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SUMMER 2005
Poultry Litter: Handle with Care
by Phil Williams

Spreading untreated chicken litter in the garden may be good for growing plants, but it’s a potential human-health hazard.

A recent study by University of Georgia scientists showed that litter in commercial poultry houses harbors microbes — in numbers much larger than previously thought — that collect and spread antibiotic-resistance genes.

At the heart of the problem are integrons, which are large clusters of distinct and inherently stable antimicrobial-resistance genes carried and exchanged by bacteria.

“Finding such a huge reservoir of integrons,” said Anne Summers, a microbiologist from UGA who led the study, “explains how clusters of resistance genes spread so rapidly and persist in bacterial communities even after antibiotic use concludes.”

Poultry litter — material spread on the floor of poultry houses — absorbs wastes such as feces, uric acid, skin and feathers; and it contains insects and small invertebrates as well. Summers and her colleagues collected litter samples from broiler houses during a 13-week period.

What the researchers discovered was startling: One integron type typically found in intestinal bacteria, such as E. coli and Salmonella, was up to 500 times more abundant in litter than these bacteria themselves.

A bit of microbial sleuthing revealed that the major source of this integron was not only the expected intestinal bacteria but also Gram-positive bacteria, which are much more abundant in litter. Close relatives of the Gram-positive bacteria found in litter include microbes that cause diphtheria and staph infections. The presence of identical integron genes in both Gram-positive bacteria and E. coli, a Gram-negative bacterium, indicates active exchange among these otherwise unrelated microorganisms, Summers said.

Summers and her colleagues are investigating whether Gram-positive bacteria from pets and people are reservoirs for these resistance-gene-clustering systems as well. The results will further scientists’ understanding of where antibiotic resistance develops and how fast and far it spreads. It also could help reform all antibiotic use, not just in agriculture.

The research, published in the May 4, 2004, issue of Proceedings of the National Academy of Sciences, was supported by a grant from the National Research Initiative of the U.S. Department of Agriculture.

For more information, contact Anne Summers at summers@uga.edu.