"They're quite specialized, the pathogenic bacteria. Each one recognizes a specific kind of host cell and does a specific thing. Each one has its own gimmick, as it were, to be successful."

The plague bacillus, which Falkow also studies, enters the cell very differently than Salmonella . First, it attaches to little spikes, called phylopodia, on the surface of a host cell. Its target is the epithelial cells that line the stomach and gut. Once attached, the cell slurps up the bacteria like spaghetti.

"The bacteria live in the gut, which is a place of privilege. It must be very nice for them, because what you see on the surface of the cell is the growth of colonies of bacteria. Even though they never go in, they subvert the host cell's biochemistry for their own betterment," says Falkow.

In the process, one strain of e. coli also secretes a potent toxin that causes bleeding in the small intestine. This strain was behind the recent deadly outbreak of food poisoning in the Pacific Northwest from eating undercooked hamburger.

By spying on the microscopic war between pathogens and their hosts, Falkow and his colleagues hope to eventually disarm the microbes' weapons by developing new vaccines or antibiotics. "It's really an exciting time now," says Falkow, "because we're getting to understand more about the strategies pathogens use during infection. In the process, we learn as much about host cell biology as we do about bacterial biology."

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