News article

Relationship between gut bacteria, blood cell development helps immune system fight infection

Posted:  Tuesday, March 18, 2014

The human body has a complicated relationship with microbial life. There are good gut bacteria and bad bacteria. Caltech professor of biology and biological engineering Sarkis Mazmanian and his team are keen on studying the thousands of bacteria that reside in the human body and keep us healthy. In his previous studies on mice, he found that restoring large populations of beneficial bacteria can help improve the symptoms of inflammatory bowel disease, multiple sclerosis, and even autism. He and his team found that the good bugs help the immune system to fight infections and harmful bacteria with the help blood cell development.

An article published in the journal Cell Host & Microbe reported that researchers have found that good gut bacteria is important for the growth of innate immune cells- specialized types of white blood cells that serve as the body's first line of defence against invading pathogens.

Blood cell development also takes place in the spleen and bone marrow. Researchers found that germ free mice (born without gut bacteria) had fewer immune cells as compared to healthy mice who had a normal population of gut microbial. The germ free mice had fewer white blood cells - specifically macrophages, monocytes, and neutrophils as compared to healthy mice.

“It's interesting to see that these microbes are having an immune effect beyond where they live in the gut,” says Arya Khosravi, a graduate student in Mazmanian's lab, and first author on the recent study. “They're affecting places like your blood, spleen, and bone marrow—places where there shouldn't be any bacteria.”

Khosravi and his team wanted to find out if the reduction in blood cell development would make the germ free mice immune system weak and unable to fight infections created by the harmful bacteria Listeria monocytogenes—a well-studied human pathogen often used to study immune responses in mice.

The germ free mice’s immune system was unable to fight the infection, whereas, healthy mice immune system fought the infection. However, when gut microbes that are normally present were injected into the germ-free mice, the immune cell population increased and were able to survive the infection.

Researchers dossed the healthy mice with brad spectrum antibiotics that killed both the good and bad bacteria and then the injected Listeria back into their immune system. However, this time around they weren’t able to fight the infection successfully.

“For example, when patients are put on antibiotics for something like hip surgery, are you damaging their gut microbe population and making them more susceptible to an infection?” said Mazmanian.

The researchers summed up the study stating that, healthy population of gut microbes can strengthen the immune system and help the immune system fight infection and provide a preventative alternative to antibiotics.

Khosravi said, "Today there are more and more antibiotic resistant superbugs out there, and we're running out of ways to treat them. Limiting our susceptibility to infection could be a good protective strategy."