In 2013, two researchers at University College Cork proposed a new word for an old observation. Timothy Dinan and John Cryan defined a psychobiotic as a live organism that could benefit mental health when a person consumes enough of it^[1]. The term combines “psyche” and “probiotic,” and it emerged because a decade of research suggested that certain gut bacteria communicate with the brain, and that this conversation may shape how people respond to stress and low mood.

The idea rests on the gut-brain axis, a two-way signaling network that connects the digestive tract and the central nervous system through the vagus nerve, immune messengers, hormones, and bacterial metabolites^[2]. Research in this area is younger than the marketing built on top of it, so the useful questions are narrow ones. Which strains have been tested in people? What did those trials measure? Where did the results hold up, and where did they fall apart?

What Are Psychobiotics?

A probiotic is a live microorganism that benefits the health of the person taking it, provided the dose is sufficient. That definition comes from an expert panel convened by the International Scientific Association for Probiotics and Prebiotics. It sets three conditions: the organisms must arrive alive, the dose must match what was studied, and the benefit must be shown for that specific strain^[3].

Psychobiotics are the subset studied for effects on the brain. Most belong to Lactobacillus and Bifidobacterium, the two most researched families of probiotic bacteria^[1][3]. In 2016, researchers widened the definition to include prebiotics, the fermentable fibers that feed beneficial bacteria, since changing what gut microbes eat also changes what they produce^[4].

Effects in this field are strain-specific, which is worth absorbing before the science. A finding for Lactobacillus rhamnosus HN001 says nothing about Lactobacillus rhamnosus GG, two strains that share a species name and little else. Careful reading means matching each claim to a named strain, and this article does that throughout.

How the Gut and Brain Communicate

The intestinal wall contains its own network of 200 to 600 million neurons, the enteric nervous system, which runs digestion largely on its own^[5]. It stays in constant contact with the brain, and gut bacteria appear to take part in that exchange through several routes^[2][6]:

• The vagus nerve carries signals in both directions between the gut and the brainstem. In a well-known mouse study, Lactobacillus rhamnosus JB-1 changed GABA receptor expression in the brain and reduced stress behaviors. When researchers cut the vagus nerve, the effect disappeared^[7].
• About 90 percent of the body’s serotonin is produced in the gut, and resident bacteria help regulate its synthesis in the intestinal lining^[8].
• Animal studies tie gut bacteria to the activity of the hypothalamic-pituitary-adrenal axis, the hormonal cascade behind the stress response^[6].
• Gut bacteria interact with the immune cells concentrated in the intestinal wall, and some strains support the tight junctions that control what passes from the gut into circulation^[6]. The Best Prebiotics and Probiotics for Leaky Gut looks at that barrier research in more detail.
• When bacteria ferment dietary fiber, they produce short-chain fatty acids that feed colon cells and carry signals well beyond the gut^[6].

What Laboratory and Animal Research Adds

A second layer of findings fills in more of the picture. Results at this level describe mechanisms, and the human trials in the next section test whether those mechanisms translate:

• Probiotic research has long tracked bowel regularity, and studied strains are tested for occasional constipation and loose stools alike. Regularity appears connected to more than digestion: in an observational study of healthy adults, stool consistency was associated with pain perception and with anxiety measures^[9].
• In rats, Bifidobacterium longum subsp. infantis raised blood levels of tryptophan, the amino acid the body uses as raw material for serotonin, shifted markers of serotonin turnover in the brain, and damped pro-inflammatory cytokines, including IL-6^[10]. Elevated inflammatory markers of that kind are among the more consistent biological findings in people with depression, which makes the combination worth studying^[11].
• Psychological stress increases the permeability of the gut lining, and in animal work certain strains supported the tight junctions that seal the space between intestinal cells^[12]. A tighter barrier keeps bacterial fragments out of circulation, one proposed route by which gut health and inflammation connect^[12].
• Several bacteria cultured from the human intestine, including Lactobacillus and Bifidobacterium species, produce GABA in laboratory conditions^[13]. GABA is the nervous system’s main inhibitory neurotransmitter, the counterweight to excitatory signals such as glutamate.
• In a widely cited experiment, mice raised germ-free, with no gut bacteria at all, released more of the stress hormones ACTH and corticosterone under stress than normal mice, and colonizing them with a single Bifidobacterium strain normalized the response^[14].
• Changing the gut microbiota of mice changed brain levels of brain-derived neurotrophic factor (BDNF), a growth protein involved in neuron health, plasticity, and memory^[15]. Average blood BDNF runs lower in people with depression, which is one reason this finding drew attention^[16].

These pathways explain why researchers ask the question. Whether swallowing a capsule changes how a person feels is a separate matter, and only human trials can answer it.

What Human Studies Show

The human evidence is a mix: some encouraging results, some null results, and many studies too small to settle anything.

Probiotics and Stress

In one of the earliest controlled trials, healthy volunteers who took a combination of Lactobacillus helveticus R0052 and Bifidobacterium longum R0175 for 30 days scored lower on measures of psychological distress, including anxiety and depression subscales, than the placebo group^[17]. In a separate line of work, Bifidobacterium longum 1714 was associated with reduced daily stress ratings and a dampened cortisol response in a small trial of healthy men^[18].

Researchers tested two Lactobacillus plantarum strains in adults selected for moderate stress. Over 12 weeks, DR7 was associated with lower stress and anxiety scores compared with placebo, with differences emerging by week eight^[19]. P8 was associated with lower stress scores in a similar design^[20].

The field also has null results worth knowing about. Lactobacillus rhamnosus JB-1, the strain from the mouse study above, was tested in healthy men and changed neither stress measures nor cognitive performance^[21]. What works in animal models does not reliably transfer to people, and any fair account of psychobiotics has to say so.

Probiotics and Low Mood

A frequently cited trial from the Netherlands gave 40 healthy young adults a multispecies probiotic or placebo for four weeks. The probiotic group reported less rumination, the repetitive negative thinking that research links to vulnerability to low mood^[22].

The largest single trial in this area followed 423 pregnant women in New Zealand, randomized to Lactobacillus rhamnosus HN001 or placebo from mid-pregnancy through six months postpartum. After birth, women in the probiotic group scored significantly lower on depression and anxiety measures than women on placebo^[23]. Mood was a secondary outcome in that trial, and the finding awaits replication, so it reads best as a strong lead.

In adults with irritable bowel symptoms, a pilot trial of Bifidobacterium longum NCC3001 reported reduced depression scores at six weeks alongside changes in brain activity on functional MRI^[24]. A small three-week Italian trial of 48 women found that a multi-strain mixture was associated with improved scores on body image and eating attitude questionnaires; three weeks and 48 participants settle very little, so this one is an early signal at most^[25].

What the Pooled Evidence Says

Meta-analyses, which combine results across trials, point to a consistent conclusion. A 2019 analysis of 34 controlled trials of probiotics for anxiety and depression measures found a small but significant benefit, larger in clinical and medically ill groups than in healthy volunteers, while prebiotics alone showed no effect^[26]. A 2025 meta-analysis covering more than 4,000 participants reached a similar conclusion: modest average reductions in anxiety and depression scores, with wide variation between studies in strains, doses, and populations^[27].

That modest, strain-dependent average effect justifies more research. It does not justify treating a capsule as therapy. Probiotics are not a treatment for depression, anxiety, or any other mental health condition, and no supplement replaces professional care. Anyone experiencing persistent low mood or anxiety should speak with a healthcare provider.

What About Lactobacillus plantarum?

An earlier version of this article singled out Lactobacillus plantarum as the psychobiotic every product should contain, crediting it with everything from surviving stomach acid to applications in serious neurological conditions. The evidence says less than that, and what it does say is more useful.

L. plantarum is a hardy, acid-tolerant species that shows up in many fermented foods, and it is one of the few species with more than one randomized trial in stressed adults: the DR7 and P8 trials described above both reported lower stress scores over 12 weeks^[19][20]. Specific L. plantarum strains have earned their place in gut-brain research on those results.

Claims tying the species to multiple sclerosis, autism, or eating disorders traced back to preliminary laboratory work and small early studies, and they have been removed from this article. When you read a label, look for the strain designation and the trial behind it. A species name alone carries less information than it appears.

Psychobiotic Foods and the Psychobiotic Diet

Researchers have also tested this idea without capsules. In 2022, the Cork group behind the original psychobiotic concept published a whole-diet trial in Molecular Psychiatry. Forty-five healthy adults with low-fiber diets were randomized to a “psychobiotic diet” or to standard dietary advice. The diet emphasized prebiotic fibers from foods like onions, leeks, apples, bananas, and oats, along with whole grains, legumes, and two to three daily servings of fermented foods such as kefir, sauerkraut, and kombucha^[28].

After four weeks, the psychobiotic diet group reported a 32 percent reduction in perceived stress, and people who followed the diet most closely saw the largest reductions^[28]. The authors acknowledged that the difference between groups did not reach statistical significance in this small sample. Still, the adherence pattern and the metabolite changes they measured make diet a sensible first lever for anyone curious about the gut-brain connection.

Fermented foods and fiber carry the strongest whole-diet signal; they cost little and support digestion, whatever future mood research concludes.

Choosing a Probiotic With Studied Strains

If you decide to try a probiotic alongside those dietary changes, the research above suggests what to look for:

• A label that names its strains. Lactobacillus plantarum DR7 or Bifidobacterium longum R0175 tells you which studies apply; a generic “Lactobacillus blend” tells you very little^[3].
• A dose in the studied range. Most trials discussed here used 1 to 10 billion CFU per day of the target strain for 4 to 12 weeks^[17][19].
• Delivery that survives stomach acid. Delayed-release capsules and acid-tolerant strains are two common approaches to delivering live organisms to the intestine^[29].
• Manufacturing you can verify. In the United States, probiotics are regulated as dietary supplements, so third-party testing and certified cGMP manufacturing are the practical quality markers^[29].
• Patience. Trials measured outcomes at 4 to 12 weeks, so a few days tells you nothing. The guide How Long Does It Take for Probiotics to Work covers realistic timelines in detail.

Those criteria shaped OmniBiotics Flora Renew Probiotics: 15 named Lactobacillus and Bifidobacterium strains totaling 100 billion CFU; inulin and apple pectin for prebiotic fiber; and vegan, delayed-release capsules, produced in the USA in an NSF-certified, cGMP facility. Many products exist, and the checklist above applies to any of them. The Physician Certified page explains the review process behind OmniBiotics formulations.

In trials, healthy adults tolerate probiotics well; the most common side effects are gas and bloating in the first few days. Medical supervision comes first for anyone severely immunocompromised or critically ill, and a conversation with a healthcare provider comes first for anyone pregnant, breastfeeding, or taking medication^[29].

Where the Science Stands

The gut-brain axis is real and well mapped. The mechanisms are plausible, and a growing set of randomized trials reports small improvements in stress and mood measures with specific strains. That same literature includes null results, small samples, and effects that vary by study, which is why researchers still call psychobiotics a promising research area and stop short of calling them proven.

A reasonable starting point costs almost nothing: eat more fiber, eat fermented foods regularly, and expect results in weeks. If you add a supplement, pick one with named, studied strains, and bring anything beyond everyday stress to a professional. For the broader picture of what probiotics are studied for, see Top 10 Probiotics Benefits.

This article is for educational purposes and is not medical advice. Dietary supplements are not intended to diagnose, treat, cure, or prevent any disease. Speak with a qualified healthcare provider before starting any supplement, particularly if you are pregnant, breastfeeding, or taking medication.

References

1. Dinan TG, Stanton C, Cryan JF. Psychobiotics: a novel class of psychotropic. Biological Psychiatry. 2013;74(10):720-726. https://pubmed.ncbi.nlm.nih.gov/23759244/
2. Carabotti M, Scirocco A, Maselli MA, Severi C. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of Gastroenterology. 2015;28(2):203-209. https://pubmed.ncbi.nlm.nih.gov/25830558/
3. Hill C, Guarner F, Reid G, et al. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology and Hepatology. 2014;11(8):506-514. https://pubmed.ncbi.nlm.nih.gov/24912386/
4. Sarkar A, Lehto SM, Harty S, Dinan TG, Cryan JF, Burnet PWJ. Psychobiotics and the manipulation of bacteria-gut-brain signals. Trends in Neurosciences. 2016;39(11):763-781. https://pubmed.ncbi.nlm.nih.gov/27793434/
5. Furness JB. The enteric nervous system and neurogastroenterology. Nature Reviews Gastroenterology and Hepatology. 2012;9(5):286-294. https://pubmed.ncbi.nlm.nih.gov/22392290/
6. Cryan JF, O’Riordan KJ, Cowan CSM, et al. The microbiota-gut-brain axis. Physiological Reviews. 2019;99(4):1877-2013. https://pubmed.ncbi.nlm.nih.gov/31460832/
7. Bravo JA, Forsythe P, Chew MV, et al. Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve. Proceedings of the National Academy of Sciences. 2011;108(38):16050-16055. https://pubmed.ncbi.nlm.nih.gov/21876150/
8. Yano JM, Yu K, Donaldson GP, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015;161(2):264-276. https://pubmed.ncbi.nlm.nih.gov/25860609/
9. Shiro Y, Arai YC, Ikemoto T, Hayashi K. Stool consistency is significantly associated with pain perception. PLoS One. 2017;12(8):e0182859. https://pubmed.ncbi.nlm.nih.gov/28793322/
10. Desbonnet L, Garrett L, Clarke G, Bienenstock J, Dinan TG. The probiotic Bifidobacteria infantis: an assessment of potential antidepressant properties in the rat. Journal of Psychiatric Research. 2008;43(2):164-174. https://pubmed.ncbi.nlm.nih.gov/18456279/
11. Dowlati Y, Herrmann N, Swardfager W, et al. A meta-analysis of cytokines in major depression. Biological Psychiatry. 2010;67(5):446-457. https://pubmed.ncbi.nlm.nih.gov/20015486/
12. Kelly JR, Kennedy PJ, Cryan JF, Dinan TG, Clarke G, Hyland NP. Breaking down the barriers: the gut microbiome, intestinal permeability and stress-related psychiatric disorders. Frontiers in Cellular Neuroscience. 2015;9:392. https://pubmed.ncbi.nlm.nih.gov/26528128/
13. Barrett E, Ross RP, O’Toole PW, Fitzgerald GF, Stanton C. Gamma-aminobutyric acid production by culturable bacteria from the human intestine. Journal of Applied Microbiology. 2012;113(2):411-417. https://pubmed.ncbi.nlm.nih.gov/22612585/
14. Sudo N, Chida Y, Aiba Y, et al. Postnatal microbial colonization programs the hypothalamic-pituitary-adrenal system for stress response in mice. The Journal of Physiology. 2004;558(Pt 1):263-275. https://pubmed.ncbi.nlm.nih.gov/15133062/
15. Bercik P, Denou E, Collins J, et al. The intestinal microbiota affect central levels of brain-derived neurotropic factor and behavior in mice. Gastroenterology. 2011;141(2):599-609. https://pubmed.ncbi.nlm.nih.gov/21683077/
16. Sen S, Duman R, Sanacora G. Serum brain-derived neurotrophic factor, depression, and antidepressant medications: meta-analyses and implications. Biological Psychiatry. 2008;64(6):527-532. https://pubmed.ncbi.nlm.nih.gov/18571629/
17. Messaoudi M, Lalonde R, Violle N, et al. Assessment of psychotropic-like properties of a probiotic formulation (Lactobacillus helveticus R0052 and Bifidobacterium longum R0175) in rats and human subjects. British Journal of Nutrition. 2011;105(5):755-764. https://pubmed.ncbi.nlm.nih.gov/20974015/
18. Allen AP, Hutch W, Borre YE, et al. Bifidobacterium longum 1714 as a translational psychobiotic: modulation of stress, electrophysiology and neurocognition in healthy volunteers. Translational Psychiatry. 2016;6(11):e939. https://pubmed.ncbi.nlm.nih.gov/27801892/
19. Chong HX, Yusoff NAA, Hor YY, et al. Lactobacillus plantarum DR7 alleviates stress and anxiety in adults: a randomised, double-blind, placebo-controlled study. Beneficial Microbes. 2019;10(4):355-373. https://pubmed.ncbi.nlm.nih.gov/30882244/
20. Lew LC, Hor YY, Yusoff NAA, et al. Probiotic Lactobacillus plantarum P8 alleviated stress and anxiety while enhancing memory and cognition in stressed adults: a randomised, double-blind, placebo-controlled study. Clinical Nutrition. 2019;38(5):2053-2064. https://pubmed.ncbi.nlm.nih.gov/30266270/
21. Kelly JR, Allen AP, Temko A, et al. Lost in translation? The potential psychobiotic Lactobacillus rhamnosus (JB-1) fails to modulate stress or cognitive performance in healthy male subjects. Brain, Behavior, and Immunity. 2017;61:50-59. https://pubmed.ncbi.nlm.nih.gov/27865949/
22. Steenbergen L, Sellaro R, van Hemert S, Bosch JA, Colzato LS. A randomized controlled trial to test the effect of multispecies probiotics on cognitive reactivity to sad mood. Brain, Behavior, and Immunity. 2015;48:258-264. https://pubmed.ncbi.nlm.nih.gov/25862297/
23. Slykerman RF, Hood F, Wickens K, et al. Effect of Lactobacillus rhamnosus HN001 in pregnancy on postpartum symptoms of depression and anxiety: a randomised double-blind placebo-controlled trial. EBioMedicine. 2017;24:159-165. https://pubmed.ncbi.nlm.nih.gov/28943228/
24. Pinto-Sanchez MI, Hall GB, Ghajar K, et al. Probiotic Bifidobacterium longum NCC3001 reduces depression scores and alters brain activity: a pilot study in patients with irritable bowel syndrome. Gastroenterology. 2017;153(2):448-459. https://pubmed.ncbi.nlm.nih.gov/28483500/
25. De Lorenzo A, Costacurta M, Merra G, et al. Can psychobiotics intake modulate psychological profile and body composition of women affected by normal weight obese syndrome and obesity? A double blind randomized clinical trial. Journal of Translational Medicine. 2017;15(1):135. https://pubmed.ncbi.nlm.nih.gov/28601084/
26. Liu RT, Walsh RFL, Sheehan AE. Prebiotics and probiotics for depression and anxiety: a systematic review and meta-analysis of controlled clinical trials. Neuroscience and Biobehavioral Reviews. 2019;102:13-23. https://pubmed.ncbi.nlm.nih.gov/31004628/
27. Zandifar A, Badrfam R, Mohammaditabar M, et al. The effect of prebiotics and probiotics on levels of depression, anxiety, and cognitive function: a meta-analysis of randomized clinical trials. Brain and Behavior. 2025;15(3):e70401. https://pubmed.ncbi.nlm.nih.gov/40038849/
28. Berding K, Bastiaanssen TFS, Moloney GM, et al. Feed your microbes to deal with stress: a psychobiotic diet impacts microbial stability and perceived stress in a healthy adult population. Molecular Psychiatry. 2023;28(2):601-610. https://pubmed.ncbi.nlm.nih.gov/36289301/
29. National Institutes of Health, Office of Dietary Supplements. Probiotics: Fact Sheet for Health Professionals. https://ods.od.nih.gov/factsheets/Probiotics-HealthProfessional/