Quorum Sensing

Quorum sensing (QS) is a communication system bacteria utilize to promote “synchronized” behaviors, many of which are virulence-related. Most  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 to study P. aeruginosa virulence led 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 in 1997 DOI: 10.1073/pnas.94.24.13245.

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 the MvfR’s key role in controlling multiple QS-regulated virulence factors mediating 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 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 controlling PA QS interplay due to its direct control of the other QS regulators, LasR and RhlR.

Our research has shown that MvfR plays a significant role in the formation of biofilm, a key factor in the establishment of P. aeruginosa chronic infections. This includes chronic pneumonia in cystic fibrosis patients, relapsing and chronic wound and ear infections, and medical device-related infections (58). The fact that MvfR controls acute, bacterial functions promoting ACRP human infections, and notably 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 inhibiting this QS system, which generated PqsA, PqsD, MvfR, and PqsBC inhibitors. 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 caused by P. aeruginosa. We have developed highly efficacious compounds that interfere in vivo with MvfR regulon activation and HAQ synthesis DOI: 10.1038/s41467-022-32833-9 (also 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 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.

Our work on 2-AA, the first demonstration of a QS molecule as a critical mediator of host tolerance/resilience to bacterial infections, has significant implications. This QS molecule is the first to promote host tolerance/resilience and permit this pathogen’s persistence via epigenetic reprogramming DOI: 10.1038/nmicrobiol.2016.174; DOI: 10.1128/mbio.00159-23. The new mechanistic insights revealed from this work may have 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 formation of persister cells that are antibiotic tolerant and LasR mutations.

HQNO: A novel bacterial programmed cell death system critical inducer of biofilm formation and antibiotic tolerance in P. aeruginosa. HQNO interferes with cell respiration in both P. aeruginosa and the 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.