Small talk--The gabfest of microbial communication

April 29, 2003

She thinks they're everywhere. What's more, she thinks they talk to each other.

But don't snicker...ONR-sponsored Bonnie Bassler won a MacArthur Foundation 'genius award' last year for her research on how some of the most deadly microbes we know - cholera, plague, TB, just to mention a few - communicate surprisingly well.

In her Princeton Lab, Bassler (and the rest of the microbiology community) calls it 'quorum sensing.' When microbes sense that there's more than just a few of them around (i.e., increases in cell population density), a sort of gabfest starts, and this can lead to the production of toxins that make us very, very sick.

Microscopic organisms must rely on simple, yet complex (depending on how you look at it) means of communication. "Quorum sensing" was first discovered in two bacteria in the belly of the cuttlefish: Vibrio harveyi and Vibrio fischeri. These fellows emit light in response to increases in cell population density - they release and detect hormone-like molecules called autoinducers that accumulate in the surrounding aquatic environment as the bacterial cell density increases.

Working with Vibrio harveyi and Vibrio fischeri Bassler and her colleagues discovered that bacteria express a gene called LuxI, which results in the release of special chemicals she calls autoinducers (AI-1) which, in turn, bind to proteins called LuxR on other bacteria nearby. Once the LuxR is activated, a multitude of cellular effects, varying by species, is performed. This is especially useful to bacteria in sensing the size of their colony. Each bacterium constantly emits AI-1, as the number of individuals in a colony grows, so does the amount of AI-1 surrounding them. Once the colony reaches a certain size, a quorum, the amount of LuxI is sufficient to trigger cellular effects. Scores of bacteria species use this quorum sensing every day.

But why would bacteria care how many others are around them? Bacteria are the biggest biomass on the planet, and in order for them to be as successful as they are, they must work together for the good of the colony. Each bacterium is not a 'lone soldier' so to speak, rather it waits until a sufficient number of others are around to begin producing toxins, or emitting light, etc. "Bacteria can talk to each other," Bassler says. "They take a roll call and the language is LuxI/LuxR."

Bassler's research is important in the fight against virulent strains of bacteria. Her team's current work is to find a way to disrupt this LuxI/LuxR language so the bacteria are deafened to the calls of their compatriots. Bassler and colleagues recently showed that cholera bacteria use quorum sensing to regulate their virulence.

"If a bacterium thinks it is alone in the world, it won't produce the toxins which make us sick, or, for that matter, make biofilms which lead to ship hull fouling" says ONR sponsor Dr. Linda Chrisey. It's only when many of them get together, and decide to "turn on", that the bacteria become a problem. "Since a disruption-type of therapy wouldn't kill the bacteria, resistance to these new drugs would be slow to develop. This research has enormous potential for rapid, accurate pathogen sensing and novel antibiotic strategies."

Harmless bacteria can also be used to sense lethal strains. V. harveyi, which uses the LuxI/LuxR language to decide when to glow, can sense LuxI from other harmful species. "These guys can sense the 'who's who' of pathogenic bacteria, like anthrax, staph, strep, E. coli, and salmonella," Bassler says. "When V. harveyi senses the AI from other species, it glows brilliantly. In fact, many diverse bacterial functions such as virulence factor production, conjugative DNA transfer, symbiosis, and antibiotic production are now known to be controlled by quorum sensing."

Ah... spoken like a true genius.
-end-


Office of Naval Research

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