Specificity of Signal-Binding via Non-AHL LuxR-Type Receptors

Assessment of the antibacterial and antioxidant activities of seaweed-derived extracts

In swine farming, animals develop diseases that require the use of antibiotics. In-feed antibiotics as growth promoters have been banned due to the increasing concern of antimicrobial resistance. Seaweeds offer bioactive molecules with antibacterial and antioxidant properties. The aim was to estimate the in vitro properties of seaweed extracts: Ascophyllum nodosum (AN), Palmaria palmata (PP), Ulva lactuca (UL), and 1:1 mixes (ANPP, ANUL, PPUL). Escherichia coli strains were used to test for growth inhibitory activity, and chemical-based assays were performed for antioxidant properties. The treatments were 2 (with/without Escherichia coli) × 2 (F4 + and F18 +) × 5 doses (0, 1.44, 2.87, 5.75, 11.50, and 23.0 mg/mL). Bacteria were supplemented with seaweed extracts, and growth was monitored. The antioxidant activity was assessed with 6 doses (0, 1, 50, 100, 200, 500, and 600 mg/mL) × 6 compounds using two chemical assays. Data were evaluated through SAS. The results showed that AN and UL significantly inhibited (p < 0.05) the growth of F4 + and F18 +. PP and mixes did not display an inhibition of the bacteria growth. AN, PP, UL extracts, and mixes exhibited antioxidant activities, with AN showing the strongest dose-response. Thus, AN and UL seaweed extracts reveal promising antibacterial and antioxidant effects and may be candidates for in-feed additives.

Bibliometric analysis and visualization of quorum sensing research over the last two decade

Background

Quorum sensing (QS) research stands as a pivotal and multifaceted domain within microbiology, holding profound implications across various scientific disciplines. This bibliometric analysis seeks to offer an extensive overview of QS research, covering the period from 2004 to 2023. It aims to elucidate the hotspots, trends, and the evolving dynamics within this research domain.

Methods

We conducted an exhaustive review of the literature, employing meticulous data curation from the Science Citation Index Extension (SCI-E) within the Web of Science (WOS) database. Subsequently, our survey delves into evolving publication trends, the constellation of influential authors and institutions, key journals shaping the discourse, global collaborative networks, and thematic hotspots that define the QS research field.

Results

The findings demonstrate a consistent and growing interest in QS research throughout the years, encompassing a substantial dataset of 4,849 analyzed articles. Journals such as Frontiers in Microbiology have emerged as significant contributor to the QS literature, highlighting the increasing recognition of QS's importance across various research fields. Influential research in the realm of QS often centers on microbial communication, biofilm formation, and the development of QS inhibitors. Notably, leading countries engaged in QS research include the United States, China, and India. Moreover, the analysis identifies research focal points spanning diverse domains, including pharmacological properties, genetics and metabolic pathways, as well as physiological and signal transduction mechanisms, reaffirming the multidisciplinary character of QS research.

Conclusion

This bibliometric exploration provides a panoramic overview of the current state of QS research. The data portrays a consistent trend of expansion and advancement within this domain, signaling numerous prospects for forthcoming research and development. Scholars and stakeholders engaged in the QS field can harness these findings to navigate the evolving terrain with precision and speed, thereby enhancing our comprehension and utilization of QS in various scientific and clinical domains.

N-acyl homoserine lactone signaling modulates bacterial community associated with human dental plaque

N-acyl homoserine lactones (AHLs) are small diffusible signaling molecules that mediate a cell density-dependent bacterial communication system known as quorum sensing (QS). AHL-mediated QS regulates gene expression to control many critical bacterial behaviors including biofilm formation, pathogenicity, and antimicrobial resistance. Dental plaque is a complex multispecies oral biofilm formed by successive colonization of the tooth surface by groups of commensal, symbiotic, and pathogenic bacteria, which can contribute to tooth decay and periodontal diseases. While the existence and roles of AHL-mediated QS in oral microbiota have been debated, recent evidence indicates that AHLs play significant roles in oral biofilm development and community dysbiosis. The underlying mechanisms, however, remain poorly characterized. To better understand the importance of AHL signaling in dental plaque formation, we manipulated AHL signaling by adding AHL lactonases or exogenous AHL signaling molecules. We find that AHLs can be detected in dental plaque grown under 5% CO2 conditions, but not when grown under anaerobic conditions, and yet anaerobic cultures are still responsive to AHLs. QS signal disruption using lactonases leads to changes in microbial population structures in both planktonic and biofilm states, changes that are dependent on the substrate preference of the used lactonase but mainly result in the increase in the abundance of commensal and pioneer colonizer species. Remarkably, the opposite manipulation, that is the addition of exogenous AHLs increases the abundance of late colonizer bacterial species. Hence, this work highlights the importance of AHL-mediated QS in dental plaque communities, its potential different roles in anaerobic and aerobic parts of dental plaque, and underscores the potential of QS interference in the control of periodontal diseases.

Exploration of Chemical Diversity in Intercellular Quorum Sensing Signalling Systems in Prokaryotes

Quorum sensing (QS) serves as a vital means of intercellular signalling in a variety of prokaryotes, which enables single cells to act in multicellular configurations. The potential to control community-wide responses has also sparked numerous recent biotechnological innovations. However, our capacity to utilize intercellular communication is hindered due to a scarcity of complementary signalling systems and a restricted comprehension of interconnections between these systems caused by variations in their dynamic range. In this study, we utilize uniform manifold approximation and projection and extended-connectivity fingerprints to explore the available chemical space of QS signalling molecules. We investigate and experimentally characterize a set of closely related QS signalling ligands, consisting of N-acyl homoserine lactones and the aryl homoserine lactone p-coumaroyl, as well as a set of more widely diverging QS ligands, consisting of photopyrones, dialkylresorcinols, 3,5-dimethylpyrazin-2-ol and autoinducer-2, and define their performance. We report on a set of six signal- and promoter-orthogonal intercellular QS signalling systems, significantly expanding the toolkit for engineering community-wide behaviour. Furthermore, we demonstrate that ligand diversity can serve as a statistically significant tool to predict much more complicated ligand-receptor interactions. This approach highlights the potential of dimensionality reduction to explore chemical diversity in microbial dynamics.

Constructions of quorum sensing signaling network for activated sludge microbial community

In wastewater treatment systems, the interactions among various microbes based on chemical signals, namely quorum sensing (QS), play critical roles in influencing microbial structure and function. However, it is challenging to understand the QS-controlled behaviors and the underlying mechanisms in complex microbial communities. In this study, we constructed a QS signaling network, providing insights into the intra- and interspecies interactions of activated sludge microbial communities based on diverse QS signal molecules. Our research underscores the role of diffusible signal factors in both intra- and interspecies communication among activated sludge microorganisms, and signal molecules commonly considered to mediate intraspecies communication may also participate in interspecies communication. QS signaling molecules play an important role as communal resources among the entire microbial group. The communication network within the microbial community is highly redundant, significantly contributing to the stability of natural microbial systems. This work contributes to the establishment of QS signaling network for activated sludge microbial communities, which may complement metabolic exchanges in explaining activated sludge microbial community structure and may help with a variety of future applications, such as making the dynamics and resilience of highly complex ecosystems more predictable.

Discovery of a LuxR-type regulator involved in isoniazid-dependent gene regulation in Mycobacterium smegmatis

OBJECTIVE

Most non-tuberculous mycobacteria exhibit intrinsic resistance against the anti-tuberculosis drug isoniazid (INH). We previously found that a pyrazinamidase/nicotinamidase of Mycobacterium smegmatis, named PzaA, has an enzymatic activity to hydrolyze INH, which may contribute to intrinsic resistance. Furthermore, PzaA expression is strongly induced by INH under nitrogen-depleted conditions, although the precise mechanism of this phenomenon remains unclear. Here, we aimed to reveal the mechanism underlying the INH-dependent induction of PzaA using a transcriptomic approach.

METHODS

RNA sequencing was performed to identify INH-inducible genes other than pzaA. 5' rapid amplification of cDNA ends analysis was employed to identify the transcription start sites of INH-induced transcription units. The function of a LuxR-like regulator gene (MSMEI_1050) found within the gene cluster containing pzaA was confirmed by gene deletion and complementation experiments involving INH hydrolysis assay and quantitative reverse transcription PCR.

RESULTS

RNA sequencing revealed 23 genes that INH strongly induced under conditions of nitrogen depletion, 17 of which were in a gene cluster containing pzaA. This cluster comprised at least three transcription units, including a non-INH-inducible monocistronic unit containing MSMEI_1050. Deletion of this gene deprived M. smegmatis of the ability to respond to INH, and complementation restored this ability.

CONCLUSIONS

MSMEI_1050 plays a key role in INH-dependent gene regulation. The precise mechanism of action is to be determined in future studies.

Antimicrobial therapeutics isolated from algal source: retrospect and prospect

In the last few decades, attention on new natural antimicrobial compounds has arisen due to a change in consumer preferences and the increase in the number of resistant microorganisms. Algae are defined as photosynthetic organisms that demonstrate a wide range of adaptability to adverse environmental conditions like temperature extremes, photo-oxidation, high or low salinity, and osmotic stress. Algae are primarily known to produce large amounts of secondary metabolite against various kinds of pathogenic microbes. Among these algae, micro and microalgae of river, lake, and algae of oceanic origin have been reported to have antimicrobial activity against the bacteria and fungi of pathogenic nature. Various polar and non- polar extracts of micro- and macro algae have been used for the suppression of these pathogenic fungi. Apart from these, certain algal derivatives have also been isolated from these having antibacterial and antifungal potential. Among the bioactive molecules of algae, polysaccharides, sulphated polysaccharides, phyco-cyanobilins polyphenols, lectins, proteins lutein, vitamin E, B₁₂ and K₁, peptides, polyunsaturated fatty acids and pigments can be highlighted. In the present review, we will discuss the biological activity of these derived compounds as antifungal/ antibacterial agents and their most promising applications. A brief outline is also given for the prospects of these isolated phytochemicals and using algae as therapeutic in the dietary form. We have also tried to answer whether alga-derived metabolites can serve as potential therapeutics for the treatment of SARS-CoV-2 like viral infections too.

Resistance Is Not Futile: The Role of Quorum Sensing Plasticity in Pseudomonas aeruginosa Infections and Its Link to Intrinsic Mechanisms of Antibiotic Resistance

Bacteria use a cell-cell communication process called quorum sensing (QS) to orchestrate collective behaviors. QS relies on the group-wide detection of extracellular signal molecules called autoinducers (AI). Quorum sensing is required for virulence and biofilm formation in the human pathogen Pseudomonas aeruginosa. In P. aeruginosa, LasR and RhlR are homologous LuxR-type soluble transcription factor receptors that bind their cognate AIs and activate the expression of genes encoding functions required for virulence and biofilm formation. While some bacterial signal transduction pathways follow a linear circuit, as phosphoryl groups are passed from one carrier protein to another ultimately resulting in up- or down-regulation of target genes, the QS system in P. aeruginosa is a dense network of receptors and regulators with interconnecting regulatory systems and outputs. Once activated, it is not understood how LasR and RhlR establish their signaling hierarchy, nor is it clear how these pathway connections are regulated, resulting in chronic infection. Here, we reviewed the mechanisms of QS progression as it relates to bacterial pathogenesis and antimicrobial resistance and tolerance.

Marine algae as efficacious bioresources housing antimicrobial compounds for preserving foods - A review

Certain synthetic chemicals used in global food industries eliminate pathogenic food microbes and prevent spoilage. Nevertheless, their toxicity precludes human consumption. This phenomenon has made scientific fraternity to look for alternative antimicrobial compounds from natural resources. In recent times, marine algae have been illustrated to be potent and rich sources of antimicrobial agents as chemical replacements for applications in food. Identifying novel antimicrobial agents from natural resources have become a worldwide research with immense significance. Marine algae are now considered as one of the most inexhaustible and unexposed sources of antimicrobial agents due to their abundance in seawaters and renewability. This review elaborated on marine algal antimicrobial agents against foodborne pathogens, mode of action and cumulated the prospective use of algal compounds in active food packaging as a natural food preservative. Due to poor solubility, unpleasant odor and ineffectiveness of plant derived antimicrobial agents against Gram-negative bacteria, researchers opted for marine algae, an ideal candidate to be used as natural food preservatives. This article elaborates and summarizes the efficient bioactive molecules in marine algae and their possible application in food preservation to extend shelf life of foods without causing any toxicity. In conclusion, marine algae have potential antimicrobial property against food pathogens and have more advantages than other natural sources of antimicrobial agents.

Quorum sensing: a new prospect for the management of antimicrobial-resistant infectious diseases

INTRODUCTION

Quorum-sensing (QS) is a microbial cell-to-cell communication system that utilizes small signaling molecules to mediates interactions between cross-kingdom microorganisms, including Gram-positive and -negative microbes. QS molecules include N-acyl-homoserine-lactones (AHLs), furanosyl borate, hydroxyl-palmitic acid methylester, and methyl-dodecanoic acid. These signaling molecules maintain the symbiotic relationship between a host and the healthy microbial flora and also control various microbial virulence factors. This manuscript has been developed based on published scientific papers.

AREAS COVERED

Furanones, glycosylated chemicals, heavy metals, and nanomaterials are considered QS inhibitors (QSIs) and are therefore capable of inhibiting the microbial QS system. QSIs are currently being considered as antimicrobial therapeutic options. Currently, the low speed at which new antimicrobial agents are being developed impairs the treatment of drug-resistant infections. Therefore, QSIs are currently being studied as potential interventions targeting QS-signaling molecules and quorum quenching (QQ) enzymes to reduce microbial virulence.

EXPERT OPINION

QSIs represent a novel opportunity to combat antimicrobial resistance (AMR). However, no clinical trials have been conducted thus far assessing their efficacy. With the recent advancements in technology and the development of well-designed clinical trials aimed at targeting various components of the, QS system, these agents will undoubtedly provide a useful alternative to treat infectious diseases.

Algal metabolites: An inevitable substitute for antibiotics

Antibiotic resistance is rising at a pace that is difficult to cope with; circumvention of this issue requires fast and efficient alternatives to conventional antibiotics. Algae inhabit a wide span of ecosystems, which contributes to their ability to synthesize diverse classes of highly active biogenic metabolites. Here, for the first time, we reviewed all possible algal metabolites with broad spectra antibacterial activity against pathogenic bacteria, including antibiotic-resistant strains, and categorized different metabolites of both freshwater and marine algae, linking them on the basis of their target sites and mechanistic actions along with their probable nanoconjugates. Algae can be considered a boon for novel drug discovery in the era of antibiotic resistance, as various algal primary and secondary metabolites possess potential antibacterial properties. The diversity of these metabolites from indigenous sources provides a promising gateway enabling researchers and pharmaceutical companies to develop novel nontoxic, cost-effective and highly efficient antibacterial medicines.

Seaweeds in Pig Nutrition

Seaweeds are macroalgae, with different sizes, colors and composition. They consist of brown algae, red algae and green algae, which all have a different chemical composition and bioactive molecule content. The polysaccharides, laminarin and fucoidan are commonly present in brown seaweeds, ulvans are found in green seaweeds and, red algae contain a large amount of carrageenans. These bioactive compounds may have several positive effects on health in livestock. In order to reduce the antimicrobials used in livestock, research has recently focused on finding natural and sustainable molecules that boost animal performance and health. The present study thus summarizes research on the dietary integration of seaweeds in swine. In particular the influence on growth performance, nutrients digestibility, prebiotic, antioxidant, anti-inflammatory, and immunomodulatory activities were considered. The review highlights that brown seaweeds seem to be a promising dietary intervention in pigs in order to boost the immune system, antioxidant status and gut health. Data on the use of green seaweeds as a dietary supplementation seems to be lacking at present and merit further investigation.

Quorum Sensing Signaling Alters Virulence Potential and Population Dynamics in Complex Microbiome-Host Interactomes

Despite the discovery of the first N-acyl homoserine lactone (AHL) based quorum sensing (QS) in the marine environment, relatively little is known about the abundance, nature and diversity of AHL QS systems in this diverse ecosystem. Establishing the prevalence and diversity of AHL QS systems and how they may influence population dynamics within the marine ecosystem, may give a greater insight into the evolution of AHLs as signaling molecules in this important and largely unexplored niche. Microbiome profiling of Stelletta normani and BD1268 sponge samples identified several potential QS active genera. Subsequent biosensor-based screening of a library of 650 marine sponge bacterial isolates identified 10 isolates that could activate at least one of three AHL biosensor strains. Each was further validated and profiled by Ultra-High Performance Liquid Chromatography Mass Spectrometry, with AHLs being detected in 8 out of 10 isolate extracts. Co-culture of QS active isolates with S. normani marine sponge samples led to the isolation of genera such as Pseudomonas and Paenibacillus, both of which were low abundance in the S. normani microbiome. Surprisingly however, addition of AHLs to isolates harvested following co-culture did not measurably affect either growth or biofilm of these strains. Addition of supernatants from QS active strains did however impact significantly on biofilm formation of the marine Bacillus sp. CH8a sporeforming strain suggesting a role for QS systems in moderating the microbe-microbe interaction in marine sponges. Genome sequencing and phylogenetic analysis of a QS positive Psychrobacter isolate identified several QS associated systems, although no classical QS synthase gene was identified. The stark contrast between the biodiverse sponge microbiome and the relatively limited diversity that was observed on standard culture media, even in the presence of QS active compounds, serves to underscore the extent of diversity that remains to be brought into culture.

In silico Analysis Reveals Distribution of Quorum Sensing Genes and Consistent Presence of LuxR Solos in the Pandoraea Species

The most common quorum sensing (QS) system in Gram-negative bacteria consists of signaling molecules called N-acyl-homoserine lactones (AHLs), which are synthesized by an enzyme AHL synthase (LuxI) and detected by a transcriptional regulator (LuxR) that are usually located in close proximity. However, many recent studies have also evidenced the presence of LuxR solos that are LuxR-related proteins in Proteobacteria that are devoid of a cognate LuxI AHL synthase. Pandoraea species are opportunistic pathogens frequently isolated from sputum specimens of cystic fibrosis (CF) patients. We have previously shown that P. pnomenusa strains possess QS activity. In this study, we examined the presence of QS activity in all type strains of Pandoraea species and acquired their complete genome sequences for holistic bioinformatics analyses of QS-related genes. Only four out of nine type strains (P. pnomenusa, P. sputorum, P. oxalativorans, and P. vervacti) showed QS activity, and C8-HSL was the only AHL detected. A total of 10 canonical luxIs with adjacent luxRs were predicted by bioinformatics from the complete genomes of aforementioned species and publicly available Pandoraea genomes. No orphan luxI was identified in any of the genomes. However, genes for two LuxR solos (LuxR2 and LuxR3 solos) were identified in all Pandoraea genomes (except two draft genomes with one LuxR solo gene), and P. thiooxydans was the only species that harbored no QS-related activity and genes. Except the canonical LuxR genes, LuxIs and LuxR solos of Pandoraea species were distantly related to the other well-characterized QS genes based on phylogenetic clustering. LuxR2 and LuxR3 solos might represent two novel evolutionary branches of LuxR system as they were found exclusively only in the genus. As a few luxR solos were located in close proximity with prophage sequence regions in the genomes, we thus postulated that these luxR solos could be transmitted into genus Pandoraea by transduction process mediated by bacteriophage. The bioinformatics approach developed in this study forms the basis for further characterization of closely related species. Overall, our findings improve the current understanding of QS in Pandoraea species, which is a potential pharmacological target in battling Pandoraea infections in CF patients.

Chemical language and warfare of bacterial natural products in bacteria-nematode-insect interactions

Covering: up to November 2017 Organismic interaction is one of the fundamental principles for survival in any ecosystem. Today, numerous examples show the interaction between microorganisms like bacteria and higher eukaryotes that can be anything between mutualistic to parasitic/pathogenic symbioses. There is also increasing evidence that microorganisms are used by higher eukaryotes not only for the supply of essential factors like vitamins but also as biological weapons to protect themselves or to kill other organisms. Excellent examples for such systems are entomopathogenic nematodes of the genera Heterorhabditis and Steinernema that live in mutualistic symbiosis with bacteria of the genera Photorhabdus and Xenorhabdus, respectively. Although these systems have been used successfully in organic farming on an industrial scale, it was only shown during the last 15 years that several different natural products (NPs) produced by the bacteria play key roles in the complex life cycle of the bacterial symbionts, the nematode host and the insect prey that is killed by and provides nutrients for the nematode-bacteria pair. Since the bacteria can switch from mutualistic to pathogenic lifestyle, interacting with two different types of higher eukaryotes, and since the full system with all players can be established in the lab, they are promising model systems to elucidate the natural function of microbial NPs. This review summarizes the current knowledge as well as open questions for NPs from Photorhabdus and Xenorhabdus and tries to assign their roles in the tritrophic relationship.

An autoinducer-independent RhlR quorum-sensing receptor enables analysis of RhlR regulation

Quorum sensing is a chemical communication process that bacteria use to coordinate group behaviors. Pseudomonas aeruginosa, an opportunistic pathogen, employs multiple quorum-sensing systems to control behaviors including virulence factor production and biofilm formation. One P. aeruginosa quorum-sensing receptor, called RhlR, binds the cognate autoinducer N-butryl-homoserine lactone (C4HSL), and the RhlR:C4HSL complex activates transcription of target quorum-sensing genes. Here, we use a genetic screen to identify RhlR mutants that function independently of the autoinducer. The RhlR Y64F W68F V133F triple mutant, which we call RhlR*, exhibits ligand-independent activity in vitro and in vivo. RhlR* can drive wildtype biofilm formation and infection in a nematode animal model. The ability of RhlR* to properly regulate quorum-sensing-controlled genes in vivo depends on the quorum-sensing regulator RsaL keeping RhlR* activity in check. RhlR is known to function together with PqsE to control production of the virulence factor called pyocyanin. Likewise, RhlR* requires PqsE for pyocyanin production in planktonic cultures, however, PqsE is dispensable for RhlR*-driven pyocyanin production on surfaces. Finally, wildtype RhlR protein is not sufficiently stabilized by C4HSL to allow purification. However, wildtype RhlR can be stabilized by the synthetic ligand mBTL (meta-bromo-thiolactone) and RhlR* is stable without a ligand. These features enabled purification of the RhlR:mBTL complex and of RhlR* for in vitro examination of their biochemical activities. To our knowledge, this work reports the first RhlR protein purification.

A novel LuxR-type solo of Sinorhizobium meliloti, NurR, is regulated by the chromosome replication coordinator, DnaA and activates quorum sensing

The genome of Sinorhizobium meliloti, a model for studying plant-bacteria symbiosis, contains eight genes coding for LuxR-like proteins. Two of these, SinR and ExpR, are essential for quorum sensing (QS). Roles and regulation surrounding the others are mostly unknown. Here, we reveal the DNA recognition sequence and regulon of the LuxR-like protein SMc00877. Unlike ExpR, which uses the long-chain acyl homoserine lactones (AHLs) as inducers, SMc00877 functioned independently of AHLs and was even functional in Escherichia coli. A target of SMc00877 is SinR, the major regulator of AHL production in S. meliloti. Disruption of SMc00877 decreased AHL production. A weaker production of AHLs resulted in smaller microcolonies, starting from single cells, and delayed AHL-dependent regulation. SMc00877 was expressed only in growing cells in the presence of replete nutrients. Therefore, we renamed it NurR (nutrient sensitive LuxR-like regulator). We traced this nutrient-sensitive expression to transcription control by the DNA replication initiation factor, DnaA, which is essential for growth. These results indicate that NurR has a role in modulating the threshold of QS activation according to growth. We propose growth behavior as an additional prerequisite to population density for the activation of QS in S. meliloti.

Engineering chemical interactions in microbial communities

Microbes living within host-associated microbial communities (microbiotas) rely on chemical communication to interact with surrounding organisms. These interactions serve many purposes, from supplying the multicellular host with nutrients to antagonizing invading pathogens, and breakdown of chemical signaling has potentially negative consequences for both the host and microbiota. Efforts to engineer microbes to take part in chemical interactions represent a promising strategy for modulating chemical signaling within these complex communities. In this review, we discuss prominent examples of chemical interactions found within host-associated microbial communities, with an emphasis on the plant-root microbiota and the intestinal microbiota of animals. We then highlight how an understanding of such interactions has guided efforts to engineer microbes to participate in chemical signaling in these habitats. We discuss engineering efforts in the context of chemical interactions that enable host colonization, promote host health, and exclude pathogens. Finally, we describe prominent challenges facing this field and propose new directions for future engineering efforts.

Computational Exploration of Putative LuxR Solos in Archaea and Their Functional Implications in Quorum Sensing

LuxR solos are unexplored in Archaea, despite their vital role in the bacterial regulatory network. They assist bacteria in perceiving acyl homoserine lactones (AHLs) and/or non-AHLs signaling molecules for establishing intraspecies, interspecies, and interkingdom communication. In this study, we explored the potential LuxR solos of Archaea from InterPro v62.0 meta-database employing taxonomic, probable function, distribution, and evolutionary aspects to decipher their role in quorum sensing (QS). Our bioinformatics analyses showed that putative LuxR solos of Archaea shared few conserved domains with bacterial LuxR despite having less similarity within proteins. Functional characterization revealed their ability to bind various AHLs and/or non-AHLs signaling molecules that involve in QS cascades alike bacteria. Further, the phylogenetic study indicates that Archaeal LuxR solos (with less substitution per site) evolved divergently from bacteria and share distant homology along with instances of horizontal gene transfer. Moreover, Archaea possessing putative LuxR solos, exhibit the correlation between taxonomy and ecological niche despite being the inhabitant of diverse habitats like halophilic, thermophilic, barophilic, methanogenic, and chemolithotrophic. Therefore, this study would shed light in deciphering the role of the putative LuxR solos of Archaea to adapt varied habitats via multilevel communication with other organisms using QS.

HexA is a versatile regulator involved in the control of phenotypic heterogeneity of Photorhabdus luminescens

Phenotypic heterogeneity in microbial communities enables genetically identical organisms to behave differently even under the same environmental conditions. Photorhabdus luminescens, a bioluminescent Gram-negative bacterium, contains a complex life cycle, which involves a symbiotic interaction with nematodes as well as a pathogenic association with insect larvae. P. luminescens exists in two distinct phenotypic cell types, designated as the primary (1°) and secondary (2°) cells. The 1° cells are bioluminescent, pigmented and can support nematode growth and development. Individual 1° cells undergo phenotypic switching after prolonged cultivation and convert to 2° cells, which lack the 1° specific phenotypes. The LysR-type regulator HexA has been described as major regulator of this switching process. Here we show that HexA controls phenotypic heterogeneity in a versatile way, directly and indirectly. Expression of hexA is enhanced in 2° cells, and the corresponding regulator inhibits 1° specific traits in 2° cells. HexA does not directly affect bioluminescence, a predominant 1° specific phenotype. Since the respective luxCDABE operon is repressed at the post-transcriptional level and transcriptional levels of the RNA chaperone gene hfq are also enhanced in 2° cells, small regulatory RNAs are presumably involved that are under control of HexA. Another phenotypic trait that is specific for 1° cells is quorum sensing mediated cell clumping. The corresponding pcfABCDEF operon could be identified as the first direct target of HexA, since the regulator binds to the pcfA promoter region and thereby blocks expression of the target operon. In summary, our data show that HexA fulfills the task as repressor of 1° specific features in 2° cells in a versatile way and gives first insights into the complexity of regulating phenotypic heterogeneity in Photorhabdus bacteria.

Specificity and complexity in bacterial quorum-sensing systems

Quorum sensing (QS) is a microbial cell-to-cell communication process that relies on the production and detection of chemical signals called autoinducers (AIs) to monitor cell density and species complexity in the population. QS allows bacteria to behave as a cohesive group and coordinate collective behaviors. While most QS receptors display high specificity to their AI ligands, others are quite promiscuous in signal detection. How do specific QS receptors respond to their cognate signals with high fidelity? Why do some receptors maintain low signal recognition specificity? In addition, many QS systems are composed of multiple intersecting signaling pathways: what are the benefits of preserving such a complex signaling network when a simple linear ‘one-to-one’ regulatory pathway seems sufficient to monitor cell density? Here, we will discuss different molecular mechanisms employed by various QS systems that ensure productive and specific QS responses. Moreover, the network architectures of some well-characterized QS circuits will be reviewed to understand how the wiring of different regulatory components achieves different biological goals.

This review focuses on the specificity and complexity of quorum-sensing circuits in both Gram-negative and Gram-positive bacterial species.

Antibacterial Derivatives of Marine Algae: An Overview of Pharmacological Mechanisms and Applications

The marine environment is home to a taxonomically diverse ecosystem. Organisms such as algae, molluscs, sponges, corals, and tunicates have evolved to survive the high concentrations of infectious and surface-fouling bacteria that are indigenous to ocean waters. Both macroalgae (seaweeds) and microalgae (diatoms) contain pharmacologically active compounds such as phlorotannins, fatty acids, polysaccharides, peptides, and terpenes which combat bacterial invasion. The resistance of pathogenic bacteria to existing antibiotics has become a global epidemic. Marine algae derivatives have shown promise as candidates in novel, antibacterial drug discovery. The efficacy of these compounds, their mechanism of action, applications as antibiotics, disinfectants, and inhibitors of foodborne pathogenic and spoilage bacteria are reviewed in this article.

Quorum Sensing and LuxR Solos in Photorhabdus

Bacterial communication via small diffusible molecules to mediate group-coordinated behaviour is commonly referred to as 'quorum sensing'. The prototypical quorum sensing system of Gram-negative bacteria consists of a LuxI-type autoinducer synthase that produces acyl-homoserine lactones (AHLs) as signals and a LuxR-type receptor that detects the AHLs to control expression of specific genes. However, many bacteria possess LuxR homologs but lack a cognate LuxI-type AHL-synthase. Those LuxR-type receptors are designated as 'LuxR orphans' or 'solos'. Entomopathogenic bacteria of the genus Photorhabdus all harbour a large number of LuxR solos, more than any other bacteria examined so far. Two novel quorum sensing systems were found to regulate cell clumping in Photorhabdus and therefore affect pathogenicity. In Photorhabdus luminescens and Photorhabdus temperata the LuxR solo PluR senses α-pyrones named 'photopyrones' instead of AHLs, which are produced by the pyrone synthase PpyS. In contrast, Photorhabdus asymbiotica, a closely related insect and human pathogen, has the PluR homolog PauR, which senses dialkylresorcinols produced by the DarABC pathway to regulate pathogenicity. All three Photorhabdus species harbour at least one LuxR solo with an intact AHL-binding motif, which might also allow sensing of exogenous AHLs. However, the majority of the LuxR solos in all Photorhabdus species have a PAS4 signal-binding domain. These receptors are assumed to detect eukaryotic compounds and are proposed to be involved in host sensing. Overall, because of the large number of LuxR solos they encode, bacteria of the genus Photorhabdus are ideal candidates to study and to identify novel bacterial communication networks.