To infect the stomach, ulcer-causing bacteria must first stick to the stomach wall, and some strains do it better than others. Now a study of various types reveals why: Infective bugs alter their tactics to adapt to different people's stomachs. The results help explain why South American Indians suffer so much from ulcer bugs, and it could help researchers develop a therapeutic vaccine that prevents ulcers and stomach cancer.
Spiral-shaped bacteria called Helicobacter pylori infect the stomachs of more than half the world's population, causing gastritis in many people and ulcers and stomach cancer in some. Antibiotics are used to banish the bug in rich countries, but researchers want less expensive ways to treat ulcer-bug infections in the developing world. One approach is to block the bug's attachment to the stomach lining. A decade ago, microbiologist Thomas Borén of Umeå University in Sweden and colleagues discovered that ulcer bugs bind the stomach by sticking to the same complex sugars that define blood types A, B, and O. Because people with blood type O are more likely to suffer from ulcers and stomach cancer, they tested how tightly Helicobacter strains from around the world bound to sugars representing the different blood types.
Bugs from Europeans, Japanese, and native Alaskans, whose populations include individuals of all blood types, were generalists: They bound A, B, and O antigens equally well. Bugs from South American Indians, however, bound only the group O antigen. Because almost all South American Indians have type O tissue, that suggested the Helicobacter in their guts had become specialists.
Probing further, the researchers found that the gene for a protein called BabA, which tightly binds the blood-group sugars, has a slightly different sequence in specialist and generalist bacteria. Swapping a small stretch of DNA turned a specialist bacterium back into a generalist, they report in the 23 July issue of Science. That means that Helicobacter relies on BabA to bind the stomach lining, Borén says. A vaccine that blocks BabA could prevent ulcers and stomach cancer, Borén says, but only if it's fine-tuned to hobble the BabA variant found in Helicobacter from the target population. "I think the study is good and solid," says gastroenterologist Nicola Jones of the Hospital for Sick Children in Toronto. It's the first study that shows ulcer bugs adapt to different human populations, and "it's an example of how bacteria can evolve to promote chronic infection."