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Probiotic bacteria block harmful microbe

A floating market in Thailand selling fruits and vegetables, which are a possible source of probiotic Bacillus spores. izzetugutmen/iStock/Thinkstock

Your body is home to thousands of species of bacteria and other microbes. Many live in the intestine, or gut. Microbes interact in the gut in complex ways. They can be helpful, neutral, or harmful depending on the conditions in your gut, which affect microbes’ growth and activity. You can host harmful bacteria without any signs of disease. But if conditions are right, an infection can develop.

Staphylococcus aureus, for example, can live in the nose or gut without causing any harm. But it can also cause a variety of diseases, from skin infections to fatal pneumonia and sepsis. An antibiotic-resistant form of this bacterium called methicillin-resistant S. aureus (MRSA) is now one of the most common infections in hospital patients. MRSA may also cause severe infections in otherwise healthy people who aren’t in a hospital.

Bacterial infections that can’t be cured with antibiotics pose the biggest threats. Researchers have been testing the use of beneficial microbes, or probiotics, to find a way to prevent S. aureus infections. Bacillus bacteria, which are found on vegetables grown in soil, are used in many probiotic formulations. They form spores that can survive the harsh environment of the gut.

An international research team led by Dr. Michael Otto at NIH’s National Institute of Allergy and Infectious Diseases (NIAID) set out to investigate whether other microbes such as Bacillus can affect the ability of S. aureus to colonize the gut. Researchers from Mahidol University and Rajamangala University of Technology in Thailand collaborated with the NIAID scientists on the project. Results appeared in Nature on October 10, 2018.

The researchers recruited 200 volunteers in rural Thailand, where processed foods and antibiotic use are less common than in urban areas. They found Bacillus in 101 of the stool samples. They found S. aureus in 25 stool samples and 26 nasal samples. However, they found no S. aureus in any of the samples from people with Bacillus.

Laboratory studies showed that Bacillus didn’t affect the ability of S. aureus to grow. Instead, it interfered with quorum sensing, the mechanism that bacteria use to detect their population levels and coordinate their activity. The Bacillus from the volunteers’ stool samples all inhibited the S. aureus quorum-sensing system in mice and prevented S. aureus from colonizing their intestines.

Further work revealed that the Bacillus made a class of substances called fengycins that inhibited S. aureus’ quorum-sensing system. The research team fed Bacillus spores to mice with S. aureus in their guts. Bacillus given every two days eliminated S. aureus. However, when the ability to produce fengycins was removed from Bacillus, it no longer stopped S. aureus from growing in the gut.

“Probiotics frequently are recommended as dietary supplements to improve digestive health,” NIAID Director Dr. Anthony S. Fauci says. “This is one of the first studies to describe precisely how they may work to provide health benefits. The possibility that oral Bacillus might be an effective alternative to antibiotic treatment for some conditions is scientifically intriguing and definitely worthy of further exploration.”

The scientists next plan to test whether a probiotic product with Bacillus can eliminate S. aureus in people. “Ultimately, we hope to determine if a simple probiotic regimen can be used to reduce MRSA infection rates in hospitals,” Otto says.