We Now Know HOW Antibiotics Kill off Good Bacteria in the Gut
Leading to a common and often fatal health problem
A single course of antibiotics can cause Clostridium difficile, or C. diff – a bacterium that can lead to a condition known as Clostridium difficile colitis – to flourish in the gut, according to researchers at North Carolina State University.
The discovery was made during experiments with mice. The scientists say the antibiotics were found to kill off bacteria responsible for altering bile acid.
C. Diff is a dormant spore bacteria that must germinate and become growing bacteria to colonize the gut. Certain antibiotics lead to a higher risk of C. diff, especially in hospital patients. For the study, Casey Theriot, an assistant professor of infectious disease at NC State, sought to find out just how C. diff spores interacted with natural bacterial environment (microbiota) of the gut.
“We know that within a healthy gut environment, the growth of C. diff is inhibited,” Theriot says. “But we wanted to learn more about the mechanisms behind that inhibitory effect.” 
Primary bile acids are created in the liver from cholesterol and help the body to digest food and absorb fat. Bile also controls the metabolism of lipoproteins, drugs, glucose, and energy. These acids travel through the intestinal tract to the large intestine where they are converted into secondary bile acids by other bacteria.
Scientists identified 26 primary and secondary bile acids in mice and recorded their levels before and after treatment with an antibiotic.
The team added C. diff spores to concentrations of the acids and discovered that primary bile acids allow spores to germinate, regardless of antibiotic treatment, including the broad-spectrum antibiotics cefoperazone, clindamycin, and vancomycin. 
When the spores passed into the mice’s large intestine, where normal gut bacteria create secondary bile acids, the researchers found that those secondary bile acids stopped C. diff from growing. After antibiotic treatment – which killed those bacteria and the secondary bile acids – the C. diff was able to quickly grow.
“These findings are a first step in understanding how the gut microbiota regulates bile acids throughout the intestine,” says Theriot. “Hopefully they will aid the development of future therapies for C. difficile infection and other metabolically relevant disorders such as obesity and diabetes.” 
In a February 2015 press release, the U.S. Centers for Disease Control and Prevention (CDC) said that C. diff caused nearly half a million infections in the United States in a single year. About 29,000 people died within 30 days of diagnosis, and about 15,000 thousand deaths were believed to have been directly caused by C. diff infections.
The agency also revealed that C. diff – which causes watery diarrhea, fever, loss of appetite, nausea and belly pain and tenderness – had become the most common microbial cause of healthcare-associated infections in U.S. hospitals, costing up to $4.8 billion each year in excess healthcare costs for acute care facilities alone.
 Science 2.0
 NC State News
Julie Fidler is a freelance writer, legal blogger, and the author of Adventures in Holy Matrimony: For Better or the Absolute Worst. She lives in Pennsylvania with her husband and two ridiculously spoiled cats. She occasionally pontificates on her blog.