Antibiotic overuse may be bad for body's good bacteria

Antibiotic overuse doesn't just lead to drug-resistant superbugs, it may also permanently wipe out the body's good bacteria.

Good bacteria in the gut help people in many ways, including helping make vitamins and boosting immunity. Some researchers think that killing them off with antibiotics may be contributing to rises in chronic health conditions such as obesity, asthma, and cancer.

A commentary published in the latest issue of Nature calls for more careful prescribing of antibiotics, particularly in pregnant women and babies, who are just establishing their colonies of good bacteria in the gut. The commentary was written by Martin Blaser, MD, who heads the department of medicine at New York University's Langone Medical Center.

Growing Antibiotic Use

Blaser points out that in a developed country like the U.S., the average child gets 10 to 20 courses of antibiotics by age 18.

Blaser says there's no question that antibiotics, when given appropriately, save lives.

But they're not always used appropriately. Studies have shown that doctors often prescribe antibiotics before they know whether an infection is viral or bacterial. If the problem is a virus, antibiotics don't help.

For example, a 2003 study published in the Journal of the American Medical Association showed that doctors prescribed antibiotics for more than 60% of adults with upper respiratory tract infections, which are usually caused by viruses.

And a 2010 study published in Infection Control and Hospital Epidemiology found that doctors sometimes kept patients on antibiotics even after tests showed they had viral infections.

Disappearing Bacteria

As antibiotic use has increased, studies have shown that the kinds of bacteria that we live with are changing.

Blaser says he thinks some bacterial species that live in our bodies are going extinct. He points to the example of H. pylori, the bacterium best known for causing ulcers.

One hundred years ago, H. pylori was the main microbe that people carried in their stomachs.

Today, studies show that less than 6% of children born in the U.S., Sweden, and Germany carry that organism. That shows that the species is disappearing from its human hosts.

The health consequences of that aren't clear. Neither is the reason for the disappearance.

Blaser's point is that if one species of gut bacteria is disappearing, others are, too.

"We know there are impacts" from antibiotic use, "impacts we don't even have the tools yet to study," says Lita Proctor, PhD, who coordinates the Human Microbiome Project at the National Institutes of Health.

Links to Chronic Disease

Blaser says it's clear those impacts go far beyond infections.

Farmers, for example, discovered decades ago that animals fed small amounts of antibiotics, below the doses used to treat infections, gain more weight.

"That works so well that it accounts for more than half of antibiotic use in the United States," Blaser says. "Since it works in chicken, turkeys, cows, and sheep, I presumed it would work in mice, and it does."

Antibiotics, he thinks, may also be contributing to obesity in humans, though Blaser says no one yet understands how.

Beyond obesity, he says studies have shown that a child's risk for inflammatory bowel disease increases with the number of courses of antibiotics taken.

He also says antibiotics may be a factor behind the unexplained rises in allergies, asthma, and type 1 diabetes in children.

But his commentary in Nature does not prove that. Blaser's paper is his opinion, not a new study.

Other research, directed by the Human Microbiome Project, which aims to catalogue and understand the microorganisms that live in the body, has suggested that a bacterial environment that's out of balance in the stomach and esophagus may contribute to cancer.

An out-of-balance bacterial environment in the digestive system may lead to inflammation, and inflammation may cause changes in cells that lead to cancer, Proctor says.

Protecting Good Bacteria

Though such research is in its early stages, some experts say Blaser's call to action is on target.

"There's really only a limited number of studies that have been done on this so far, but I think we're going to see more because I think it's going to be a big deal for us to understand this," says George Weinstock, PhD, a professor at the Genome Institute at Washington University in St. Louis.

But "the evidence is all circumstantial," Alexander Khoruts, MD, associate professor of medicine at the University of Minnesota, tells WebMD in an email. He says there is an "urgent need" to see if overuse of antibiotics are causing the problems Blaser describes, and that there is "little doubt that antibiotics have been overused in clinical and veterinary medicine, and farming practices." But Khoruts says he is "less than convinced that we have enough data to revise guidelines for solid indications for antibiotic usage."

Because studies have already shown that antibiotic overuse is contributing to the problem of drug resistance, Blaser believes it's not rash to act in advance of more definitive science.

"We all know that there's antibiotic overuse early in life, and I'm giving us yet another reason why we have to control it," he says.

Among measures he'd like to see in place are better diagnostic tests that would help doctors more quickly pinpoint the cause of an infection.

Blaser says it's critical to swap the widely used broad-spectrum antibiotics, which can kill many different kinds of bacteria, for agents that more narrowly target the bug that's causing the disease.

And he says effective probiotics are needed to replace lost good bacteria.