Probiotics and stress: which strain, which trial, and who was actually studied

We sell probiotics. The topic came from a 2026 continuing-education module in the Australian Journal of Pharmacy. We used its title and learning outcomes as a reading list and went to the published trials ourselves, though we could read only the summaries of the trial papers, not the full text. We do not stock R0052 with R0175, 1714 or Shirota, the strains below that reported a result against placebo. You can weigh all of that as you read.

In short: your gut and brain signal each other through nerves, immune messengers and chemicals made by gut microbes. Four studies in healthy adults have tested named probiotic strains on stress measures. Three reported lower measures than placebo, and one found no difference. The letters and numbers after a probiotic's name identify the strain, and the strain is what a trial tests.

The same bacterium, two different results

In 2011, researchers fed mice a strain of Lactobacillus rhamnosus called JB-1. The mice showed less anxiety-like behaviour, and the receptors for a calming brain chemical changed. When the researchers cut the vagus nerve, the nerve that links gut and brain, those effects were not seen (Bravo and colleagues, PNAS, 2011).

Six years later, another group gave the same strain to 29 healthy men in an eight-week study that switched each man between the strain and a placebo. On the stress, hormone, inflammation and thinking measures they used, there was no difference (Kelly and colleagues, Brain, Behavior, and Immunity, 2017).

The gut–brain link is real enough to study. A mouse result is a reason to run a human trial, not a reason to buy a product.

How the gut and brain talk

The connection runs both ways. Your brain changes what your gut does through the automatic nerve signals that run your organs. Your gut, and the microbes in it, signal back through three main channels (Cryan and colleagues, Physiological Reviews, 2019):

• Nerves. The vagus nerve carries traffic in both directions, but about 80 per cent of its fibres run upward, from gut to brain. That figure comes from animal fibre counts, so treat it as approximate (Breit and colleagues, Frontiers in Psychiatry, 2018).
• Immune messengers. Immune cells in the gut wall release signalling proteins called cytokines.
• Microbial chemicals. Gut bacteria ferment fibre into small molecules called short-chain fatty acids, and they help process tryptophan, the amino acid the body uses to make serotonin.

Species, strain, and the code after the name

Bifidobacterium longum is a species. Bifidobacterium longum 1714 is a strain: one specific member of that species, isolated in a laboratory and given a code. The code is what a trial tests.

The international consensus body on probiotics puts it this way: some effects are shared across a whole genus, a group of related species, and some are found in only a few strains (Hill and colleagues, Nature Reviews Gastroenterology and Hepatology, 2014). For stress measures, every human study we found tested a named strain.

Which strains have been tested on stress measures

These are the studies in healthy adults that we could trace to a published paper.

L. helveticus R0052 with B. longum R0175

• Who was studied: 55 healthy men and women aged 30 to 60.
• What was measured: stress and mood questionnaires; urinary cortisol.
• What happened: lower scores than placebo on several scales.
• How long: 30 days.
• Source: Messaoudi and colleagues, British Journal of Nutrition, 2011 (doi:10.1017/S0007114510004319).

B. longum 1714

• Who was studied: 22 healthy volunteers.
• What was measured: cortisol and self-rated anxiety during a cold-water stress test; a daily stress diary; a memory task.
• What happened: smaller cortisol and anxiety rise; lower daily stress; a small memory change.
• How long: 4 weeks on the probiotic.
• Source: Allen and colleagues, Translational Psychiatry, 2016 (doi:10.1038/tp.2016.191).

L. casei Shirota

• Who was studied: medical students in the weeks before a national exam (three pooled trials).
• What was measured: salivary cortisol; physical symptoms.
• What happened: cortisol rose less than with placebo; fewer physical symptoms.
• How long: 8 weeks.
• Source: Takada and colleagues, Neurogastroenterology and Motility, 2016 (doi:10.1111/nmo.12804).

L. rhamnosus JB-1

• Who was studied: 29 healthy men.
• What was measured: stress measures; stress hormones; inflammation; thinking tests.
• What happened: no difference from placebo.
• How long: 8 weeks, switching between strain and placebo.
• Source: Kelly and colleagues, Brain, Behavior, and Immunity, 2017 (PubMed 27865949).

Three things to notice.

Who paid. We could not read the full papers, so we could not confirm who funded each study. Some of the Shirota paper's authors work for Yakult, the company that owns the strain. Read the funding statement before you weigh a result.

Who was studied. None of these studies was designed around women in midlife. The closest is the 2011 study, which enrolled men and women aged 30 to 60. The 1714 study measured the same 22 people after a placebo period and after a probiotic period, so there was no separate placebo group. We could not find a study of these strains that reported results for women in their forties and fifties specifically.

What we have not read. We know of at least one later trial of the 1714 strain that we could not read, and there may be others. The list above shows what we could check, not everything that exists.

What the evidence does not show

• None of these studies enrolled people with a diagnosed condition. They enrolled healthy volunteers.
• They ran four to eight weeks, so they say nothing about longer use.
• The 2011 study enrolled 55 people and the 1714 study 22, so each result rests on a few dozen people.

How to read your own bottle

1. 1Find the strain code. Species name, then letters or numbers: R0052, 1714, Shirota. No code means no traceable trial.
2. 2Match the code to a trial. The strain on the label should be the strain in the study.
3. 3Read the CFU line. CFU is the count of live organisms. Look for a count that holds to the end of shelf life, not only at manufacture.
4. 4Read the storage instructions. They tell you whether it needs the fridge.
5. 5Find the AUST L or AUST R number. It shows the product is on the Australian Register of Therapeutic Goods. For a listed (AUST L) medicine, the TGA does not assess whether it works before it is listed.

Where a pharmacist fits

If a naturopath or nutritionist chose your probiotic, keep working with them. Practitioners draw on traditional use and clinical experience as well as trials, and this post covers one kind of evidence only. The strain code is simply a good question to bring to whoever chose it, and a good one to bring to us.

If you already take a probiotic for your digestion, nothing here is a reason to stop. This post is only about stress. For the basics of how probiotics are used for gut health, see our probiotics guide.

We make no stress claim for the probiotics we stock.

You can look a strain code up yourself. The papers are searchable. If you would like a second set of eyes on what you find, ask in the chat on this site.

Questions people ask

What is the gut–brain axis? The two-way signalling between the gut, its microbes and the brain, through the vagus nerve and gut nervous system, immune messengers and microbial chemicals such as short-chain fatty acids.

Do probiotics help with anxiety? None of the studies in this post enrolled people with anxiety. They enrolled healthy adults. If anxiety is affecting you, see your GP.

Is there a best probiotic for stress? None of the studies above compared strains against each other. The list shows what has been tested, not what to take.

Does the brand matter, or just the strain? The strain code tells you whether a trial exists. The maker decides whether the strain is still alive at the stated dose when you take it, which is what the CFU and storage lines are about. If the same strain, dose and shelf-life count cost less elsewhere, and the rest of the label matches, you are comparing like with like, and the cheaper one is the better buy.

Pharmacist or naturopath for probiotic advice? Both, for different things. Your practitioner knows your history and what you are taking it for. A pharmacist can check the strain, the duration and the trial behind it, and check it against your other medicines.

Sources

• Bravo and colleagues, PNAS, 2011. doi:10.1073/pnas.1102999108
• Kelly and colleagues, Brain, Behavior, and Immunity, 2017. PubMed 27865949
• Cryan and colleagues, Physiological Reviews, 2019. doi:10.1152/physrev.00018.2018
• Breit and colleagues, Frontiers in Psychiatry, 2018. doi:10.3389/fpsyt.2018.00044
• Hill and colleagues, Nature Reviews Gastroenterology and Hepatology, 2014. doi:10.1038/nrgastro.2014.66
• Messaoudi and colleagues, British Journal of Nutrition, 2011. doi:10.1017/S0007114510004319
• Allen and colleagues, Translational Psychiatry, 2016. doi:10.1038/tp.2016.191
• Takada and colleagues, Neurogastroenterology and Motility, 2016. doi:10.1111/nmo.12804
• Starting point: AJP Advancing Your Practice CPD, "The gut–brain axis and probiotic applications", AJP26071, 1 July 2026.