Microbes and Magic Spot: Conjuring cures for a mysterious malady
Medical investigators were mystified in 1976, when some 200 American Legion members came down with pneumonia-like symptoms and 34 died after attending a convention in Philadelphia.
In time, the blame was pinned on a previously unknown bacterium christened Legionella pneumophila, and the illness it causes came to be known as Legionnaires’ Disease. The bug, it turns out, thrives in water and is just as happy in the cooling towers of hotel air conditioning systems as in the ponds it normally inhabits. People at the convention had become infected by breathing bacteria-laden water droplets, researchers eventually deduced.
The mystery was solved, but the case wasn’t closed, and more than 30 years later, L. pneumophila has yet to be conquered. Outbreaks still occur in hospitals, hotels, cruise ships and other settings where water in air conditioning systems or hot tubs can become contaminated. Even fountains, vaporizers and the misters that keep produce fresh in grocery store bins can harbor Legionella, which sickens an estimated 20,000 Americans annually and is especially dangerous to older people, smokers and people whose immune systems have been weakened by diseases such as cancer and diabetes.
The unsolved mysteries of Legionella intrigue Michele Swanson, who studies interactions between the microbe and white blood cells in the people it infects. The insights she gleans will be useful not only in thwarting Legionella, but also in battling other disease-causing bacteria with similar lifestyles.
One reason Legionella remains a public health problem, in spite of accurate diagnostic tests and effective antibiotics, is the wily way it hides out, sheltered from water treatment chemicals that kill less secretive bacteria. In truth, it isn’t so much craftiness as circumstance that gives Legionella the edge: like Jonah in the whale’s belly, the bug gets swallowed up and spends a good bit of its life inside another water-dweller, the amoeba—a single-celled blob of a creature that Swanson calls a “professional eater.”
“The same tactics that allow Legionella to survive and reproduce in amoebae also allow it to survive and reproduce in human white blood cells known as macrophages (big-eaters), sentries of the immune system that—like amoebae—crawl around looking for bacteria and other particles to engulf,” Swanson says.
Using modern molecular biology and genetic tools, combined with the technique of fluorescence microscopy, Swanson’s research group is looking in detail at Legionella’s neatest trick: the ability to avoid being digested and destroyed when eaten by an amoeba or a white blood cell. The trick is part of the bug’s mercurial lifestyle, which involves dramatic switches between two forms: a benign homebody that builds itself a cozy compartment in which to reproduce, and a restless and virulent “transmissive” form that breaks free, swims away and takes shelter in another amoeba or white blood cell.
The whole process is coordinated by a molecule with a fanciful name: Magic Spot. Because both this molecule and the habit of switching between reproductive and transmissive forms are common to many bacteria, Swanson believes that focusing on Magic Spot and the processes it directs is the key to keeping Legionella—and other disease-causing microbes—in check.
