What do bacteria in the stomach do?

The human stomach is generally free from bacteria. Most bacteria which are swallowed either with food or in mucus from the upper repiratory tract are killed due to the low pH (acid) conditions. Some lactobaccili can survive in the stomach. Helicobacter pylori may grow in the stomach and cause stomach ulcers. It does this by producing urease which forms ammonium to neutralise the acid around itself. Mycobacterium tuberculosis may survive in the stomach and be avilable to recolonise the lungs.