The gut contains billions of bacteria. These play a role in the normal functioning of the digestive system, the lowering of blood cholesterol levels, the promotion of the immune response and the synthesis of certain vitamins. There is a delicate balance between these 'good' bacteria and harmful bacteria. Stress, antibiotics, certain medications and illnesses can cause a bacteria imbalance, resulting in gastrointestinal problems. Probiotics can be used to re-populate the gut with 'good' bacteria.