Some species have been found living in the lining of the upper gastrointestinal tract, as well as the liver of mammals and some birds. The most widely known species of the genus is H. pylori which infects up to 50% of the human population. Some strains of this bacterium are pathogenic to humans as it is strongly associated with peptic ulcers, chronic gastritis, duodenitis, and stomach cancer. It also serves as the type species of the genus.
Helicobacter spp. are able to thrive in the very acidic mammalian stomach by producing large quantities of the enzyme urease, which locally raises the pH from ~2 to a more biocompatible range of 6 to 7. Bacteria belonging to this genus are usually susceptible to antibiotics such as penicillin, are microaerophilic (require small amounts of oxygen) capnophiles, and are fast-moving with their flagella.
Read more about Helicobacter: Molecular Signatures, Non-pylori Helicobacter Species