Quorum Sensing

Quorum sensing (QS) is a communication system utilized by bacteria to promote “synchronized” behaviors many of which are virulence-related. Most of the P. aeruginosa virulence genes are controlled by the three interconnected QS master regulators, LasR, RhlR, and MvfR also called PqsR.  The use of non-vertebrate model hosts we developed lead to the discovery of the QS system,  MvfR (multiple virulence factor regulator) a LysR-type transcriptional regulator, which we first identified using the plant Arabidopsis thaliana.

Our MvfR regulon studies have revealed an unprecedented P. aeruginosa virulence mechanism, and identified a new indispensable player in P. aeruginosa’s cell density-dependent QS virulence network. We have demonstrated MvfR’s key role in the control of multiple QS-regulated virulence factors that mediate acute, chronic, or relapsing/persistent (ACRP) infections. We uncovered that MvfR controls the synthesis of ∼60 distinct low-molecular-weight compounds via the transcriptional regulation of the  pqsABCDE operon whose encoded proteins catalyze the biosynthesis of the 4-hydroxyl-2-alkyl- quinolines (HAQs), including MvfR activating ligand 4-hydroxy-2-heptyl-quinoline (HHQ), which is later hydroxylated into the second MvfR activating ligand 3,4-dihydroxy-2-heptyl-quinoline (PQS) by PqsH, and the non-HAQ molecule 2-amino-acetophenone (2-AA). HHQ is the major MvfR co-ligand in vivo. Positive and negative regulatory loops fine-tune the MvfR regulon via the multi-layered interdependent regulation of MvfR.  MvfR plays a central role in the control of PA QS interplay, as a result of its direct control of the other QS regulators LasR and RhlR. We recently showed that MvfR contributes to biofilm formation.  It is well documented that biofilms greatly contribute to the establishment of P. aeruginosa chronic infections, including chronic pneumonia in cystic fibrosis patients, relapsing and chronic wound and ear infections, as well as medical device related infections (58). The ability of MvfR to control acute, bacterial functions promoting ACRP human infections, and importantly the fact that, as opposed to LasR, no clinical isolates with frequent mutations in MvfR have been reported to date, make it a highly desirable target for drug discovery and underscores its great importance in P. aeruginosa pathogenesis. Indeed many groups have been focusing on the inhibition of this QS system, that generated inhibitors of PqsA, PqsD, MvfR and PqsBC. However, MvfR remains the best target in this pathway and holds great promise. 

That MvfR can intercept opioid compounds released during host stress and integrate them into core elements of QS circuitry leading to enhanced virulence, further corroborates the importance of the MvfR  as an excellent target  against human infections cause by P. aeruginosa. We have identified a series of highly efficacious compounds that interfere with MvfR regulon activation and/or HAQ synthesis (please see anti-infectives- anti virulence page). These compounds limit P. aeruginosa infections and virulence with high efficacy in mice, and thus are potential therapeutics against human-P. aeruginosa pathogenicity.

Although the biological function of most of MvfR-regulated small molecules remains elusive, the most recent novel discoveries we reported on the MvfR-regulated small molecules include  the physiological role of these excreted molecules as new players in the bacterial cell-to-cell signaling (HHQ, 2-AA and HQNO), antibiotic tolerance/persister cell formation (2-AA), programmed cell death (HQNO), and inter-kingdom signaling (2-AA and HQNO) that lead to the modulation of critical host immune and metabolic functions:

2-AA: The first demonstration of a QS molecule to act as a critical mediator (training agent) of host-tolerance/resilience to bacterial infections, and the first to promote such effects via epigenetic reprogramming. The new mechanistic insights revealed from this work may prove of enormous potential for developing preventive treatments that may train hosts to become tolerant to pathogen damage. In bacteria, 2-AA promotes phenotypes critical for chronic/persistent infections, including the the formation of persister cells that are antibiotic tolerant.

HQNO: A novel bacterial programmed cell death system that is a critical inducer of biofilm formation and antibiotic tolerance in P. aeruginosa. HQNO interferes with cell respiration in both P. aeruginosa and host. This newly identified pathway suggests intriguing mechanistic similarities with the initial mitochondrial-mediated steps of eukaryotic apoptosis, providing exciting new directions for future work.