Joining the Resistance: Drug-immune microbes waft over hogs

The air in a modern farm building contains many bacteria that are invulnerable to several antibiotics, according to a new report. The finding suggests that drug-resistant microbes can spread by air from animals to people.

WHEN PATHOGENS FLY. Drug-resistant bacteria can permeate the air in livestock facilities, where animals are fed antibiotics. K. Hammond/USDA

Bacteria on farms develop resistance to antibiotics commonly used to accelerate growth of or prevent and treat infections, says environmental microbiologist Kellogg Schwab of Johns Hopkins University in Baltimore. Those bacteria threaten public health, he says, because the same drugs or chemically related ones are administered to people.

In past research, scientists found drug-resistant microbes in workers who had contact with animals, in streams near dumping areas for animal waste, and in meat. Recently, some researchers working at swine-growing facilities also identified antibiotics in settled dust, which includes feed and excrement particles, and others found one drug—and bacteria resistant to it—in exhaust air emitted from buildings containing livestock (SN: 7/5/03, p. 5: Available to subscribers at Suspended Drugs: Antibiotics fed to animals drift in air).

To better understand the potential for aerial spread of drug-resistant microbes, Schwab and his four Johns Hopkins colleagues collected air samples in two mid-Atlantic buildings each housing about 1,500 swine. Schwab declined to identify the exact locations of the facilities.

The researchers isolated airborne bacteria within the buildings and cultivated the microbes using techniques that support several types of bacteria. They then exposed the microbes to five antibiotics: erythromycin, clindamycin, tetracycline, virginiamycin, and vancomycin. Except for vancomycin, each drug is given to animals and it or a chemical relative is used for treating people.

The researchers found no microbe that was resistant to vancomycin. However, all 124 tested microbes were resistant to at least one of the other drugs, and 93 percent of the samples were resistant to three or all four compounds, Schwab and his colleagues report in an upcoming Environmental Health Perspectives.

The bacteria that Schwab’s group detected aren’t highly pathogenic, but they can cause ear, nose, and throat infections, comments James A. Zahn of Dow AgroSciences in Indianapolis.

Kelley J. Donham of the University of Iowa in Iowa City points out that once these microbes get into a person’s body, they could transfer resistance-conferring genes to more-dangerous pathogens.

The greatest risks face workers within swine houses, but drug-resistant bacteria may also threaten people downwind of farms, say Zahn and Donham. One drawback of the study is that it provides no data on which drugs had been used—or how much—in the facilities tested, they note.

Some farms cut down on the odors emitted from swine houses by directing exhaust through bales of hay or other materials. Such filters might also trap airborne drug-resistant bacteria, Zahn says.

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