Bonnie L. Bassler

The research in my laboratory focuses on the molecular mechanisms that bacteria use for intercellular communication. Our goal is to understand how bacteria detect multiple environmental cues, and how the integration and processing of this information results in the precise regulation of gene expression.

The bacterial communication phenomenon that we study is called quorum sensing, which is a process that allows bacteria to communicate using secreted chemical signaling molecules called autoinducers. This process enables a population of bacteria to collectively regulate gene expression and, therefore, behavior. In quorum sensing, bacteria assess their population density by detecting the concentration of a particular autoinducer, which is correlated with cell density. This "census-taking" enables the group to express specific genes only at particular population densities. Quorum sensing is widespread; it occurs in numerous Gram-negative and Gram-positive bacteria. In general, processes controlled by quorum sensing are ones that are unproductive when undertaken by an individual bacterium but become effective when undertaken by the group. For example, quorum sensing controls bioluminescence, secretion of virulence factors, sporulation, and conjugation. Thus, quorum sensing is a mechanism that allows bacteria to function as multi-cellular organisms.

We have shown that the model luminous bacterium Vibrio harveyi and the related pathogen Vibrio cholerae each produce two different autoinducers, called AI-1 and AI-2, each of which is detected by its own sensor protein. Both sensors transduce information to a shared integrator protein to control the output, light emission in V. harveyi and virulence in V. cholerae. We have cloned the genes for signal production, detection and response in both species and have shown that the mechanism of signal relay is a phosphorylation/dephosphorylation cascade. Our recent studies combining genetics and bioinformatics (in collaboration with the Wingreen lab) show that the small RNA chaperone protein Hfq, together with multiple small regulatory RNAs (sRNAs), act at the center of these quorum sensing cascades. They function as an ultrasensitive regulatory switch that controls the critical transition into and out of quorum sensing mode.

V. harveyi and V. cholerae use the AI-1 quorum sensing circuit for intra-species communication and the AI-2 quorum sensing circuit for inter-species communication. To investigate the mechanism of AI-2 signaling, we constructed mutants and cloned the gene responsible for AI-2 production from several bacteria. The gene we identified in each case is highly homologous, and we named it luxS. We found that luxS homologues and AI-2 production are widespread in the bacterial world, suggesting that communication via an AI-2 signal response system could be a common mechanism that bacteria employ for inter-species interaction in natural environments. We determined the biosynthetic pathway for AI-2 production as well as the AI-2 identity by solving the crystal structures of the V. harveyi and S. typhimurium sensor proteins in complex with their cognate AI-2 signals. The structural work was performed in collaboration with the Hughson lab. The V. harveyi AI-2 is a furanosylborate diester. Finding boron in the active molecule was surprising because boron, while widely available in nature has almost no known role in biology. The S. typhimurium crystal showed that its receptor binds a chemically distinct AI-2 that lacks borate. Importantly, the active signal molecules spontaneously inter-convert upon release from their respective receptors, revealing a surprising level of sophistication in the chemical lexicon used by bacteria for inter-species cell-cell communication.

Finally, we are focused on developing molecules that are structurally related to AI-2. Such molecules have potential use as anti-microbial drugs aimed at bacteria that use AI-2 quorum sensing to control virulence. Similarly, the biosynthetic enzymes involved in AI-2 production and the AI-2 detection apparatuses are viewed as potential targets for novel anti-microbial drug design.