Quorum sensing in bacteria: from mechanisms to applications in foods

Quorum sensing (QS) is a cell-to-cell communication system, which is used by many bacteria to regulate diverse gene expression in response to changes in population density. Bacteria recognize the differences in cell density by sensing the concentration of signal molecules such as N-acyl-homoserine lactones (AHL) and autoinducer-2 (AI-2). In particular, QS plays a key role in biofilm formation, which is a specific bacterial group behavior. Biofilms are dense aggregates of packed microbial communities that grow on surfaces, and are embedded in a self-produced matrix of extracellular polymeric substances (EPS). QS regulates biofilm dispersal as well as the production of EPS. In some bacteria, biofilm formations are regulated by c-di-GMP-mediated signaling as well as QS, thus the two signaling systems are mutually connected. Biofilms are one of the major virulence factors in pathogenic bacteria. In addition, they cause numerous problems in industrial fields, such as the biofouling of pipes, tanks and membrane bioreactors (MBR). Therefore, the interference of QS, referred to as quorum quenching (QQ) has received a great deal of attention. To inhibit biofilm formation, several strategies to disrupt bacterial QS have been reported, and many enzymes which can degrade or modify the signal molecule AHL have been studied. QQ enzymes, such as AHL-lactonase, AHL-acylase, and oxidoreductases may offer great potential for the effective control of biofilm formation and membrane biofouling in the future. This review describes the process of bacterial QS, biofilm formation, and the close relationship between them. Finally, QQ enzymes and their applications for the reduction of biofouling are also discussed.

Quorum sensing (QS) is a chemical communication system by which bacteria coordinate gene expression and social behaviors. Quorum quenching (QQ) refers to processes of inhibiting the QS pathway. Deep-sea hydrothermal vents are extreme marine environments, where abundant and diverse microbial communities live. However, the nature of chemical communication in bacteria inhabiting the hydrothermal vent is poorly understood. In this study, the QS and QQ activities with N-acyl homoserine lactones (AHLs) as the autoinducer were detected in bacteria isolated from hydrothermal vents in the Okinawa Trough. A total of 18 and 108 isolates possessed AHL-producing and AHL-degrading abilities, respectively. Bacteria mainly affiliated with Rhodobacterales, Hyphomicrobiales, Enterobacterales and Sphingomonadales showed QS activities; QQ was mainly associated with Bacillales, Rhodospirillales and Sphingomonadales. The results showed that the bacterial QS and QQ processes are prevalent in hydrothermal environments in the Okinawa Trough. Furthermore, QS significantly affected the activities of extracellular enzymes represented by β-glucosidase, aminopeptidase and phosphatase in the four isolates with higher QS activities. Our results increase the current knowledge of the diversity of QS and QQ bacteria in extreme marine environments and shed light on the interspecific relationships to better investigate their dynamics and ecological roles in biogeochemical cycling.

Quorum sensing (QS) is a molecular communication mechanism among bacterial cells. It is critical in regulating virulence factors, motility, antibiotic resistance, and biofilm formation. Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen linked to healthcare-associated infections, food poisoning, and biofilm formation. Treating infections caused by pathogenic bacteria has become a challenge due to the development of multi-antibiotic resistance upon continuous exposure of bacteria to antibiotics. An alternative strategy to conventional antimicrobials to decrease the bacterial pathogenicity is QS inhibition, also known as quorum quenching. Using plant-derived compounds is an environmentally friendly strategy to block the bacterial QS and inhibit bacterial growth. Portulaca oleracea is a popular plant in different countries and is also used in traditional medicine. It is widely consumed raw in salads and as garnishes, though it can be cooked as a vegetarian dish. This study evaluates the antimicrobial activity of the methanolic extract of P. oleracea and its effectiveness in blocking or attenuating the QS of P. aeruginosa. The agar well diffusion method used for screening the antibacterial activity showed a significant growth inhibition of P. aeruginosa by the extract at 500 mg/mL with a minimum inhibitory concentration of 31.25 mg/mL. A bioindicator bacterium, Chromobacterium violaceum CV026, was used to determine the effect of the methanolic extract on the QS of P. aeruginosa. The results indicated a significant reduction in biofilm formation, pyocyanin production, and LasA staphylolytic activity. The phytochemical analysis by Gas Chromatography-Mass Spectrometry showed that the methanolic extract contained several phenols, alkaloids, esters, and other compounds previously reported to have antibacterial and antioxidant effects. These findings highlight the effectiveness of P. oleracea methanolic extract in attenuating the QS and virulence factors of P. aeruginosa. This study suggests that P. oleracea is an important source of natural antimicrobials and its use would be beneficial in food and pharmaceutical applications.

BackgroundBacterial cells communicate via small extracellular molecules that facilitate gene expression which is dependent on cell density and this mechanism is known as Quorum Sensing (QS). At low cell density, these bacteria show a single cellular type of behavior but once they reach the threshold level they alter to a multicellular type and hence a QS is established by the transfer of signalling molecules called autoinducers. Quorum sensing inhibitors (QSI) are those that hinder the quorum sensing pathway.Main body of the abstractThe emergence of antimicrobial resistance has become a threat to mankind with quorum sensing being one of the mechanisms responsible for this resistance. Hence Quorum Quenching can be considered to interrupt bacterial communication. This review focuses on the effects of different synthetic and natural quorum-sensing inhibitors on different organisms and how it affects their gene regulation.ConclusionDifferent natural and synthetic agents can quench quorum sensing by various mechanistic pathways. The various quorum-sensing inhibitors against both Gram-positive and Gram-negative bacteria provide a wider scope to prevent emerging antimicrobial resistance.

Chapter 8 - Effect of Polyphenols on Microbial Cell-Cell Communications

Quorum sensing (QS) is the process by which bacteria communicate with each other through small signaling molecules such as N-acylhomoserine lactones (AHLs). Certain bacteria can degrade AHL molecules by a process called quorum quenching (QQ); therefore, QQ can be used to control bacterial infections and biofilm formation. In this study, we aimed to identify new species of bacteria with QQ activity. Red Sea sediments were collected either from the close vicinity of seagrass or from areas with no vegetation. We isolated 72 bacterial strains, which were tested for their ability to degrade/inactivate AHL molecules. Chromobacterium violaceum CV026-based bioassay was used for the initial screening of isolates with QQ activity. QQ activity was further quantified using high-performance liquid chromatography-tandem mass spectrometry. We found that these isolates could degrade AHL molecules of different acyl chain lengths as well as modifications. 16S-rRNA sequencing of positive QQ isolates showed that they belonged to three different genera. Specifically, two isolates belonged to the genus Erythrobacter; four, Labrenzia; and one, Bacterioplanes. The genome of one representative isolate from each genus was sequenced, and potential QQ enzymes, namely, lactonases and acylases, were identified. The ability of these isolates to degrade the 3OXOC12-AHLs produced by Pseudomonas aeruginosa PAO1 and hence inhibit biofilm formation was investigated. Our results showed that the isolate VG12 (genus Labrenzia) is better than other isolates at controlling biofilm formation by PAO1 and degradation of different AHL molecules. Time-course experiments to study AHL degradation showed that VG1 (genus Erythrobacter) could degrade AHLs faster than other isolates. Thus, QQ bacteria or enzymes can be used in combination with an antibacterial to overcome antibiotic resistance.

Aeromonas hydrophila, a Gram-negative bacterium, poses significant threats to aquaculture, leading to substantial economic losses. Its pathogenicity is primarily driven by a sophisticated quorum sensing (QS) system that regulates virulence factors. This study investigates saponins extracted from the sea cucumber Holothuria leucospilota as potential natural inhibitors of QS in A. hydrophila, offering a novel disease management strategy for aquaculture. Specimens of H. leucospilota were collected, and saponins were extracted from their Cuvierian tubules through a process of homogenization, solvent extraction, and purification. The saponin extract's minimum bactericidal concentration (MBC) and minimum inhibitory concentration (MIC) against A. hydrophila were found to be 80 μg/mL and 30 μg/mL, respectively. Hemolysin, lipase, and protease are examples of QS-regulated virulence factors whose activities were found to be significantly reduced by sub-MIC levels of saponins. Additionally, swarming motility and biofilm formation were notably inhibited. A significant downregulation of the QS genes ahyI and ahyR was observed, indicating an effective disruption of the QS system.These findings suggest that saponins from H. leucospilota can inhibit the QS system in A. hydrophila, thereby reducing its pathogenicity. This disruption offers a promising method for controlling bacterial infections without directly killing the bacteria, potentially mitigating antibiotic resistance. The study highlights the potential of marine-derived compounds as natural QS inhibitors, contributing to environmentally friendly aquaculture practices. Overall, it provides evidence that sea cucumber saponins could serve as a novel class of anti-QS agents, presenting a new perspective on disease management in aquaculture and other bacterial infection contexts.

Many bacteria use quorum sensing (QS), a bacterial communication system based on the diffusion and perception of small signaling molecules, to synchronize their behavior in a cell-density dependent manner. QS regulates the expression of many genes associated with virulence factor production and biofilm formation. This latter is known to be involved in antibiotic and phage resistance mechanisms. Therefore, disrupting QS, a strategy known as quorum quenching (QQ), appears to be an interesting way to reduce bacterial virulence and increase antibiotic and phage treatment efficiency. In this study, the ability of the QQ enzyme SsoPox-W263I, a lactonase able to degrade acyl-homoserine lactones, was investigated for quenching both virulence and biofilm formation in clinical isolates of Pseudomonas aeruginosa from diabetic foot ulcers, as well as in the PA14 model strain. These strains were further evolved to resist to bacteriophage cocktails. Overall, 10 antibiotics or bacteriophage resistant strains were evaluated and SsoPox-W263I was shown to decrease pyocyanin, protease and elastase production in all strains. Furthermore, a reduction of more than 70% of biofilm formation was achieved in six out of ten strains. This anti-virulence potential was confirmed in vivo using an amoeba infection model, showing enhanced susceptibility toward amoeba of nine out of ten P. aeruginosa isolates upon QQ. This amoeba model was further used to demonstrate the ability of SsoPox-W263I to enhance the susceptibility of sensitive and phage resistant bacteria to bacteriophage and antibiotic.

Amid growing concerns about antibiotic resistance, innovative strategies are imperative in addressing bacterial infections in aquaculture. Quorum quenching (QQ), the enzymatic inhibition of quorum sensing (QS), has emerged as a promising solution. This study delves into the QQ capabilities of the probiotic strain Bacillus velezensis D-18 and its products, particularly in Vibrio anguillarum 507 communication and biofilm formation. Chromobacterium violaceum MK was used as a biomarker in this study, and the results confirmed that B. velezensis D-18 effectively inhibits QS. Further exploration into the QQ mechanism revealed the presence of lactonase activity by B. velezensis D-18 that degraded both long- and short-chain acyl homoserine lactones (AHLs). PCR analysis demonstrated the presence of a homologous lactonase-producing gene, ytnP, in the genome of B. velezensis D-18. The study evaluated the impact of B. velezensis D-18 on V. anguillarum 507 growth and biofilm formation. The probiotic not only controls the biofilm formation of V. anguillarum but also significantly restrains pathogen growth. Therefore, B. velezensis D-18 demonstrates substantial potential for preventing V. anguillarum diseases in aquaculture through its QQ capacity. The ability to disrupt bacterial communication and control biofilm formation positions B. velezensis D-18 as a promising eco-friendly alternative to conventional antibiotics in managing bacterial diseases in aquaculture.

Quorum sensing (QS), a type of bacterial cell–cell communication, produces autoinducers which help in biofilm formation in response to cell population density. In this review, biofilm formation, the role of QS in biofilm formation and development with reference to biological wastewater treatment are discussed. Autoinducers, for example, acyl-homoserine lactones (AHLs), auto-inducing oligo-peptides (AIPs) and autoinducer 2, present in both Gram-negative and Gram-positive bacteria, with their mechanism, are also explained. Over the years, wastewater treatment (WWT) by QS-regulated biofilms and their optimization for WWT have gained much attention. This article gives a comprehensive review of QS regulation methods, QS enrichment methods and QS inhibition methods in biological waste treatment systems. Typical QS enrichment methods comprise adding QS molecules, adding QS accelerants and cultivating QS bacteria, while typical QS inhibition methods consist of additions of quorum quenching (QQ) bacteria, QS-degrading enzymes, QS-degrading oxidants, and QS inhibitors. Potential applications of QS regulated biofilms for WWT have also been summarized. At last, the knowledge gaps present in current researches are analyzed, and future study requirements are proposed.

Quorum sensing (QS) is a bacterial intercellular communication system that can regulate the expression of various virulence genes coordinate the group behaviors of the bacteria by sensing the concentration of signaling molecules in the surrounding environment. An increase in bacterial drug-resistance has been caused by the widespread use of antibiotics, making it urgent to identify safe and effective alternatives to antibiotics. Quorum quenching (QQ) is a strategy to control bacterial infections by disrupting the QS system, which reduces pathogenicity or increases biofilm susceptibility to antibiotics. Several natural agents with QQ activity have been identified, including small molecular inhibitors and QQ enzymes that disrupt bacterial QS by degrading or modifying the QS signal molecules. In the present study, We performed heterologous recombinant expression of the potential QQ enzyme-encoding gene RmmLII from Tritonibacter mobilis YJ3. The degradation activity of RmmLII against AHLs was assessed in vitro using the A136 liquid X-Gal assay and a plate detection method. Furthermore, the degradation mechanism of RmmLII was analyzed via high-performance liquid chromatography-mass spectrometry (HPLC-MS). The effects of RmmLII on extracellular proteases production, pyocyanin synthesis, rhamnolipids secretion, biofilm formation, and motility of Pseudomonas aeruginosa PAO1 were evaluated in vitro. Additionally, a mouse infection model was established using P. aeruginosa PAO1 to investigate the impact of RmmLII on the production of inflammatory cytokines IL-1β, IL-6, and TNF-α, as well as mouse survival rates. A novel N-acylhomoserine (AHL) lactonase RmmLII was identified and characterized from T. mobilis YJ3, which was isolated from healthy shrimp in our previous work. Through amino acid sequence alignment, a conserved "HXHXDH" domain was detected in RmmLII, indicating that RmmLII belongs to the phosphotriesterase (PTE) family. Recombinant RmmLII could effectively degrade AHLs in vitro, both long-chain and short-chain AHLs, ranging from C6 to C14. It exhibited the strongest quenching effect on C6-HSL, C8-HSL, C10-HSL, 3-oxo-C8-HSL, 3-oxo-C10-HSL, 3-oxo-C12-HSL, and 3-oxo-C14-HSL, while the quenching effect on C14-HSL and 3-oxo-C6-HSL was relatively weaker, especially with more notable degradation activity towards long-chain AHLs with a substitution of oxo-group at the C-3 position. HPLC-MS analysis revealed that RmmLII could hydrolyze the ester bond of AHLs. In addition, RmmLII significantly inhibited the production of extracellular proteases, pyocyanin, rhamnolipids, biofilm formation, as well as motility of P. aeruginosa PAO1 in vitro. It also reduced the production of inflammatory factors IL-1β, IL-6, and TNF-α, thereby improving the survival rates of mice infected with PAO1 in vivo. This study demonstrates the potential application of RmmLII in controlling PAO1 infections, offering new insights for the development of novel antibiotic alternatives. RmmLII has the potential as a therapeutic agent for application in the mitigating PAO1 infections.

Two chemical signaling systems, quorum sensing (QS) and 3',5'-cyclic diguanylic acid (c-di-GMP), reciprocally control biofilm formation in Vibrio cholerae. QS is the process by which bacteria communicate via the secretion and detection of autoinducers, and in V. cholerae, QS represses biofilm formation. c-di-GMP is an intracellular second messenger that contains information regarding local environmental conditions, and in V. cholerae, c-di-GMP activates biofilm formation. Here we show that HapR, a major regulator of QS, represses biofilm formation in V. cholerae through two distinct mechanisms. HapR controls the transcription of 14 genes encoding a group of proteins that synthesize and degrade c-di-GMP. The net effect of this transcriptional program is a reduction in cellular c-di-GMP levels at high cell density and, consequently, a decrease in biofilm formation. Increasing the c-di-GMP concentration at high cell density to the level present in the low-cell-density QS state restores biofilm formation, showing that c-di-GMP is epistatic to QS in the control of biofilm formation in V. cholerae. In addition, HapR binds to and directly represses the expression of the biofilm transcriptional activator, vpsT. Together, our results suggest that V. cholerae integrates information about the vicinal bacterial community contained in extracellular QS autoinducers with the intracellular environmental information encoded in c-di-GMP to control biofilm formation.

It is now apparent that bacteria utilize regulatory systems called quorum sensing (QS) to sense their population density. Such systems are dependant on the production of signaling molecules that activate specific genes when the signal reaches a critical threshold concentration. Such QS-regulated genes produce phenotypes that require coordinate behavior to convey competitive advantage to the population (such as biofilm formation and pathogenesis). The best-characterized QS system is that driven by acylated homoserine lactone (AHL) molecules. Quorum sensing-regulated phenotypes are diverse; however, their evolutionary selection is based on the competitive advantage conveyed by coordinating gene expression with the establishment of a quorum. Population density and coordinated gene expression are coupled for either (1) the multicellular characteristic of behaviors such as cell differentiation (for example swarming, biofilm formation), or (2) the fitness benefit of many individual cells simultaneously expressing the same phenotype (for example virulence factors, luminescence). QS enables a population to differentiate under favorable conditions where the population is dense enough to support the division and coordination of labor into subpopulations. In undifferentiated populations, QS coordinates gene expression so that it is simultaneous for cells within the population. In both scenarios, having QS regulation provides a competitive advantage for a population to both produce and respond to QS molecules. A selective pressure also exists for non-QS bacteria to sense and respond to QS molecules produced within the community. Examples of QS bacteria and bacteria able to detect and respond to exogenous signals are found in the literature; however, the frequency of QS and QS cheaters in the environment is poorly documented. With the growing number of bacterial genomes sequenced, especially genomes of nonclinically isolated bacteria, it may not be surprising that the number of genomes containing homologs of AHL-QS circuitry is ever growing. In this article, we use all current bacterial genomes to examine the frequency of AHL-QS among these bacteria, and the surprising number of bacteria with the genetic potential for eavesdropping on AHL signals from other bacteria.

Quorum sensing (QS), a cell-to-cell communication, regulates a variety of social beha-viors, such as biofilm formation, public goods produce and gene horizontal transfer of bacteria. In the process of quorum sensing, public goods could be utilized by any members in the population, which was termed as cooperation. Notably, public goods also could be shared by the individuals who could not produce them, which was termed as cheating. Once cheaters come up, they possibly maintain equilibrium with cooperators, meanwhile they also possibly induce the collapse of population due to their rapid growth and shortage of public goods. Therefore, invasion of cheaters arouses wide attentions in medicine, agriculture, food science and so on regarded as a new strategy to control pathogens. In this study, based on the introduction about the theory of bacterial quorum sensing cooperation and cheating, we analyzed the factors influencing the formation and development of the relationship between cooperator and cheater. Moreover, we discussed the mechanism of stabilization in the relationship between cooperator and cheater, including kin selection, metabolic prudence, metabolic constraint (gene pleiotropy) and policing quorum sensing. Finally, some problems in current researches of quorum sensing cooperation and cheating were presented as well as the future research directions. We hoped this paper could deepen the understanding of bacterial quorum sen-sing and ecology of bacterial population.

With the rise of antibiotic resistance, finding novel methods of targeting bacteria and treating disease is in high demand. One target system of interest is the bacterial quorum sensing system. This process involves bacteria communicating using small signaling molecules. This communication allows bacteria to engage in group behaviors such as biofilm formation and release of infective agents. It has been demonstrated that synthetic small molecules can be used to inhibit such behaviors. Although some success has been had, the design of more diverse libraries could lead to more efficient modulators, easier synthetic routes, and increased understanding of the quorum sensing pathway. The research presented here focuses on the design and synthesis of a library of adamantyl‐derived triazoles. The triazole scaffold was chosen because of its ease of synthesis, and a wide selection of terminal and di‐substituted alkynes were chosen based on similarities to natural quorum sensing molecules. Some functionalities include, a five membered ring with heteroatoms and carbon chains of varying lengths. Upon synthesis and purification, these molecules are assayed for their ability to inhibit quorum sensing in Vibrio fischeri. Vibrio fischeri is used because its quorum sensing system is similar to other Gram‐negative bacteria that are known to cause human infections (i.e. Pseudomonas aeruginosa), but it is non‐pathogenic. Additionally, this species' bioluminescence is controlled by a quorum‐dependent pathway, which allows for easy detection of quorum sensing inhibition via quantification of light production. To date, the molecules that have been tested have not been found to inhibit bacterial quorum sensing, however, the library continues to grow with different functionalities. Herein, we will describe the design and synthesis of a growing triazole‐based library as well as data regarding the ability of these molecules to inhibit quorum sensing in Vibrio fischeri.Support or Funding InformationCEK received partial funding from the Pennsylvania Academy of Sciences.Mercyhurst University provided the remaining funding.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.