There are beneficial and pathogenic flora in human. Various factors such as environmental, nutritional and/or metabolic changes favor pathogen proliferation and that disturb equilibrium between them leading to disease conditions.
Synthetic antibiotics may be used to control pathogenic overgrowth; but also may induce multi-drug resistance in pathogenic bacteria. Hence, the utilization of beneficial bacteria (probiotics) has emerged as an alternative based on the beneficial good results obtained with it.
There are some probiotics used in the commercial marketing, such as: Baccilus, Lactobacillus and Bifidobacterium. However, there are some major challenges related to manufacturing probiotic formulations containing Lactobacillus and Bifidobacterium, such as: Their handling and production is complex and could be a challenge because both of them are microaerophilic or strict anaerobic; slow growers, sensitive to temperature; many of Lactobacillus and Bifidobacterium are sensitive to gastric juice during gastrointestinal (GI) tract transit.
Meanwhile, Bacillus, especially B. subtillis spore, has many outstanding advantages such as: highly resistant to temperature, extreme pH, gastric acid; bile and solvents, hence keep viability in the gut., B. subtilis can be stored for long time periods without refrigeration.
B. subtilis has unique properties such as spore formation, versatility of growth nutrients utilization, high level of enzymes production, fast growth rate, and growth in aerobic and anaerobic conditions.
Strong scientific data about using Bacillus, especially B. subtillis, as probiotic have made significant progress to elucidate the activity spectrum which makes this bacterium the most attractive probiotic for clinical use.
Lamya Rhayat et al. studied effect of Bacillus subtilis on intestinal barrier function and inflammatory response. The data show that Bacillus subtilis probiotics may indeed improve digestive health by strengthening intestinal barrier and limiting inflammatory responses. 
In clinical study in patients with acute enteric infections, the results showed the pronounced curative effect of Bacillus probiotics manifested by the rapid normalization of stool, abdominal pain relief, and decrease in intestinal dysbiosis. Bacillus probiotics found to be safe and well tolerated.
B. subtilis and B. licheniformis has been also evaluated for effect on intestine microflora in acute digestive disorders and dysbacterioses in 53 newborn children with perinatal pathology. Results showed high therapeutic and prophylactic efficiency for dysbacterioses and diarrheas in the newborn children without side effects.
One of the most common side effects of antibiotic therapy is antibiotic-associated diarrhea (AAD). In clinical trial, 11 children aged 3–24 months for 5 days along with antibiotic were provided B. subtilis spores and 8 subjects was given alone antibiotic. Results showed that number of stools increased in alone antibiotic group while in B. subtilis along with antibiotic group no such changes were observed. Bacteriotherapy along with antibiotic also increased saccharolytic flora, aerobic and anaerobic flora.
Horosheva et al. carried out a randomized, double-blind, placebo-controlled clinical trial on outpatients aged ≥45 years who were prescribed ≥1 oral or intravenous antibiotics for at least 5 days. Results showed that B. subtilis significantly reduced the incidence of nausea, vomiting, bloating, and abdominal pain.
There have been 23 clinical trials involving over 1800 patients for probiotic preparation containing a combination of B. subtilis and other probiotic. It has been used in the improvement of symptoms associated with chronic diarrhea and irritable bowel syndrome, as a coadjuvant therapy with sulfasalazine and mesalazine to improve remission times in mild to moderate ulcerative colitis and to improve compliance with conventional triple therapy for Helicobacter pylori eradication.
With so many studies done, Bacillus subtilis is actually a highly effective and potential probiotic product.
Mechanisms of action:
+ Antimicrobial effect by synthesis of antimicrobial substances,
+ Antidiarrheal effect
+ Immunostimulatory effect, competitive exclusion of pathogens, prevention of intestinal inflammation, and stimulation of growth of intestinal normal flora.
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