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Animals, antibiotics and resistance

Livestock growers add almost 25 million pounds of antibiotics and similar medications to livestock feed each year.
Feeding tons of antibiotics to healthy animals, a practice under review in the United States and banned in the European Union, might increase the growth of farm animals.
In a classic biology textbook example of evolution through natural selection, such use of antibiotics often increases drug resistance in the targeted bacteria.
Just as Jonathan Swift noted in 1733 that fleas have fleas that “bite ’em … ad infinitum,” bacteria have microbes that bite ’em, too. And a new study indicates that antibiotics in livestock feed might affect those microbes as well, indirectly increasing antibiotic resistance in the bacteria they infect.
The “fleas” biting the bacteria are phages, small clumps of genes that invade bacteria and other host cells. Phages start life — if it can be called that — as prophages inside bacteria. They replicate with their host bacterium and can incorporate genes from it in their genome. They also can insert their genes into the bacterium’s genome. Various environmental triggers can cause prophages to hijack their host bacterium’s DNA-replication process, induce production of multiple copies of themselves, rupture the bacterium and explode into its environment. The new phages then can invade other bacteria, initiating the process once again.
Bacteria and phages can replicate in a farm animal — or a farmer for that matter — thousands of times in a few days. Not all phages from a bacterium carry the same genes. Some might include a gene or two from the host bacterium’s chromosome.
Phages contain only a handful of genes, and a little slop can occur in the process.
A phage’s extra gene from a recent bacterial host might be a godsend for its new host. If that extra gene finds its way into the genome of the new bacterial host, the result can be what microbiologists refer to as “horizontal gene transfer.”
This is when genes from one individual end up in another that is not its offspring. The recipient might not even be the same species.
The new research suggests that some antibiotics in livestock feed can somehow enhance the prophase hijacking process. This can lead to increased horizontal transfer of genes — including ones for antibiotic resistance — among bacteria in a pig, for example, or maybe a whole food lot.
Genome sequencing reveals that horizontal gene transfer occurs regularly among microbes. Recognition of the process as a powerful force of evolution has grown with our discovery of its frequency now and throughout history.
In terms of public health, the process can generate new strains of common disease microorganisms. These new strains can infect us and our food organisms.
Heather Allen and her colleagues at the U.S. Department of Agriculture’s National Animal Disease Center described this work last week in mBio, an online publication of the American Society for Microbiology.
Allen explained that the work suggests vigilance not only toward the bacteria exposed to antibiotics in livestock feed but also of the flea-like phages that bite ’em ad infinitum.

Steve Rissing is a biology professor at Ohio State University.
steverissing@hotmail.com

http://www.dispatch.com/content/stories/science/2011/12/04/animals-antibiotics-and-resistance.html

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