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Cell-to-Cell Communication in Bacteria

Research Summary

Bonnie Bassler studies the molecular mechanisms that bacteria use to communicate with one another. This process is called quorum sensing. Her goal is to discover how bacteria evolved collective behaviors. She aims to develop anti-quorum-sensing strategies to combat deadly bacterial diseases.

Our lab wants to understand quorum sensing: the process of cell-cell communication in bacteria. Quorum sensing involves the production, release, and subsequent detection of chemical signal molecules called autoinducers. This process enables populations of bacteria to synchronously regulate gene expression, and therefore behavior, on a community-wide scale. Quorum sensing is widespread in the bacterial world, so understanding this process is fundamental to all of microbiology, including industrial and clinical microbiology, and ultimately to understanding the development of higher organisms. Our studies of quorum sensing are providing insight into intra- and inter-species communication, population-level cooperation, and the design principles underlying signal transduction and information processing at the cellular level. These investigations are also leading to synthetic strategies for controlling quorum sensing. Our objectives include development of strategies to disrupt quorum sensing in pathogenic bacteria, and pro-quorum-sensing strategies to improve production of products of medical or industrial importance. We are pursuing our goal of understanding bacterial communication by combining genetics, biochemistry, structural biology, chemistry, microarray studies, bioinformatics, modeling, and engineering approaches.

The basic steps involved in detecting and responding to fluctuations in cell number are analogous in all known quorum-sensing systems. First, low molecular weight molecules called autoinducers are synthesized intracellularly. Second, these molecules are either passively released or actively secreted outside of the cells. As the number of cells in a population increases, the extracellular concentration of autoinducer likewise increases. Third, when autoinducers accumulate above the minimal threshold level required for detection, cognate receptors bind the autoinducers and trigger signal transduction cascades that result in population-wide changes in gene expression. Thus, quorum sensing enables cells in a population to function in unison and, in so doing, bacteria carry out behaviors as a collective.

We focus on marine vibrios as model systems for quorum sensing because the first observation indicating that bacteria could communicate with multiple quorum-sensing autoinducers came with our definition of the quorum-sensing system of Vibrio harveyi. The V. harveyi quorum-quorum sensing system consists of three autoinducers and three cognate receptors functioning in parallel to channel information into a shared regulatory pathway (Figure 1). We showed that beyond controlling gene expression on a global scale, quorum sensing allows bacteria to communicate within and between species. We demonstrated that HAI-1 is only produced by V. harveyi so it is used for intra-species communication. We discovered CAI-1 and showed that it is made by all vibrios and thus is used for intra-genera communication. By contrast, AI-2 is made and detected by a vast array of bacterial species. Thus, AI-2 is used for inter-species communication (Figure 1).

By studying the signal transduction pathway that mediates responses to autoinducers, we discovered a set of redundant small RNAs (sRNAs), which we named the Qrr sRNAs that lie at the core of vibrio quorum-sensing cascades. Gene dosage compensation, coupled with a set of feedback loops, adjusts the total Qrr sRNA pool such that small perturbations in Qrr levels have profound effects on the output of the system. Changes in the Qrr sRNA levels track with changes in extracellular autoinducer levels (Figure 2). Precisely maintained Qrr levels are required to direct the proper timing and correct patterns of expression of quorum-sensing-regulated target genes. In the quorum-sensing regulon, the genes that are controlled by the Qrr sRNAs are the most rapid to respond to quorum-sensing autoinducers and presumably encode components required to instigate subsequent phases of the quorum-sensing program (Figure 2).

A more applied side of our research is focused on developing pro- and anti-quorum sensing molecules to be used as new therapeutics. Toward this end, we work on the human pathogen Vibrio cholerae, the causative agent of the endemic diarrheal disease cholera, and Pseudomonas aeruginosa, a pathogen that is devastating in cystic fibrosis patients, hospital burn units, and on sub-epithelial inserted medical devices such as intubation tubes, stents, and other long-term submerged prosthetics. In the case of V. cholerae, we showed that it possesses a quorum-sensing network similar to that of V. harveyi except the circuit controls virulence and biofilm formation. Quorum sensing controls virulence in many bacterial pathogens and, typically, activation of virulence factor expression occurs at high cell density. Surprisingly, quorum sensing promotes V. cholerae virulence factor expression and biofilm formation at low cell density and represses these traits at high cell density. This opposite pattern of regulation in the case of V. cholerae can be understood in terms of the specific disease the bacterium causes. Following a successful V. cholerae infection, the ensuing diarrhea washes huge numbers of bacteria from the human intestine into the environment. Repression of virulence factor production and biofilm formation genes at high cell density promotes dissemination of V. cholerae. We showed that pro-quorum-sensing molecules, which signal the high cell density state, repress production of virulence factors in V. cholerae. Our findings suggest that manipulating quorum sensing could be used as a therapy to prevent cholera infection and, furthermore, that such strategies hold promise in the clinical arena.

In the case of P. aeruginosa, quorum-sensing-directed virulence factor production and biofilm formation appear to be critical for infection. We developed molecules that inhibit P. aeruginosa quorum sensing and these molecules are effective at saving animals and human tissue culture cells from killing by this pathogen (Figure 3). The anti-Pseudomonas quorum-sensing molecules also prevent biofilm formation and clogging in microfluidics models of natural conditions for Pseudomonas such as soil, water filtration devices, and stents (Figure 4). Together the V. cholerae and P. aeruginosa work validates our notion that targeting quorum sensing has potential for antimicrobial drug development. 

This work is supported in part by the National Institutes of Health and the National Science Foundation. 

As of November 22, 2013