Research

Overview

Cell-cell communication

Many bacteria communicate using small chemical molecules or ‘signals’ that transmit messages between cells. One of the mechanisms by which bacteria communicate is quorum sensing. Quorum sensing is used by many bacteria to sense and respond to their own cell density (hence the term “quorum”) and in turn regulate changes in behavior. There are many types of quorum sensing systems; our studies focus on quorum sensing involving acyl-homoserine lactone (AHL) signals. AHLs vary in length and substitution of the acyl side group, conferring specificity of these systems. AHL quorum sensing was first identified in the marine bacterium Vibrio fischeri in which the synthase, LuxI, produces a diffusible AHL that is detected by LuxR to activate bioluminescence. At sufficient concentrations, the AHLs bind LuxR and cause it to activate target genes. Quorum sensing in other bacteria use analogous systems to activate production of extracellular factors such as biofilm matrix components, toxins, and proteases, which are thought to benefit bacteria in groups or complex communities. Our studies focus on two model bacteria, Pseudomonas aeruginosa (a human pathogen) and Chromobacterium subtsugae (a soil bacterium).

 

Illustration of quorum sensing at the population level (left panel) and at the mechanistic level (right panel). Quorum sensing signals (red circles) increase with increasing cell density until they reach a sufficient concentration to bind to the LuxR receptor and cause it to activate expression of a few to dozens of genes in the genome.

Projects:

Quorum sensing and antibiotic resistance:Quorum sensing increases antibiotic resistance in several bacteria, such as the pathogen Pseudomonas aeruginosa. Surprisingly, natural P. aeruginosa isolatesfrequently lose quorum sensing by mutation even in patients treated with antibiotics, where these mutations are expected to be unfavorable. There is vast genetic diversity of natural and clinical P. aeruginosa, and our ongoing work has shown that some of these mutations can have unexpected consequences on antibiotic selection of quorum sensing. There is currently a gap in knowledge of how genetic diversity modulates quorum sensing behaviors and evolution. We are using a combination of genetic, genomic, biochemistry and experimental evolution approaches to better understand how genetic diversity alters the landscape of quorum sensing functions and evolutionary patterns.

 

Quorum sensing and cooperative behavior. Quorum sensing commonly regulates cooperative behaviors. Cooperative behaviors involve public goods that can be shared by all of the members of a population, including those that do not produce it. For example, NPR is an example of a public good. Public goods are available to everyone. Those individuals that pay for the public goods are called cooperators. Those that utilize the public goods without paying are called cheaters. In the case of NPR and other true public goods, a certain number of cooperators are required to maintain production of the public good - if there are too many cheaters, the system will fail. In bacteria, common examples of public goods include extracellular factors such as proteases or antibiotics. Many public goods are regulated by quorum sensing, but the reasons why are poorly understood. We have established experimental evolution models that can be used to study cooperation and cheating in real time, and we use these models to ask questions about cooperation and cheating and what factors lead to the maintenance (or loss) of quorum sensing in dynamically evolving populations.

 

Quorum sensing and cross-species eavesdropping. Quorum sensing is thought to be used by bacteria to sense a “quorum” of their own population, or kin cells. However, many LuxR homologs can detect AHLs from other species, because they have promiscuous selectivity for a wide range of AHLs. Our ongoing efforts are focused on understanding cross-species signaling, which we call “eavesdropping,” and the potential advantages of eavesdropping in complex microbial communities. To this end, we developed a dual-species coculture model using two soil bacteria, Chromobacterium subtsugae and Burkholderia thailandensis. Using this model, we found that C. subtsugae uses eavesdropping to activate production of antimicrobials, which provides an advantage during competition. We are now using this model, and a combination of genomics and genetics approaches, to study how C. subtsugae regulates genes in response to AHLs produced by other species, and the resulting behavior changes induced by eavesdropping.