ intestinal bacteria contained almost entirely tetracycline-resistant organisms.  The intestinal bacteria of the people who worked on the farm also started to become resistant, but more slowly – it took about 3-5 months to see the changes start to occur.  Within 5-6 months however, 31.3% of weekly fecal samples from farm dwellers contained almost entirely (> 80%)  tetracycline-resistant bacteria.

The research group also evaluated the effects of removal of the tetracycline-supplemented food from the farm.  Astoundingly, within six months after removal of the antibiotic in the feed, no detectable tetracycline-resistant organisms were found (less than 1%) in the vast majority of the farm workers (8 of 10).

This study illustrated how the simple addition of tetracycline antibiotics into chicken feed can breed antibiotic resistant bacteria at first in the chicken, then the farmers, and how the removal of the antibiotic can significantly decrease the number of resistant bacteria. 

There is another aspect of this study that is intriguing:  After three to four months’ exposure to tetracycline on the farm, the chickens and humans excreted bacteria that were resistant to multiple antibiotics including four OTHER types of antibiotics (streptomycin, ampicillin, carbenicillin and sulfonamides).  Resistance to multiple antibiotics was found in more than 50% of the E. coli strains from chickens that ate the tetracycline feed for more than ten weeks. The study found that the genes that provided the resistance for the OTHER antibiotics were linked to the genes that provided resistance to the tetracycline antibiotics.  So tetracycline selected for not only tetracycline resistance, but for resistance to other antibiotics.

The authors concluded, and rightfully so, that “The rise in frequency of resistant organisms in our environment is the obvious result of antibiotic usage”, that “The only means to curtail this trend is to control the indiscriminate use of these drugs” and that these data speak “strongly against the unqualified and unlimited use of drug feeds in animal husbandry and speak for re-evaluation of this form of widespread treatment of animals.”

Thirty-five years later, these statements are truer than ever.


You can access the study here (abstract free, full-text requires a subscription): S.B. Levy, G.B. FitzGErald, A. B. Macone. 1976. Changes in intestinal flora of farm personnel after introduction of a tetracycline-supplemented feed on a farm.  New England Journal of Medicine. 295 (11): 583-588.

More information on this topic is available on the NRDC website and at Peter Lehner's blog.

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Kari O'BrienMay 31 2011 04:38 PM

This is an excellent blog! I will be coming back to find more great articles for our Twitter feed.

Thank you for your thorough and thoughtful writing.

Kari O'Brien
Community Manager
The Science Advisory Board

Michael MJun 1 2011 12:40 PM

Finally some focus on the real source of the problem! Too often antibiotic resistance is blamed on over-presc­ription to sick people. But the reality is that these industrial uses dwarf the use of antibiotic­s in human medicine. There is a good summary of all the factors involved in the developmen­t and spread of antibiotic resistance @ http://the­scienceofa­­w-do-bacte­ria-become­-resistant­-to-antibi­otics/

SusanKJun 3 2011 11:03 AM

So how did antibiotic resistance jump to the farmers? Were they eating the chickens that were fed the tetracycline feed? Thank you for explaining this as it is not clear in the article.

Courtney CJun 3 2011 12:18 PM

Is the population at risk of becoming resistant to antibiotics from the consumption of chickens from the store that have been treated with the antibiotics?

Gina SolomonJun 3 2011 02:53 PM

This study didn't evaluate exactly how the antibiotic resistance jumped to the farmers and to the local community members in this particular case -- the study just showed that it happened.

Other studies have found antibiotic-resistant bacteria in the animal feces, in the air downwind of confined animal facilities, in the nearby soil and water, and in the meat sold in stores, so there is no one pathway by which these 'super-bugs' escape from the facilities.

The population is most at risk of getting sick from an antibiotic-resistant form of bacteria if they eat chicken from the store that was treated with antibiotics. Studies have shown that antibiotic-treated chicken are far more likely to have contaminated meat compared to organically-raised chicken, and that the contamination is more likely to be drug-resistant.

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