For the first time, scientists have found strong evidence that some types of cancer may be caused by bacteria.
Stanford University researchers reported in today's edition of the Journal of the National Cancer Institute that virtually all their patients with the most common type of stomach cancer are infected with a bacterium that has previously been linked to inflammation of the stomach and ulcers -- strong evidence that the infectious agent plays a role in the development of the disease.
Although stomach cancer strikes only about 20,000 Americans each year, it kills nearly 14,000 and is common elsewhere, reaching epidemic status in some areas of Asia and South America.
At least two other groups of researchers also have linked the bacterium to stomach cancer, although neither has published findings yet.
"All the evidence is consistent, and it is all moving in the same direction -- that [the bacterium] does cause stomach cancer," said epidemiologist Martin J. Blaser of Vanderbilt University.
A few human cancers are known to be caused by viruses, and one or two by carcinogenic chemicals produced by molds and fungi, but to date none has been clearly linked to bacteria. The majority of cancers are thought to be caused by carcinogens in the environment, radiation and genetic susceptibility.
Linking a bacterium to the disease is critical because it should then be possible to make sharp inroads into the incidence of stomach cancer by developing vaccines to prevent the bacterial infections.
"I'm optimistic because, if it is correct, this is the kind of thing one could do something about," said oncologist John Laszlo, medical director of the American Cancer Society in Atlanta. "Many people will follow it up quickly."
But he cautioned that previous research linking bacteria to other forms of cancer, such as Hodgkin's disease, subsequently has been disproved. In such cases, he said, the association was found to be only coincidental. "We need confirmation with much larger numbers of patients to be convinced," he said.
The bacterium in question is called Heliobacter pylori. The spiral-shaped microorganism was first isolated and identified in 1982 by Australian researchers and has since been the subject of intensive study. It is the only microorganism that can live in the acidic environment of the stomach and is rarely found elsewhere in the body, according to Stanford epidemiologist Julie Parsonnet, who conducted the study.
Most people who are infected by it show no symptoms and are not harmed by the microorganism. But scientists are now convinced that a Heliobacter infection is necessary for peptic ulcers to develop, and a growing body of evidence indicates that it also contributes to gastritis, an inflammation of the stomach lining. It can be killed with powerful antibiotics, but for most people the risks of side effects from the antibiotics are greater than the risk of ulcers or cancer.