Chemical Conversations With Bacteria

Howard Hughes Medical Institute, November 15, 2007

Bonnie L. Bassler PhD

Researchers have deciphered the molecular language that cholera bacteria use to coordinate their infectivity. The bacteria use this chemical communication to signal their presence to one another, so that they can plan as a group when to be most virulent and when to escape their host to find new victims.

Although cholera is rare in the U.S., it is epidemic in parts of Africa, Asia and Latin America, and the severe diarrhea it causes can lead to death if not treated. The researchers say that by interrupting the bacterium’s chemical conversation, they may be able to stop cholera virulence. Their findings also offer hope that similar approaches may form the basis of effective treatments for a wide range of other bacterial diseases.

“If our studies with cholera demonstrate that it is possible to trick bacteria into reducing virulence, they constitute the first demonstration that manipulating such bacterial conversations can be a useful treatment.”
Bonnie L. Bassler

Howard Hughes Medical Institute investigator Bonnie Bassler and her colleagues at Princeton University reported their findings in the November, 15, 2007, issue of the journal Nature.

Bassler and her colleagues have long studied a type of bacterial chemical conversation known as quorum sensing. This process depends on the bacteria releasing signaling chemicals called autoinducers into their environment, and subsequently detecting and responding to the build up of these molecules to coordinate with one another to ensure maximum infectivity and other group behaviors.

“We had shown that cholera had quorum sensing, and we had produced a mutant form of cholera that couldn’t perform quorum sensing properly, which affected virulence,” said Bassler. “This finding told us that there must be an autoinducer molecule that this mutant couldn’t make that had a role in virulence, but we had no idea what that molecule was.”

Bassler explained that the way the cholera bacteria use that molecule suggested it could make a useful treatment. “When people first get cholera, the bacteria immediately stick to the intestine in a structure called a biofilm and they release toxins,” she said. “During this time, they are multiplying rapidly and also releasing the autoinducer molecule. When the bacteria reach high cell numbers, the high concentration of the autoinducer molecule represses virulence and stops biofilm formation, enabling the bacteria to escape into the environment to spread to other people. So, if we could isolate and purify this molecule, and supply it to the bacteria to get them to prematurely terminate virulence, we thought it could be used as a treatment approach.”

Through their mutational studies, the researchers had identified the gene that codes for the enzyme that makes the unknown molecule. They inserted that gene into the gut bacterium E. coli, transforming the bacterium into a biological factory for large amounts of the chemical. That strategy allowed them to purify the chemical, which they called CAI-1, and analyze its molecular structure.

“This structure produced a real surprise,” said Bassler. “CAI-1 turned out to be a molecule brand new to biology. What’s more, it was a simple molecule, almost like one you could buy at a chemical supply house. Because there was no precedent for this molecule, we felt we had to go to a lot of effort to demonstrate that this really was the correct molecule.” To do so, the researchers created synthetic CAI-1 and introduced it into cultures of cholera bacteria. The synthetic CAI-1 repressed virulence in those cells exactly like the natural molecule did. Carrying their studies further, the researchers are now exploring how CAI-1 is made by analyzing the function of the enzyme that produces it.

CAI-1’s success in terminating virulence in cultures of cholera has encouraged Bassler and her colleagues to test the chemical as a treatment. “Next, we want to see whether we can cure a mouse of cholera using CAI-1,” she said. “These experiments also will enable us to answer some important questions about the properties of the molecule. For example, does it last in the gut? Is it stable? What should be the dose? Do we have to adjust the structure to make it more potent or less potent?”

The discovery of CAI-1 may also inspire efforts to control quorum sensing to treat other bacterial diseases, said Bassler. “Cholera uses quorum sensing in a different way than most other bacteria,” she said. “Cholera causes an acute infection; it gets into the host and then has to get out, so its strategy is to use quorum sensing to repress virulence when the bacterial cells reach high numbers. But other bacteria that cause persistent infections use quorum sensing to turn on virulence only when they reach high numbers—which makes biological sense because they want to hide from the immune system until they have successfully reproduced and then launch their attack en masse. Thus, treatments for other bacteria that target quorum sensing are focused on developing drugs that block autoinducers. These drugs are very hard to make, and such efforts have not yet been very successful.

“If our studies with cholera demonstrate that it is possible to trick bacteria into reducing virulence, they constitute the first demonstration that manipulating such bacterial conversations can be a useful treatment. It will give the field solid evidence that quorum sensing is a viable new therapeutic target, which is especially important given the failure of so many traditional antibiotics.”

Also, emphasized Bassler, whose team has solved the structures of other quorum sensing molecules, the discovery of diverse quorum-sensing molecules such as CAI-1 represents another step in a promising and productive effort to decipher and manipulate the chemical language of bacteria.

“We know that there are molecules analogous to CAI-1 that are very species-specific, and we also understand that there are molecules that are generic and enable inter-species communication. Together, they give bacteria a multicellular character. And the fact that we are coming to understand this communication and even learn how to manipulate it both for medical and industrial purposes makes this a very exciting time for this research field.”

image006.gifUntil recently, the ability of bacteria to communicate with one another was considered an anomaly that occurred only among a few marine bacteria. It is now clear that group talk is the norm in the bacterial world, and understanding this process is important for fighting deadly strains of bacteria and for understanding communication between cells in the human body.

Bonnie Bassler has discovered that bacteria communicate with a chemical language. This process, called quorum sensing, allows bacteria to count their numbers, determine when they have reached a critical mass, and then change their behavior in unison to carry out processes that require many cells acting together to be effective.

For example, one process commonly controlled by quorum sensing is virulence. Virulent bacteria do not want to begin secreting toxins too soon, or the host’s immune system will quickly eliminate the nascent infection. Instead, Bassler explained, using quorum sensing, the bacteria count themselves and when they reach a sufficiently high number, they all launch their attack simultaneously. This way, the bacteria are more likely to overpower the immune system. Quorum sensing, Bassler says, allows bacteria to act like enormous multicellular organisms. She has shown that this same basic mechanism of communication exists in some of the world’s most virulent microbes, including those responsible for cholera and plague.

Working with Vibrio harveyi, a harmless marine bacterium that glows in the dark, Bassler and her colleagues discovered that this bacterium communicates with multiple chemical signaling molecules called autoinducers (AIs). Some of these molecules allow V. harveyi to talk to its own kind, while one molecule—called AI-2—allows the bacterium to talk to other bacterial species in its vicinity. Bassler showed that a gene called luxS is required for production of AI-2, and that hundreds of species of bacteria have this gene and use AI-2 to communicate. This work suggests that bacteria have a universal chemical language, a type of “bacterial Esperanto” that they use to talk between species.

Bassler’s research opens up the possibility for new strategies for combating important world health problems. Her team is currently working to find ways to disrupt the LuxS/AI-2 discourse so the bacteria either cannot talk or cannot listen to one another. Such strategies have potential use as new antimicrobial therapies.

Her interest in bacterial communication grew from her curiosity about how information flows among cells in the human body, and she is convinced she will find parallels between the bacterial systems and those in higher organisms. “We have a chance to learn something fundamental in bacteria about chemical communication,” Bassler said. “If we can understand the rules or paradigms governing the process in bacteria, what we learn could hold true in higher organisms.”

Bassler won a 2002 MacArthur Fellowship, which she said provided tremendous validation for her group’s research, recognizing that they are working on a problem that is much larger than a glow-in-the-dark bacterium. She was also chosen as the 2004 Inventor of the Year by the New York Intellectual Property Law Association for her idea that interfering with the AI-2 language could form the basis of a new type of broad-spectrum antibiotic. “The fantasy is to make one pill that works against all kinds of bacteria,” she said.

Dr. Bassler is also Squibb Professor and Director of Graduate Studies in the Department of Molecular Biology at Princeton University.

RESEARCH ABSTRACT SUMMARY:

Bonnie Bassler studies the molecular mechanisms that bacteria use to communicate with one another, and her aims include combating deadly bacterial diseases and understanding cell signaling in higher organisms.