UniSci - Daily University Science News

   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 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
   condition, 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 wide spread 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.

   The American Society for Microbiology is the largest single life
   science society, composed of over 42,000 scientists, teachers,
   physicians, and health professionals. Its mission is to promote
   research and training in the microbiological sciences and to assist
   communication between scientists, policymakers, and the public to
   improve health, economic well being, and the environment.

   20-Feb-2001

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