Five probiotic species and the research behind each
Probiotic research is strain-specific. A finding about Lactobacillus rhamnosus JB-1 in mice is not a finding about every L. rhamnosus. With that rule in front of us, here are five species that carry the gut-brain story, with the strains the studies actually used.
Lactobacillus helveticus
Half of the most-studied psychobiotic pairing. In a 30-day randomised, double-blind, placebo-controlled trial, healthy adults taking L. helveticus R0052 with Bifidobacterium longum R0175 had lower psychological distress on the Hopkins Symptom Checklist (global severity, somatisation, depression and anger-hostility scales), lower Hospital Anxiety and Depression Scale scores, and lower 24-hour urinary free cortisol than placebo; the same formulation reduced anxiety-like behaviour in rats. (Messaoudi et al., British Journal of Nutrition, 2011, PMID 20974015.)
Lactobacillus rhamnosus
The species behind the vagus nerve study that started the field (Bravo 2011, PMID 21876150, described in the previous article). In people, the New Zealand strain HN001 was given to 423 women from mid-pregnancy to six months after birth in a randomised, placebo-controlled trial designed around infant eczema; as a secondary outcome, the mothers on HN001 reported lower postnatal depression scores (7.7 against 9.0, p = 0.037) and anxiety scores (12.0 against 13.0, p = 0.014). Fonterra co-funded the study. (Slykerman et al., EBioMedicine, 2017, PMID 28943228.)
Lactobacillus plantarum
The species with a randomised, placebo-controlled trial in children. In Taiwan, boys aged 7 to 15 with autism spectrum disorder took L. plantarum PS128 or placebo for four weeks. PS128 improved opposition and defiance behaviours, and the total score on the SNAP-IV rating scale improved significantly against placebo in the younger children, aged 7 to 12. The authors called for larger studies. (Liu et al., Nutrients, 2019, PMID 30979038.) In adults, L. plantarum DR7 at one billion CFU a day for 12 weeks lowered stress and anxiety scores on the DASS-42 in 111 stressed adults against placebo, with lower plasma cortisol and pro-inflammatory cytokines. (Chong et al., Beneficial Microbes, 2019, PMID 30882244.)
Bifidobacterium breve
In a 16-week randomised, double-blind, placebo-controlled trial of 80 older adults with suspected mild cognitive impairment, B. breve A1 (MCC1274) improved the RBANS total cognitive score against placebo by 11.3 points (95% CI 6.7 to 15.8, p < 0.0001), with no adverse events. (Xiao et al., Journal of Alzheimer's Disease, 2020, PMID 32623402.) In a 4-week trial of 45 patients with major depressive disorder, B. breve CCFM1025 produced larger reductions in depression ratings than placebo (HDRS-24 10.40 against 6.44, p = 0.036) alongside changes in serotonin turnover and gut tryptophan metabolism. (Tian et al., Brain, Behavior, and Immunity, 2022, PMID 34875345.)
Lactobacillus brevis
The GABA maker. When Irish researchers screened 91 bacterial strains from the human gut for their ability to make GABA, the brain's main calming neurotransmitter, L. brevis DPC6108 was the most efficient, converting up to 90% of its glutamate feedstock, and it raised GABA in a faeces-based fermentation. (Barrett et al., Journal of Applied Microbiology, 2012, PMID 22612585.) L. brevis W63 was one of eight strains in a multispecies product that reduced cognitive reactivity to sad mood in 40 healthy adults over four weeks. (Steenbergen et al., Brain, Behavior, and Immunity, 2015, PMID 25862297.)
Probiotics as a group
A 2025 meta-analysis of 72 randomised trials (3,319 people on a probiotic, prebiotic or synbiotic, 2,778 on placebo) found moderate improvements in depression scores (SMD -0.53) and anxiety scores (SMD -0.44), and a smaller improvement in sleep quality with probiotics (SMD -0.39), with high heterogeneity and limited study quality. (Zhang et al., BMC Psychiatry, 2025, PMID 41310510.) An earlier analysis found probiotics worked when added to antidepressants but not alone. (Nikolova et al., Journal of Clinical Medicine, 2021, PMID 33567631.) Almost all of this is in adults.
This article reports published research for parents and adults who want to read it for themselves. It is not medical advice, it does not describe or recommend any product, and it never replaces a conversation with your GP or your child's clinicians.