Nobody celebrates when human activity drives a species of whale or songbird toward extinction. We see those losses as threats to the biological diversity that preserves our own well-being.
But plenty of people are cheering as Helicobacter pylori vanishes from the teeming ecosystem where it has lived for thousands of years.
H. pylori is a squid-shaped bacterium that lurks in the human stomach, and scientists have discovered that it can play a key role in the development of gastric ulcers and stomach cancer - a malignancy that killed more than 12,000 Americans in 1999. Earlier this year, it killed children's TV host Fred Rogers.
"This is a major and serious pathogen," said Dr. David Y. Graham, chief of gastroenterology at the Veterans Administration Medical Center Hospital in Houston. He says H. pylori "should be eradicated whenever and wherever it is discovered."
But others defend the diversity of our innards. We might exterminate this particular pathogen at our peril, they say, because it might protect us from other deadly illnesses, such as cancer of the esophagus.
Prominent among those defenders is Dr. Martin J. Blaser, chairman and professor of microbiology at the New York University School of Medicine, who helped identify the links between the bacterium and stomach disease.
"The evidence is that Helicobacter has colonized the human stomach since our origins as humans," he said, "and like other indigenous organisms, I would presume it has both costs and benefits to humans."
As H. pylori has disappeared in the United States, there have been increases in gastro-esophageal reflux disease (GERD) - the intrusion of stomach acid into the esophagus - as well as cancer of the esophagus. Although still rare, cases of this cancer are increasing 8 percent to 9 percent a year. It's one of the fastest-rising cancers in the United States.
Not everyone agrees that esophageal disease is increasing because H. pylori is disappearing. But Blaser believes the accumulating evidence is "strong and consistent and biologically plausible."
The proposed link is acid. H. pylori suppresses acid production by attacking cells in the stomach's lining. Remove the bacteria, and the risks of GERD and esophageal cancer go up. "I would be loath to remove Helicobacter from human stomachs, certainly from people who are asymptomatic," Blaser said.
Graham insists that the scientific debate is over: "The only good H. pylori is a dead one." But Dr. John T. Bartlett, chief of the infectious diseases division at the Johns Hopkins School of Medicine, said it's still "a big gray area, and it will be a long time before anyone does a serious study to decipher it."
"The consensus, and the only consensus," he said, "is that people with peptic ulcer disease and H. pylori should be treated to get rid of H. pylori."
Most of the 500 to 1,000 species of bacteria that live in the human gut are welcome - we couldn't survive without them. They are "a life form that's perfectly insinuated into our own biology ... and major contributors to our biology," said Dr. Jeffrey I. Gordon, a gastroenterologist at Washington University in St. Louis.
Each of us feeds and shelters tens of trillions of bacteria. They serve our digestion and health by breaking down complex sugars in our food and manufacturing vitamins K and B12. They also defend us against dangerous pathogens by crowding them out, killing them off or stimulating defensive mechanisms in our intestines.
In 1983, Australian researcher Barry Marshall discovered H. pylori living happily in the mucus layer that protects the stomach lining from its own digestive acids. Few other microbes can survive there.
By infecting himself, Marshall proved that it's not always a benign resident. It gave him gastritis - inflammation of the stomach - which can lead to ulcers and cancer. But doctors scoffed for years. They believed ulcers were caused by stress or spicy food, not infections. Still, as they began prescribing antibiotics instead of antacids for ulcer patients, many more got well.
It turned out that half the world's population is infected with H. pylori, passed down from parents and older siblings like family genes. Its various genetic strains are so closely affiliated with specific human populations that scientists have used H. pylori genetics to trace ancient migrations. For example, some American Indians, whose ancestors crossed over from Siberia 13,000 years ago, still carry East Asian strains of H. pylori.
In some Third World countries, Blaser said, 70 percent to 90 percent of the children are infected with H. pylori by age 10. In the United States and other developed countries, however, infections have declined to less than 20 percent of the population - thanks to improved hygiene, reduced crowding and smaller families.