What is Helicobacter pylori?
Helicobacter pylori, or H. pylori, is a spiral-shaped bacterium that grows in the mucus layer that coats the inside of the human stomach.
To survive in the harsh, acidic environment of the stomach, H. pylori secretes an enzyme called urease, which converts the chemical urea to ammonia. The production of ammonia around H. pylori neutralizes the acidity of the stomach, making it more hospitable for the bacterium. In addition, the helical shape of H. pylori allows it to burrow into the mucus layer, which is less acidic than the inside space, or lumen, of the stomach. H. pylori can also attach to the cells that line the inner surface of the stomach.
Although immune cells that normally recognize and attack invading bacteria accumulate near sites of H. pylori infection, they are unable to reach the stomach lining. In addition, H. pylori has developed ways of interfering with local immune responses, making them ineffective in eliminating this bacterium.1,2
H. pylori has coexisted with humans for many thousands of years, and infection with this bacterium is common. The Centers for Disease Control and Prevention (CDC) estimates that approximately two-thirds of the world’s population harbors the bacterium, with infection rates much higher in developing countries than in developed nations.
Although H. pylori infection does not cause illness in most infected people, it is a major risk factor for peptic ulcer disease and is responsible for the majority of ulcers of the stomach and upper small intestine. More information about H. pylori and peptic ulcer disease is available from the National Institute of Diabetes and Digestive and Kidney Diseases.
In 1994, the International Agency for Research on Cancer classified H. pylori as a carcinogen, or cancer-causing agent, in humans, despite conflicting results at the time. Since then, it has been increasingly accepted that colonization of the stomach with H. pylori is an important cause of gastric cancer and of gastric mucosa-associated lymphoid tissue (MALT) lymphoma. Infection with H. pylori is also associated with a reduced risk of esophageal adenocarcinoma.
H. pylori is thought to spread through contaminated food and water and through direct mouth-to-mouth contact. In most populations, the bacterium is first acquired during childhood. Infection is more likely in children living in poverty, in crowded conditions, and in areas with poor sanitation.
What is gastric cancer?
Gastric cancer, or cancer of the stomach, was once considered a single entity. Now, scientists divide this cancer into two main classes: gastric cardia cancer (cancer of the top inch of the stomach, where it meets the esophagus) and non-cardia gastric cancer (cancer in all other areas of the stomach).
According to NCI’s Surveillance, Epidemiology, and End Results (SEER) Program, an estimated 21,600 people in the United States will be diagnosed with gastric cancer and 10,990 people will die of this cancer during 2013. Gastric cancer is the second most common cause of cancer-related deaths in the world, killing approximately 738,000 people in 2008.3 Gastric cancer is less common in the United States and other Western countries than in countries in Asia and South America.
Overall gastric cancer incidence is decreasing. However, this decline is mainly in the rates of non-cardia gastric cancer.4 Gastric cardia cancer, which was once very uncommon, has risen in incidence in recent decades.5
Infection with H. pylori is the primary identified cause of gastric cancer. Other risk factors for gastric cancer include chronic gastritis; older age; male sex; a diet high in salted, smoked, or poorly preserved foods and low in fruits and vegetables; tobacco smoking; pernicious anemia; a history of stomach surgery for benign conditions; and a family history of stomach cancer.6,7
H. pylori has different associations with the two main classes of gastric cancer. Whereas people infected with H. pylori have an increased risk of non-cardia gastric cancer, their risk of gastric cardia cancer is not increased and may even be decreased.