Research Overview

We are devising novel chemical tools to decode and interfere with bacterial communication pathways. There is a growing appreciation that bacteria communicate with each other using chemical signals. An imperative to understand these bacterial conversations arises from their roles in both pathogenic and symbiotic processes. At high cell densities, bacteria use these chemical signals (or autoinducers) to switch from a nomadic existence to that of multicellular community in a process referred to as “quorum sensing” (QS; see Fig. 1). This lifestyle switch is significant. Numerous pathogenic bacteria use QS to turn on virulence pathways and form communities called biofilms that are resistant to antibiotics. In contrast, various symbiotic bacteria use QS to commence beneficial relationships with their hosts. To initiate these diverse and important processes, bacteria use a chemical language of low molecular weight organic compounds. We recognized that we could use synthetic chemistry to intercept and redirect these bacterial conversations by creating our own small molecule languages. Using novel synthesis methods, my research group has generated a suite of synthetic ligands that selectively inhibit or activate QS processes. These compounds are active in wild-type bacteria on their native hosts and are amongst the most potent QS modulators known.

Figure 1. Schematic of the QS process in bacteria. Pentagons represent autoinducer signals. A. LuxR/LuxI-type QS in Gram-negative bacteria (such as P. aeruginosa). B. QS system in many Gram-positive bacteria (such as the agr system in S. aureus). Graphic modified from: Org. Biomol. Chem. 2012.10, 8189–8199.

Methods Development

At the outset of our research, relatively few non-native compounds were known to strongly activate or inhibit bacterial QS, and this made the design of non-native QS modulators somewhat difficult a priori. The native ligands utilized by most bacteria for QS are small molecules or peptides. To expedite our ligand discovery, we develop a series of synthetic approaches for generating small molecules and peptides en masse. These approaches entail the parallel solid-phase and solution-phase synthesis of arrays of discrete compounds, allowing us to prepare milligrams to grams of compound per batch. We have generated hundreds of novel ligands using these routes to date. We also have prepared most of the native QS signals as essential control compounds.

QS Interception

We first applied our synthetic methods to the identification of non-native ligands that interfere with QS in Gram-negative bacteria. Our compounds mimicked the structures of the native signals used by these bacteria – N-acyl L-homoserine lactones (AHLs). Gram-negative bacteria use AHLs and cognate cytoplasmic AHL receptors (LuxR-type proteins) as their primary QS signaling network (Fig. 1A), and the LuxR/LuxI circuit represents the best-characterized QS system to date. We reasoned that interception of AHL:LuxR-type protein binding with a non-native molecule would represent a general strategy to alter QS outcomes. Despite their structural similarity to QS activators, we showed that our AHL analogs were highly potent inhibitors of QS. Several of these compounds can block the formation of biofilms, swarming, and virulence factor production in wild-type pathogens. Their activity against biofilms is notable, as these aggregates of microorganisms are endemic in infections and resistant to antibiotics. We have leveraged our success to design more potent compounds. For example, we recently identified a class of 2-aminobenzimazoles that can both strongly inhibit the formation of and disperse preformed biofilms of Pseudomonas aeruginosa, one of the most notorious Gram-negative pathogens. These compounds are some of the most potent biofilm inhibitors known and provide a novel pathway for infection control (e.g., in wounds and on surfaces).

Understanding how mixed microbial communities respond to QS signals is also critical for devising strategies to target these signaling pathways in clinical and environmental settings. Our synthetic modulators of QS provide new means to explore these phenomena. We have examined the activities of a set of over ~100 AHL analogs across eight clinically, environmentally, and agriculturally relevant Gram-negative species. Prior to this work, such a comparative analysis of small molecule QS modulators across multiple species had not been attempted. These investigations revealed a family of compounds that were selective antagonists or agonists of key QS receptors in each species, along with a subset that were broad spectrum antagonists of at least seven of receptors. These findings have significant implications for understanding how bacterial QS networks can be intercepted or diverted in complex, biologically relevant settings.

We are also investigating the use of synthetic ligands to explore QS pathways in Gram-positive bacteria (Fig. 1B). We have recently developed highly potent peptide-based QS inhibitors in the pathogen Staphylococcus aureus (with low picomolar IC50 values). S. aureus uses macrocyclic autoinducing peptide (AIP) signals to mediate QS and thereby regulate virulence. Our peptide QS inhibitors are structural mimetics of the AIP ligand used by group-III S. aureus. Group-III strains are responsible for toxic shock syndrome (TSS) and have been underestimated in many human infections to date. Our compounds are active in clinical isolates of MRSA and completely block TSS toxin production. We are gearing up to use these compounds to explore the role of QS in the colonization of dissimilar environments by different groups of S. aureus, which is currently unknown.

Outlook

The crisis in antibiotic resistance requires new targets and new approaches. The use of compounds that block QS, and thereby virulence, has significant advantages over the use of conventional antibiotics. Most notably, bactericidal agents put selective pressure on bacteria to survive – compounds like ours, however, only block the most deleterious effects of the bacteria. Thus, there is less incentive for bacteria to become resistant to QS inhibitors. Notably, we have recently demonstrated that this is indeed the case in a P. aeruginosa model system. Our long-term goal is to test our QS inhibitors and associated hypotheses in the clinic and environmental settings.

Last Updated: August 20, 2014