Narrator:        This is Science Today. Certain disease-causing microbes like the food-borne bacterium, listeria monocytogenes, invade and replicate inside the walls of the host's cells. This makes it hard for the immune system to attack and kill these pathogens, but microbiologist Daniel Portnoy of the University of California, Berkeley discovered a molecular alarm system inside the cell that sends out signals that can trigger a response from the immune system.

Portnoy:         The observation is that the immune system is able to detect something - a small molecule that's being pumped out of bacteria. Now, the implication here is that bacteria have to be alive to be pumping out these molecules and previously, most of the studies of this nature had looked at the response of the immune system to molecules that can be taken from a killed organism and represent more of a static picture of the response of the host. Our study suggests that the immune system has a mechanism to detect live pathogenic organisms.

Narrator:        For Science Today, I'm Larissa Branin.