Researchers have discovered one way the bacterium that causes Legionnaires’ disease successfully lives in contaminated water supplies: by killing off its neighbors. The study, published today in the journal eLife by scientists at Fred Hutchinson Cancer Research Center, reveals the unexpected molecular poison the bacterium uses to do so — and suggests a potential way to prevent an illness that strikes about 6,000 people each year in the U.S.
Lead scientist Dr. Tera Levin’s interest in the project was piqued when she grew Legionella pneumophila on culture plates and saw that nearby bacteria died off. Levin, a postdoctoral fellow in Hutch geneticist Dr. Harmit Malik’s laboratory, was surprised to discover that L. pneumophila uses a common molecule called HGA to kill off neighboring bacteria, ensuring more nutrients (and better growing conditions) for itself. It just so happens that a compound that inhibits L. pneumophila’s ability to produce HGA already exists, although it’s too soon to know if it could play a role in public health, the researchers said.
Levin added that HGA is made by many other bacterial species, and it may be that some of them also use HGA as a weapon and could be similarly inhibited. She also found that L. pneumophila only produces HGA under specific conditions, suggesting that it may use a previously unknown strategy to “count” the number of kin, or genetically identical bacterial cells, nearby.
Legionnaires’ disease, an atypical form of pneumonia, is named after U.S. veteran members of the American Legion, among whom it was first diagnosed in the 1970s. The bacteria that causes it, L. pneumophila, is naturally found in freshwater lakes and streams and doesn’t usually infect people. However, it can contaminate human-built water-supply systems. If people, particularly those with reduced immune function like cancer patients and the elderly, breathe in L. pneumophila-riddled water droplets spewed from these systems, they can become infected. Legionnaires’ disease is usually easily treated with antibiotics, so the major public health challenge is preventing outbreaks in the first place, Levin said.
The bacterium has a complex life cycle. In one stage, it grows inside a water-borne amoeba. In another, it grows within slimy, complex layers made up of many different bacterial species. These layers, called biofilms, can form in fountains and water-cooling systems. Life within a biofilm is complicated. Each species of bacteria tries to outgrow its neighbor, and success depends on a delicate balance of cooperation and competition between them.
Prior to this study, L. pneumophila wasn’t known to be dangerous to nearby bacteria. But there were hints from other research that it did use some kind of strategy to succeed in the face of its neighbors’ failures, the scientists said.
Levin and co-author Brian Goldspiel, then a Malik Lab technician and now a graduate student at the University of Pennsylvania, used a combination of genetic and biochemical studies to track down the molecule responsible for the death of bacteria that approach L. pneumophila too closely. Their discovery that the killer molecule was homogentisic acid, or HGA, was the first time this common byproduct of cell metabolism has been shown to have any bactericidal activity. Prior to this study, HGA was only thought to have beneficial effects on bacterial growth.
Counterintuitively, Levin and Goldspiel also found that L. pneumophila itself is sensitive to HGA produced by neighboring, genetically identical Legionella cells. But its sensitivity depended on its environmental conditions. In uncrowded conditions, when L. pneumophila cells have little to no contact with each other, the bug is sensitive to HGA. And in these conditions, it doesn’t produce any.
But it turns out that there’s safety in numbers. Levin found that when L. pneumophila finds itself cheek-by-jowl with genetically identical, or kin, cells, it resists HGA’s toxic effects. And it’s only under these safe, crowded conditions that L. pneumophila churns out HGA. Any single bacteria passing by, even other L. pneumophila, will be killed off by the HGA produced by a bacterial clump.
“It’s a very strange mode of determining of all the cells out there, which ones are the same as me — and so we should work together — and which ones are different, and so I want to kill you and take all your stuff,” said Levin.
The way bacteria typically sense how many genetically related kin are nearby is called quorum sensing. Levin tested the only quorum-sensing mechanism known in L. pneumophila, but it made no difference to the bacteria’s susceptibility to HGA.
This means, said Levin, “Either there’s another quorum-sensing mechanism, or there’s a totally separate way of sensing density that nobody knows about. Our discovery might be a handle to pull that out.”
Though Levin has yet to discover how L. pneumophila sense that their living conditions make HGA production safe, she’s keen to work it out.
Her postdoctoral mentor, Malik, added, “Tera did not embark on this project to discover a new antibacterial intervention strategy. Instead, she followed her intuition and initially surprising findings that Legionella may have a mechanism to police its surroundings. It led us to a possible new means by which many bacteria may count their neighbors and establish a “gated” community when the conditions are right.”
Sabrina Richards, a senior editor and writer at Fred Hutch Cancer Center, has written about scientific research and the environment for The Scientist and OnEarth Magazine. She has a PhD in immunology from the University of Washington, an MA in journalism and an advanced certificate from the Science, Health and Environmental Reporting Program at New York University. Reach her at srichar2@fredhutch.org.
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