Two Gut Bacteria Strip the Colon's Moisture Barrier, Driving Chronic Constipation

Chronic constipation affects roughly 15% of the global population and costs healthcare systems billions of dollars each year in diagnostics, laxatives, and specialist visits. For most patients, the cause is labeled idiopathic - meaning clinicians cannot find one. A study from Nagoya University offers a more specific answer for at least some of those cases: a pair of gut bacteria that systematically strip the colon of its moisture-retaining mucus layer.

The research, published in Gut Microbes, identifies Akkermansia muciniphila and Bacteroides thetaiotaomicron as the bacterial duo behind what the team has called "bacterial constipation." Individually, neither microbe is uniformly harmful - Akkermansia in particular is often discussed in the research literature as a marker of gut health. But together, in excess, the pair appears to degrade the intestinal mucus coating faster than the colon can replenish it, with cascading consequences for stool consistency and motility.

The Mucus Layer: More Than a Passive Lining

The large intestine is lined with a gel-like mucus layer secreted by goblet cells. This layer does two things relevant to constipation: it lubricates the passage of fecal material, and it retains water in stool. When the layer is intact, stool remains hydrated and moves through the colon with appropriate ease. When it degrades, water absorption by the colon wall goes unchecked, stool becomes dense and dry, and transit slows dramatically.

A. muciniphila is a specialist mucin-degrader - its entire metabolic niche involves breaking down the mucus glycoproteins. B. thetaiotaomicron contributes carbohydrate-cleaving enzymes that further process the fragments A. muciniphila produces. In moderate amounts, this degradation is part of normal microbial ecology. The Nagoya team found that when both species are present at elevated levels simultaneously, their combined enzymatic activity overwhelms the colon's regenerative capacity.

Experimental Evidence in Mice and Human Samples

The researchers worked with germ-free mice - animals raised without any gut bacteria - which allowed them to introduce specific bacterial combinations and observe the effects without interference from a native microbiome. Mice colonized with both A. muciniphila and B. thetaiotaomicron together showed significantly reduced mucus thickness, measurably drier stool, and slower colonic transit compared with control animals colonized with either bacterium alone or with neither.

The team also examined stool samples from human patients with chronic constipation, finding elevated levels of both bacteria relative to individuals with normal bowel habits. The correlation was not absolute - not all constipated patients showed the pattern, and not all individuals with high levels of both species reported constipation symptoms - indicating that this mechanism explains some but not all cases of the condition.

This is an important caveat. The mouse model uses germ-free animals with artificially simplified microbiomes, which differ fundamentally from the complex, hundreds-of-species ecosystems in human guts. Results in germ-free mouse models do not always translate cleanly to clinical populations.

Implications for Treatment

Current treatments for chronic constipation center on osmotic laxatives, stimulant laxatives, fiber supplementation, and gut motility drugs. None of these address the microbial composition of the colon. If bacterial mucus degradation is a root cause in a subset of patients, those interventions treat symptoms while leaving the underlying mechanism intact.

The Nagoya findings point toward several potential therapeutic approaches. Probiotics designed to outcompete or suppress A. muciniphila and B. thetaiotaomicron at elevated densities could theoretically restore mucus integrity. Dietary strategies that support goblet cell function and mucus production - including certain prebiotic fibers - represent another angle. Fecal microbiome transplantation, already in clinical use for Clostridioides difficile infections, could potentially correct the bacterial imbalance more broadly.

Clinical translation requires considerably more work. A reliable diagnostic test that identifies which constipated patients have this specific microbial signature would be a prerequisite for targeted treatment. Large-scale human trials demonstrating that reducing these bacteria actually resolves symptoms have not yet been conducted.

Reframing a Common Diagnosis

The broader significance of this work lies in its mechanistic specificity. "Idiopathic constipation" is a diagnostic category that effectively means the cause is unknown. The Nagoya study adds evidence that for at least a subset of patients, a specific, biologically describable mechanism is operating - one that could, in principle, be detected and targeted.

That shift from symptom management to mechanistic understanding has been one of the driving themes in gut microbiome research over the past decade. Constipation joins a growing list of conditions - from irritable bowel syndrome to metabolic disease to depression - where the bacterial ecosystem in the colon appears to play a causal, rather than merely correlational, role.