Antibiotic Crisis

"If antibiotics are overused or especially if they're misused then there's an increased selection for bacteria to become resistant," explains microbiologist Abigail Salyers, from University of Illinois at Urbana-Champaign.

Salyers's work has shown that decades of antibiotic use have bred — through the transfer of genetic material between bacteria — a frightening degree of drug resistance into our intestinal flora. This resistance might be harmless while the bacteria remain confined to their normal habitat, but it can prove deadly when those bacteria contaminate an open wound or cause an infection after surgery.

"If you're carrying a population of bacteria that is resistant to many antibiotics then your chances of acquiring a post-surgical infection, that would be very difficult to treat, are increased," she says.

As reported in Discover magazine, her team used DNA sequences to trace the spread of resistance genes among different types of human colon bacteria. "There's a very large population of bacteria there, in fact about half of the contents of your colon is bacteria," Salyers explains. "It's a very nice warm wet nutrient rich environment for them, and so conditions are very conducive to the transfer of genes." The researchers took a kind of forensic approach to answering the question of "how much of this resistance is going on and how widespread is it? How many different bacteria are involved?" So they searched for individual genes that conferred antibiotic resistance. "We have a fingerprint of those genes," Salyers explains. "And then we went and looked in many different bacteria that were isolated from the human intestine, and also some of the same bacteria that were isolated from human infections, and we asked the question how many of these bacteria have this particular resistance gene?"

They found the exact same DNA fingerprint sequence in bacteria of different species. "That almost had to have occurred as a result of transfer of the DNA from one species to the other," she says, comparing it to tracking currency. "If a dollar bill was in one person's hands and then a few years later it's in another person's hands you know that somehow the dollar bill went from one person to another if it has the same serial number," she explains.

They found our normally good gut bacteria are major gene traffickers.

"It's kind of like an Ebay for bacteria where you have bacteria trading antibiotic resistance genes very freely with each other," Salyers says.

In the case of one resistance gene, the researchers found that about 20 percent of the bacteria before 1970 carried this resistance gene, but by the 1990's over 80 percent were carrying it.

Gut bacteria image: CDC

What's more, the research team also discovered that one common antibiotic — tetracycline — promotes gene transfer. "If you think of the conjugative transfer of resistance genes as bacterial sex, you have to think of tetracycline as the aphrodisiac," Salyers says. "It raises the question of whether there are stimulating conditions that make it more likely that bacteria will go into this gene transfer mode."

She hopes understanding what conditions help resistance genes to spread will lead to ways to prevent it. "In a sense the genie is out of the bottle once the resistance gene starts circulating, and so the thing to do is to keep the genie in the bottle and not to have it get out in the first place," she says. "And so we need to worry about not just having new antibiotics to keep coming onto the market to solve the resistance problem, but also to preserve the antibiotics that we have."

In the meantime, the researchers say doctors are becoming more cautious about prescribing antibiotics, and stress that it's important to follow your doctor's instructions about taking them very carefully.

In addition, Salyers continues to advise government agencies such as the Food and Drug Administration and the Department of Agriculture to reduce the use of antibiotics in livestock feed, a practice banned throughout the European Union.