[DigitalToday AI Reporter] Gut microbes, previously seen only as indirect helpers of intestinal cells, are in fact a key factory that directly produces serotonin, known as the “happiness hormone,” it has been found.

A unique cooperative mechanism has been identified in which serotonin is synthesised only when two probiotic bacteria that showed no reaction on their own combine in a “1+1” form. This is adding weight to calls to manage the gut ecosystem itself rather than individual strains.

Seokjin Kim (김석진), head of the Good Bacteria Research Institute, recently introduced in a video the latest findings from a research team led by Professor Fredrik Backhed at the University of Gothenburg in Sweden, published in the international journal Cell Reports in October 2025.

About 90 percent of serotonin, which is involved not only in emotion regulation but also intestinal movement, sleep rhythm and appetite control, is produced in intestinal mucosal cells. The academic view has been that short-chain fatty acids produced when gut bacteria break down dietary fibre stimulate intestinal cells and indirectly promote serotonin production. The study has now, for the first time, demonstrated that gut bacteria play the role of a “production factory” that directly synthesises serotonin.

After culturing and tracking gut microbes from healthy people, the team found that two Lactobacillus-related probiotic strains, Limosilactobacillus mucosae and Ligilactobacillus ruminis, carry out a reaction that directly converts serotonin. Notably, no serotonin was detected when the two strains were cultured separately. For the mucosae strain, which has a serotonin synthesis enzyme, to function normally, cooperation with the ruminis strain was essential.

The mechanism was reproduced in animal experiments. When the two strains were administered together to germ-free mice without gut bacteria, serotonin levels in faeces rose, the density of enteric nerve cells increased, and abnormally slow gut transit time was observed to normalise.

The study also provides an important clue for identifying and treating the cause of irritable bowel syndrome (IBS), which 7 to 10 percent of South Korea’s population experiences. IBS, which is accompanied by abdominal pain, diarrhoea and constipation, is closely linked to abnormal regulation of serotonin in the gut. Analysis of faeces from IBS patients showed the proportion of the mucosae strain was significantly lower than in healthy people. This suggests a lack of a key strain that directly makes serotonin may have led to abnormal intestinal movement.

Kim said the study clearly shows that the microbiome works as an ecosystem rather than the superiority of individual strains. He advised that rather than focusing on one specific strain, it is most important to steadily consume dietary fibre and fermented foods to keep the entire gut ecosystem healthy and create an environment in which beneficial bacteria can cooperate with one another.