Changes in our microbiome – all the microbes that live in or on us – are thought to play a role in autoimmune diseases like lupus and Crohn’s disease. Several scientific studies have identified a few prime microbial suspects that can cause disease if they acquire new traits or end up in parts of the body where they shouldn’t. One such bacteria, called Enterococcus gallinarum, is normally found in our gut but can be harmful if it spreads elsewhere in the body. 

A recent study from Yale School of Medicine found that E. gallinarum can evolve inside mice at risk for autoimmune diseases and gain the ability to spread from the gut to the liver. In some mice, two different populations of E. gallinarum developed over time – one in the liver and the other in the gut. Liver bacteria were more difficult to detect and kill by the immune system. Mice with liver bacteria also had more liver inflammation and weaker intestinal walls, which can allow more bacteria to leave the gut and cause inflammation in other organs. These results also held true when the researchers transplanted the liver and gut bacteria to mice that had never encountered bacteria before, called “germ-free mice.” 

This study helps explain how some harmless bacteria can become harmful later in a person’s life through random changes in the bacterial genetic code. Autoimmune diseases usually take many years to develop, and not everyone who is at risk will develop them. The good news is that most of the gut bacteria could not migrate through the body. This means that bacteria that spread between people via stool are unlikely to make you sick right away. Scientists are still investigating the environmental factors that affect how fast E. gallinarum makes the transition from harmless to harmful and if that transition can be blocked. 

The lead author, Yi Yang, is a graduate student at Yale University. Dr. Noah Palm is an associate professor of immunobiology and a member of the Human and Translational Immunology Program at Yale School of Medicine. 

Managing Correspondent: Sanjana Kulkarni

Journal Article: Within-host evolution of a gut pathobiont facilitates liver translocationNature

Press Article: How Gut Microbes Can Evolve and Become DangerousTechnology Networks

Image Credit: National Human Genome Research Institute

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