Bacillus clausii is a probiotic bacterium that forms spores. The spores survive stomach acid and pass through the gut intact. You'll find it sold as a dietary supplement in spore form.
People take it to keep gut flora in balance, especially alongside antibiotics. Many use it to help maintain a healthy microbial community. It may also promote normal immune function.
You can buy it as an oral suspension, capsule, or sachet. Take one or two doses daily with food. The spores stay active in the stomach.
Check with a doctor before use if you have a serious illness or a weak immune system. Avoid the product if you are allergic to any of its ingredients.