Microbiota as a Trainer: Bacteria That Grew Muscle Fibers

A study was published in Scientific Reports in which scientists "reassembled" the microbiota in mice and found specific intestinal bacteria that can significantly improve strength performance and muscle composition. After transplanting human microflora into mice and subsequently testing candidates, the authors identified two species - Lactobacillus johnsonii and Limosilactobacillus reuteri. Long-term administration of these bacteria to aging mice improved strength test results, increased skeletal muscle mass and muscle fiber cross-sectional area, and at the molecular level increased the expression of myoregenerative markers FST (follistatin) and IGF-1. The work was published on August 18, 2025.

Background of the study

Sarcopenia—the age-related decline in skeletal muscle strength and quality—increases the risk of falls, disability, and mortality. Classic interventions (resistance training, adequate protein) work, but the effect is limited in many older adults, so attention is shifting to new targets, including the gut microbiome. Accumulating evidence links microbiota composition to muscle metabolism and function, and even suggests that probiotic supplementation can modestly improve strength and exercise performance, although results are mixed across studies.

The idea of a “gut-muscle axis” relies on several mechanisms: short-chain fatty acids synthesized by microbes influence muscle energy metabolism; microbiota modulates inflammation and the integrity of the intestinal barrier; and growth and plasticity signals are altered via neuroendocrine pathways. Physical activity, in turn, also “restructures” the microbial composition - a two-way relationship. This creates the basis for searching for strains that specifically support muscle function in aging organisms.

Until recently, however, we had a lot of associations and little causal evidence at the level of specific bacteria. A new paper in Scientific Reports closes part of this gap: the authors first transplanted human microbiota into mice and showed that its variations affected strength tests differently, and then functionally tested the candidates and identified two key species, Lactobacillus johnsonii and Limosilactobacillus reuteri. Long-term administration of these strains to aging mice increased muscle strength, mass, and cross-sectional area, and at the molecular marker level, it increased the expression of FST and IGF-1, indicating a growth-promoting effect.

The practical conclusion is cautious so far: this is a convincing preclinical study and a step towards strain-specific “anti-sarcopenic” probiotics, but translation to humans requires randomized trials with powered endpoints and mechanistic biomarkers. Current reviews highlight the potential of lactobacilli as adjuvant therapy, but also the need for standardization of strains, doses, and duration before making broad recommendations.

How was this tested?

The researchers first “zeroed” the gut flora of 9-month-old mice with antibiotics and performed a fecal transplant: for three months, the animals were given a mixture of feces from 10 healthy adults (donors without chronic diseases and without recent intake of antibiotics/probiotics). Strength and agility were assessed using two independent tests: rotarod (time to fall from a rotating rod) and wire suspension (holding time). Already at this stage, it became clear that different bacterial profiles affect muscle function in different ways. Comparative analysis of the gastrointestinal tract and feces microbiota showed that the composition in the intestinal lumen is more diverse and more accurately associated with strength metrics than the “fecal cast”. From a set of different species, L. johnsonii, L. reuteri and Turicibacter sanguinis statistically consistently “floated up”; the first two authors chose for functional testing.

Next, a direct experiment on 12-month-old mice: after a short intestinal sanitation, the animals were given L. johnsonii, L. reuteri or a combination of them daily for three months. The result was an increase in the time on the rotarod and suspension already from the first month in the "bacterial" groups, with the combination giving the most pronounced dynamics. Histologically, the transverse area of the fibers (soleus, gastrocnemius and long extensor of the fingers) was greater than in the control; at the same time, body weight as a whole decreased, and muscle mass increased, indicating an improvement in body composition. At the level of mRNA expression, follistatin in the L. johnsonii group almost doubled, IGF-1 was also higher in all "bacterial" branches.

Why might this be necessary?

With age, muscle strength and quality decline (sarcopenia), and the risks of falls, fractures, and loss of independence increase. The concept of a “gut-muscle axis” has long been debated, but here we present direct functional evidence for specific strains: L. johnsonii and L. reuteri are not simply associated with better performance, but also improve strength and muscle morphology in the experiment. The authors suggest that the effect may occur via several pathways simultaneously - from the production of short-chain fatty acids and the modulation of mitochondrial function to the regulation of muscle growth pathways (via FST/IGF-1).

What's new in science (and carefully - about the "power pill")

• The strain itself matters. We are not talking about “probiotics in general,” but about two specific strains, independently confirmed in two different behavioral tests and identified using differential analysis (DESeq2).
• Synergy in a Pair: Co-administration of L. johnsonii + L. reuteri produced the greatest gains in both strength and fiber area, hinting at potential multi-strain formulas.
• Gut is more important than feces. The gastrointestinal microbiota "portrait" is more informative than fecal samples - a practical hint for future design strategies.

How it works (authors' hypotheses)

In the discussion, the researchers linked improved muscle function to:

• possible normalization of mitochondria in muscles (reduction of damage by cytochrome C in previously described works for these species);
• increased production of short-chain fatty acids, which improve muscle anabolism and metabolism;
• activation of growth-promoting pathways - growth of FST (myostatin antagonist) and IGF-1.
  The combination of these factors can shift the balance towards greater strength and oxidative potential of fibers. The mechanisms need to be detailed at the "omics" levels - metabolomics, transcriptomics, proteomics.

Caution first

This is a mouse model; transferring the results "as is" to humans is premature. The authors explicitly write about the need for testing in humans - from organoids and ex vivo models to population and clinical trials. It is also important that the effect depended on long-term administration (months), and the initial changes in the microbiota in animals were achieved by aggressive sanitation - this is not what we do in the clinic. Finally, the third often "companion" species Turicibacter sanguinis in this work did not undergo functional validation, although its enrichment consistently coincided with an increase in strength - a possible target for future experiments.

What does this mean "in practice" today?

• "any probiotic" supplements do not equal L. johnsonii and L. reuteri supplements - real world product composition varies greatly;
• the path to an "anti-sarcopenic" probiotic requires human RCTs with strength endpoints (dynamometer grip, stand-up and go test, walking speed), muscle morphometry and metabolic markers;
• If the hypothesis is confirmed, the target is obvious: older age groups, patients at risk of sarcopenia/weakening after immobilization, and athletes in rehabilitation phases. For now, this is an interesting preclinical study and a basis for carefully designed trials.

Source: Ahn JS., Kim HM., Han EJ., Hong ST., Chung HJ. Discovery of intestinal microorganisms that affect the improvement of muscle strength. Scientific Reports. 2025;15:30179. https://doi.org/10.1038/s41598-025-15222-2