Great Increase Seen In Antibiotic-Resistant Bacteria In Human Colon

Since less than a third of colonic bacteria collected 30 years ago contained the gene, the new finding shows how rapidly resistance genes can spread through the bacterial population of the colon, said Abigail Salyers, a professor of microbiology. Her research -- not yet published -- also shows that other resistance genes are now moving along with the tetracycline resistance gene.

Speaking at the Sixth Annual Midwest Microbial Pathogenesis Meeting in Milwaukee in September, Salyers said her findings should be a wake-up call for medical researchers to survey how extensive the problem has become. The government, she said, must decide what agency or agencies will fund such research, which can be done rapidly and cheaply.

Salyers' lab focused on Bacteroides -- a strain of bacteria not harmful in the lower intestines but the cause of often life-threatening infection when they escape during surgery or trauma. Bacteria, she said, have produced a blueprint for resistance that can be used to spread resistance to newer antibiotics. To show the resistance gene sequence was the same in contemporary and pre-1960s samples, Salyers used a process called DNA hybridization. A copy of a sequence is labeled with a fluorescent molecule and watched as it seeks out its complement in other bacteria. Because it finds a fit, "this proves the gene is being transferred as opposed to mutation," she said. "The DNA sequences were the same."

In Salyers' research, funded by the National Institutes of Health, tetracycline proved to be the vehicle of transfer. It its absence, bacteria did not mix their DNA during initial exposure, but they did so rapidly when minuscule amounts of the antibiotic were added to the mix.

"The exchange appears to be a form of bacterial sex, a very promiscuous exchange of genetic material that can involve different species of bacteria," Salyers said. "They don't transfer their entire genome, but they will transfer segments of their DNA."

Concern about antibiotic resistance is growing worldwide. Resistance to tetracycline has existed for many years, but it and other antibiotics still are prescribed widely, especially in operating rooms and nursing homes, and they are used extensively in agricultural production. Although the spread of resistance genes is being driven mainly by the overuse of antibiotics in human medicine, Salyers said, questions now are being raised about the widespread use of antibiotics in agriculture.

"There is evidence that resistance genes in bacteria from both cows and humans have the same gene sequence, suggesting they can exchange these genes," she said. "Resistant bacteria are now entering our food supply and might donate their resistance genes to human bacteria. I am not saying that the agricultural use of antibiotics is causing the problem, just that it is a possible factor. So let's find out."