Wednesday 19 May 2021

When beneficial bacteria start to cause harm


Research reveals insights into how the body maintains balance with 'good' gut bacteria that allows these microbes to flourish in the intestine but keeps them out of tissues and organs where they're not supposed to be.


The research reveals insights into how the body maintains this balance. Investigations with mice demonstrate that early life is a critical time when the immune system learns to recognize gut bacteria and sets up surveillance that keeps them in check. Defects in these mechanisms could help explain why the immune system sometimes attacks good bacteria in the wrong place, causing the chronic inflammation that's responsible for inflammatory bowel disease.


The scientists found that specialized immune cells capture pieces of bacteria and carry them over long distances, from the gut to the thymus. Located in the chest, above the heart, the thymus is a gland responsible for "educating" immune T cells. Delivery of the cargo prompts the thymus to produce T cells that are targeted to the microbiota. Then, the T cells exit the thymus to surveil lymph nodes, the gut, and other sites in order to keep the bacteria under control.


Finding DNA from the bacteria in the thymus and lymph nodes was the first clue that microbiota migrated to those sites. To trace their journey, the researchers used specially engineered mice whose cells fluoresced red after being exposed to a laser. In the two days following photoactivation, red gut cells eventually made their way to the thymus, lymph nodes, and spleen.


These processes were robust within the first weeks of life but waned significantly by the time the mice reached adulthood. The results suggest that building immunity to microbiota also builds protection against harmful bacteria the body has yet to encounter.




Daniel F. Zegarra-Ruiz, Dasom V. Kim, Kendra Norwood, Myunghoo Kim, Wan-Jung H. Wu, Fatima B. Saldana-Morales, Andrea A. Hill, Shubhabrata Majumdar, Stephanie Orozco, Rickesha Bell, June L. Round, Randy S. Longman, Takeshi Egawa, Matthew L. Bettini, Gretchen E. Diehl. Thymic development of gut-microbiota-specific T cells. Nature, 2021; DOI: 10.1038/s41586-021-03531-1


Posted by Dr. Tim Sandle, Pharmaceutical Microbiology Resources (

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