Chicken litter promotes resistance

Researchers may need to take a hard leek at what appear to be certain myths concerning the spread of antibiotic-resistant genes, according to a study by scientists at the University of Georgia, Athens. They found that poultry litter--a ubiquitous part of large broiler operations--harbors a vastly greater number of microbial agents that collect and express resistance genes than was previously thought. Apparently, waste left behind by flocks raised in industrial chicken houses is rich in genes called integrons that promote the spread and persistence of such clusters.

This is a serious and growing problem for farm animal operations and human health. Antibiotic use in treating disease and increasing feed efficiency has been a common part of, industrial farms for more than half a century. When antibiotic-resistant bacteria began to show up in hospitals in the 1950s, researchers initially believed that simply restricting the use of antibiotics on farms could reduce their prevalence among humans, but it has not been that easy.

"Over the past 30 years, we have learned this hope was unrealistic because we share both pathogenic and benign bacteria with other humans and animals, and because bacteria transfer genes among themselves," says microbiologist Anne Summers.

At the heart of the dilemma are integrons, which scientist--until now--have studied exclusively in such pathogenic bacteria as Salmonella and E. coli. The researchers wondered, however: Does the poultry production environment also harbor integrons that assemble these large groups of distinct genes? To find out, they collected samples of poultry litter from broiler houses regularly over a 13-week period. Utter begins as a bedding material of softwood shavings. By the time the flock is harvested, the shavings have become mixed with chicken feces, uric acid, skin, feathers, insects, and small invertebrates. Rich in minerals, poultry litter often is recycled for fertilizer, among other uses.

What they discovered was startling: One integron type, called intl1 (typically found in E. coli and Salmonella) was up to 500 times more abundant than these bacteria themselves were in litter. A bit of microbial sleuthing revealed that integrons also are carried by so-called Gram positive bacteria that are much more abundant in litter than the E. coli-type bugs, called Gram negative bacteria.

"... Integrons and resistance genes were abundant regardless of antibiotic use on the farms, suggesting that, once acquired, integrons are inherently stable, even without continual exposure to antibiotics."

Humans and animals have billions of bacteria in and on their bodies at any time, and even if resistance to a single antibiotic arises in a few of them through mutation, there still are several other antibiotics that can eliminate them. However, if bacteria in the same environment already are equipped with masses of genes conferring resistance to many antibiotics and readily can exchange these clusters, then the treatment options are limited.

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