Scientists have been warning doctors for years to cut down on the amount of antibiotics they prescribe because overuse reduces the effectiveness of drugs to kill bacteria. Now, researchers at the University of Virginia School of Medicine have found another powerful reason to curb the use of antibiotics — they reduce the variety of microbes in the body's gut, which interferes with the immune system's ability to fight disease.

Researchers found that the use of antibiotics reduces the effectiveness of neutrophils, a type of immune cell that fights infections, and also weakens the body's intestinal barrier against invading organisms.

"Neutrophils play an important role as a first-line 'innate immune response' when foreign pathogens invade," said researcher Koji Watanabe, Ph.D. "We found that antibiotic disruption of the natural microbes in the gut prevented this from happening properly, leaving the gut susceptible to severe infection."

The researchers found that children with severe cases of ameobic colitis, a potentially deadly parasitic infection, had less diversity in their gut microbiome than children with less severe cases.

They then used lab mice to determine how the decrease in gut bacteria might be worsening the disease. They found that antibiotics disrupted the mice's gut microbiomes, decreasing the activity of neutrophils and blocking these important white blood cells from responding when needed.

As a result, the gut wasn't adequately protected, and invading microbes had a clear path into the body.

"I think the take-home is that this is another important reason not to use antibiotics unless they are clearly needed," said researcher Bill Petri, M.D., Ph.D., the chief of UVA's Division of Infectious Diseases.

"Unwise use of antibiotics not only increases the risk of multi-drug resistant bacteria and the risk of C. difficile infection but also impairs white blood cell function."

Two new British studies investigated ways to reduce antibiotic consumption. In one study, a patient was prescribed antibiotics but told not to take them immediately in hopes the infection would get better on its own and the antibiotics wouldn't be needed. The study found that the delay was as effective as giving them the drugs immediately.

The second study found that a five-day course of antibiotics was as effective as a 10-day course and was shown to be a successful way of reducing the use of antibiotics.

More than 150 million prescriptions for antibiotics are written in the U.S. each year, according to the Centers for Disease Control and Prevention. About 2 million people develop infections that are resistant to antibiotics, and at least 23,000 die as a direct result.