24 February 2006

Prof Ponders Bacterial Benefits

by Kate Melville

Destroy all bacteria, bacteria are bad; goes the mantra of mothers everywhere and the cleaning product industry. But they could be incorrect, says Stanford University microbiologist Stanley Falkow. In an essay in the latest issue of the journal Cell, he canvases the intriguing idea that persistent bacterial and viral infections may have very real health benefits.

"Organisms that cause disease are usually considered in the context of harm and epidemics and so on," explained Falkow. "But the fact is that a great number of organisms that infect humans come in and set up housekeeping as it were. There are no clinical symptoms of anything wrong and people take the organisms with them to their graves." Falkow is quick to point out that it's not that the organisms in question - such as the bacteria that cause pneumonia or meningitis - are harmless, but that most individuals do not contract a disease from being infected.

Falkow cites the example of H. pylori, which was implicated as a cause of ulcers and stomach cancers by last year's Nobel Prize in Medicine winners. As clean water and pasteurization are adopted in developing nations, the prevalence of H. pylori infection has declined. Understandably, this has been accompanied by a drop in the incidence of gastric cancer and ulcers. But intriguingly, there has been an increase in esophageal cancer. Could H. pylori be protecting against cancer of the esophagus? Is it possible that "there might be something about persistent infection that might be protective," wonders Falkow.

Falkow and fellow researchers have been studying the phenomenon of persistent infection for decades, in particular with H. pylori and Salmonella. They have shown that when infected with these organisms, mice initially show an inflammatory response that then settles down and stays with them for the rest of their lives. Although very few of these organisms remain in the mice, it's enough to cause the immune system to have an ongoing response. "It's not so much that the immune system has failed," Falkow explained, "but that the organisms have manipulated the immune system in such a way that they can't be cleared." If the infections are cleared by antibiotic intervention, the mice are highly susceptible to re-infection, and the re-infection is more likely to progress to disease than the initial infection.

"The continued presence of these organisms in human society may actually be beneficial to the host, and that is why they are tolerated by the immune system," said Falkow. "We can guess that since a persistently infected host is constantly having its immune system stimulated and refined, that may provide it with resistance to other things."

The fact that a decrease in infectious diseases over recent years has been accompanied by a rise in autoimmune diseases, such as diabetes, lends weight to the idea that we may be wrong-headed in our thinking about bacteria.

As with H. pylori, Falkow wonders whether humans are unknowingly causing the disappearance of other things that have been with us for all of our evolution, and that may well play some role in our health. He believes that further research with animal models may provide answers.

Source: Stanford University Medical Center