Tal-Gan Research Group
Chemistry Department
University of Nevada, Reno
Research
Background
The rise in infections caused by multi-drug resistant pathogens is of major concern to public health worldwide. The increased rate of antibiotic resistance development is attributed to the massive use of antibiotics, all of which target essential pathways for bacterial survival. The use of these targets imposes a direct selection pressure on the bacteria for the development of antibiotic resistance. This existential crisis is propelling research endeavors toward the discovery and development of novel solutions to multi-drug resistant bacteria.
Figure 1: Selection for antibiotic-resistant bacteria. (a) Antibiotic treatment begins with the introduction of an antibiotic into an environment with both antibiotic-susceptible (blue) and antibiotic-resistant (red) bacterial species. (b) The antibiotic decimates antibiotic-susceptible species, while allowing antibiotic-resistant species to survive. (c) This resistant species thrives and proliferates in a now resource-rich environment free of antibiotic-susceptible species. (d) Resistant strains maintain the ability to transfer resistance genes to neighboring species.
The Tal-Gan lab group explores Streptococcal quorum sensing through a multifaceted lens. Quorum sensing is a cell-cell signaling mechanism that enables bacteria to assess their cell density in a given environment; and at a high population density, synchronize the transcription of genes associated with group-behavior phenotypes. Streptococcal QS circuits are centered around the production, secretion, and detection of peptide signals (termed autoinducers). The peptide signal concentration is directly proportionate to the population density. As the bacteria reach a critical concentration indicative of a high population density, quorum sensing peptides activate a membrane-bound receptor leading to the transcription of group behavior genes. Because many pathogens utilize quorum sensing to initiate and synchronize their attack on the host, in a process called virulence, quorum sensing interference can be used to reduce or eliminate bacterial pathogenicity without leading to resistance.
Figure 2: Streptococci quorum sensing circuit: a) ComC is processed by ComAB. b) The mature competence stimulating peptide (CSP) is secreted. c) CSP binds the transmembrane receptor ComD. d) ComD phosphorylates ComE. e) ComE autoactivates the CSP quorum sensing circuit and upregulates the expression of genes involved in biofilm formation and attaining genetic competence, through ComX.
Alternative approaches for antimicrobial therapy
Our interdisciplinary work resides at the intersection of organic chemistry, biochemistry, microbiology, and molecular biology. Our research efforts are focused on the development and analysis of peptide-based probes to study quorum sensing in streptococci. Using organic chemical methods, we have developed QS peptide analogs of native streptococcal QS peptides that effectively silence QS signaling. These molecules are potential candidates for therapeutic alternatives to antibiotics. Additionally, using molecular approaches, we analyze and manipulate quorum sensing peptide molecules and circuitry to understand binding dynamics that mediate downstream bacterial virulence and survival processes. Finally, using immunological approaches, we investigate how these systems influence the behaviors of host immune cells.
Figure 3: Image from Lella et al., 2022. A schematic workflow of our work with quorum sensing peptides (QSP) from Streptococcus pneumoniae. First, a peptide library is created with various substitutions in the native peptide sequence to see which has the highest affinity for the QSP receptor. Additionally, the peptides are synthesized with cyclic bridges which allow for higher stability of the peptide. Lead peptide candidates show a competitive inhibition of the QSP receptor. The helical structure is probed using CD spectroscopy, and IC50 and EC50 (the concentration at which the QSP causes 50% inhibition or 50% efficacy, respectively) are assayed. Finally, the lead peptide candidates are used in ‘in vivo’ mouse infection models to assess in vivo stability and survivability of the host.