Quorum sensing in vibrios: complexity for diversification

Exopolysaccharides in microbial interactions: signalling, quorum sensing, and community dynamics

Microbial interactions within diverse ecosystems are intricately governed by the dynamic interplay of exopolysaccharides (EPSs) produced by microorganisms. This review delves into the multifaceted roles of EPS in microbial signalling, quorum sensing (QS), and community dynamics, highlighting their significance in orchestrating cooperative behaviours and shaping community structures. EPSs serve as pivotal signalling molecules, influencing chemical communication and promoting intricate interactions among microorganisms. The integration of EPS into QS mechanisms adds an additional layer of complexity, allowing microorganisms to assess population density and synchronise communal responses. Furthermore, EPSs actively contribute to community dynamics by influencing spatial organisation, adhesion, and resistance to environmental stressors. By providing comprehensive knowledge of EPS dynamics, this review offers valuable insights into microbial ecology, serving as a foundational resource for future research. It will benefit the research community by advancing our understanding of microbial ecosystems, with broad applications in biotechnology, environmental science, and beyond.

Attenuating Vibrio harveyi Virulence Through Quorum Sensing Interference Using Piperine: An In Vitro and In Silico Approach

Aquaculture diseases caused by pathogens such as Vibrio spp., amplified by the emergence of antibiotic resistance, threaten the aquaculture industry. Due to its critical role in regulating the expression of genes related to antibiotic resistance and virulence, quorum sensing (QS) was proved to be an ideal target in disease control. In this study, we investigated the effects of the alkaloid piperine on the QS system of a fish pathogenic Vibrio harveyi strain. In vitro assays showed that piperine inhibits biofilm formation of V. harveyi without affecting their growth. Moreover, piperine specifically reduced QS activity in V. harveyi, as evident in the inhibition of biofilm and bioluminescence, likely through the AI-2 pathway. Molecular docking simulations showed significant binding energies between piperine and QS proteins-LuxP, LuxQ, LuxR and LuxS-revealing competitive inhibitory effects against LuxP, LuxR, and LuxS, and non-competitive interactions with LuxQ. This study demonstrated the effects of piperine against V. harveyi and elucidated its mechanism of action against V. harveyi QS, implying its potential application in aquaculture systems.

Role of bacterial quorum sensing in plant growth promotion

Quorum sensing (QS) also known as bacterial cell-cell communication or bacterial crosstalk is a phenomenon regulating various bacterial traits that can affect plant growth and defence. Similarities in the structure of root exudates and bacterial signalling molecules have tremendous implications governing the plant heath. The rhizosphere ecosystem being an excellent example of plant-microbe and microbe-microbe interactions harbours a variety of microorganisms exhibiting quorum sensing. Phytochemicals present in plant root exudates and QS signal molecules as well as volatile organic compounds (VOCs) produced by microorganisms work in coordination to establish intra- and inter-species communications. Interestingly, a number of plant growth promoting rhziobacterial (PGPR) activities like effective/enhanced root colonization, nutrient uptake, nodulation, nitrogen fixation, production of plant hormones, antimicrobial compounds and induction of plant defences can be attributed directly or indirectly to their quorum sensing and quenching abilities. Although not completely understood, root development, stress tolerance and defence against phytopathogens are some of the implications of such abilities which might prove beneficial for sustainable agriculture. Deciphering the mechanism of these interactions would be instrumental in improving crop health. Plant beneficial microorganisms employing QS and QS inhibition (QSI) strategies have been discussed in this review.

Advances in Vibrio-related infection management: an integrated technology approach for aquaculture and human health

Vibrio species pose significant threats worldwide, causing mortalities in aquaculture and infections in humans. Global warming and the emergence of worldwide strains of Vibrio diseases are increasing day by day. Control of Vibrio species requires effective monitoring, diagnosis, and treatment strategies at the global scale. Despite current efforts based on chemical, biological, and mechanical means, Vibrio control management faces limitations due to complicated implementation processes. This review explores the intricacies and challenges of Vibrio-related diseases, including accurate and cost-effective diagnosis and effective control. The global burden due to emerging Vibrio species further complicates management strategies. We propose an innovative integrated technology model that harnesses cutting-edge technologies to address these obstacles. The proposed model incorporates advanced tools, such as biosensing technologies, the Internet of Things (IoT), remote sensing devices, cloud computing, and machine learning. This model offers invaluable insights and supports better decision-making by integrating real-time ecological data and biological phenotype signatures. A major advantage of our approach lies in leveraging cloud-based analytics programs, efficiently extracting meaningful information from vast and complex datasets. Collaborating with data and clinical professionals ensures logical and customized solutions tailored to each unique situation. Aquaculture biotechnology that prioritizes sustainability may have a large impact on human health and the seafood industry. Our review underscores the importance of adopting this model, revolutionizing the prognosis and management of Vibrio-related infections, even under complex circumstances. Furthermore, this model has promising implications for aquaculture and public health, addressing the United Nations Sustainable Development Goals and their development agenda.

Genomic evidence of two-staged transmission of the early seventh cholera pandemic

The seventh cholera pandemic started in 1961 in Indonesia and spread across the world in three waves in the decades that followed. Here, we utilised genomic evidence to detail the first wave of the seventh pandemic. Genomes of 22 seventh pandemic Vibrio cholerae isolates from 1961 to 1979 were completely sequenced. Together with 152 publicly available genomes from the same period, they fell into seven phylogenetic clusters (CL1-CL7). By multilevel genome typing (MGT), all were assigned to MGT2 ST1 (Wave 1) except three isolates in CL7 which were typed as MGT2 ST2 (Wave 2). The Wave 1 seventh pandemic expanded in two stages, with Stage 1 (CL1-CL5) spread across Asia and Stage 2 (CL6 and CL7) spread to the Middle East and Africa. Three non-synonymous mutations, one each, in three regulatory genes, csrD (global regulator), acfB (chemotaxis), and luxO (quorum sensing) may have critically contributed to its pandemicity. The three MGT2 ST2 isolates in CL7 were the progenitors of Wave 2 and evolved from within Wave 1 with acquisition of a novel IncA/C plasmid. Our findings provide new insight into the evolution and transmission of the early seventh pandemic, which may aid future cholera prevention and control.

Evidence for immune priming specificity and cross-protection against sympatric and allopatric Vibrio splendidus strains in the oyster Magalana (Crassostrea) gigas

Infections with pathogenic Vibrio strains are associated with high summer mortalities of Pacific oysters Magalana (Crassostrea) gigas, affecting production worldwide. This raises the question of how M. gigas cultures can be protected against deadly Vibro infection. There is increasing experimental evidence of immune priming in invertebrates, where previous exposure to a low pathogen load boosts the immune response upon secondary exposure. Priming responses, however, appear to vary in their specificity across host and parasite taxa. To test priming specificity in the Vibrio - M. gigas system, we used two closely related Vibrio splendidus strains with differing degrees of virulence towards M. gigas. These V. splendidus strains were either isolated in the same location as the oysters (sympatric, opening up the potential for co-evolution) or in a different location (allopatric). We extracted cell-free haemolymph plasma from infected and control oysters to test the influence of humoral immune effectors on bacterial growth in vitro. While addition of haemolypmph plasma in general promoted growth of both strains, priming by an exposure to a sublethal dose of bacterial cells lead to inhibitory effects against a subsequent challenge with a potentially lethal dose in vitro. Inhibitory effects and immune priming was strongest when oysters had been primed with the sympatric Vibrio strain, but inhibitory effects were seen both when challenged with the sympatric as well as against allopatric V. splendidus, suggesting some degree of cross protection. The stronger immune priming against the sympatric strain suggests that priming could be more efficient against matching local strains potentially adding a component of local adaptation or co-evolution to immune priming in oysters. These in vitro results, however, were not reflected in the in vivo infection data, where we saw increased bacterial loads following an initial challenge. This discrepancy might suggests that that it is the humoral part of the oyster immune system that produces the priming effects seen in our in vitro experiments.

Contamination of Vibrio parahaemolyticus in crayfish for sale

Crayfish (Procambarus clarkii) are economically important freshwater crustaceans. With the growth of the crayfish industry, the associated food-safety risks should be seriously considered. Although Vibrio parahaemolyticus is commonly recognized as a halophilic foodborne pathogen associated with seafood, it has been found to be a major pathogen in crayfish-associated food poisoning cases. In this study, the V. parahaemolyticus contamination level in crayfish production-sale chain was investigated using crayfish and environmental samples collected from crayfish farms and markets. Serious V. parahaemolyticus contamination (detection rate of 66%) was found in the entire crayfish production-sale chain, while the V. parahaemolyticus contamination level of the market samples was extremely high (detection rate of 92%). The V. parahaemolyticus detection rate of crayfish surface was similar to that of whole crayfish, indicating that crayfish surface was important for V. parahaemolyticus contamination. The simulation experiments of crayfish for sale being contaminated by different V. parahaemolyticus sources were performed. All the contamination sources, containing V. parahaemolyticus-positive tank, water, and crayfish, were found to be efficient to contaminate crayfish. The crayfish tank displayed the most significant contaminating role, while the water seemed to inhibit the V. parahaemolyticus contamination. The contamination extent of the crayfish increased with the number of V. parahaemolyticus cells the tank carried and the contact time of the crayfish and the tank, but decreased with the time that the crayfish were maintained in the water. It was also confirmed that the crayfish surface was more susceptible to V. parahaemolyticus contamination than the crayfish intestine. Furthermore, the adsorption of V. parahaemolyticus onto the crayfish shell was analyzed. Over 90% of the V. parahaemolyticus cells were adsorbed onto the crayfish shell in 6 h, indicating a significant adsorption effect between V. parahaemolyticus and the crayfish shell. In conclusion, within a water-free sale style, the fresh crayfish for sale in aquatic products markets uses its shell to capture V. parahaemolyticus cells from the V. parahaemolyticus-abundant environments. The V. parahaemolyticus contamination in crayfish for sale exacerbates the crayfish-associated food-safety risk. This study sheds light on V. parahaemolyticus control and prevention in crayfish industry.

Ocimum sanctum as a Source of Quorum Sensing Inhibitors to Combat Antibiotic Resistance of Human and Aquaculture Pathogens

Biofilms play a decisive role in the infectious process and the development of antibiotic resistance. The establishment of bacterial biofilms is regulated by a signal-mediated cell-cell communication process called "quorum sensing" (QS). The identification of quorum sensing inhibitors (QSI) to mitigate the QS process may facilitate the development of novel treatment strategies for biofilm-based infections. In this study, the traditional medicinal plant Ocimum sanctum was screened for QS inhibitory potential. Sub-MICs of the extract significantly affected the secretion of EPS in Gram-negative human pathogens such as Escherichia coli, Pseudomonas aeruginosa PAO1, Proteus mirabilis, and Serratia marcescens, as well as aquaculture pathogens Vibrio harveyi, V. parahaemolyticus, and V. vulnificus, which render the bacteria more sensitive, leading to a loss of bacterial biomass from the substratum. The observed inhibitory activity of the O. sanctum extract might be attributed to the presence of eugenol, as evidenced through ultraviolet (UV)-visible, gas chromatography-mass spectroscopy (GC-MS), Fourier transformer infrared (FTIR) spectroscopy analyses, and computational studies. Additionally, the QSI potential of eugenol was corroborated through in vitro studies using the marker strain Chromobacterium violaceum.

Unveiling the positive impact of biofloc culture on Vibrio parahaemolyticus infection of Pacific white shrimp by reducing quorum sensing and virulence gene expression and enhancing immunity

This study aimed to evaluate and unveil the positive impact of biofloc culture on Vibrio parahaemolyticus infection of Pacific white shrimp by reducing quorum sensing (QS) and virulence gene expression and enhancing shrimp's immunity. The shrimp with an average body weight of 0.50 ± 0.09 g were reared in containers with a volume of 2.5 L, 21 units, and a density of 20 shrimp L-1. The shrimp were cultured for 5 days, with each treatment including biofloc system maintenance with a C/N ratio of 10 and a control treatment without biofloc, followed by a challenge test through immersion using V. parahaemolyticus at densities of 103, 105, and 107 CFU mL-1 initially. The results of the in vitro experiment showed that biofloc suspension can inhibit and disperse biofilm formation, as well as reduce the exo-enzyme activity (amylase, protease, and chitinase) of V. parahaemolyticus. Furthermore, the biofloc treatment significantly reduced the expression of the QS regulatory gene OpaR, the PirB toxin gene, and the virulence factor genes T6SS1 and T6SS2 in both in vitro and in vivo. The biofloc system also increased the expression of shrimp immunity-related genes (LGBP, proPO, SP, and PE) and the survival rate of white shrimp challenged with V. parahaemolyticus.

Characterization of tryptanthrin as an antibacterial reagent inhibiting Vibrio splendidus

Isolates of Vibrio splendidus are ubiquitously presented in various marine environments, and they can infect diverse marine culture animals, leading to high mortality and economic loss. Therefore, a control strategy of the infection caused by V. splendidus is urgently recommended. Tryptanthrin is a naturally extracted bioactive chemical with antimicrobial activity to other bacteria. In this study, the effects of tryptanthrin on the bacterial growth and virulence-related factors of one pathogenic strain V. splendidus AJ01 were determined. Tryptanthrin (10 μg/mL) could completely inhibit the growth of V. splendidus AJ01. The virulence-related factors of V. splendidus AJ01 were affected in the presence of tryptanthrin. Tryptanthrin resulted an increase in biofilm formation, but lead to reduction in the motility and hemolytic activity of V. splendidus cells. In the cells treated with tryptanthrin, two distinctly differentially expressed extracellular proteins, proteases and flagellum, were identified using SDS-PAGE combined with LC-MS. Real-time reverse transcriptase PCR confirmed that the genes involved in the flagellar formation and hemolysin decreased, whereas specific extracellular proteases and the genes involved in the biofilm formation were upregulated. Two previously annotated luxOVs genes were cloned, and their expression levels were analyzed at different cell densities. Molecular docking was performed to predict the interaction between LuxOVs and ATP/tryptanthrin. The two sigma-54-dependent transcriptional regulators showed similar ATP or tryptanthrin binding capacity but with different sites, and the direct competitive binding between ATP and tryptanthrin was present only in their binding to LuxO1. These results indicated that tryptanthrin can be used as a bactericide of V. splendidus by inhibiting the growth, bacterial flagella, and extracellular proteases, but increasing the biofilm. Sigma-54-dependent transcriptional regulator, especially the quorum sensing regulatory protein LuxO1, was determined to be the potential target of tryptanthrin. KEY POINTS: • Tryptanthrin inhibited the growth of V. splendidus in a dose-dependent manner. • The effect of tryptanthrin on the virulence factors of V. splendidus was characterized. • LuxO was the potential target for tryptanthrin based on molecular docking.

Quorum sensing in bacteria: in silico protein analysis, ecophysiology, and reconstruction of their evolutionary history

BACKGROUND

Quorum sensing (QS) is a sophisticated cell-to-cell signalling mechanism that allows the coordination of important processes in microbial populations. The AI-1 and AI-2 autoinducer systems are among the best characterized bacterial QS systems at the genetic level.

RESULTS

In this study, we present data derived from in silico screening of QS proteins from bacterial genomes available in public databases. Sequence analyses allowed identifying candidate sequences of known QS systems that were used to build phylogenetic trees. Eight categories were established according to the number of genes from the two major QS systems present in each genome, revealing a correlation with specific taxa, lifestyles or metabolic traits. Many species had incomplete QS systems, encoding the receptor protein but not the biosynthesis of the quorum sensing molecule (QSMs). Reconstruction of the evolutionary history of the LuxR family and prediction of the 3D structure of the ancestral protein suggested their monomeric configuration in the absence of the signal molecule and the presence of a cavity for its binding.

CONCLUSIONS

Here we correlate the taxonomic affiliation and lifestyle of bacteria from different genera with the QS systems encoded in their genomes. Moreover, we present the first ancestral reconstruction of the LuxR QS receptors, providing further insight in their evolutionary history.

AHL-Based Quorum Sensing Regulates the Biosynthesis of a Variety of Bioactive Molecules in Bacteria

Bacteria are social microorganisms that use communication systems known as quorum sensing (QS) to regulate diverse cellular behaviors including the production of various secreted molecules. Bacterial secondary metabolites are widely studied for their bioactivities including antibiotic, antifungal, antiparasitic, and cytotoxic compounds. Besides playing a crucial role in natural bacterial niches and intermicrobial competition by targeting neighboring organisms and conferring survival advantages to the producer, these bioactive molecules may be of prime interest to develop new antimicrobials or anticancer therapies. This review focuses on bioactive compounds produced under acyl homoserine lactone-based QS regulation by Gram-negative bacteria that are pathogenic to humans and animals, including the Burkholderia, Serratia, Pseudomonas, Chromobacterium, and Pseudoalteromonas genera. The synthesis, regulation, chemical nature, biocidal effects, and potential applications of these identified toxic molecules are presented and discussed in light of their role in microbial interactions.

Genomic and Proteomic Analyses of Extracellular Products Reveal Major Virulence Factors Likely Accounting for Differences in Pathogenicity to Bivalves between Vibrio mediterranei Strains

Vibrio mediterranei, a bacterial pathogen of bivalves, has exhibited strain-dependent virulence. The mechanisms behind the variations in bivalve pathogenicity between V. mediterranei strains have remained unclear. However, a preliminary analysis of the extracellular product (ECP) proteomes has revealed differences in protein compositions between low- and high-virulence strains; in addition to 1265 shared proteins, 127 proteins have been identified to be specific to one low-virulence strain and 95 proteins to be specific to two high-virulence strains. We further studied the ECP proteins of the three V. mediterranei strains from functional perspectives using integrated genomics and proteomics approaches. The results showed that lipid metabolism, transporter activity and membrane transporter pathways were more enriched in the ECPs of the two high-virulence strains than in those of the low-virulence strain. Additionally, 73 of the 95 high-virulence strain-specific proteins were found to have coding genes in the genome but were not expressed in the low-virulence strain. Moreover, comparisons with known virulence factors in the Virulence Factor Database (VFDB) and the Pathogen-Host Interactions Database (PHI-base) allowed us to predict more than 10 virulence factors in the categories of antimicrobial activity/competitive advantage, the effector delivery system and immune modulation, and the high-virulence strain-specific ECP proteins consisted of a greater percentage of known virulence factors than the low-virulence strain. Particularly, two virulence factors, MtrC and KatG, were identified in the ECPs of the two high-virulence strains but not in those of the low-virulence strain. Most coding genes of the ECP proteins including known virulence factors were identified on chromosome 1 of V. mediterranei. Our findings indicate that variations in virulence factor composition in the bacterial ECPs may partially account for the differences in the bivalve pathogenicity between V. mediterranei strains.

Remodulation of bacterial transcriptome after acquisition of foreign DNA: the case of irp-HPI high-pathogenicity island in Vibrio anguillarum

The high-pathogenicity island irp-HPI is widespread in Vibrionaceae and encodes the siderophore piscibactin, as well as the regulator PbtA that is essential for its expression. In this work, we aim to study whether PbtA directly interacts with irp-HPI promoters. Furthermore, we hypothesize that PbtA, and thereby the acquisition of irp-HPI island, may also influence the expression of other genes elsewhere in the bacterial genome. To address this question, an RNAseq analysis was conducted to identify differentially expressed genes after pbtA deletion in Vibrio anguillarum RV22 genetic background. The results showed that PbtA not only modulates the irp-HPI genes but also modulates the expression of a plethora of V. anguillarum core genome genes, inducing nitrate, arginine, and sulfate metabolism, T6SS1, and quorum sensing, while repressing lipopolysaccharide (LPS) production, MARTX toxin, and major porins such as OmpV and ChiP. The direct binding of the C-terminal domain of PbtA to piscibactin promoters (PfrpA and PfrpC), quorum sensing (vanT), LPS transporter wza, and T6SS structure- and effector-encoding genes was demonstrated by electrophoretic mobility shift assay (EMSA). The results provide valuable insights into the regulatory mechanisms underlying the expression of irp-HPI island and its impact on Vibrios transcriptome, with implications in pathogenesis.IMPORTANCEHorizontal gene transfer enables bacteria to acquire traits, such as virulence factors, thereby increasing the risk of the emergence of new pathogens. irp-HPI genomic island has a broad dissemination in Vibrionaceae and is present in numerous potentially pathogenic marine bacteria, some of which can infect humans. Previous works showed that certain V. anguillarum strains exhibit an expanded host range plasticity and heightened virulence, a phenomenon linked to the acquisition of the irp-HPI genomic island. The present work shows that this adaptive capability is likely achieved through comprehensive changes in the transcriptome of the bacteria and that these changes are mediated by the master regulator PbtA encoded within the irp-HPI element. Our results shed light on the broad implications of horizontal gene transfer in bacterial evolution, showing that the acquired DNA can directly mediate changes in the expression of the core genome, with profounds implications in pathogenesis.

Quorum Sensing Regulates the Production of Methanethiol in Vibrio harveyi

Methanethiol (MeSH) and dimethyl sulfide (DMS) are important volatile organic sulfur compounds involved in atmospheric chemistry and climate regulation. However, little is known about the metabolism of these compounds in the ubiquitous marine vibrios. Here, we investigated MeSH/DMS production and whether these processes were regulated by quorum-sensing (QS) systems in Vibrio harveyi BB120. V. harveyi BB120 exhibited strong MeSH production from methionine (Met) (465 nmol mg total protein-1) and weak DMS production from dimethylsulfoniopropionate (DMSP) cleavage. The homologs of MegL responsible for MeSH production from L-Met widely existed in vibrio genomes. Using BB120 and its nine QS mutants, we found that the MeSH production was regulated by HAI-1, AI-2 and CAI-1 QS pathways, as well as the luxO gene located in the center of this QS cascade. The regulation role of HAI-1 and AI-2 QS systems in MeSH production was further confirmed by applying quorum-quenching enzyme MomL and exogenous autoinducer AI-2. By contrast, the DMS production from DMSP cleavage showed no significant difference between BB120 and its QS mutants. Such QS-regulated MeSH production may help to remove excess Met that can be harmful for vibrio growth. These results emphasize the importance of QS systems and the MeSH production process in vibrios.

ArcB orchestrates the quorum-sensing system to regulate type III secretion system 1 in Vibrio parahaemolyticus

In many Vibrio species, virulence is regulated by quorum sensing, which is regulated by a complex, multichannel, two-component phosphorelay circuit. Through this circuit, sensor kinases transmit sensory information to the phosphotransferase LuxU via a phosphotransfer mechanism, which in turn transmits the signal to the response regulator LuxO. For Vibrio parahaemolyticus, type III secretion system 1 (T3SS1) is required for cytotoxicity, but it is unclear how quorum sensing regulates T3SS1 expression. Herein, we report that a hybrid histidine kinase, ArcB, instead of LuxU, and sensor kinase LuxQ and response regulator LuxO, collectively orchestrate T3SS1 expression in V. parahaemolyticus. Under high oxygen conditions, LuxQ can interact with ArcB directly and phosphorylates the Hpt domain of ArcB. The Hpt domain of ArcB phosphorylates the downstream response regulator LuxO instead of ArcA. LuxO then activates transcription of the T3SS1 gene cluster. Under hypoxic conditions, ArcB autophosphorylates and phosphorylates ArcA, whereas ArcA does not participate in regulating the expression of T3SS1. Our data provides evidence of an alternative regulatory path involving the quorum sensing phosphorelay and adds another layer of understanding about the environmental regulation of gene expression in V. parahaemolyticus.

A review of mechanism analysis methods in multi-species biofilm of foodborne pathogens

Biofilms are an aggregation of microorganisms that have high resistance to antimicrobial agents. In the food industry, it has been widely studied that foodborne pathogens on both food surfaces and food-contact surfaces can form biofilms thereby threatening the safety of the food. In the natural environment, multi-species biofilms formed by more than two different microorganisms are abundant. In addition, the resistance of multi-species biofilms to antimicrobial agents is higher than that of mono-species biofilms. Therefore, studies to elucidate the mechanisms of multi-species biofilms formed by foodborne pathogens are still required in the food industry. In this review paper, we summarized the novel analytical methods studied to evaluate the mechanisms of multi-species biofilms formed by foodborne pathogens by dividing them into four categories: spatial distribution, bacterial interaction, extracellular polymeric substance production and quorum sensing analytical methods.

Perturbation of Quorum Sensing after the Acquisition of Bacteriophage Resistance Could Contribute to Novel Traits in Vibrio alginolyticus

Bacteria employ a wide range of molecular mechanisms to confer resistance to bacteriophages, and these mechanisms are continuously being discovered and characterized. However, there are instances where certain bacterial species, despite lacking these known mechanisms, can still develop bacteriophage resistance through intricate metabolic adaptation strategies, potentially involving mutations in transcriptional regulators or phage receptors. Vibrio species have been particularly useful for studying the orchestrated metabolic responses of Gram-negative marine bacteria in various challenges. In a previous study, we demonstrated that Vibrio alginolyticus downregulates the expression of specific receptors and transporters in its membrane, which may enable the bacterium to evade infection by lytic bacteriophages. In our current study, our objective was to explore how the development of bacteriophage resistance in Vibrio species disrupts the quorum-sensing cascade, subsequently affecting bacterial physiology and metabolic capacity. Using a real-time quantitative PCR (rt-QPCR) platform, we examined the expression pattern of quorum-sensing genes, auto-inducer biosynthesis genes, and cell density regulatory proteins in phage-resistant strains. Our results revealed that bacteriophage-resistant bacteria downregulate the expression of quorum-sensing regulatory proteins, such as LuxM, LuxN, and LuxP. This downregulation attenuates the normal perception of quorum-sensing peptides and subsequently diminishes the expression of cell density regulatory proteins, including LuxU, aphA, and LuxR. These findings align with the diverse phenotypic traits observed in the phage-resistant strains, such as altered biofilm formation, reduced planktonic growth, and reduced virulence. Moreover, the transcriptional depletion of aphA, the master regulator associated with low cell density, was linked to the downregulation of genes related to virulence. This phenomenon appears to be phage-specific, suggesting a finely tuned metabolic adaptation driven by phage-host interaction. These findings contribute to our understanding of the role of Vibrio species in microbial marine ecology and highlight the complex interplay between phage resistance, quorum sensing, and bacterial physiology.

Quorum sensing signals: Aquaculture risk factor

Bacteria produce several virulence factors and cause massive mortality in fish and crustaceans. Abundant quorum sensing (QS) signals and high cell density are essentially required for the production of such virulence factors. Although several strategies have been developed to control aquatic pathogens through antibiotics and QS inhibition, the impact of pre‐existing QS signals in the aquatic environment has been overlooked. QS signals cause detrimental effects on mammalian cells and induce cell death by interfering with multiple cellular pathways. Moreover, QS signals not only function as a messenger, but also annihilate the functions of the host immune system which implies that QS signals should be designated as a major virulence factor. Despite QS signals' role has been well documented in mammalian cells, their impact on aquatic organisms is still at the budding stage. However, many aquatic organisms produce enzymes that degrade and detoxify such QS signals. In addition, physical and chemical factors also determine the stability of the QS signals in the aqueous environment. The balance between QS signals and existing QS signals degrading factors essentially determines the disease progression in aquatic organisms. In this review, we highlight the impact of QS signals on aquatic organisms and further discussed potential alternative strategies to control disease progression.

Quorum sensing signal synthases enhance Vibrio parahaemolyticus swarming motility

Vibrio parahaemolyticus is a significant food-borne pathogen that is found in diverse aquatic habitats. Quorum sensing (QS), a signaling system for cell-cell communication, plays an important role in V. parahaemolyticus persistence. We characterized the function of three V. parahaemolyticus QS signal synthases, CqsAvp , LuxMvp , and LuxSvp , and show that they are essential to activate QS and regulate swarming. We found that CqsAvp , LuxMvp , and LuxSvp activate a QS bioluminescence reporter through OpaR. However, V. parahaemolyticus exhibits swarming defects in the absence of CqsAvp , LuxMvp , and LuxSvp , but not OpaR. The swarming defect of this synthase mutant (termed Δ3AI) was recovered by overexpressing either LuxOvp D47A , a mimic of dephosphorylated LuxOvp mutant, or the scrABC operon. CqsAvp , LuxMvp , and LuxSvp inhibit lateral flagellar (laf) gene expression by inhibiting the phosphorylation of LuxOvp and the expression of scrABC. Phosphorylated LuxOvp enhances laf gene expression in a mechanism that involves modulating c-di-GMP levels. However, enhancing swarming requires phosphorylated and dephosphorylated LuxOvp which is regulated by the QS signals that are synthesized by CqsAvp , LuxMvp , and LuxSvp . The data presented here suggest an important strategy of swarming regulation by the integration of QS and c-di-GMP signaling pathways in V. parahaemolyticus.

Strain-specific quorum-sensing responses determine virulence properties in Vibrio anguillarum

Bacterial populations communicate using quorum-sensing (QS) molecules and switch on QS regulation to engage in coordinated behaviour such as biofilm formation or virulence. The marine fish pathogen Vibrio anguillarum harbours several QS systems, and our understanding of its QS regulation is still fragmentary. Here, we identify the VanT-QS regulon and explore the diversity and trajectory of traits under QS regulation in Vibrio anguillarum through comparative transcriptomics of two wildtype strains and their corresponding mutants artificially locked in QS-on (ΔvanO) or QS-off (ΔvanT) states. Intriguingly, the two wildtype populations showed different QS responses to cell density changes and operated primarily in the QS-on and QS-off spectrum, respectively. Examining 27 V. anguillarum strains revealed that ~11% were QS-negative, and GFP-reporter measurements of nine QS-positive strains revealed a highly strain-specific nature of the QS responses. We showed that QS controls a plethora of genes involved in processes such as central metabolism, biofilm formation, competence, T6SS, and virulence properties in V. anguillarum, with large strain-specific differences. Moreover, we demonstrated that the QS state is an important driver of virulence towards fish larvae in one of two V. anguillarum strains. We speculate that infections by mixed-strain communities spanning diverse QS strategies optimize the infection efficiency of the pathogen.

Anti-Vibrio potential of natural products from marine microorganisms

The emergence of drug-resistant Vibrio poses a serious threat to aquaculture and human health, thus there is an urgent need for the discovery of new related antibiotics. Given that marine microorganisms (MMs) are evidenced as important sources of antibacterial natural products (NPs), great attention has been gained to the exploration of potential anti-Vibrio agents from MMs. This review summarizes the occurrence, structural diversity, and biological activities of 214 anti-Vibrio NPs isolated from MMs (from 1999 to July 2022), including 108 new compounds. They were predominantly originated from marine fungi (63%) and bacteria (30%) with great structural diversity, including polyketides, nitrogenous compounds, terpenoids, and steroids, among which polyketides account for nearly half (51%) of them. This review will shed light on the development of MMs derived NPs as potential anti-Vibrio lead compounds with promising applications in agriculture and human health.

Evaluation of the pathogenesis of non-typical strain with α-hemolysin, Vibrio parahaemolyticus 353, isolated from Chinese seafood through comparative genome and transcriptome analysis

Vibrio parahaemolyticus outbreaks frequently occur, causing gastrointestinal sickness owing to the consumption of aquatic foods by various virulence factors; however, the mechanism of pathogenesis is still unknown. In this study, a non-typical strain of V. parahaemolyticus, named VP353, was isolated from shrimp in China. Its comparative genome and transcriptome after infection with Caco-2 cells were examined to illustrate the mechanisms of its pathogenesis. VP353 was a tdh-trh- strain but uncommonly manifested robust cytotoxicity towards Caco-2 cells. Compared with the standard strain RIMD2210633, VP353 harbored alpha-hemolysins (hlyA, hlyB, hlyC, and hlyD) was first reported in V. parahaemolyticus and showed high diversity in the T3SS2 gene cluster. Moreover, the expression of flagella, T2SS, quorum sensing-related genes, hlyA, hlyC were up-regulated, and hlyB, hlyD were down-regulated. In summary, our results demonstrate that some novel virulence factors contribute to the pathogenesis of V. parahaemolyticus infection.

Natural products as antivibrio agents: insight into the chemistry and biological activity

Vibriosis causes serious problems and economic loss in aquaculture and human health. Investigating natural products as antivibrio agents has gained more attention to combat vibriosis. The present review highlights the chemical diversity of antivibrio isolated from bacteria, fungi, plants, and marine organisms. Based on the study covering the literature from 1985-2021, the chemical diversity ranges from alkaloids, terpenoids, polyketides, sterols, and peptides. The mechanisms of action are included inhibiting growth, interfering with biofilm formation, and disrupting of quorum sensing. Relevant summaries focusing on the source organisms and the associated bioactivity of different chemical classes are also provided. Further research on in vivo studies, toxicity, and clinical is required for the application in aquaculture and human health.

Accurate Identification of Diverse N-acyl Homoserine Lactones in Marine Vibrio fluvialis by UHPLC-MS/MS

Vibrio fluvialis is a marine opportunistic pathogen that frequently causes diseases in aquatic animals and humans. V. fluvialis can produce quorum sensing signaling molecules to coordinate cell density-dependent behavioral changes, including N-acyl homoserine lactone (AHL), which acts as a vital mediator of virulence-associated gene expression. Currently, several AHL molecules in V. fluvialis have been detected via biological and physicochemical methods, although different detection approaches have generated diverse AHL profiles. Here, we describe the AHL-producing bacterium, V. fluvialis BJ-1, which was isolated from marine sediments from the East China Sea. V. fluvialis BJ-1 could stimulate AHL-mediated β-galactosidase synthesis of the biosensor Agrobacterium tumefaciens NTL4 (pZLR4) but could not induce violacein production in the AHL reporter strain, Chromobacterium violaceum CV026. This bacterial isolate exhibited strong AHL-producing activity at low cell density; however, the AHL activity declined when population density remained at high levels. Analysis of the AHLs by Ultra-High-Performance Liquid Chromatography tandem Mass Spectrometry demonstrated that V. fluvialis BJ-1 produced five different AHL signaling molecules, including two linear chain AHL products (C₈- and C₁₀-HSL), and three β-carbon-oxidative AHL products (3-O-C₈-, 3-O-C₁₀- and 3-O-C₁₂-HSL). Significantly, the present study is the first to accurately define the AHL profile of marine V. fluvialis. In future, the coupling of UHPLC to ESI-MS/MS is expected to be utilized for the accurate determination of AHL profiles in marine Vibrio.

Protective effects of the biofloc system in Pacific white shrimp (Penaeus vannamei) culture against pathogenic Vibrio parahaemolyticus infection

The mitigation of vibriosis in shrimp culture could be accomplished by reducing the virulence of the pathogen or by increasing the immune response of the shrimp. This study aims to evaluate the application of a biofloc system in protecting Pacific white shrimp (Penaeus vannamei) from pathogenic Vibrio parahaemolyticus infection. Shrimp post-larvae (PL 20) with an average body weight of 0.041 ± 0.019 g were reared in an aquarium with a working volume of 33 L at a density of 3 L^-1 for 21 days using two rearing systems, i.e., the biofloc system and the regular clear water system as control. In each system, the shrimp post-larvae were challenged by adding V. parahaemolyticus at different densities, 10³, 10⁵, and 10⁷ CFU mL^-1, while the negative control was performed by maintaining shrimp post-larvae in the clear water system without the addition of V. parahaemolyticus. The results of the in vitro experiment showed that the density and biofilm activity of V. parahaemolyticus reared in biofloc suspension was lower than that of the positive control (p < 0.05). The density of V. parahaemolyticus in shrimp rearing water was lower than that in the control at 10⁵ CFU mL^-1, especially on the 3rd day post-challenge, but there was no significant difference in the total presumptive Vibrio count between the biofloc treatment and the control. The survival, growth, and immune response parameters, such as total hemocyte count, phagocytic activity, respiratory burst, and phenoloxidase activity, of the shrimp, reared in the biofloc system were also higher than those of the positive control (p < 0.05), regardless of the density of V. parahaemolyticus. The present study demonstrated that the application of biofloc could significantly protect and increase the resistance of Pacific white shrimp against pathogenic V. parahaemolyticus infection.

Quorum Sensing Controls the CRISPR and Type VI Secretion Systems in Aliivibrio wodanis 06/09/139

For bacteria to thrive in an environment with competitors, phages and environmental cues, they use different strategies, including Type VI Secretion Systems (T6SSs) and Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) to compete for space. Bacteria often use quorum sensing (QS), to coordinate their behavior as the cell density increases. Like other aliivibrios, Aliivibrio wodanis 06/09/139 harbors two QS systems, the main LuxS/LuxPQ system and an N-acyl homoserine lactone (AHL)-mediated AinS/AinR system and a master QS regulator, LitR. To explore the QS and survival strategies, we performed genome analysis and gene expression profiling on A. wodanis and two QS mutants (ΔainS and ΔlitR) at two cell densities (OD600 2.0 and 6.0) and temperatures (6 and 12°C). Genome analysis of A. wodanis revealed two CRISPR systems, one without a cas loci (CRISPR system 1) and a type I-F CRISPR system (CRISPR system 2). Our analysis also identified three main T6SS clusters (T6SS1, T6SS2, and T6SS3) and four auxiliary clusters, as well about 80 potential Type VI secretion effectors (T6SEs). When comparing the wildtype transcriptome data at different cell densities and temperatures, 13-18% of the genes were differentially expressed. The CRISPR system 2 was cell density and temperature-independent, whereas the CRISPR system 1 was temperature-dependent and cell density-independent. The primary and auxiliary clusters of T6SSs were both cell density and temperature-dependent. In the ΔlitR and ΔainS mutants, several CRISPR and T6SS related genes were differentially expressed. Deletion of litR resulted in decreased expression of CRISPR system 1 and increased expression of CRISPR system 2. The T6SS1 and T6SS2 gene clusters were less expressed while the T6SS3 cluster was highly expressed in ΔlitR. Moreover, in ΔlitR, the hcp1 gene was strongly activated at 6°C compared to 12°C. AinS positively affected the csy genes in the CRISPR system 2 but did not affect the CRISPR arrays. Although AinS did not significantly affect the expression of T6SSs, the hallmark genes of T6SS (hcp and vgrG) were AinS-dependent. The work demonstrates that T6SSs and CRISPR systems in A. wodanis are QS dependent and may play an essential role in survival in its natural environment.

Profiling Signal Transduction in Global Marine Biofilms

Microbes use signal transduction systems in the processes of swarming motility, antibiotic resistance, virulence, conjugal plasmid transfer, and biofilm formation. However, the signal transduction systems in natural marine biofilms have hardly been profiled. Here we analyzed signal transduction genes in 101 marine biofilm and 91 seawater microbial metagenomes. The abundance of almost all signal transduction-related genes in biofilm microbial communities was significantly higher than that in seawater microbial communities, regardless of substrate types, locations, and durations for biofilm development. In addition, the dominant source microbes of signal transduction genes in marine biofilms were different from those in seawater samples. Co-occurrence network analysis on signal communication between microbes in marine biofilms and seawater microbial communities revealed potential inter-phyla interactions between microorganisms from marine biofilms and seawater. Moreover, phylogenetic tree construction and protein identity comparison displayed that proteins related to signal transductions from Red Sea biofilms were highly similar to those from Red Sea seawater microbial communities, revealing a possible biological basis of interspecies interactions between surface-associated and free-living microbial communities in a local marine environment. Our study revealed the special profile and enrichment of signal transduction systems in marine biofilms and suggested that marine biofilms participate in intercellular interactions of the local ecosystem where they were seeded.

Quorum sensing and iron-dependent coordinated control of autoinducer-2 production via small RNA RyhB in Vibrio vulnificus

Roles for the non-coding small RNA RyhB in quorum-sensing and iron-dependent gene modulation in the human pathogen V. vulnificus were assessed in this study. Both the quorum sensing master regulator SmcR and the Fur-iron complex were observed to bind to the region upstream of the non-coding small RNA RyhB gene to repress expression, which suggests that RyhB is associated with both quorum-sensing and iron-dependent signaling in this pathogen. We found that expression of LuxS, which is responsible for the biosynthesis of autoinducer-2 (AI-2), was higher in wild type than in a ryhB-deletion isotype. RyhB binds directly to the 5′-UTR (untranslated region) of the luxS transcript to form a heteroduplex, which not only stabilizes luxS mRNA but also disrupts the secondary structure that normally obscures the translational start codon and thereby allows translation of LuxS to begin. The binding of RyhB to luxS mRNA requires the chaperone protein Hfq, which stabilizes RyhB. These results demonstrate that the small RNA RyhB is a key element associated with feedback control of AI-2 production, and that it inhibits quorum-sensing signaling in an iron-dependent manner. This study, taken together with previous studies, shows that iron availability and cell density signals are funneled to SmcR and RyhB, and that these regulators coordinate cognate signal pathways that result in the proper balance of protein expression in response to environmental conditions.

Unraveling the complex regulatory networks in biofilm formation in bacteria and relevance of biofilms in environmental remediation

Biofilms are assemblages of bacteria embedded within a matrix of extracellular polymeric substances (EPS) attached to a substratum. The process of biofilm formation is a complex phenomenon regulated by the intracellular and intercellular signaling systems. Various secondary messenger molecules such as cyclic dimeric guanosine 3',5'-monophosphate (c-di-GMP), cyclic adenosine 3',5'-monophosphate (cAMP), and cyclic dimeric adenosine 3',5'-monophosphate (c-di-AMP) are involved in complex signaling networks to regulate biofilm development in several bacteria. Moreover, the cell to cell communication system known as Quorum Sensing (QS) also regulates biofilm formation via diverse mechanisms in various bacterial species. Bacteria often switch to the biofilm lifestyle in the presence of toxic pollutants to improve their survivability. Bacteria within a biofilm possess several advantages with regard to the degradation of harmful pollutants, such as increased protection within the biofilm to resist the toxic pollutants, synthesis of extracellular polymeric substances (EPS) that helps in the sequestration of pollutants, elevated catabolic gene expression within the biofilm microenvironment, higher cell density possessing a large pool of genetic resources, adhesion ability to a wide range of substrata, and metabolic heterogeneity. Therefore, a comprehensive account of the various factors regulating biofilm development would provide valuable insights to modulate biofilm formation for improved bioremediation practices. This review summarizes the complex regulatory networks that influence biofilm development in bacteria, with a major focus on the applications of bacterial biofilms for environmental restoration.

CqsA-introduced quorum sensing inhibits type VI secretion system 2 through an OpaR-dependent pathway in Vibrio parahaemolyticus

The well-known food-borne pathogen Vibrio parahaemolyticus employs at least three quorum sensing signals to maintain its high environmental adaptability. V. parahaemolyticus CqsA, the synthase involved in 3-hydroxyundecan-4-one quorum sensing signal, introduces a quorum sensing network. The V. parahaemolyticus virulent factor type VI secretion system 2 (T6SS2), which is associated with adhesion to host cells, was previously reported to be regulated by a quorum sensing system. Herein, we set out to determine the role of CqsA-introduced quorum sensing (CIQS) in T6SS2-associated virulent regulation. Using a tandem mass tag (TMT)-based quantitative proteomics assay, 17 T6SS2 proteins were found having significantly higher abundances in the ΔcqsA strain than in the wild type strain. TMT proteomics assay results were confirmed by a parallel reaction-monitoring (PRM)-based proteomics assay. Two T6SS2 up-regulators, OpaR and CalR, were found under control of CIQS in the TMT proteomics assay, while OpaR was down-regulated and CalR was up-regulated by CIQS. Thus, it was hypothesized that CIQS would inhibit T6SS2 with an OpaR-dependent mechanism. Epistasis experiment with quantitative PCR was designed to analyze the role of OpaR in the process of CIQS inhibiting T6SS2 production. The mRNA levels of T6SS2 genes were up-regulated in the ΔcqsA strain while down-regulated in the ΔopaR strain and in the ΔcqsAΔopaR mutant, indicating that OpaR plays a predominant role in the regulation of T6SS2 by CIQS. Using a cell adhesion assay, we further found that the T6SS2-dependent adhesion activity of V. parahaemolyticus to Hela cells was also inhibited by CIQS and the inhibition was OpaR-dependent. In this study, we confirmed that V. parahaemolyticus CIQS inhibited T6SS2 through an OpaR-dependent pathway. It enriches the knowledge of how V. parahaemolyticus quorum sensing regulates its virulence.

Quorum Sensing Regulation of Virulence Gene Expression in Vibrio harveyi during its Interaction with Marine Diatom Skeletonema marinoi

Communication between species from different kingdoms may be as important as intra-kingdom communication. It has recently been confirmed that co-existing bacteria and phytoplankton in aquatic ecosystems do cross-talk. This study examined the signs of possible cross signalling between V. harveyi, one of the predominant bacterial species of the marine ecosystem and a dominant diatom species, S.marinoi, to understand communication over species borders. It is known that V.harveyi employ quorum sensing for cell-to-cell communication, bioluminescence (luxR), and the regulation of the virulence gene (vhp, chiA). Former studies have also shown, this kind of interactions being disrupted by compounds secreted by a few algal species existing in the aquatic ecosystem. We investigated the QS communication by quantifying the expression levels of virulence regulator luxR and virulence factors metalloprotease (vhp) and chitinase (chiA) in four different V. harveyi strains grown in the presence of S. marinoi strain. Results obtained in this study indicate that quorum sensing was activated in strains of V. harveyi analysed but did not regulate the expressions of vhp and chiA virulence factors. This observation suggests that the existence of S. marinoi did not interfere with the QS behaviour of V. harveyi and its interaction with marine diatom; it may be due to the commensalism relationship.

Bacillus spp. Inhibit Edwardsiella tarda Quorum-Sensing and Fish Infection

The disruption of pathogen communication or quorum-sensing (QS) via quorum-quenching (QQ) molecules has been proposed as a promising strategy to fight bacterial infections. Bacillus spp. have recognizable biotechnology applications, namely as probiotic health-promoting agents or as a source of natural antimicrobial molecules, including QQ molecules. This study characterized the QQ potential of 200 Bacillus spp., isolated from the gut of different aquaculture fish species, to suppress fish pathogens QS. Approximately 12% of the tested Bacillus spp. fish isolates (FI). were able to interfere with synthetic QS molecules. Ten isolates were further selected as producers of extracellular QQ-molecules and their QQ capacity was evaluated against the QS of important aquaculture bacterial pathogens, namely Aeromonas spp., Vibrio spp., Photobacterium damselae, Edwardsiela tarda, and Shigella sonnei. The results revealed that A. veronii and E. tarda produce QS molecules that are detectable by the Chr. violaceum biosensor, and which were degraded when exposed to the extracellular extracts of three FI isolates. Moreover, the same isolates, identified as B. subtilis, B. vezelensis, and B. pumilus, significantly reduced the pathogenicity of E. tarda in zebrafish larvae, increasing its survival by 50%. Taken together, these results identified three Bacillus spp. capable of extracellularly quenching aquaculture pathogen communication, and thus become a promising source of bioactive molecules for use in the biocontrol of aquaculture bacterial diseases.

Virulence of Vibrio harveyi ORM4 towards the European abalone Haliotis tuberculata involves both quorum sensing and a type III secretion system

Environmental Vibrio strains represent a major threat in aquaculture, but the understanding of their virulence mechanisms heavily relies on the transposition of knowledge from human-pathogen vibrios. Here, the genetic bases of the virulence of Vibrio harveyi ORM4 towards the European abalone Haliotis tuberculata were characterized. We demonstrated that luxO, encoding a major regulator of the quorum sensing system, is crucial for the virulence of this strain, and that its deletion leads to a decrease in swimming motility, biofilm formation, and exopolysaccharide production. Furthermore, the biofilm formation by V. harveyi ORM4 was increased by abalone serum, which required LuxO. The absence of LuxO in V. harveyi ORM4 yielded opposite phenotypes compared with other Vibrio species including V. campbellii (still frequently named V. harveyi). In addition, we report a full type III secretion system (T3SS) gene cluster in the V. harveyi ORM4 genome. LuxO was shown to negatively regulate the promoter activity of exsA, encoding the major regulator of the T3SS genes, and the deletion of exsA abolished the virulence of V. harveyi ORM4. These results unveil virulence mechanisms set up by this environmentally important bacterial pathogen and pave the way for a better molecular understanding of the regulation of its pathogenicity.

LuxT controls specific quorum-sensing-regulated behaviors in Vibrionaceae spp. via repression of qrr1, encoding a small regulatory RNA

Quorum sensing (QS) is a process of chemical communication bacteria use to transition between individual and collective behaviors. QS depends on the production, release, and synchronous response to signaling molecules called autoinducers (AIs). The marine bacterium Vibrio harveyi monitors AIs using a signal transduction pathway that relies on five small regulatory RNAs (called Qrr1-5) that post-transcriptionally control target genes. Curiously, the small RNAs largely function redundantly making it difficult to understand the necessity for five of them. Here, we identify LuxT as a transcriptional repressor of qrr1. LuxT does not regulate qrr2-5, demonstrating that qrr genes can be independently controlled to drive unique downstream QS gene expression patterns. LuxT reinforces its control over the same genes it regulates indirectly via repression of qrr1, through a second transcriptional control mechanism. Genes dually regulated by LuxT specify public goods including an aerolysin-type pore-forming toxin. Phylogenetic analyses reveal that LuxT is conserved among Vibrionaceae and sequence comparisons predict that LuxT represses qrr1 in additional species. The present findings reveal that the QS regulatory RNAs can carry out both shared and unique functions to endow bacteria with plasticity in their output behaviors.

Bacteria communicate and count their cell numbers using a process called quorum sensing (QS). In response to changes in cell density, QS bacteria alternate between acting as individuals and participating in collective behaviors. Vibrio harveyi is used as a model organism to understand QS-mediated communication. Five small RNAs lie at the heart of the V. harveyi QS system, and they regulate the target genes that underlie the QS response. The small RNAs largely function redundantly making it difficult to understand why V. harveyi requires five of them. Here, we discover a regulator, called LuxT, that exclusively represses the gene encoding one of the QS small RNAs. LuxT regulation of one QS small RNA enables unique control of a specific subset of QS target genes. LuxT is broadly conserved among Vibrionaceae. Our findings show how redundant regulatory components can possess both common and unique roles that provide bacteria with plasticity in their behaviors.

Quorum Sensing Inhibition or Quenching in Acinetobacter baumannii: The Novel Therapeutic Strategies for New Drug Development

Acinetobacter baumannii is a Gram-negative opportunistic nosocomial pathogen, which can cause ventilator-related and blood infection in critically ill patients. The resistance of A. baumannii clinical isolates to common antimicrobials and their tolerance to desiccation have emerged as a serious problem to public health. In the process of pathogenesis, bacteria release signals, which regulate virulence and pathogenicity-related genes. Such bacteria coordinate their virulent behavior in a cell density-dependent phenomenon called quorum sensing (QS). In contrast, the two main approaches of QS interference, quorum sensing inhibitors (QSIs) and quorum quenching (QQ) enzymes, have been developed to reduce the virulence of bacteria, thus reducing the pressure to produce bacterial drug resistance. Therefore, QSIs or QQ enzymes, which interfere with these processes, might potentially inhibit bacterial QS and ultimately biofilm formation. In this review, we aim to describe the state-of-art in the QS process in A. baumannii and elaborate on the use of QSIs or QQ enzymes as antimicrobial drugs in various potential sites of the QS pathway.

Resistance risk induced by quorum sensing inhibitors and their combined use with antibiotics: Mechanism and its relationship with toxicity

The abuse of antibiotics has brought out serious bacterial resistance, which threatens the ecological environment and human health. Quorum sensing inhibitors (QSIs), as a new kind of potential antibiotic substitutes that are theoretically difficult to trigger bacterial resistance, are recommended to individually use or jointly use with traditional antibiotics. However, there are few studies on the resistance risk in the use of QSIs. In this study, the influence of QSIs alone or in combination with sulfonamides (SAs) on conjugative transfer and mutation of Escherichia coli (E. coli) was investigated to explore whether QSIs have the potential to induce bacterial resistance. The results show that QSIs may facilitate plasmid RP4 conjugative transfer by binding with SdiA protein to regulate pilus expression, and interact with LsrR protein to increase SOS gene expression, inducing gene mutation. The QSIs-SAs mixtures could promote plasmid RP4 conjugative transfer and mutation in E. coli, and the main joint effects are synergism and antagonism. Furthermore, there is a good correlation among conjugative transfer, mutation, and growth inhibition of QSIs-SAs to E. coli. It could be speculated that bacteria may delay cell division to provide sufficient energy and time for regulating conjugative transfer and mutation under the stress of QSIs and their combined exposure with antibiotics, which is essentially a balance between bacterial resistance and toxicity. This study provides a reference for the resistance risk assessment of QSIs and benefits the clinical application of QSIs.

Use of Quorum Sensing Inhibition Strategies to Control Microfouling

Interfering with the quorum sensing bacterial communication systems has been proposed as a promising strategy to control bacterial biofilm formation, a key process in biofouling development. Appropriate in vitro biofilm-forming bacteria models are needed to establish screening methods for innovative anti-biofilm and anti-microfouling compounds. Four marine strains, two Pseudoalteromonas spp. and two Vibrio spp., were selected and studied with regard to their biofilm-forming capacity and sensitivity to quorum sensing (QS) inhibitors. Biofilm experiments were performed using two biofilm cultivation and quantification methods: the xCELLigence^® system, which allows online monitoring of biofilm formation, and the active attachment model, which allows refreshment of the culture medium to obtain a strong biofilm that can be quantified with standard staining methods. Although all selected strains produced acyl-homoserine-lactone (AHL) QS signals, only the P. flavipulchra biofilm, measured with both quantification systems, was significantly reduced with the addition of the AHL-lactonase Aii20J without a significant effect on planktonic growth. Two-species biofilms containing P. flavipulchra were also affected by the addition of Aii20J, indicating an influence on the target bacterial strain as well as an indirect effect on the co-cultured bacterium. The use of xCELLigence^® is proposed as a time-saving method to quantify biofilm formation and search for eco-friendly anti-microfouling compounds based on quorum sensing inhibition (QSI) strategies. The results obtained from these two in vitro biofilm formation methods revealed important differences in the response of biosensor bacteria to culture medium and conditions, indicating that several strains should be used simultaneously for screening purposes and the cultivation conditions should be carefully optimized for each specific purpose.

High cell densities favor lysogeny: induction of an H20 prophage is repressed by quorum sensing and enhances biofilm formation in Vibrio anguillarum

Temperate ϕH20-like phages are repeatedly identified at geographically distinct areas as free phage particles or as prophages of the fish pathogen Vibrio anguillarum. We studied mutants of a lysogenic isolate of V. anguillarum locked in the quorum-sensing regulatory modes of low (ΔvanT) and high (ΔvanO) cell densities by in-frame deletion of key regulators of the quorum-sensing pathway. Remarkably, we find that induction of the H20-like prophage is controlled by the quorum-sensing state of the host, with an eightfold increase in phage particles per cell in high-cell-density cultures of the quorum-sensing-deficient ΔvanT mutant. Comparative studies with prophage-free strains show that biofilm formation is promoted at low cell density and that the H20-like prophage stimulates this behavior. In contrast, the high-cell-density state is associated with reduced prophage induction, increased proteolytic activity, and repression of biofilm. The proteolytic activity may dually function to disperse the biofilm and as a quorum-sensing-mediated antiphage strategy. We demonstrate an intertwined regulation of phage-host interactions and biofilm formation, which is orchestrated by host quorum-sensing signaling, suggesting that increased lysogeny at high cell density is not solely a strategy for phages to piggy-back the successful bacterial hosts but is also a host strategy evolved to take control of the lysis-lysogeny switch to promote host fitness.

Current and future perspectives for controlling Vibrio biofilms in the seafood industry: a comprehensive review

The contamination of seafood with Vibrio species can have severe repercussions in the seafood industry. Vibrio species can form mature biofilms and persist on the surface of several seafoods such as crabs, oysters, mussels, and shrimp, for extended duration. Several conventional approaches have been employed to inhibit the growth of planktonic cells and prevent the formation of Vibrio biofilms. Since Vibrio biofilms are mostly resistant to these control measures, novel alternative methods need to be urgently developed. In this review, we propose environmentally friendly approaches to suppress Vibrio biofilm formation using a hypothesized mechanism of action.

NarL, a Novel Repressor for CYP108j1 Expression during PAHs Degradation in Rhodococcus sp. P14

Rhodococcus sp. P14 was isolated from crude-oil-contaminated sediments, and a wide range of polycyclic aromatic hydrocarbons (PAHs) could be used as the sole source of carbon and energy. A key CYP450 gene, designated as cyp108j1 and involved in the degradation of PAHs, was identified and was able to hydroxylate various PAHs. However, the regulatory mechanism of the expression of cyp108j1 remains unknown. In this study, we found that the expression of cyp108j1 is negatively regulated by a LuxR (helix-turn-helix transcription factors in acyl-homoserine lactones-mediated quorum sensing) family regulator, NarL (nitrate-dependent two-component regulatory factor), which is located upstream of cyp108j1. Further analysis revealed that NarL can directly bind to the promoter region of cyp108j1. Mutational experiments demonstrated that the binding site between NarL and the cyp108j1 promoter was the palindromic sequence GAAAGTTG-CAACTTTC. Together, the finding reveal that NarL is a novel repressor for the expression of cyp108j1 during PAHs degradation.

Quorum Quenching Properties and Probiotic Potentials of Intestinal Associated Bacteria in Asian Sea Bass Lates calcarifer

Quorum quenching (QQ), the enzymatic degradation of N-acyl homoserine lactones (AHLs), has been suggested as a promising strategy to control bacterial diseases. In this study, 10 AHL-degrading bacteria isolated from the intestine of barramundi were identified by 16S rDNA sequencing. They were able to degrade both short and long-chain AHLs associated with several pathogenic Vibrio species (spp.) in fish, including N-[(RS)-3-Hydroxybutyryl]-l-homoserine lactone (3-oh-C₄-HSL), N-Hexanoyl-l-homoserine lactone (C₆-HSL), N-(β-Ketocaproyl)-l-homoserine lactone (3-oxo-C₆-HSL), N-(3-Oxodecanoyl)-l-homoserine lactone (3-oxo-C₁₀-HSL), N-(3-Oxotetradecanoyl)-l-homoserine lactone (3-oxo-C₁₄-HSL). Five QQ isolates (QQIs) belonging to the Bacillus and Shewanella genera, showed high capacity to degrade both synthetic AHLs as well as natural AHLs produced by Vibrio harveyi and Vibrio alginolyticus using the well-diffusion method and thin-layer chromatography (TLC). The genes responsible for QQ activity, including aiiA, ytnP, and aaC were also detected. Analysis of the amino acid sequences from the predicted lactonases revealed the presence of the conserved motif HxHxDH. The selected isolates were further characterized in terms of their probiotic potentials in vitro. Based on our scoring system, Bacillus thuringiensis QQ1 and Bacillus cereus QQ2 exhibited suitable probiotic characteristics, including the production of spore and exoenzymes, resistance to bile salts and pH, high potential to adhere on mucus, appropriate growth abilities, safety to barramundi, and sensitivity to antibiotics. These isolates, therefore, constitute new QQ probiotics that could be used to control vibriosis in Lates calcalifer.

Vibrio parahaemolyticus cqsA controls production of quorum sensing signal molecule 3-hydroxyundecan-4-one and regulates colony morphology

In order to adapt to different environments, Vibrio parahaemolyticus employed a complicated quorum sensing system to orchestrate gene expression and diverse colony morphology patterns. In this study, the function of the putative quorum sensing signal synthase gene cqsA (VPA0711 in V. Parahaemolyticus strain RIMD2210633 genome) was investigated. The cloning and expression of V. parahaemolyticus cqsA in Escherichia coli system induced the production of a new quorum sensing signal that was found in its culture supernatant. The signal was purified by high performance liquid chromatography methods and determined to be 3-hydroxyundecan- 4-one by indirect and direct mass spectra assays. The deletion of cqsA in RIMD2210633 changed V. parahaemolyticus colony morphology from the classical 'fried-egg' shape (thick and opaque in the center, while thin and translucent in the edge) of the wild-type colony to a 'pancake' shape (no significant difference between the centre and the edge) of the cqsA deleted colony. This morphological change could be restored by complementary experiment with cqsA gene or the signal extract. In addition, the expression of opaR, a well-known quorum sensing regulatory gene, could be up-regulated by cqsA deletion. Our results suggested that V. parahaemolyticus used cqsA to produce 3-hydroxyundecan-4-one signal and thereby regulated colony morphology and other quorum sensing-associated behaviors.

Quorum sensing in Vibrio spp.: the complexity of multiple signalling molecules in marine and aquatic environments

Quorum sensing (QS) is a density-dependent mechanism enabling bacteria to coordinate their actions via the release of small diffusible molecules named autoinducers (AIs). Vibrio spp. are able to adapt to changing environmental conditions by using a wide range of physiological mechanisms and many species pose a threat for human health and diverse marine and estuarine ecosystems worldwide. Cell-to-cell communication controls many of their vital functions such as niche colonization, survival strategies, or virulence. In this review, I summarize (1) the different known QS pathways (2) the diversity of AIs as well as their biological functions, and (3) the QS-mediated interactions between Vibrio and other organisms. However, the current knowledge is limited to a few pathogenic or bioluminescent species and in order to provide a genus-wide view an inventory of QS genes among 87 Vibrio species has been made. The large diversity of signal molecules and their differential effects on a particular physiological function suggest that the complexity of multiple signalling systems within bacterial communities is far from being fully understood. I question here the real level of specificity of such communication in the environment and discuss the different perspectives in order to better apprehend QS in natural habitats.

Relationship Between Quorum Sensing and Secretion Systems

Quorum sensing (QS) is a communication mechanism between bacteria that allows specific processes to be controlled, such as biofilm formation, virulence factor expression, production of secondary metabolites and stress adaptation mechanisms such as bacterial competition systems including secretion systems (SS). These SS have an important role in bacterial communication. SS are ubiquitous; they are present in both Gram-negative and Gram-positive bacteria and in Mycobacterium sp. To date, 8 types of SS have been described (T1SS, T2SS, T3SS, T4SS, T5SS, T6SS, T7SS, and T9SS). They have global functions such as the transport of proteases, lipases, adhesins, heme-binding proteins, and amidases, and specific functions such as the synthesis of proteins in host cells, adaptation to the environment, the secretion of effectors to establish an infectious niche, transfer, absorption and release of DNA, translocation of effector proteins or DNA and autotransporter secretion. All of these functions can contribute to virulence and pathogenesis. In this review, we describe the known types of SS and discuss the ones that have been shown to be regulated by QS. Due to the large amount of information about this topic in some pathogens, we focus mainly on Pseudomonas aeruginosa and Vibrio spp.

Marine biofilms: diversity of communities and of chemical cues

Surfaces immersed in seawater are rapidly colonized by various microorganisms, resulting in the formation of heterogenic marine biofilms. These communities are known to influence the settlement of algae spores and invertebrate larvae, triggering a succession of fouling events, with significant environmental and economic impacts. This review covers recent research regarding the differences in composition of biofilms isolated from different artificial surface types and the influence of environmental factors on their formation. One particular phenomenon - bacterial quorum sensing (QS) - allows bacteria to coordinate swarming, biofilm formation among other phenomena. Some other marine biofilm chemical cues are believed to modulate the settlement and the succession of macrofouling organisms, and they are also reviewed here. Finally, since the formation of a marine biofilm is considered to be an initial, QS-dependent step in the development of marine fouling events, QS inhibition is discussed on its potential as a tool for antibiofouling control in marine settings.