Helicobacter pylori is a conventional gram-negative bacteria that causes an infection of the gastric mucosa. The organism is sensitive to most common antibiotics and theoretically should be easy to eradicate, provided that the patient takes antibiotics to which the organism is sensitive using a sufficient dose and duration of therapy. Unfortunately, the infection has proved difficult to cure. There are many other examples of bacterial infections that resist treatment with antibiotics even when the individual organisms demonstrate antibiotic sensitivity in vitro (e.g., Salmonella in the biliary tract). Failure can be due to the presence of antibiotic-resistant organisms, features of the infection may make it inaccessible to the antibiotics despite having antibiotic-sensitive organisms (a resistant infection), or both. Acquired resistance of H. pylori to clarithromycin, metronidazole, tetracycline, and amoxicillin have all been described.