Introduction
Society faces a significant problem in modern health care, where many antibiotics have lost their effectiveness in treating life-threatening and debilitating diseases due to the emergence of multi-drug resistant bacteria. Because antibiotics inhibit the growth or kill bacteria, they place strong selective pressures on microbes to become resistant. An alternative strategy to protect from pathogen attack is to disarm their ability to cause disease, but to do so without causing bacterial death. In theory, this strategy places lower selective pressure on bacteria to develop resistance to treatments.
Cell-to-cell communication in bacteria is a fascinating area of biological research, but its significance to human health lies in the potential to develop technologies that harness bacterial behavior by communication modulation. Our laboratory is committed to understanding how bacteria coordinate gene expression and behavior across microbial populations through chemical communication. This process, referred to as quorum sensing (QS), is an established mechanism by which bacteria control activities, which include defending microbial communities, coordinating assaults on competitors or host immune systems, and acquiring new genetic information by horizontal gene transfer.
We have contributed to the field of quorum sensing by identifying new intercellular communication pathways in Gram-positive bacterial pathogens and by identifying small molecules that interfere with signaling. We hypothesize that interrupting pheromone-receptor interactions will block communication pathways that coordinate events leading to pathogenesis. Our long-term goal is to develop new therapeutics that will prevent and treat bacterial infections, or that promote a robust and healthy microflora, by modulating communication networks.
Current Projects
Identify communication pathways in Gram-positive pathogens and elucidate mechanisms of signaling
We have helped in the discovery of peptide-based quorum sensing regulatory systems that utilize transcription factors of the Rgg protein family (a subset of the larger RRNPP family). Our work provides a molecular-level understanding of how signaling is propagated, received, and acted upon by bacteria. However, several important questions remain unanswered, which if known, would contribute to a general biological knowledge of cellular signaling, or would identify new targets within bacteria that might serve a therapeutic role if inhibited. Among such questions: How can we better identify unknown peptide pheromones? How are pheromones produced, stabilized, and degraded? How do pheromones interact with their receptors? What are the determinants of signaling specificity? Do peptide-driven allosteric changes in receptor conformations follow a conserved path? Though we have identified several active pheromones that serve as examples in searches for new peptides, the existing paradigm is likely far too limited. Most rgg receptor genes do not have recognizable affiliated peptide genes; we are pursuing their discovery.
Exploring QS-mediated suppression of host cells
Our team has identified that the Rgg2/Rgg3 QS system in Streptococcus pyogenes results in a diminished inflammatory response from host cells. This effect persists even when other pro-inflammatory agonists are present. Research suggests that carbohydrate-based alterations on the bacterial cell wall could be responsible for this effect. The precise host cell mechanisms being triggered or suppressed, leading to the immunosuppressed phenotype, are still unclear. Additionally, ongoing investigations aim to understand the implications of this suppression on in vivo infections and potential fitness benefits to the bacteria. Understanding the mechanism of this host suppression system could reveal ways of how this organism can reside asymptomatically in humans.
Deciphering the role, structure, and regulation of a novel pigment in S. pyogenes
Our lab has observed that under certain conditions, S. pyogenes produces an orange-pink pigment, previously undocumented in this species. Despite requiring QS-enabled media to produce this pigment, QS itself appears to inhibit its production. Current research aims to identify this pigment, its production process, and its potential function as a virulence factor.
Investigate interspecies communication among pathogenic and commensal bacteria
We have found several conserved and disparate QS pathways among pathogenic Gram-positive bacteria, including Groups A, B, and G Streptococcus, S. pneumoniae, S. mutans, S. sobrinus, and S. bovis. We have even demonstrated that communication is possible between species of bacteria. Recently, we have identified a novel QS-regulated antimicrobial peptide that is produced by S. mutans and S. ferus, common commensal organisms found in humans and animals. Very little remains understood how these pathways contribute to pathogenesis, to human transmission, or to carriage. We are investigating the roles that Rgg-QS pathways play in host-pathogen interactions by interrogating these regulons in bacterial and cell cultures, and in animal models of carriage and infection.
Develop inhibitors of communication
We have developed several reporter systems to monitor and test components of the Rgg QS systems. These reporters provide the means to test gene and compound libraries using classic genetic and high-throughput screening methodologies to identify peptides and molecules that specifically inhibit components of the communication pathways. We anticipate that peptide and compound hits found by these screening systems will serve as leads in a therapeutic design process that will eventually block communication pathways in bacteria that contribute to pathogenic development. By these methods we identified compounds like cyclosporin A and valspodar as specific inhibitors of Rgg2/3 and compound P516-0475 as a novel inhibitor of the bacterial endopeptidase PepO. Collaborations with natural product chemists are revealing extracts of environmental microbes that also display inhibitory activities in our bioassays.
Past projects
Further Characterization of QS-Induced Cell Wall Modifications
Our team has demonstrated that the Rgg2/Rgg3 QS system in S. pyogenes induces carbohydrate-based changes to the bacterial cell wall. These changes include alterations in O-acetylation of peptidoglycan and the production of a wall-teichoic-acid-like structure. This structure seems to induce an immunosuppressive phenotype in host cells. However, the complete characterization of this structure and the genes and proteins possibly contributing to its formation remain incomplete. There is also evidence indicating the presence of other modifications. These aspects constitute the focus of potential future research in our lab.
Elucidate QS regulation of HGT among streptococci
Among the several quorum-sensing pathways that we have identified, one is conserved among several groups of streptococci. This system, called ComRS, is a master regulator of genes involved in genetic transformation, which is a form of horizontal gene transfer (HGT) that works through the acquisition of extracellular DNA. HGT is an important way bacteria evolve rapidly and is a common mechanism by which antibiotic resistance emerges. Genomic analysis of several pathogenic Streptococcus species reveals high rates of HGT yet scientists remain unable to determine how and when these events occur. Our work has advanced an understanding of how this phenomenon takes place, and we are in pursuit of identifying conditions and genes contributing to this process, especially in species that are the most recalcitrant to transformation in the laboratory.