Human immune signal sets off bacterial attack.

A chemical secreted by immune cells when people are sick or stressed causes a common gut bacterium to go on the offensive against its host, according to a new study.

Pseudomonas aeruginosa lives harmlessly in the intestines of about 3 percent of healthy people. However, during and after major surgery, its presence can turn into a dangerous infection, says John Alverdy of the University of Chicago.

Scientists had previously proposed that such opportunistic infections develop when the immune system is weakened. Alverdy and his colleagues hypothesized that the microbes themselves may behave differently when they sense the body is stressed.

To investigate that idea, Alverdy's team isolated various biomolecules secreted by lab-grown human immune cells, called T cells that were artificially stimulated to battle an infection. The same cells are typically activated when people are under stress or ill.

The scientists then placed the biomolecules into petri dishes of P. aeruginosa that had been genetically altered to glow green when certain virulence genes are activated. Tests revealed that one of the T cell secretions, interleukin-gamma, flipped on the genes. This protein is known to stimulate healthy cells to ward off a bacterial attack.

The results, published in the July 29 Science, suggest that "the bacterium itself can intercept a signal by the host that says 'I'm stressed' and use the same [signal] to turn virulence genes on," says Alverdy. He adds that if scientists can find some agent to keep this signal from reaching infectious microbes, they may be able to prevent opportunistic infections of P. aeruginosa and other bacteria.