Pseudomonas aeruginosa has broken the immune system's code

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 host – usually during stress, such as after major surgery – let them know there's a problem."

For Pseudomonas, detecting interferon-gamma, "is like receiving a demolition notice from your landlord," Alverdy said. "It lets them know they need to find a new home. They don't take that news any better than we would."

A vulnerable host, like a condemned home, is a liability, a threat to its tenants' survival. Pseudomonas, however, has the tools to engineer its own escape – by killing off the host.

This wily pathogen can evade a host's immune system. It can repel antibiotics, secrete toxins similar to those used by anthrax, latch onto the bowel wall, bore its way through, and flow into the blood stream. As a consequence, patients with widespread Pseudomonas infection often die within a few days.

Alverdy and colleagues were able to pinpoint key early steps of this lethal process. The transformation starts when a weakened host tries to boost its defenses against any possible invasion. The host's T cells release chemical signals that activate the immune system. One of those signals, interferon-gamma, is intercepted by a protein, called OprF, found on the outer membrane surface of Pseudomonas. This serves an early warning system.

Once Pseudomonas detects the first signs of a brewing immune response, they also begin to prepare for battle, gathering information and responding with their own counteroffensive.

Their first move is a process called quorum sensing, which bacteria use to gauge their own numbers. When interferon-gamma binds with OprF on the bacterial cell surface, it activates a gene called rhII. RhII triggers synthesis and secretion of a bacterial signaling molecule called C4-HSL. By measuring the amount of C4-SHL in their environment these bacteria can estimate their own numbers and density.

If they feel they are sufficiently numerous, they produce two virulence factors, molecular weapons known as PA-I and Pyocyanin. PA-I causes the barrier cells that line the host's bowel to become more permeable, which renders them more susceptible to the microbe's toxins. Pyocyanin enhances the germ's ability to pass through the weakened bowel wall, enter the bloodstream and invade tissue.

"Our goal," Alverdy said, "is to understand the many steps in this process and use that knowledge to find novel ways to intervene, to stop the infection before it starts rather than trying to kill all the germs."

Many harmful bacteria have already learned how to resist the drugs developed to treat them. Scientists are now looking at alternatives, such as ways to block or scramble the chemical messages that allow microbes to eavesdrop on their hosts or to conspire together to mount an attack.

"We chose to study this in Pseudomonas because it is one of the deadliest infections for patients who undergo major surgery," said Alverdy. "We suspect something very similar, however, occurs in all sorts of infections."

Inflammatory bowel disease patients, for example, have elevated cytokines – the chemical messengers that trigger an immune response – in the bowel. "These could signal the bug," said Alverdy, "then the bug strikes back and then the inflammation process snowballs." Because the bacteria in this case are "normal flora," people with no real infection develop a chronic disease.

The battles between pathogens and their hosts have been going on for millions of years, Alverdy said, with each side constantly devising novel measures, countermeasures, and counter-countermeasures, including sophisticated mutual espionage.

The discovery of antibiotics gave human hosts a temporary advantage, "but that seems to be waning a bit," he added. "We need to learn new ways to understand our germs and think about how to placate rather than annihilate them."