Specificity and complexity in bacterial quorum-sensing systems

Targeting quorum sensing in Pseudomonas aeruginosa with high-affinity inhibitors: A high-throughput screening and in-silico analysis

Pseudomonas aeruginosa is a major pathogen in clinical settings, notorious for its intrinsic resistance to multiple antibiotics and ability to form biofilms, therefore, complicating treatment. This study reporting the results of a high-throughput screen of an antibacterial library targeting LasR, a key QS regulator in P. aeruginosa MB638, isolated from an infected surgical implant. The species identity was confirmed as P. aeruginosa via 16S rDNA analysis (accession number MT643188). The strain demonstrated strong biofilm formation and multidrug resistance, along with significant production of the quorum sensing signalling molecule N-(3-oxododecanoyl)-L-homoserine lactone (OdDHL). We screened a ∼1400-compound library and identified inhibitors compounds that surpass the binding affinity of LasR's native ligand. The ADMET analysis revealed that among these, compounds Inh-1, Inh-2 and Inh-3 demonstrated favourable absorption, permeability and broader bioactivity profiles. Inh-1 exhibited a suitable profile, being non-toxic, non-hepatotoxic, and non-mutagenic, with low carcinogenic and immunotoxin potential. Molecular docking studies using GLIDE identified key binding interactions and residues within the LasR ligand-binding domain (LBD), with Inh-1, Inh-2 and Inh-3 showing the highest binding affinities and favourable docking scores of -14.587, -13.645 and -12.967, respectively. Structural Interaction Molecular dynamics simulation at 100 ns showed Inh-1 maintained stable hydrophobic and hydrophilic contacts within the active site. RMSD analysis confirmed the stability of the Inh-1 complex, while RMSF indicated conformational adaptability. Inh-1 stands out as promising lead for LasR inhibition, warranting further experimental study. These inhibitors hold promise for disrupting quorum sensing in P. aeruginosa and may serve as potential therapeutic agent against resistant infections.

Surfactin Activating KinB to Sense and Utilize Carbon Sources Against Cannibalism in Bacillus amyloliquefaciens

Bacillus species are ubiquitous microorganisms with broad applications. The lipopeptide surfactin serves as a signaling molecule to activate histidine kinases, subsequently activating the global regulatory protein Spo0A to modulate quorum sensing like cannibalism in response to specific environmental conditions such as carbon source limitation. Our previous studies revealed that deletion of the surfactin synthesis gene in Bacillus amyloliquefaciens resulted in rapid cell death during the logarithmic growth phase. In this study, by employing gene knockout strains involved in the surfactin signaling pathway, we found that B. amyloliquefaciens primarily employs histidine kinase KinB to perceive surfactin signals, enabling the utilization of environmental carbon sources. Surfactin signal transduction is not exclusively dependent on histidine kinases; cells utilize alternative pathways (e.g., cation-induced Spo0A activation) to regulate carbon source utilization. Adding glucose, NADH, and ATP all alleviated cell death in ΔsrfA cultures. Both the wild-type and ΔsrfA mutant produce polypeptide/protein-like autotoxins during growth, but ΔsrfA produced more toxins earlier. Carbon sources such as glucose inhibit autotoxin production, while surfactin initiates detoxifying against these toxins. The absence of Spo0A results in both the loss of autotoxin production and antitoxin capabilities, indicating that Spo0A regulates both toxin production and detoxification mechanisms. Collectively, we firstly reported that in B. amyloliquefaciens, cells utilize surfactin as a signaling molecule to activate KinB, which senses environmental carbon sources and thereby inhibits rather than promotes the production of autotoxins as seen in Bacillus subtilis. However, similar to B. subtilis, KinB activates Spo0A to combat autotoxins.

Quorum quenching mediated virulence factor impairment by Pandanus tectorius leaf extract against Chromobacterium violaceum

This study investigated the anti-quorum sensing ability of Pandanus tectorius (PT) leaf extract in suppressing virulence factors including biofilm formation in Chromobacterium violaceum under in vitro conditions. The extract significantly inhibited the biosynthesis of violacein, acyl homoserine lactone, chitinase, exopolysaccharides, haemolysin, and outer membrane vesicles at sub-MIC concentrations of 1.5, 1, and 0.5 mg/ml. Biofilm formation was reduced by 81.46% at a concentration of 1.5 mg/ml. PT leaf extract also downregulated the quorum sensing-associated genes, including cviI, cviR, and vioA of C. violaceum. GC-MS analysis identified 15 major bioactive compounds, which were docked with the cviR master regulator protein. Among those, stigmasterol demonstrated efficient binding at the active site of cviR, with a binding energy of -8.5 kcal/mol. Toxicity assays confirmed the non-toxic nature of the PT extract, highlighting its potential as a safe anti-quorum sensing agent.

Molecular structural arrangement in quorum sensing and bacterial metabolic production

Quorum sensing (QS) regulates bacterial behaviors such as biofilm formation, virulence, and metabolite production through signaling molecules like acyl-homoserine lactones (AHLs), peptides, and AI-2. These signals are pivotal in bacterial communication, influencing pathogenicity and industrial applications. This review explores the molecular architecture of QS signals and their role in metabolite production, emphasizing structural modifications that disrupt bacterial communication to control virulence and enhance industrial processes. Key findings highlight the development of synthetic QS analogs, engineered inhibitors, and microbial consortia as innovative tools in biotechnology and medicine. The review underscores the potential of molecular engineering in managing microbial behaviors and optimizing applications like biofuel production, bioplastics, and anti-virulence therapies. Additionally, cross-species signaling mechanisms, particularly involving AI-2, reveal new opportunities for regulating interspecies cooperation and competition. This synthesis aims to bridge molecular insights with practical applications, showcasing how QS-based technologies can drive advancements in microbial biotechnology and therapeutic strategies.

Interspecies Crosstalk via LuxI/LuxR-Type Quorum Sensing Pathways Contributes to Decreased Nematode Survival in Coinfections of Pseudomonas aeruginosa and Burkholderia multivorans

Quorum sensing (QS) is a prominent chemical communication mechanism used by common bacteria to regulate group behaviors at high cell density, including many processes important in pathogenesis. There is growing evidence that certain bacteria can use QS to sense not only themselves but also other species and that this crosstalk could alter collective behaviors. In the current study, we report the results of culture-based and in vivo coinfection experiments that probe interspecies interactions between the opportunistic pathogens Pseudomonas aeruginosa and Burkholderia multivorans involving their LuxI/LuxR-type QS circuits. Using a Caenorhabditis elegans infection model, we show that infections with both species result in poorer host outcomes compared with monoinfections. We use genetic mutants and a transwell infection assay to establish that crosstalk via LuxR-type receptors and signals is important for this coinfection pathogenicity. Using laboratory cocultures with cell-based reporter systems, we show that the RhlR and CepR receptors in P. aeruginosa and B. multivorans, respectively, can each recognize a QS signal produced by the other species. Lastly, we apply chemical biology to complement our genetic approach and demonstrate the potential to regulate interspecies interactions between the wild-type strains of P. aeruginosa and B. multivorans through the application of synthetic compounds that modulate RhlR and CepR activities. Overall, this study reveals that interspecies interaction via QS networks is possible between P. aeruginosa and B. multivorans and that it can contribute to coinfection severity with these two species.

Protocol for in silico characterization of natural-based molecules as quorum-sensing inhibitors

The search and development of new quorum-sensing (QS) inhibitors are ongoing processes for biofilm control. Here, we present a protocol for in silico characterization of natural-based molecules as QS inhibitors. We describe steps for preparing models of protein receptors for virtual screening. We then detail procedures for construction and virtual screening of phytochemical libraries and hit picking to be experimentally validated by in vitro assays. This protocol allows exploration of a broad range of potential inhibitors for a specific target. For complete details on the use and execution of this protocol, please refer to Fernandes et al.1.

Contribution of the infection ecosystem and biogeography to antibiotic failure in vivo

The acquisition of antibiotic resistance in bacteria, though a deeply concerning international issue, is reasonably well-understood at a mechanistic level. Less well-understood is why bacteria that are sensitive in vitro to well-established and widely-used antibiotics sometimes fail to respond to these agents in vivo. This is a particularly common problem in chronic, polymicrobial infection scenarios. Here, we discuss this in vitro-in vivo disconnect from the perspective of the bacterium, focusing in particular on how infection micro/macro-environment, biogeography, and the presence of co-habiting species affect the response to antibiotics. Using selected exemplars, we also consider interventions that might improve treatment outcomes, as well as ecologically 'eubiotic' approaches that have less of an impact on the patient's commensal microflora. In our view, the accrued data strongly suggest that we need a more comprehensive understanding of the in situ microbiology at infection sites.

Bactericidal and anti-quorum sensing activity of repurposing drug Visomitin against Staphylococcus aureus

With the growing antibiotic resistance in Staphylococcus aureus, it is imperative to develop innovative therapeutic strategies against new targets to reduce selective survival pressures and incidence of resistance. In S. aureus, interbacterial communication relies on a quorum sensing system that regulates gene expression and physiological activities. Here, we identified that Visomitin, an antioxidant small molecule, exhibited bactericidal efficacy against methicillin-resistant S. aureus and its high tolerance phenotypes like intracellular bacteria and persister cells without inducing resistance. Critically, sub-minimal inhibitory concentrations (sub-MICs) of Visomitin could serve as a potent quorum-quencher reducing virulence production (such as haemolysin and staphyloxanthin), along with inhibiting biofilm formation, self-aggregation, and colony spreading of S. aureus. These effects were probably mediated by interfering with the S. aureus accessory gene regulator quorum sensing system. In summary, our findings suggest that Visomitin shows dual antimicrobial effects, including bactericidal effects at the concentrations above MIC and quorum sensing inhibition effects at sub-MICs, which holds promise for treating MRSA-related refractory infections.

Autoinducer-2 relieves soil stress-induced dormancy of Bacillus velezensis by modulating sporulation signaling

The collective behavior of bacteria is regulated by quorum sensing (QS). Autoinducer-2 (AI-2) is a common QS signal that regulates the behavior of both Gram-positive and Gram-negative bacteria. Despite the plethora of processes described to be influenced by AI-2 in diverse Gram-negative bacteria, the AI-2-regulated processes in Bacilli are relatively unexplored. Here, we describe a novel function for AI-2 in Bacillus velezensis SQR9 related to the sporulation. AI-2 inhibited the initiation of sporulation through the phosphatase RapC and the DNA binding regulator ComA. Using biochemistry experiments, we demonstrated that AI-2 interacts with RapC to stimulate its binding to ComA, which leads to an inactive ComA and subsequently a sporulation inhibition. The AI-2 molecule could be shared across species for inhibiting Bacillus sporulation and it also plays the same role in different soil conditions. Our study revealed a novel function and regulatory mechanism of AI-2 in inhibiting sporulation in Bacilli.

Metabolite from supernatant of soil and plant-associated bacteria control biofilm of fish pathogens

OBJECTIVES

This research aimed to identify and quantify the antibiofilm activity of bioactive compounds from bacteria isolated from rhizosphere and nodule butterfly pea (Clitoria ternatea), rhizosphere clove afo 3 (Syzygium aromaticum), nodule mimosa (Mimosa pudica L.), and soil from gold mining land which were recovered from Ternate, Tidore, Obi Island, and Marotai Island, Eastern part of Indonesia.

RESULTS

Eight supernatants from soil and plant-associated bacteria were found to have quorum quenching activity against Chromobacterium violaceum. All supernatants exhibited antibiofilm activity against biofilm formed by Aeromonas hydrophila and Vibrio harveyi. The supernatant of FT5 showed the highest activity in disrupting (66.59%) and inhibiting (85.63%) the biofilm of A. hydrophila. For V. harveyi, the supernatant of PTM3 showed the highest disruption activity (72.61%), whileRCA7 showed the highest inhibition activity(75.68%). The Gas Chromatography-Mass Spectrometry (GC-MS) identified fatty acids, ester, and diketopiperazine as the compounds related to the antibiofilm activity. Molecular identification revealed that the isolates belong to the genera Bacillus, Priestia, and Chryseobacterium.

An insight on the plausible biological and non-biological detoxification of heavy metals in tannery waste: A comprehensive review

A key challenge for the tannery industries is the volume of tannery waste water (TWW) generated during the processing of leather, releasing various forms of toxic heavy metals resulting in uncontrolled discharge of tannery waste (TW) into the environment leading to pollution. The pollutants in TW includes heavy metals such as chromium (Cr), cadmium (Cd), lead (Pb) etc, when discharged above the permissible limit causes ill effects on humans. Therefore, several researchers have reported the application of biological and non-biological methods for the removal of pollutants in TW. This review provides insights on the global scenario of tannery industries and the harmful effects of heavy metal generated by tannery industry on micro and macroorganisms of the various ecological niches. It also provides information on the process, advantages and disadvantages of non-biological methods such as electrochemical oxidation, advanced oxidation processes, photon assisted catalytic remediation, adsorption and membrane technology. The various biological methods emphasised includes strategies such as constructed wetland, vermitechnology, phytoremediation, bioaugmentation, quorum sensing and biofilm in the remediation of heavy metals from tannery wastewater (TWW) with special emphasize on chromium.

The past, present and future of polymicrobial infection research: Modelling, eavesdropping, terraforming and other stories

Over the last two centuries, great advances have been made in microbiology as a discipline. Much of this progress has come about as a consequence of studying the growth and physiology of individual microbial species in well-defined laboratory media; so-called "axenic growth". However, in the real world, microbes rarely live in such "splendid isolation" (to paraphrase Foster) and more often-than-not, share the niche with a plethora of co-habitants. The resulting interactions between species (and even between kingdoms) are only very poorly understood, both on a theoretical and experimental level. Nevertheless, the last few years have seen significant progress, and in this review, we assess the importance of polymicrobial infections, and show how improved experimental traction is advancing our understanding of these. A particular focus is on developments that are allowing us to capture the key features of polymicrobial infection scenarios, especially as those associated with the human airways (both healthy and diseased).

Exploring the dynamics of mixed-species biofilms involving Candida spp. and bacteria in cystic fibrosis

Cystic fibrosis (CF) is an inherited disease that results from mutations in the gene responsible for the cystic fibrosis transmembrane conductance regulator (CFTR). The airways become clogged with thick, viscous mucus that traps microbes in respiratory tracts, facilitating colonization, inflammation and infection. CF is recognized as a biofilm-associated disease, it is commonly polymicrobial and can develop in biofilms. This review discusses Candida spp. and both Gram-positive and Gram-negative bacterial biofilms that affect the airways and cause pulmonary infections in the CF context, with a particular focus on mixed-species biofilms. In addition, the review explores the intricate interactions between fungal and bacterial species within these biofilms and elucidates the underlying molecular mechanisms that govern their dynamics. Moreover, the review addresses the multifaceted issue of antimicrobial resistance in the context of CF-associated biofilms. By synthesizing current knowledge and research findings, this review aims to provide insights into the pathogenesis of CF-related infections and identify potential therapeutic approaches to manage and combat these complex biofilm-mediated infections.

Nanomaterials Regulate Bacterial Quorum Sensing: Applications, Mechanisms, and Optimization Strategies

Anti-virulence therapy that interferes with bacterial communication, known as "quorum sensing (QS)", is a promising strategy for circumventing bacterial resistance. Using nanomaterials to regulate bacterial QS in anti-virulence therapy has attracted much attention, which is mainly attributed to unique physicochemical properties and excellent designability of nanomaterials. However, bacterial QS is a dynamic and multistep process, and there are significant differences in the specific regulatory mechanisms and related influencing factors of nanomaterials in different steps of the QS process. An in-depth understanding of the specific regulatory mechanisms and related influencing factors of nanomaterials in each step can significantly optimize QS regulatory activity and enhance the development of novel nanomaterials with better comprehensive performance. Therefore, this review focuses on the mechanisms by which nanomaterials regulate bacterial QS in the signal supply (including signal synthesis, secretion, and accumulation) and signal transduction cascade (including signal perception and response) processes. Moreover, based on the two key influencing factors (i.e., the nanomaterial itself and the environment), optimization strategies to enhance the QS regulatory activity are comprehensively summarized. Collectively, applying nanomaterials to regulate bacterial QS is a promising strategy for anti-virulence therapy. This review provides reference and inspiration for further research on the anti-virulence application of nanomaterials.

Competition quenching strategies reduce antibiotic tolerance in polymicrobial biofilms

Bacteria typically live in dense communities where they are surrounded by other species and compete for a limited amount of resources. These competitive interactions can induce defensive responses that also protect against antimicrobials, potentially complicating the antimicrobial treatment of pathogens residing in polymicrobial consortia. Therefore, we evaluate the potential of alternative antivirulence strategies that quench this response to competition. We test three competition quenching approaches: (i) interference with the attack mechanism of surrounding competitors, (ii) inhibition of the stress response systems that detect competition, and (iii) reduction of the overall level of competition in the community by lowering the population density. We show that either strategy can prevent the induction of antimicrobial tolerance of Salmonella Typhimurium in response to competitors. Competition quenching strategies can thus reduce tolerance of pathogens residing in polymicrobial communities and could contribute to the improved eradication of these pathogens via traditional methods.

Identifying LasR Quorum Sensors with Improved Signal Specificity by Mapping the Sequence-Function Landscape

Programmable intercellular signaling using components of naturally occurring quorum sensing can allow for coordinated functions to be engineered in microbial consortia. LuxR-type transcriptional regulators are widely used for this purpose and are activated by homoserine lactone (HSL) signals. However, they often suffer from imperfect molecular discrimination of structurally similar HSLs, causing misregulation within engineered consortia containing multiple HSL signals. Here, we studied one such example, the regulator LasR from Pseudomonas aeruginosa. We elucidated its sequence-function relationship for ligand specificity using targeted protein engineering and multiplexed high-throughput biosensor screening. A pooled combinatorial saturation mutagenesis library (9,486 LasR DNA sequences) was created by mutating six residues in LasR's β5 sheet with single, double, or triple amino acid substitutions. Sort-seq assays were performed in parallel using cognate and noncognate HSLs to quantify each corresponding sensor's response to each HSL signal, which identified hundreds of highly specific variants. Sensor variants identified were individually assayed and exhibited up to 60.6-fold (p = 0.0013) improved relative activation by the cognate signal compared to the wildtype. Interestingly, we uncovered prevalent mutational epistasis and previously unidentified residues contributing to signal specificity. The resulting sensors with negligible signal crosstalk could be broadly applied to engineer bacteria consortia.

Signals behind Listeria monocytogenes virulence mechanisms

The tight and coordinated regulation of virulence gene expression is crucial to ensure the survival and persistence of bacterial pathogens in different contexts within their hosts. Considering this, bacteria do not express virulence factors homogenously in time and space, either due to their associated fitness cost or to their detrimental effect at specific infection stages. To efficiently infect and persist into their hosts, bacteria have thus to monitor environmental cues or chemical cell-to-cell signaling mechanisms that allow their transition from the external environment to the host, and therefore adjust gene expression levels, intrinsic biological activities, and appropriate behaviors. Listeria monocytogenes (Lm), a major Gram-positive facultative intracellular pathogen, stands out for its adaptability and capacity to thrive in a wide range of environments. Because of that, Lm presents itself as a significant concern in food safety and public health, that can lead to potentially life-threatening infections in humans. A deeper understanding of the intricate bacterial virulence mechanisms and the signals that control them provide valuable insights into the dynamic interplay between Lm and the host. Therefore, this review addresses the role of some crucial signals behind Lm pathogenic virulence mechanisms and explores how the ability to assimilate and interpret these signals is fundamental for pathogenesis, identifying potential targets for innovative antimicrobial strategies.

A review of chemical signaling pathways in the quorum sensing circuit of Pseudomonas aeruginosa

Pseudomonas aeruginosa, an increasingly common competitive and biofilm organism in healthcare infection with sophisticated, interlinked and hierarchic quorum systems (Las, Rhl, PQS, and IQS), creates the greatest threats to the medical industry and has rendered prevailing chemotherapy medications ineffective. The rise of multidrug resistance has evolved into a concerning and potentially fatal occurrence for human life. P. aeruginosa biofilm development is assisted by exopolysaccharides, extracellular DNA, proteins, macromolecules, cellular signaling and interaction. Quorum sensing is a communication process between cells that involves autonomous inducers and regulators. Quorum-induced infectious agent biofilms and the synthesis of virulence factors have increased disease transmission, medication resistance, infection episodes, hospitalizations and mortality. Hence, quorum sensing may be a potential therapeutical target for bacterial illness, and developing quorum inhibitors as an anti-virulent tool could be a promising treatment strategy for existing antibiotics. Quorum quenching is a prevalent technique for treating infections caused by microbes because it diminishes microbial pathogenesis and increases microbe biofilm sensitivity to antibiotics, making it a potential candidate for drug development. This paper examines P. aeruginosa quorum sensing, the hierarchy of quorum sensing mechanism, quorum sensing inhibition and quorum sensing inhibitory agents as a drug development strategy to supplement traditional antibiotic strategies.

Modulation of quorum sensing and biofilm of Gram-negative bacterial pathogens by Cinnamomum zeylanicum L

The development of antibiotic resistant microbial pathogens has become a global health threat and a major concern in modern medicine. The problem of antimicrobial resistance (AMR) has majorly arisen due to sub-judicious use of antibiotics in health care and livestock industry. A slow progress has been made in last two decades in discovery of new antibiotics. A new strategy in combatting AMR is to modulate or disarm the microbes for their virulence and pathogenicity. Plants are considered as promising source for new drugs against AMR pathogens. In this study, fraction-based screening of the Cinnamomum zeylanicum extract was performed followed by detailed investigation of antiquorum sensing and antibiofilm activities of the most active fraction that is, C. zeylanicum hexane fraction (CZHF). More than 75% reduction in violacein pigment of C. violaceum 12472 was overserved. CZHF successfully modulated the virulence of Pseudomonas aeruginosa PAO1 by 60.46%-78.35%. A similar effect was recorded against Serratia marcescens MTCC 97. A broad-spectrum inhibition of biofilm development was found in presence of sub-MICs of CZHF. The colonization of bacteria onto the glass coverslips was remarkably reduced apart from the reduction in exopolymeric substances. Alkaloids and terpenoids were found in CZHF. GC/MS analysis revealed the presence of cinnamaldehyde dimethyl acetal, 2-propenal, coumarin, and α-copaene as major phytocompounds. This study provides enough evidence to support potency of C. zeylanicum extract in targeting the virulence of Gram -ve pathogenic bacteria. The plant extract or active compounds can be developed as successful drugs after careful in vivo examination to target microbial infections. RESEARCH HIGHLIGHTS: Hexane fraction of Cinnamomum zeylanicum is active against QS and biofilms. The broad-spectrum antibiofilm activity was further confirmed by microscopic analysis. Dimethyl acetal, 2-propenal, coumarin, α-copaene, and so forth are major phytocompounds.

Microscopy and modelling investigations on the morphology of the biofilm exopolysaccharide produced by Burkholderia multivorans strain C1576

Bacteria form very often biofilms where they embed in a self-synthesized matrix exhibiting a gel-like appearance. Matrices offer several advantages, including defence against external threats and the easiness of intercellular communication. In infections, biofilm formation enhances bacteria resistance against antimicrobials, causing serious clinical problems for patients' treatments. Biofilm matrices are composed of proteins, extracellular DNA, and polysaccharides, the latter being the major responsible for matrix architecture. The repeating unit of the biofilm polysaccharide synthesized by Burkholderia multivorans strain C1576 contains two mannoses and two sequentially linked rhamnoses, one of them 50 % methylated on C-3. Rhamnose, a 6-deoxysugar, has lower polarity than other common monosaccharides and its methylation further reduces polarity. This suggests a possible role of this polysaccharide in the biofilm matrix; in fact, computer modelling and atomic force microscopy studies evidenced intra- and inter-molecular non-polar interactions both within polysaccharides and with aliphatic molecules. In this paper, the polysaccharide three-dimensional morphology was investigated using atomic force microscopy in both solid and solution states. Independent evidence of the polymer conformation was obtained by transmission electron microscopy which confirmed the formation of globular compact structures. Finally, data from computer dynamic simulations were used to model the three-dimensional structure.

Targeting microbial quorum sensing: the next frontier to hinder bacterial driven gastrointestinal infections

Bacteria synchronize social behaviors via a cell-cell communication and interaction mechanism termed as quorum sensing (QS). QS has been extensively studied in monocultures and proved to be intensively involved in bacterial virulence and infection. Despite the role QS plays in pathogens during laboratory engineered infections has been proved, the potential functions of QS related to pathogenesis in context of microbial consortia remain poorly understood. In this review, we summarize the basic molecular mechanisms of QS, primarily focusing on pathogenic microbes driving gastrointestinal (GI) infections. We further discuss how GI pathogens disequilibrate the homeostasis of the indigenous microbial consortia, rebuild a realm dominated by pathogens, and interact with host under worsening infectious conditions via pathogen-biased QS signaling. Additionally, we present recent applications and main challenges of manipulating QS network in microbial consortia with the goal of better understanding GI bacterial sociality and facilitating novel therapies targeting bacterial infections.

Development of a dual fluorescent reporter system to identify inhibitors of Staphylococcus aureus virulence factors

IMPORTANCE

Staphylococcus aureus is a formidable pathogen responsible for a wide range of infections, and the emergence of antibiotic-resistant strains has posed significant challenges in treating these infections. In this study, we have established a novel dual reporter system capable of concurrently monitoring the activities of two critical virulence regulators in S. aureus. By incorporating both reporters into a single screening platform, we provide a time- and cost-efficient approach for assessing the activity of compounds against two distinct targets in a single screening round. This innovative dual reporter system presents a promising strategy for the identification of molecules capable of modulating virulence gene expression in S. aureus, potentially expediting the development of antivirulence therapies.

Crosstalk enables mutual activation of coupled quorum sensing pathways through "jump-start" and "push-start" mechanisms

Many quorum sensing microbes produce more than one chemical signal and detect them using interconnected pathways that crosstalk with each other. While there are many hypotheses for the advantages of sensing multiple signals, the prevalence and functional significance of crosstalk between pathways are much less understood. We explore the effect of intracellular signal crosstalk using a simple model that captures key features of typical quorum sensing pathways: multiple pathways in a hierarchical configuration, operating with positive feedback, with crosstalk at the receptor and promoter levels. We find that crosstalk enables activation or inhibition of one output by the non-cognate signal, broadens the dynamic range of the outputs, and allows one pathway to modulate the feedback circuit of the other. Our findings show how crosstalk between quorum sensing pathways can be viewed not as a detriment to the processing of information, but as a mechanism that enhances the functional range of the full regulatory system. When positive feedback systems are coupled through crosstalk, several new modes of activation or deactivation become possible.

Quorum sensing autoinducers AHLs protect Shewanella baltica against phage infection

Quorum sensing (QS) plays an important role in phage-host interactions. Shewanella baltica can't produce the N-acyl-homoserine lactones (AHLs) signal molecules but can eavesdrop on exogenous AHLs through its LuxR receptor. However, no clear evidence exists regarding the involvement of AHLs-mediated QS systems in S. baltica in regulating phage infection. Here, we report that AHLs modulated the phage resistance of S. baltica OS155. Specifically, we characterized a S. baltica phage vB_Sb_QDWS and preliminarily identified that lipopolysaccharide (LPS) is an important receptor for phage vB_Sb_QDWS. AHLs could protect S. baltica against phage infection by decreasing LPS-mediated phage adsorption. The expression of genes galU and tkt, which are essential for LPS synthesis, down-regulated significantly in response to AHLs autoinducers. Our finding confirms the important roles of QS in virus-host interactions and would be helpful to develop novel phage strategies for food spoilage control.

Molecular modeling study of micro and nanocurcumin with in vitro and in vivo antibacterial validation

Repurposing natural compounds as inhibitory targets to combat bacterial virulence is an important potential strategy to overcome resistance to traditional antibiotics, in the present study, the antibacterial activity of micro-curcumin and nano-sized curcumin was investigated against four predominant bacterial pathogens, namely, Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, and Bacillus subtilis. Curcumin bactericidal susceptibility could be summarized as the order, P. aeruginosa > B. subtilis > S. aureus > E. coli. Molecular docking analysis was conducted to confirm the impact of curcumin on the most vital and positively identified quorum-sensing pathway signaling proteins SecA-SecY, LsrR, PqsR (MvfR), AgrA which act as key players in the bacterial communication systems. The in silico physicochemical properties revealed that curcumin as a nutraceutical can be classified as a drug-like compound. An in vivo infected wound model was employed in four groups of albino rats. Topical application of nano-curcumin lotion showed a marked reduction in wound area (98.8%) as well as nearly 100% reduction in total bacterial viable count compared to the control group, on the fifteenth day post-treatment post-injury. The obtained data suggested that curcumin nanoparticles exhibited superior antibacterial activity and may possess clinical utility as a novel topical antimicrobial and wound healing agent.

Quorum sensing interference by phenolic compounds - A matter of bacterial misunderstanding

Over the past decade, numerous publications have emerged in the literature focusing on the inhibition of quorum sensing (QS) by plant extracts and phenolic compounds. However, there is still a scarcity of studies that delve into the specific mechanisms by which these compounds inhibit QS. Thus, our question is whether phenolic compounds can inhibit QS in a specific or indirect manner and to elucidate the underlying mechanisms involved. This study is focused on the most studied QS system, namely, autoinducer type 1 (AI-1), represented by N-acyl-homoserine lactone (AHL) signals and the AHL-mediated QS responses. Here, we analyzed the recent literature in order to understand how phenolic compounds act at the cellular level, at sub-inhibitory concentrations, and evaluated by which QS inhibition mechanisms they may act. The biotechnological application of QS inhibitors holds promising prospects for the pharmaceutical and food industries, serving as adjunct therapies and in the prevention of biofilms on various surfaces.

A review of filamentous sludge bulking controls from conventional methods to emerging quorum quenching strategies

Filamentous bulking, which results from the overgrowth of filamentous microorganisms, is a common issue that frequently disrupts the stable operation of activated sludge processes. Recent literature has paid attention to the relationship between quorum sensing (QS) and filamentous bulking highlighting that the morphological transformations of filamentous microbes are regulated by functional signal molecules in the bulking sludge system. In response to this, a novel quorum quenching (QQ) technology has been developed to control sludge bulking effectively and precisely by disturbing QS-mediated filamentation behaviors. This paper presents a critical review on the limitations of classical bulking hypotheses and traditional control methods, and provides an overview of recent QS/QQ studies that aim to elucidate and control filamentous bulking, including the characterization of molecule structures, the elaboration of QS pathways, and the precise design of QQ molecules to mitigate filamentous bulking. Finally, suggestions for further research and development of QQ strategies for precise bulking control are put forward.

Advances in mechanisms and engineering of electroactive biofilms

Electroactive biofilms (EABs) are electroactive microorganisms (EAMs) encased in conductive polymers that are secreted by EAMs and formed by the accumulation and cross-linking of extracellular polysaccharides, proteins, nucleic acids, lipids, and other components. EABs are present in the form of multicellular aggregates and play a crucial role in bioelectrochemical systems (BESs) for diverse applications, including biosensors, microbial fuel cells for renewable bioelectricity production and remediation of wastewaters, and microbial electrosynthesis of valuable chemicals. However, naturally occurred EABs are severely limited owing to their low electrical conductivity that seriously restrict the electron transfer efficiency and practical applications. In the recent decade, synthetic biology strategies have been adopted to elucidate the regulatory mechanisms of EABs, and to enhance the formation and electrical conductivity of EABs. Based on the formation of EABs and extracellular electron transfer (EET) mechanisms, the synthetic biology-based engineering strategies of EABs are summarized and reviewed as follows: (i) Engineering the structural components of EABs, including strengthening the synthesis and secretion of structural elements such as polysaccharides, eDNA, and structural proteins, to improve the formation of biofilms; (ii) Enhancing the electron transfer efficiency of EAMs, including optimizing the distribution of c-type cytochromes and conducting nanowire assembly to promote contact-based EET, and enhancing electron shuttles' biosynthesis and secretion to promote shuttle-mediated EET; (iii) Incorporating intracellular signaling molecules in EAMs, including quorum sensing systems, secondary messenger systems, and global regulatory systems, to increase the electron transfer flux in EABs. This review lays a foundation for the design and construction of EABs for diverse BES applications.

Investigating quorum-quenching marine bacilli as potential biocontrol agents for protection of shrimps against Early Mortality Syndrome (EMS)

Early Mortality Syndrome (EMS) has been a major problem for shrimp aquaculture in Southeast Asia due to its epizootic prevalence within the region since the first reported case in 2009. This study explores the application of halophilic marine bacilli isolated from coral mucus and their quorum-quenching abilities as potential biocontrol agents in aquaculture systems to combat the causative agent of EMS, Vibrio parahaemolyticus. N-acylhomoserine lactone (AHL)-degrading (AiiA) activity was first screened by PCR then confirmed by bio-reporter assay, and a combination of 16S rDNA sequence analysis and quantitative phenotype assays including biofilm-formation and temperature-growth responses were used to demonstrate diversity amongst these quorum-quenching isolates. Three phenotypically distinct strains showing notable potential were chosen to undergo co-cultivation as a method for strain improvement via long term exposure to the pathogenic V. parahaemolyticus. The novel approach taken led to significant improvements in antagonism and quorum quenching activities as compared to the ancestral wild-type strains and offers a potential solution as well as pathway to improve existing beneficial microbes for one of the most pressing issues in shrimp aquacultures worldwide.

Antimicrobial and Antibiofilm Photodynamic Action of Photosensitizing Nanoassemblies Based on Sulfobutylether-β-Cyclodextrin

Developing new broad-spectrum antimicrobial strategies, as alternatives to antibiotics and being able to efficiently inactivate pathogens without inducing resistance, is one of the main objectives in public health. Antimicrobial photodynamic therapy (aPDT), based on the light-induced production of reactive oxygen species from photosensitizers (PS), is attracting growing interest in the context of infection treatment, also including biofilm destruction. Due to the limited photostability of free PS, delivery systems are increasingly needed in order to decrease PS photodegradation, thus improving the therapeutic efficacy, as well as to reduce collateral effects on unaffected tissues. In this study, we propose a photosensitizing nanosystem based on the cationic porphyrin 5,10,15,20-tetrakis (N-methyl- 4-pyridyl)-21H,23H-porphyrin (TMPyP), complexed with the commerical sulfobutylether-beta-cyclodextrin (CAPTISOL®), at a 1:50 molar ratio (CAPTISOL®/TMPyP)50_1. Nanoassemblies based on (CAPTISOL®/TMPyP)50_1 with photodynamic features exhibited photo-antimicrobial activity against Gram-negative and Gram-positive bacteria. Moreover, results from P. aeruginosa reveal that CAPTISOL® alone inhibits pyocyanin (PYO) production, also affecting bacterial biofilm formation. Finally, we obtained a synergistic effect of inhibition and destruction of P. aeruginosa biofilm by using the combination of CAPTISOL® and TMPyP.

Determinants of synergistic cell-cell interactions in bacteria

Bacteria are ubiquitous and colonize virtually every conceivable habitat on earth. To achieve this, bacteria require different metabolites and biochemical capabilities. Rather than trying to produce all of the needed materials by themselves, bacteria have evolved a range of synergistic interactions, in which they exchange different commodities with other members of their local community. While it is widely acknowledged that synergistic interactions are key to the ecology of both individual bacteria and entire microbial communities, the factors determining their establishment remain poorly understood. Here we provide a comprehensive overview over our current knowledge on the determinants of positive cell-cell interactions among bacteria. Taking a holistic approach, we review the literature on the molecular mechanisms bacteria use to transfer commodities between bacterial cells and discuss to which extent these mechanisms favour or constrain the successful establishment of synergistic cell-cell interactions. In addition, we analyse how these different processes affect the specificity among interaction partners. By drawing together evidence from different disciplines that study the focal question on different levels of organisation, this work not only summarizes the state of the art in this exciting field of research, but also identifies new avenues for future research.

Targeting Peptide-Based Quorum Sensing Systems for the Treatment of Gram-Positive Bacterial Infections

Bacteria utilize a cell density-dependent communication system called quorum sensing (QS) to coordinate group behaviors. In Gram-positive bacteria, QS involves the production of and response to auto-inducing peptide (AIP) signaling molecules to modulate group phenotypes, including pathogenicity. As such, this bacterial communication system has been identified as a potential therapeutic target against bacterial infections. More specifically, developing synthetic modulators derived from the native peptide signal paves a new way to selectively block the pathogenic behaviors associated with this signaling system. Moreover, rational design and development of potent synthetic peptide modulators allows in depth understanding of the molecular mechanisms that drive QS circuits in diverse bacterial species. Overall, studies aimed at understanding the role of QS in microbial social behavior could result in the accumulation of significant knowledge of microbial interactions, and consequently lead to the development of alternative therapeutic agents to treat bacterial infectivity. In this review, we discuss recent advances in the development of peptide-based modulators to target QS systems in Gram-positive pathogens, with a focus on evaluating the therapeutic potential associated with these bacterial signaling pathways.

N-acetylcysteine (NAC) attenuates quorum sensing regulated phenotypes in Pseudomonas aeruginosa PAO1

The expression of many virulence genes in bacteria is regulated by quorum sensing (QS), and the inhibition of this mechanism has been intensely investigated. N-acetylcysteine (NAC) has good antibacterial activity and is able to interfere with biofilm-related respiratory infections, but little is known whether this compound has an effect on bacterial QS communication. This work aimed to evaluate the potential of NAC as a QS inhibitor (QSI) in Pseudomonas aeruginosa PAO1 through in silico and in vitro analyses, as well as in combination with the antibiotic tobramycin. Initially, a molecular docking analysis was performed between the QS regulatory proteins, LasR and RhlR, of P. aeruginosa with NAC, 3-oxo-C12-HSL, C4-HSL, and furanone C30. The NAC sub-inhibitory concentration was determined by growth curves. Then, we performed in vitro tests using the QS reporter strains P. aeruginosa lasB-gfp and rhlA-gfp, as well as the expression of QS-related phenotypes. Finally, the synergistic effect of NAC with the antibiotic tobramycin was calculated by fractional inhibitory concentrations index (FIC_i) and investigated against bacterial growth, pigment production, and biofilm formation. In the molecular docking study, NAC bound to LasR and RhlR proteins in a similar manner to the AHL cognate, suggesting that it may be able to bind to QS receptor proteins in vivo. In the biosensor assay, the GFP signal was turned down in the presence of NAC at 1000, 500, 250, and 125 μM for lasB-gfp and rhlA-gfp (p < 0.05), suggesting a QS inhibitory effect. Pyocyanin and rhamnolipids decreased (p < 0.05) up to 34 and 37%, respectively, in the presence of NAC at 125 μM. Swarming and swimming motilities were inhibited (p < 0.05) by NAC at 250 to 10000 μM. Additionally, 2500 and 10000 μM of NAC reduced biofilm formation. NAC-tobramycin combination showed synergistic effect with FIC_i of 0.8, and the best combination was 2500-1.07 μM, inhibiting biofilm formation up to 60%, besides reducing pyocyanin and pyoverdine production. Confocal microscopy images revealed a stronger, dense, and compact biofilm of P. aeruginosa PAO1 control, while the biofilm treated with NAC-tobramycin became thinner and more dispersed. Overall, NAC at low concentrations showed promising anti-QS properties against P. aeruginosa PAO1, adding to its already known effect as an antibacterial and antibiofilm agent.

Curcumin and 10-undecenoic acid as natural quorum sensing inhibitors of LuxS/AI-2 of Bacillus subtilis and LasI/LasR of Pseudomonas aeruginosa

The quorum sensing (QS) system is related to cell-to-cell communication as a function of population density, which regulates several physiological functions including biofilm formation and virulence gene expression. QS inhibitors have emerged as a promising strategy to tackle virulence and biofilm development. Among a wide variety of phytochemicals, many of them have been described as QS inhibitors. Driven by their promising clues, this study aimed to identify active phytochemicals against LuxS/autoinducer-2 (AI-2) (as the universal QS system) from Bacillus subtilis and LasI/LasR (as a specific QS system) of Pseudomonas aeruginosa, through in silico analysis followed by in vitro validation. The optimized virtual screening protocols were applied to screen a phytochemical database containing 3479 drug-like compounds. The most promising phytochemicals were curcumin, pioglitazone hydrochloride, and 10-undecenoic acid. In vitro analysis corroborated the QS inhibitory activity of curcumin and 10-undecenoic acid, however, pioglitazone hydrochloride showed no relevant effect. Inhibitory effects on LuxS/AI-2 QS system triggered reduction of 33-77% by curcumin (at 1.25-5 µg/mL) and 36-64% by 10-undecenoic acid (at 12.5-50 µg/mL). Inhibition of LasI/LasR QS system was 21% by curcumin (at 200 µg/mL) and 10-54% by 10-undecenoic acid (at 15.625-250 µg/mL). In conclusion, in silico analysis allowed the identification of curcumin and, for the first time, 10-undecenoic acid (showing low cost, high availability, and low toxicity) as alternatives to counteract bacterial pathogenicity and virulence, avoiding the imposition of selective pressure usually related to classic industrial disinfection and antibiotics therapy.

Microbiome engineering for bioremediation of emerging pollutants

Axenic microbial applications in the open environment are unrealistic and may not be always practically viable. Therefore, it is important to use mixed microbial cultures and their interactions with the microbiome in the targeted ecosystem to perform robust functions towards their sustainability in harsh environmental conditions. Emerging pollutants like phthalates and hydrocarbons that are toxic to several aquatic and terrestrial life forms in the water bodies and lands are an alarming situation. The present review explores the possibility of devising an inclusive eco-friendly strategy like microbiome engineering which proves to be a unique and crucial technology involving the power of microbial communication through quorum sensing. This review discusses the interspecies and intra-species communications between different microbial groups with their respective environments. Moreover, this review also envisages the efforts for designing the next level of microbiome-host engineering concept (MHEC). The focus of the review also extended toward using omics and metabolic network analysis-based tools for effective microbiome engineering. These approaches might be quite helpful in the future to understand such microbial interactions but it will be challenging to implement in the real environment to get the desired functions. Finally, the review also discusses multiple approaches for the bioremediation of toxic chemicals from the soil environment.

Sustainable Chitosan/Polybenzoxazine Films: Synergistically Improved Thermal, Mechanical, and Antimicrobial Properties

Polybenzoxazines (Pbzs) are considered as an advanced class of thermosetting phenolic resins as they overcome the shortcomings associated with novolac and resole type phenolic resins. Several advantages of these materials include curing without the use of catalysts, release of non-toxic by-products during curing, molecular design flexibility, near-zero shrinkage of the cured materials, low water absorption and so on. In spite of all these advantages, the brittleness of Pbz is a knotty problem that could be solved by blending with other polymers. Chitosan (Ch), has been extensively investigated in this context, but its thermal and mechanical properties rule out its practical applications. The purpose of this work is to fabricate an entirely bio-based Pbz films by blending chitosan with benzoxazine (Bzo), which is synthesized from curcumin and furfuryl amine (curcumin-furfurylamine-based Bzo, C-fu), by making use of a benign Schiff base chemistry. FT-IR and ¹H-NMR spectroscopy were used to confirm the structure of C-fu. The impact of chitosan on benzoxazine polymerization was examined using FT-IR and DSC analyses. Further evidence for synergistic interactions was provided by DSC, SEM, TGA, and tensile testing. By incorporating C-fu into Ch, Ch-grafted-poly(C-fu) films were obtained with enhanced chemical resistance and tensile strength. The bio-based polymer films produced inhibited the growth of Staphylococcus aureus and Escherichia coli, by reversible labile linkages, expanding Ch galleries, and releasing phenolic species, which was 125 times stronger than bare Ch. In addition, synthesized polybenzoxazine films [Ch/Poly(C-fu)] showed significant dose-dependent antibiofilm activity against S. aureus and E. coli as determined by confirmed by confocal laser scanning microscopy (CLSM). This study suggests that bio-based Ch-graft-polymer material provide improved anti-bacterial property and characteristics that may be considered as a possibility in the near future for wound healing and implant applications.

Conversations in the Gut: The Role of Quorum Sensing in Normobiosis

An imbalance in gut microbiota, termed dysbiosis, has been shown to affect host health. Several factors, including dietary changes, have been reported to cause dysbiosis with its associated pathologies that include inflammatory bowel disease, cancer, obesity, depression, and autism. We recently demonstrated the inhibitory effects of artificial sweeteners on bacterial quorum sensing (QS) and proposed that QS inhibition may be one mechanism behind such dysbiosis. QS is a complex network of cell-cell communication that is mediated by small diffusible molecules known as autoinducers (AIs). Using AIs, bacteria interact with one another and coordinate their gene expression based on their population density for the benefit of the whole community or one group over another. Bacteria that cannot synthesize their own AIs secretly "listen" to the signals produced by other bacteria, a phenomenon known as "eavesdropping". AIs impact gut microbiota equilibrium by mediating intra- and interspecies interactions as well as interkingdom communication. In this review, we discuss the role of QS in normobiosis (the normal balance of bacteria in the gut) and how interference in QS causes gut microbial imbalance. First, we present a review of QS discovery and then highlight the various QS signaling molecules used by bacteria in the gut. We also explore strategies that promote gut bacterial activity via QS activation and provide prospects for the future.

Cellular mechanisms and molecular pathways linking bitter taste receptor signalling to cardiac inflammation, oxidative stress, arrhythmia and contractile dysfunction in heart diseases

Heart diseases and related complications constitute a leading cause of death and socioeconomic threat worldwide. Despite intense efforts and research on the pathogenetic mechanisms of these diseases, the underlying cellular and molecular mechanisms are yet to be completely understood. Several lines of evidence indicate a critical role of inflammatory and oxidative stress responses in the development and progression of heart diseases. Nevertheless, the molecular machinery that drives cardiac inflammation and oxidative stress is not completely known. Recent data suggest an important role of cardiac bitter taste receptors (TAS2Rs) in the pathogenetic mechanism of heart diseases. Independent groups of researchers have demonstrated a central role of TAS2Rs in mediating inflammatory, oxidative stress responses, autophagy, impulse generation/propagation and contractile activities in the heart, suggesting that dysfunctional TAS2R signalling may predispose to cardiac inflammatory and oxidative stress disorders, characterised by contractile dysfunction and arrhythmia. Moreover, cardiac TAS2Rs act as gateway surveillance units that monitor and detect toxigenic or pathogenic molecules, including microbial components, and initiate responses that ultimately culminate in protection of the host against the aggression. Unfortunately, however, the molecular mechanisms that link TAS2R sensing of the cardiac milieu to inflammatory and oxidative stress responses are not clearly known. Therefore, we sought to review the possible role of TAS2R signalling in the pathophysiology of cardiac inflammation, oxidative stress, arrhythmia and contractile dysfunction in heart diseases. Potential therapeutic significance of targeting TAS2R or its downstream signalling molecules in cardiac inflammation, oxidative stress, arrhythmia and contractile dysfunction is also discussed.

Nutrient encryption and the diversity of cobamides, siderophores, and glycans

Encryption makes information available only to those with the decoding key. We propose that microbes, living in a chemical environment, encrypt nutrients, thereby making them available only to those with the decoding enzymes, such as their kin. Examples of encrypted nutrients include cobamides, which are expensive to make and valuable for microbial fitness. Furthermore, we propose that hosts encrypt nutrients to encourage desirable colonizers. For instance, plant root exudates and breast milk oligosaccharides encourage beneficial microbes.

Cross-species activation of hydrogen cyanide production by a promiscuous quorum-sensing receptor promotes Chromobacterium subtsugae competition in a dual-species model

Many saprophytic bacteria have LuxR-I-type acyl-homoserine lactone (AHL) quorum-sensing systems that may be important for competing with other bacteria in complex soil communities. LuxR AHL receptors specifically interact with cognate AHLs to cause changes in expression of target genes. Some LuxR-type AHL receptors have relaxed specificity and are responsive to non-cognate AHLs. These promiscuous receptors might be used to sense and respond to AHLs produced by other bacteria by eavesdropping. We are interested in understanding the role of eavesdropping during interspecies competition. The soil saprophyte Chromobacterium subtsugae has a single AHL circuit, CviR-I, which produces and responds to N-hexanoyl-HSL (C6-HSL). The AHL receptor CviR can respond to a variety of AHLs in addition to C6-HSL. In prior studies we have utilized a coculture model with C. subtsugae and another soil saprophyte, Burkholderia thailandensis. Using this model, we previously showed that promiscuous activation of CviR by B. thailandensis AHLs provides a competitive advantage to C. subtsugae. Here, we show that B. thailandensis AHLs activate transcription of dozens of genes in C. subtsugae, including the hcnABC genes coding for production of hydrogen cyanide. We show that hydrogen cyanide production is population density-dependent and demonstrate that the cross-induction of hydrogen cyanide by B. thailandensis AHLs provides a competitive advantage to C. subtsugae. Our results provide new information on C. subtsugae quorum sensing and are the basis for future studies aimed at understanding the role of eavesdropping in interspecies competition.

A critical review on microbial ecology in the novel biological nitrogen removal process: Dynamic balance of complex functional microbes for nitrogen removal

The novel biological nitrogen removal process has been extensively studied for its high nitrogen removal efficiency, energy efficiency, and greenness. A successful novel biological nitrogen removal process has a stable microecological equilibrium and benign interactions between the various functional bacteria. However, changes in the external environment can easily disrupt the dynamic balance of the microecology and affect the activity of functional bacteria in the novel biological nitrogen removal process. Therefore, this review focuses on the microecology in existing the novel biological nitrogen removal process, including the growth characteristics of functional microorganisms and their interactions, together with the effects of different influencing factors on the evolution of microbial communities. This provides ideas for achieving a stable dynamic balance of the microecology in a novel biological nitrogen removal process. Furthermore, to investigate deeply the mechanisms of microbial interactions in novel biological nitrogen removal process, this review also focuses on the influence of quorum sensing (QS) systems on nitrogen removal microbes, regulated by which bacteria secrete acyl homoserine lactones (AHLs) as signaling molecules to regulate microbial ecology in the novel biological nitrogen removal process. However, the mechanisms of action of AHLs on the regulation of functional bacteria have not been fully determined and the composition of QS system circuits requires further investigation. Meanwhile, it is necessary to further apply molecular analysis techniques and the theory of systems ecology in the future to enhance the exploration of microbial species and ecological niches, providing a deeper scientific basis for the development of a novel biological nitrogen removal process.

Potential therapeutic targets for combating Mycoplasma genitalium

Mycoplasma genitalium (M. genitalium) has emerged as a sexually transmitted infection (STI) all over the world in the last three decades. It has been identified as a cause of male urethritis, and there is now evidence that it also causes cervicitis and pelvic inflammatory disease in women. However, the precise role of M. genitalium in diseases such as pelvic inflammatory disease, and infertility is unknown, and more research is required. It is a slow-growing organism, and with the advent of the nucleic acid amplification test (NAAT), more studies are being conducted and knowledge about the pathogenicity of this organism is being elucidated. The accumulation of data has improved our understanding of the pathogen and its role in disease transmission. Despite the widespread use of single-dose azithromycin in the sexual health field, M. genitalium is known to rapidly develop antibiotic resistance. As a result, the media frequently refer to this pathogen as the "new STI superbug." Despite their rarity, antibiotics available today have serious side effects. As the cure rates for first-line antimicrobials have decreased, it is now a challenge to determine the effective antimicrobial therapy. In this review, we summarise recent M. genitalium research and investigate potential therapeutic targets for combating this pathogen.

Quorum Sensing and Antimicrobial Production Orchestrate Biofilm Dynamics in Multispecies Bacterial Communities

Microbial interactions are often mediated by diffusible small molecules, including secondary metabolites, that play roles in cell-to-cell signaling and inhibition of competitors. Biofilms are often "hot spots" for high concentrations of bacteria and their secondary metabolites, which make them ideal systems for the study of small-molecule contributions to microbial interactions. Here, we use a five-member synthetic community consisting of Roseobacteraceae representatives to investigate the role of secondary metabolites on microbial biofilm dynamics. One synthetic community member, Rhodobacterales strain Y4I, possesses two acylated homoserine lactone (AHL)-based cell-to-cell signaling systems (pgaRI and phaRI) as well as a nonribosomal peptide synthase gene (igi) cluster that encodes the antimicrobial indigoidine. Through serial substitution of Y4I with mutants deficient in single signaling molecule pathways, the contribution of these small-molecule systems could be assessed. As secondary metabolite production is dependent upon central metabolites, the influence of growth substrate (i.e., complex medium versus defined medium with a single carbon substrate) on these dynamics was also considered. Depending on the Y4I mutant genotype included, community dynamics ranged from competitive to cooperative. The observed interactions were mostly competitive in nature. However, the community harboring a Y4I variant that was both impaired in quorum sensing (QS) pathways and unable to produce indigoidine (pgaR variant) shifted toward more cooperative interactions over time. These cooperative interactions were enhanced in the defined growth medium. The results presented provide a framework for deciphering complex, small-molecule-mediated interactions that have broad application to microbial biology. IMPORTANCE Microbial biofilms play critical roles in marine ecosystems and are hot spots for microbial interactions that play a role in the development and function of these communities. Roseobacteraceae are an abundant and active family of marine heterotrophic bacteria forming close associations with phytoplankton and carrying out key transformations in biogeochemical cycles. Group members are aggressive primary colonizers of surfaces, where they set the stage for the development of multispecies biofilm communities. Few studies have examined the impact of secondary metabolites, such as cell-to-cell signaling and antimicrobial production, on marine microbial biofilm community structure. Here, we assessed the impact of secondary metabolites on microbial interactions using a synthetic, five-member Roseobacteraceae community by measuring species composition and biomass production during biofilm growth. We present evidence that secondary metabolites influence social behaviors within these multispecies microbial biofilms, thereby improving understanding of bacterial secondary metabolite production influence on social behaviors within marine microbial biofilm communities.

Natural and synthetic inhibitors of a phage-encoded quorum-sensing receptor affect phage-host dynamics in mixed bacterial communities

Viruses that infect bacteria, called phages, shape the composition of bacterial communities and are important drivers of bacterial evolution. We recently showed that temperate phages, when residing in bacteria (i.e., prophages), are capable of manipulating the bacterial cell-to-cell communication process called quorum sensing (QS). QS relies on the production, release, and population-wide detection of signaling molecules called autoinducers (AI). Gram-negative bacteria commonly employ N-acyl homoserine lactones (HSL) as AIs that are detected by LuxR-type QS receptors. Phage ARM81ld is a prophage of the aquatic bacterium Aeromonas sp. ARM81, and it encodes a homolog of a bacterial LuxR, called LuxRARM81ld. LuxRARM81ld detects host Aeromonas-produced C4-HSL, and in response, activates the phage lytic program, triggering death of its host and release of viral particles. Here, we show that phage LuxRARM81ld activity is modulated by noncognate HSL ligands and by a synthetic small molecule inhibitor. We determine that HSLs with acyl chain lengths equal to or longer than C8 antagonize LuxRARM81ld. For example, the C8-HSL AI produced by Vibrio fischeri that coexists with Aeromonads in aquatic environments, binds to and inhibits LuxRARM81ld, and consequently, protects the host from lysis. Coculture of V. fischeri with the Aeromonas sp. ARM81 lysogen suppresses phage ARM81ld virion production. We propose that the cell density and species composition of the bacterial community could determine outcomes in bacterial-phage partnerships.

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.

Quorum sensing in human gut and food microbiomes: Significance and potential for therapeutic targeting

Human gut and food microbiomes interact during digestion. The outcome of these interactions influences the taxonomical composition and functional capacity of the resident human gut microbiome, with potential consequential impacts on health and disease. Microbe-microbe interactions between the resident and introduced microbiomes, which likely influence host colonisation, are orchestrated by environmental conditions, elements of the food matrix, host-associated factors as well as social cues from other microorganisms. Quorum sensing is one example of a social cue that allows bacterial communities to regulate genetic expression based on their respective population density and has emerged as an attractive target for therapeutic intervention. By interfering with bacterial quorum sensing, for instance, enzymatic degradation of signalling molecules (quorum quenching) or the application of quorum sensing inhibitory compounds, it may be possible to modulate the microbial composition of communities of interest without incurring negative effects associated with traditional antimicrobial approaches. In this review, we summarise and critically discuss the literature relating to quorum sensing from the perspective of the interactions between the food and human gut microbiome, providing a general overview of the current understanding of the prevalence and influence of quorum sensing in this context, and assessing the potential for therapeutic targeting of quorum sensing mechanisms.

Comparative genomic and functional analysis of Arthrobacter sp. UMCV2 reveals the presence of luxR-related genes inducible by the biocompound N, N-dimethylhexadecilamine

Quorum sensing (QS) is a bacterial cell-cell communication system with genetically regulated mechanisms dependent on cell density. Canonical QS systems in gram-negative bacteria possess an autoinducer synthase (LuxI family) and a transcriptional regulator (LuxR family) that respond to an autoinducer molecule. In Gram-positive bacteria, the LuxR transcriptional regulators "solo" (not associated with a LuxI homolog) may play key roles in intracellular communication. Arthrobacter sp. UMCV2 is an actinobacterium that promotes plant growth by emitting the volatile organic compound N, N-dimethylhexadecylamine (DMHDA). This compound induces iron deficiency, defense responses in plants, and swarming motility in Arthrobacter sp. UMCV2. In this study, the draft genome of this bacterium was assembled and compared with the genomes of type strains of the Arthrobacter genus, finding that it does not belong to any previously described species. Genome explorations also revealed the presence of 16 luxR-related genes, but no luxI homologs were discovered. Eleven of these sequences possess the LuxR characteristic DNA-binding domain with a helix-turn-helix motif and were designated as auto-inducer-related regulators (AirR). Four sequences possessed LuxR analogous domains and were designated as auto-inducer analogous regulators (AiaR). When swarming motility was induced with DMHDA, eight airR genes and two aiaR genes were upregulated. These results indicate that the expression of multiple luxR-related genes is induced in actinobacteria, such as Arthrobacter sp. UMCV2, by the action of the bacterial biocompound DMHDA when QS behavior is produced.

3-Phenylpropan-1-Amine Enhanced Susceptibility of Serratia marcescens to Ofloxacin by Occluding Quorum Sensing

Serratia marcescens (S. marcescens) is an environmental bacterium that causes infections with high morbidity and mortality. Notably, infections caused by multidrug-resistant S. marcescens have become a global public health issue. Therefore, the discovery of promising compounds to reduce the virulence of pathogens and restore antibiotic activity against multidrug-resistant bacteria is critical. Quorum sensing (QS) regulates virulence factors and biofilm formation of microorganisms to increase their pathogenicity and is, therefore, an important factor in the formation of multidrug resistance. In this study, we found that 3-phenylpropan-1-amine (3-PPA) inhibited S. marcescens NJ01 biofilm formation and virulence factors, including prodigiosin, protease, lipase, hemolysin, and swimming. The combination of 3-PPA (50.0 μg/mL) and ofloxacin (0.2 μg/mL) enhanced S. marcescens NJ01 sensitivity to ofloxacin. Based on crystalline violet staining, scanning electron microscopy (SEM), and confocal laser scanning microscopy (CLSM), 3-PPA (50.0 μg/mL) reduced S. marcescens NJ01 biofilm formation by 48%. Quantitative real-time PCR (qRT-PCR) showed that 3-PPA regulated the expression of virulence- and biofilm-related genes fimA, fimC, bsmB, pigP, flhC, flhD, and sodB. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) indicated that 3-PPA affected intracellular metabolites of S. marcescens NJ01, leading to reduce metabolic activity. These results suggested that 3-PPA inhibits the pathogenicity of S. marcescens NJ01 by occluding QS. Thus, 3-PPA is feasible as an ofloxacin adjuvant to overcome multidrug-resistant S. marcescens and improve the treatment of intractable infections. IMPORTANCE Multidrug-resistant bacteria have become a major threat to global public health, leading to increased morbidity, mortality, and health care costs. Bacterial virulence factors and biofilms, which are regulated by quorum sensing (QS), are the primary causes of multidrug resistance. In this study, 3-PPA reduced virulence factors and eliminated biofilm formation by inhibiting QS in S. marcescens NJ01 bacteria, without affecting bacterial growth, thus restoring sensitivity to ofloxacin. Thus, the discovery of compounds that can restore antibiotic activity against bacteria is a promising strategy to mitigate multidrug resistance in pathogens.