18 February 2001

Human Gut Breeding Ground For Antibiotic Resistance

by Kate Melville

Bacteria in your gut could be exchanging genetic material, including antibiotic resistance genes, with bacteria that are simply passing through on your food, say researchers from the University of Illinois. The study, which appears in the February 2001 issue of the journal Applied and Environmental Microbiology, is the first to provide evidence of this phenomenon in the human digestive tract.

"What we've shown is antibiotic resistance genes in nature can move about in the human colon," says Abigail Saylers, the senior investigator. "A surprising amount of gene transfer is occurring in the human colon. There's a lot of bacterial hanky-panky going on in there." These findings are important given recent concerns over the safety implications of antibiotic-resistant bacteria in foods and the likelihood that such bacteria may transfer resistance genes to human intestinal bacteria.

Scientists have long believed that bacteria in the intestines, known as Bacteroides, could exchange genetic information. Under certain conditions bacteria might copy and pass specific genes on to other bacteria which incorporate them into their genetic makeup, a process known as conjugation or horizontal gene transfer. Laboratory experiments in the past few years have supported this theory. "The question we asked is to what extent is there gene transfer in nature? In the lab you are doing these experiments under what you hope are ideal conditions," says Salyers. "Just because it transfers in the lab it doesn't mean it will transfer in nature."

Salyers and her colleagues compared Bacteroides strains collected before 1980 by the Anaerobe Laboratory at the Virginia Polytechnic Institute with ones collected from ordinary people and medical centers across the United States in the late 1990s, focusing on antibiotic resistance genes. They found a significant increase in resistance to the antibiotic tetracycline was caused by a single gene, from 23% of the samples in the 1970s to more than 80% in the 1990s. They also found a significant, though smaller, increase in erythromycin resistance due to only two genes.

"Because the same resistance gene was found in a variety of Bacteroides species, we believe that the increase over the past three decades is due to horizontal gene transfer," says Salyers.

These findings raise the question of whether antibiotic resistance genes in bacteria in the food supply could be transferred to bacteria in the human gut.

"For example, you feed a pig antibiotics for a large part of its life. The bacteria in the pig's digestive tract become resistant to antibiotics. You slaughter that pig and send it to market. The bacteria end up on the meat products. The consumer then takes that product home and consumes those bacteria," says Salyers. "Horizontal transfer can take place in as little as an hour."

Antibiotic resistance in Bacteroides does pose a threat to human health. These bacteria often cause post-surgical infections, and these infections are increasingly becoming resistant to antibiotics commonly used to treat them, such as clindamycin, which is in the same family of antibiotics as erythromycin. Another concern is that Bacteroides may pass these antibiotic-resistance genes on to other bacteria that can cause human disease.

"Once these genes get loose it's like letting a genie out of a bottle," says Salyers. One possible cause for all this "bacterial sex" could be the antibiotics themselves, says Salyers. "We know from laboratory studies that one catalyst that triggers horizontal gene transfer is the antibiotic tetracycline. Tetracycline is like an aphrodisiac for Bacteroides, causing it to transfer its resistance genes. This suggests to us that this orgy of horizontal gene transfer may have been due to widespread of tetracycline in humans over the last several decades." Salyers warns that her group's research should not signal alarm but should be a starting point for futher research.

"Studies of this sort should be done on other types of bacteria. We need to see if this is something peculiar to Bacteriodes or, if it's as I suspect, we're going to find this level of horizontal gene transfer is taking place in other types of bacteria, both in and out of the human intestines," says Salyers, who is also president-elect of the American Society for Microbiology.