Immune system's distress signal tells bacteria when to strike back

The human opportunistic pathogen, Pseudomonas aeruginosa, has broken the immune system's code, report researchers from the University of Chicago, enabling the bacteria to recognize when its host is most vulnerable and to launch an attack before the weakened host can muster its defenses.

In the 29 July 2005 issue of Science, the researchers show how this lethal organism detects interferon-gamma, a chemical messenger the immune system uses to coordinate its efforts to get rid of bacteria. When these bacteria intercept this message, they recognize it as a threat, assess their own numbers and, if they have sufficient strength, activate genes that quickly transform them from benign passengers in the bowel into deadly blood-stream invaders.

"Most of the time these microbes are content to live and grow in our intestines," said John Alverdy, M.D., professor of surgery at the University of Chicago and director of the study. "They don't feel the need or even look for the opportunity to attack. But when they detect a threat, they have a remarkably sophisticated defense plan, based, unfortunately, on the notion that the best defense is an overwhelming offense."

Pseudomonas aeruginosa is ubiquitous. It lives in all sorts of moist places, including damp soil and on the surface of vegetables, as well as in streams, faucets and drinking fountains. It is often a long-term bowel tenant, colonizing the intestines of about three percent of healthy people.

In the bowel this germ is usually harmless, but it can turn deadly, causing gut-derived sepsis. It is also a frequent cause of infections after major surgery.

Physicians have theorized, said Alverdy, that germs such as Pseudomonas are always "probing for a weakness in the host and are ready and willing to strike whenever they find one." He and his colleagues, however, are testing an alternative theory: that "bacteria are perfectly content in their niche until signals from the

Contact: John Easton
University of Chicago Medical Center

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