Trading floor

Natural History, July-August, 2004 by Stephan Reebs

There's more going on in a chicken house than just the raising of chickens. Anne O. Summers and Sobhan Nandi, both microbiologists at the University of Georgia in Athens, and their colleagues decided to check out the bacterial residents of the wood shavings that cover the floor for the six-week lifespan of a flock of broiler chickens. What they found was ecological bad news.

Since the 1940s, the feed given to farm animals has routinely been dosed with antibiotics, which promote weight gain and seem to minimize chronic infections among the crowded animals. As with humans, however, the antibiotics have also spawned drug resistance in bacteria hosted by the animals. In addition, as investigators have learned in recent years, bacteria--even unrelated bacteria--do exchange genes, and so one microbe's genes for drug resistance can get passed around to its neighbors. And a floor covered in six-week-old wood shavings brimming with chicken waste is loaded with multiple species of microbes, and is thus a perfect place for a swap meet.

In the chicken-litter bacteria, the microbiologists found a huge reservoir of mobile genetic organizers called integrons, which assemble genes into clusters--including the genes that code for antibiotic resistance.

Carrying out their appointed task in both the chickens and the chicken litter, the integrons enable the bacteria to become resistant to many antibiotics. The vast majority of those bacteria are harmless, but those that are pathogens can cause obdurate infections. ("Gram-positive bacteria are a major reservoir of Class 1 antibiotic resistance integrons in poultry litter," Proceedings of the National Academy of Sciences 101:7118-22, May 4, 2004 --S.R.