L. Plantarum is a beneficial and versatile probiotic strain, able to survive stomach acid with ease, allowing it to reach the colon fully active and intact. Here, the probiotic strain is effective at maintaining, restoring, and balancing the gut microbiota.
Studies have shown that L. Plantarum can support long-term memory by increasing levels of the all-star brain growth hormone, brain-derived neurotrophic factor (BDNF). The culture has also been shown to improve cognitive abilities in rats by lowering pressure and exerting neuroprotective properties in brain cells. 
Regular consumption of L. Plantarum may also reduce anxiety. The strain was found to increase levels of dopamine and serotonin while reducing anxiety symptoms, depressive symptoms, and inflammatory cytokines associated with mental and physical stress. [2 3]
L. Plantarum also plays a key role in regulating inflammation. Daily supplementation was found to reduce production of inflammatory cytokines and reduce oxidative stress in animal models .
L. Plantarum supports the health and survival of brain cells by suppressing activation of the protein NF-κB, the single most important regulator of inflammation in the body and brain. NF-κB inhibition is linked with brain growth and development. More specifically, inhibition of this protein is associated with improved learning, intelligence, neural plasticity, and memory. synaptic plasticity, BDNF levels are also increased, which enhances cognitive function and the growth of new neurons. [5, 6, 7]
L. Brevis is one of the most well-studied probiotic strains known to influence human health and cognition. The predominant culture in sauerkraut and pickled vegetables, this strain is well known to increase immune function and fight bacterial infection, in addition to its unique effects on the central nervous system.
L. Brevis provides support against inflammatory compounds associated with impaired memory and cognitive decline. It is one of the essential probiotics for brain health because it increases levels of BDNF, the brain’s all-star growth hormone that controls the health and growth of new neurons. In one study, old rats were given L. Brevis and young rats were given a placebo. The results found that in addition to lowering inflammatory markers TNF, IL1β, and NF-κB L. Brevis significantly increased expression of BDNF to levels greater than the healthy young rats. 
This suggests that L. Brevis may restore healthy brain function in elderly populations, possibly to levels greater than healthy young individuals.
L. Brevis has also been shown to have a positive effect on reducing anxiety, depressive symptoms, and restoring a positive mood. 
Lactobacillus Helveticus & Bifidobacterium Longum
These are two of the most effective probiotic strains to promote a positive mood, reduce stress, and relieve anxiety and depressive symptoms.
In one study, consuming a mixture of three Lactobacillus and Bifidobacteria strains for eight weeks significantly reduced depressive symptoms. The probiotic also reduced inflammation in the patients. 
A handful of other studies have examined how probiotics affect depressive symptoms in healthy subjects. In healthy people these strains have been found to reduce:
Symptoms of anxiety [11, 12, 13]
Depressive symptoms 
Psychological distress 
Academic stress 
Researchers also found that taking probiotics reduced participants’ negative thoughts associated with a sad mood.