Challenges and Limitations of Anti-quorum Sensing Therapies

2,4-Di-Tert-Butylphenol Isolated From an Endophytic Fungus, Daldinia eschscholtzii, Reduces Virulence and Quorum Sensing in Pseudomonas aeruginosa

Pseudomonas aeruginosa is among the top three gram-negative bacteria according to the WHO’s critical priority list of pathogens against which newer antibiotics are urgently needed and considered a global threat due to multiple drug resistance. This situation demands unconventional antimicrobial strategies such as the inhibition of quorum sensing to alleviate the manifestation of classical resistance mechanisms. Here, we report that 2,4-di-tert-butylphenol (2,4-DBP), isolated from an endophytic fungus, Daldinia eschscholtzii, inhibits the quorum-sensing properties of P. aeruginosa. We have found that treating P. aeruginosa with 2,4-DBP substantially reduced the secretion of virulence factors as well as biofilm, and its associated factors that are controlled by quorum sensing, in a dose-dependent manner. Concomitantly, 2,4-DBP also significantly reduced the expression of quorum sensing-related genes, i.e., lasI, lasR, rhlI, and rhlR significantly. Importantly, 2,4-DBP restricted the adhesion and invasion of P. aeruginosa to the A549 lung alveolar carcinoma cells. In addition, bactericidal assay with 2,4-DBP exhibited synergism with ampicillin to kill P. aeruginosa. Furthermore, our computational studies predicted that 2,4-DBP could bind to the P. aeruginosa quorum-sensing receptors LasR and RhlR. Collectively, these data suggest that 2,4-DBP can be exploited as a standalone drug or in combination with antibiotic(s) as an anti-virulence and anti-biofilm agent to combat the multidrug resistant P. aeruginosa infection.

Human Single-chain Variable Fragments Neutralize Pseudomonas aeruginosa Quorum Sensing Molecule, 3O-C12-HSL, and Prevent Cells From the HSL-mediated Apoptosis

The quorum sensing (QS) signaling molecule, N-(3-oxododecanoyl)-L-homoserine lactone (3O-C12-HSL), contributes to the pathogenesis of Pseudomonas aeruginosa by regulating expression of the bacterial virulence factors that cause intense inflammation and toxicity in the infected host. As such, the QS molecule is an attractive therapeutic target for direct-acting inhibitors. Several substances, both synthetic and naturally derived products, have shown effectiveness against detrimental 3O-C12-HSL activity. Unfortunately, these compounds are relatively toxic to mammalian cells, which limits their clinical application. In this study, fully human single-chain variable fragments (HuscFvs) that bind to P. aeruginosa haptenic 3O-C12-HSL were generated based on the principle of antibody polyspecificity and molecular mimicry of antigenic molecules. The HuscFvs neutralized 3O-C12-HSL activity and prevented mammalian cells from the HSL-mediated apoptosis, as observed by Annexin V/PI staining assay, sub-G1 arrest population investigation, transmission electron microscopy for ultrastructural morphology of mitochondria, and confocal microscopy for nuclear condensation and DNA fragmentation. Computerized homology modeling and intermolecular docking predicted that the effective HuscFvs interacted with several regions of the bacterially derived ligand that possibly conferred neutralizing activity. The effective HuscFvs should be tested further in vitro on P. aeruginosa phenotypes as well as in vivo as a sole or adjunctive therapeutic agent against P. aeruginosa infections, especially in antibiotic-resistant cases.

Chloroquine fumardiamides as novel quorum sensing inhibitors

Quorum sensing inhibitors (QSIs) that specifically interfere with bacterial cell-to-cell communication are considered as an alternative approach to conventional antibacterial therapy. In our study, a set of twenty-six fumardiamides with a quinoline head-group were evaluated as potential QSIs. Two strains of Gram-negative Chromobacterium violaceum (violacein-producing strain ATCC31532 and violacein-negative, mini-Tn5 mutant derivative CV026) were used as QS reporters for testing anti-QS and bactericidal activity of various quinoline fumardiamides. The initial screening of eighteen fumardiamides with primaquine, mefloquine and chloroquine scaffolds identified chloroquine derivatives as the most promising QSIs. Tail-group optimization of chloroquine fumardiamides led to the most active compounds 27, 29 and 30 bearing aminoethyl or piperidine moieties. At 400 µM concentration, these compounds inhibited the QS of C. violaceum strains in a manner similar to quercetin (the model QSI), while at the 40 µM concentration their inhibitory effect was twice less than that of quercetin. As none of the compounds displayed a bactericidal effect and that the QS inhibition was specific to the CV026 strain, our findings indicate that the structurally optimized chloroquine derivatives could function as quorum quenching (QQ) agents with a potential to block the signaling without entering the cell. In conclusion, our finding provides an important step toward the further design of agents targeting cell-to-cell communication.

Plant Phenolics and Phenolic-Enriched Extracts as Antimicrobial Agents against Food-Contaminating Microorganisms

Phenolic compounds and extracts with bioactive properties can be obtained from many kinds of plant materials. These natural substances have gained attention in the food research as possible growth inhibitors of foodborne pathogenic and spoilage bacteria. Many phenolic-enriched plant extracts and individual phenolics have promising anti-quorum sensing potential as well and can suppress the biofilm formation and toxin production of food-related pathogens. Various studies have shown that plant phenolics can substitute or support the activity of synthetic food preservatives and disinfectants, which, by the way, can provoke serious concerns in consumers. In this review, we will provide a brief insight into the bioactive properties, i.e., the antimicrobial, anti-quorum sensing, anti-biofilm and anti-enterotoxin activities, of plant phenolic extracts and compounds, with special attention to pathogen microorganisms that have food relation. Carbohydrase aided applications to improve the antimicrobial properties of phenolic extracts are also discussed.

2,4-Di-Tert-Butylphenol Isolated From an Endophytic Fungus, Daldinia eschscholtzii, Reduces Virulence and Quorum Sensing in Pseudomonas aeruginosa

Pseudomonas aeruginosa is among the top three gram-negative bacteria according to the WHO's critical priority list of pathogens against which newer antibiotics are urgently needed and considered a global threat due to multiple drug resistance. This situation demands unconventional antimicrobial strategies such as the inhibition of quorum sensing to alleviate the manifestation of classical resistance mechanisms. Here, we report that 2,4-di-tert-butylphenol (2,4-DBP), isolated from an endophytic fungus, Daldinia eschscholtzii, inhibits the quorum-sensing properties of P. aeruginosa. We have found that treating P. aeruginosa with 2,4-DBP substantially reduced the secretion of virulence factors as well as biofilm, and its associated factors that are controlled by quorum sensing, in a dose-dependent manner. Concomitantly, 2,4-DBP also significantly reduced the expression of quorum sensing-related genes, i.e., lasI, lasR, rhlI, and rhlR significantly. Importantly, 2,4-DBP restricted the adhesion and invasion of P. aeruginosa to the A549 lung alveolar carcinoma cells. In addition, bactericidal assay with 2,4-DBP exhibited synergism with ampicillin to kill P. aeruginosa. Furthermore, our computational studies predicted that 2,4-DBP could bind to the P. aeruginosa quorum-sensing receptors LasR and RhlR. Collectively, these data suggest that 2,4-DBP can be exploited as a standalone drug or in combination with antibiotic(s) as an anti-virulence and anti-biofilm agent to combat the multidrug resistant P. aeruginosa infection.