Dallas ? When two Australian scientists won a Nobel Prize this month for discovering that a bacterium causes ulcers, the judges praised the men for keeping faith in an idea dismissed by the scientific mainstream as crazy.

True to Helicobacter pylori’s unconventional past, some scientists are now exploring another theory just as radical: that wiping out those bacteria with antibiotics, thus curing all those ulcers, could unintentionally make people fat.

“It’s speculative, but intriguing,” said Dr. Martin Blaser of New York University School of Medicine. Blaser began his research more than 20 years ago with the belief that H. pylori was a stomach-wrecking nuisance. Yet the more he has studied it, the more he has come to think that causing disease is not H. pylori’s day job.

Scientists believe that the bacterium, though declining in developed nations, has inhabited the human stomach for more than 60,000 years. It is usually passed from person to person, and doesn’t make its home in any other animal. “The evidence of its ancientness suggests that it is normal,” he said.

Weak evidence, but with profound implications, implies a role in obesity. Most striking to Blaser and others are data suggesting that H. pylori might tinker with the stomach’s production of hormones that affect appetite and energy regulation.

It’s a proposal that some scientists view with caution or even outright skepticism.

Blaser and his colleagues are quick to say they don’t think that H. pylori alone could be responsible for a person’s weight.

“I’m a scientist. I know causation is not black and white; it’s all gray,” Blaser said.

Americans are getting larger for a complex tangle of reasons, of which microbes may be just one. Bacteria are not responsible for cheap fast food and minimal physical activity.

H. pylori could just be along for the ride, Blaser also says, while some other, undiscovered bug might be driving any effect on body weight.