Over a century ago,a Russian researcher Ilya Ilyich (Élie) Metchnikoff (1908 Nobel Prize ) did research in Immunology. He theorized that health could be enhanced, and also that senility could be delayed, by manipulating the intestinal microbiome with host-friendly bacteria found in yogurt now known as Probiotics (translated from gr. "For Life"). His theory was so significant that L.Pasteur invited him to Paris and they experimented together. This theory flourished and in the mid-1990s gained mainstream medical attention. 

Bacillus subtilis is one of the oldest bacterium used widely in industry and medicine. 

Cultures of B. subtilis were popular worldwide, before the introduction of antibiotics, as an immunostimulatory agent to aid treatment of gastrointestinal and urinary tract diseases.
This is is a Gram-positive, rod-shaped bacterium that forms heat-resistant spores. It is commonly found in the soil. It is nonpathogenic. It received its name in 1872 from Ferdinand Cohn, who also demonstrated its ability to form spores that were heat-resistant. It produces several commercially important products, most notably proteases and amylases. In part because of its commercial importance, and more because of the ease of its genetic manipulation, B. subtilis has been intensively studied.  A strain of B. subtilis formerly known as Bacillus natto is used in the commercial production of the Japanese food nattō, as well as the similar Korean food cheonggukjang.

Bacillus Subtilis 250 CFU + Coagulans 250 CFU Tablets - Daily Probiotic, Healthy Gut, UK | Natural Foundation Supplements (120 Tablets)s 

Lactobacillus reuteri is named after Gerhard Reuter, a German microbiologist who spent most of his career attempting to understand the human intestinal microbiota with a focus on lactobacilli and bifidobacteria.His lab undertook a challenge that we still face in microbial ecology today—how do we access the small intestine with minimal disruption in order to understand the ecology of the normal small intestine? Using regulated capsule sampling technology and cultivationfrom sudden death victims, Reuter found that lactobacilli were abundant in the small intestine and present in feces of many of the individuals they sampled, which included children and adults. In addition, they found significant levels of lactobacilli in the stomach and duodenum, which for a long time were considered sterile sites of the GI tract. From  R.A. Britton, in The Microbiota in Gastrointestinal Pathophysiology, 2017.

in 1985 Walter Dobrogosz and colleague Sven Lindgren discovered that L. reuteri produces a potent anti-microbial substance, which they termed "reuterin." Based on this, they hypothesized that the human-specific strain of L. reuteri had the potential to be a protective probiotic for human and animal health. L. reuteri is now known to maintain intestinal health, prevent fungal, bacterial, and protozoal infections, and mediate the body's immune response. Trughout the years of research a few L.reuteri strains proved beneficial in more than 200 clinical studies like:
Lactobacillus reuteri, in experimental models (aged mice) generated some fascinating effects, including thicker hair, no weight gain (compared to obese controls), more muscle preservation,
increased dermal (skin) thickness and increased testosterone. They observed that the
L. reuteri-supplemented mice looked and acted more youthful. What made the findings even more surprising was the magnitude of these effects:
- the increase in dermal thickness, for instance, was an unprecedented 35%,
- the increase in skin thickness 100%,
- the increase in testosterone nearly 8-fold.

Another study show additional fascinating observations supplementing L. reuteri (strain ATCC PTA 6475) to aged mice like :
- Wound healing time was cut in half
- Improvement in Collagen deposition
- Oxytocin blood levels in females was more than doubled

In other studies, oxytocin has been demonstrated to exert substantial weight-reducing effects 

We have to bear in mind that these observations were made in mice and there was no parallel human study. ( from: L. reuteri: It gets even better)

Some of the strains mentioned in th list below are available to grow and make your own yogurt or try as a food supplement: