Probiotic or commensal, good or bad?

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Researchers at the University of Vermont (UVM) have found that a type of bacteria called Lactobacillus reuteri, commonly used in probiotics, can increase the severity of disease in a mouse model of multiple sclerosis (MS), an autoimmune disease that is the leading cause of non- Sclerosis is traumatic neurological disability in young adults. But don’t throw away your yogurt just yet.

Recent evidence suggests that the gut microbiome modulates risk of MS, but the specific types of bacteria and the role of host genetics are not clear. In many chronic diseases, scientists have found that the state of the gut microbiome is changing, increasing the possibility of rebalancing the gut microbiome for the treatment or even prevention of disease, and further boosting the popularity of probiotic products, which are typically made from live bacteria with alleged health benefits; presumably to “sow” or “restore” the gut microbiome.

The results, published in the journal Proceedings of the National Academy of Sciences, show that complex interactions between host genetics and environmental factors increase susceptibility to multiple sclerosis, and suggest that antibiotic or probiotic strategies to prevent or treat multiple sclerosis should consider host genetics, the gut microbiome already present and the timing or type of intervention.

“Our gut bacteria are part of a complex ecosystem that has great potential for preventing, treating and / or diagnosing chronic diseases. However, many scientists have taken a unified approach, and our research suggests that this is unlikely to work,” said Dimitry Krementsov, who led the study. “Instead, a more personal approach is required.”

Krementsov, assistant professor in the Department of Biomedical and Health Sciences at the UVM, and researchers modulated the gut microbiome of genetically different mice with high or low susceptibility to experimental autoimmune encephalomyelitis (EAE), a common animal model of multiple sclerosis. To determine which characteristics of these microbiomes made them more susceptible to MS-like diseases, the researchers isolated different types of bacteria from the complex mixture and transferred them one by one to new mice.

The results were surprising: one of the guilty parties identified was Lactobacillus reuteri, a normal commensal inhabitant in the intestines of humans and mice, which is also a widely used probiotic. This seemed to label this bacterium as the “bad guy,” but the researchers also found that this bacterium was originally from a genetically distinct mouse host that was usually resistant to MS-like diseases. This suggests that the host’s genetics determine whether a particular microbiome status is “good” or “bad”. The results of the study raise a critical question: are probiotics generally good or bad? The results suggest that the answer most likely depends on the context, especially the person receiving the probiotic.

The study’s authors point out reservations: They did not examine probiotics per se, but rather how normal commensal bacteria interact with the host. Probiotic bacteria are usually grown in a laboratory or food production facility and taken orally in massive daily doses. These probiotic bacteria are also very different genetically from their counterparts, and the authors are currently investigating this possibility. In addition, what is “bad” for an autoimmune disease like MS may actually be “good” in other contexts, such as a stronger immune response to infections. There are numerous examples of genes that promote “good” immune responses to infections or vaccinations, but which in turn lead to a higher risk of autoimmune diseases. Finally, other scientists have used high-dose daily treatment with probiotic strains of lactobacillus to reduce MS-like illnesses in mice, although recent studies show that the probiotic bacteria don’t even have to be alive for this to work.

So the answer is, probiotics can be good or bad depending on the context. The next step is to find out what that “context” actually is.

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More information:
Theresa L. Montgomery et al., “Interactions Between Host Genetics and Gut Microbiota Determine Susceptibility to CNS Autoimmunity”, PNAS (2020). Provided by the University of Vermont

Quote: Intestinal bacteria in multiple sclerosis: probiotic or commensal, good or bad? (2020, October 19) Retrieved October 19, 2020 from

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