Amyotrophic lateral sclerosis has long been treated as a disease of the brain and spinal cord. A study published in May 2026 in Cell Reports suggests that for one genetic subtype, the trouble may start much further south: in the gut, where certain bacteria churn out sugar molecules that hijack the immune system and send inflammation roaring toward the central nervous system.
The research, conducted by a team at Harvard Medical School led by immunologist Aaron Burberry, identifies bacterial glycogen-like polysaccharides as a concrete trigger for the runaway immune response seen in mice engineered to lack the C9orf72 gene. C9orf72 repeat expansions are the most common genetic cause of ALS, accounting for roughly 40 percent of familial cases in populations of European descent. If the pathway holds up in human research, it could shift therapeutic strategy away from the brain alone and toward the trillions of microbes living in the intestine.
A single gut bacterium sets off a chain reaction
ALS destroys the motor neurons that control voluntary movement, gradually stripping away the ability to walk, speak, swallow, and eventually breathe. About 30,000 Americans are living with the disease at any given time, and most patients survive only two to five years after diagnosis. Treatments remain limited, which is why any new angle on the disease’s origins draws intense interest from patients, families, and clinicians alike.
The central experiment used germ-free mice, animals raised without any microbes, that had been engineered to lack C9orf72. When Burberry’s team introduced a single gut bacterium called Parabacteroides merdae, the mice developed a surge of inflammatory cytokines, including interferons and interleukins tied to innate immune activation. That inflammation did not stay in the gut. It spread to the brain and spinal cord.
When the same bacterium colonized genetically normal mice, the immune response was far more restrained. The difference came down to the host’s genetics: without functional C9orf72, the immune system lost its ability to tolerate a microbial product that healthy cells would simply ignore.
The culprit: bacterial sugar molecules tied to glycogen metabolism
To pinpoint what the bacteria were actually producing, the team used metatranscriptomic sequencing, a technique that reads the active RNA inside gut microbes. Genes involved in glycogen synthesis were highly expressed in the bacterial strains that provoked inflammation. Follow-up biochemical assays showed that glycogen-like polysaccharides isolated from these bacteria could directly activate myeloid cells, the white blood cells that serve as the immune system’s first responders, in laboratory dishes.
Multiple bacterial strains, not just P. merdae, triggered cytokine release in a pattern that depended on whether the host cells carried functional C9orf72. That breadth matters: it suggests the problem is not one rogue species but a class of microbial sugar molecules that many gut bacteria can produce.
The gene itself acts as a brake on innate immune activation. In myeloid cells, C9orf72 normally restrains the STING signaling pathway, a molecular alarm system that detects foreign DNA and ramps up inflammation. Without that restraint, microbial glycogen engages pattern-recognition receptors on immune cells, primes STING, and unleashes a cytokine cascade. Separate research from a group at the Broad Institute of MIT and Harvard has shown that C9orf72 also governs IL-17A-related signaling across both myeloid and lymphoid immune cell lineages, meaning the gene’s loss disrupts multiple branches of immune regulation at once.
Earlier work laid the groundwork
The new findings did not emerge from nowhere. A 2020 study published in Nature demonstrated that C9orf72-deficient mice developed broad immune dysfunction, including leukocyte expansion and autoantibody production. Critically, when researchers in that study suppressed the gut microbiome with antibiotics, immune cell infiltration into the spinal cord stopped. That result established that microbes in the intestine were necessary for central nervous system immune activation in this genetic context.
What the 2020 work could not answer was which microbes, and which of their products, were responsible. The 2026 Cell Reports paper from Burberry’s group narrows the trigger from the microbiome in general to a specific class of bacterial sugar molecules linked to glycogen metabolism. By using gnotobiotic mice, animals colonized with a defined microbial community, the researchers could isolate the effect of a single species and then trace the molecular chain from bacterial gene expression to immune activation.
Major questions the study cannot yet answer
All of the mechanistic data come from animal models and cell-based experiments. Whether P. merdae or its glycogen products provoke the same immune cascade in humans carrying C9orf72 mutations has not been tested. Mouse immune systems share broad architecture with human immunity, but the specific thresholds for cytokine release, the composition of the gut microbiome, and the permeability of the blood-brain barrier all differ between species.
Reviews of gut dysbiosis in ALS patients have cataloged intestinal permeability changes, altered microbial diversity, and candidate toxin mechanisms, but the evidence remains observational. It does not yet confirm that microbial glycogen is a driver in human disease.
There is also the question of scope. C9orf72 mutations account for a meaningful share of familial ALS, but most ALS is sporadic. Recent large-scale genetic studies have identified risk variants in sporadic cases, yet whether the glycogen-immune pathway matters for patients without C9orf72 mutations is entirely unresolved. The mouse models used here involve complete gene knockout, a more extreme loss of function than the partial reduction seen in most human carriers. People with intermediate C9orf72 activity might experience a subtler or more context-dependent response to microbial glycogen than the dramatic inflammation observed in knockout mice.
Even within C9orf72-associated ALS, variation in age of onset and disease progression suggests that additional genetic and environmental factors shape risk. Diet, prior infections, antibiotic exposure, and other medications can all remodel the gut microbiome and its metabolic output. How these variables interact with bacterial glycogen synthesis and host immunity has not been mapped. Longitudinal human studies tracking microbiome composition, glycogen-related genes, and inflammatory markers in C9orf72 carriers over time would be needed to determine whether the mouse findings translate.
The relationship between bacterial glycogen and other proposed gut-derived triggers also needs sorting out. ALS researchers have studied short-chain fatty acids, lipopolysaccharides, and other microbial metabolites as possible contributors to neuroinflammation. Where glycogen fits in that hierarchy, whether it acts alone or amplifies other signals, remains an open question. The new study shows that glycogen biosynthesis genes are active in the bacteria that provoke inflammation, but it does not demonstrate that blocking glycogen production alone would prevent disease in an animal model.
Gut-targeted therapies now have a specific molecular target
No clinical intervention based on these findings is available or recommended yet. Routine microbiome testing and empiric antibiotic use are not supported by current evidence for ALS patients. But the research sharpens a testable hypothesis: that targeted inhibition of bacterial glycogen biosynthesis in the gut could reduce immune activation in C9orf72 carriers.
If future work in human-derived cell models or patient cohorts confirms the pathway, several gut-targeted strategies could enter clinical testing. Precision antibiotics designed to suppress glycogen-producing bacteria without wiping out beneficial species are one possibility. Engineered probiotics that outcompete harmful strains or that lack glycogen synthesis genes are another. Dietary interventions designed to shift the microbiome’s metabolic output could offer a less invasive approach.
The convergence of multiple research groups on the same gene-immune axis, from STING pathway regulation to IL-17A signaling to the antibiotic experiments that halted spinal cord inflammation, strengthens the overall case that C9orf72 shapes systemic inflammatory tone far beyond the nervous system. The 2026 study adds the most specific piece yet: a microbial product that can be isolated, measured, and potentially blocked.
For the roughly 10 percent of ALS patients whose disease runs in families, and especially for those who carry C9orf72 mutations, this line of research offers something that has been scarce: a mechanistic explanation that points toward an intervention target outside the nervous system. The work does not guarantee new therapies, but it opens a door that was largely closed five years ago, reframing at least one form of ALS as a disease shaped by a dialogue between gut bacteria and a misfiring immune system.
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*This article was researched with the help of AI, with human editors creating the final content.
