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To understand how bacterial populations function and evolve, it is essential to identify socially significant subpopulations, including previously unrecognized types of cheaters. In this study, we uncover an unexpected role of RNA polymerase (RNAP) in regulating quorum sensing (QS) and QS-associated social behaviors in P. aeruginosa . Specifically, we demonstrate that the α subunit of RNAP (RpoA) is a key regulatory component in this process. A single-nucleotide mutation within the C-terminal domain of RpoA was found to alter QS activity, driving an environment-dependent transition between cooperative and cheating phenotypes. This discovery of this novel, noncanonical QS cheater mutant offers new insights into intra-population interactions, population stability, and evolutionary dynamics. These findings carry significant implications for microbial ecology and deepen our understanding of social evolution in bacterial communities.

SmcR family proteins were discovered in the 1990s as central regulators of quorum-sensing gene expression and later discovered to be conserved in all studied Vibrio species. SmcR homologs regulate a wide range of genes involved in pathogenesis, including but not limited to genes involved in biofilm production and toxin secretion. As archetypal members of the broad class of TetR-type transcription factors, each SmcR-type protein has a predicted ligand-binding pocket. However, no native ligand has been identified for these proteins that control their function as regulators. Here, we used SmcR-specific chemical inhibitors to determine that ligand binding drives proteolytic degradation in vivo , providing the first demonstration of SmcR function connected to ligand binding for this historical protein family.

The discovery of quorum-sensing responsive linear plasmid phages has transformed understanding of phage-bacterial interactions by demonstrating inter-domain chemical communication. To date, however, examples of quorum-sensing responsive phages have been sparse. The founding example of such a phage, φVP882, detects a chemical communication signal molecule called DPO that is produced by diverse bacterial species. We investigated whether a family of VP882-like phages might exist that detect and respond to DPO. We find that indeed, VP882-like phages reside in DPO-producing bacterial species isolated at different times and geographic locations, suggesting their wide circulation in the environment. This discovery strengthens the evidence for the generality of phage-bacterial inter-domain chemical communication.

Quorum sensing is a physiological phenomenon of microbial cell-to-cell information exchange, which relies on the quorum sensing signal molecules (QSSMs) to communicate and coordinate collective processes. Quorum sensing enables bacteria to alter their behavior as the population density and species composition of the bacterial community change. Effective detection of QSSMs is paramount for regulating microbial community behavior. However, traditional detection methods face the shortcomings of complex operation, high costs, and lack of portability. By combining the advantage of biosensing and nanomaterials, the biosensors play a pivotal significance in QSSM detection. In this review, we first briefly describe the QSSM classification and common detection techniques. Then, we provide a comprehensive summary of research progress in biosensor-based QSSM detection according to the transduction mechanism. Finally, challenges and development trends of biosensors for QSSM detection are discussed. We believe it offers valuable insights into this burgeoning research area. Graphical Abstract Biosensors such as electrochemical and optical sensors have been widely used in quorum sensing signal molecules (QSSMs) detection. QSSM classification and traditional detection techniques such as liquid chromatography, gas chromatography, mass spectrometry, and ELISA have also been described.

Biological control is being considered as an alternative or a supplemental way of reducing the use of chemicals in agriculture. An endophytic strain G3 with potential as a biocontrol agent was isolated from the stems of Triticum aestivum L. It was classified by 16S rDNA sequencing as a member of Serratia. Strain G3 displayed a broad spectrum of antifungal activity in vitro against a number of phytopathogens such as Botrytis cinerea, Cryphonectria parasitica, Rhizoctonia cerealis and Valsa sordida. Molecular mechanisms involved in biocontrol by Serratia sp. G3 was investigated for its potential application to plant health management. The results showed that G3 produces an array of antimicrobial exoproducts, including chitinase, protease, antibiotic pyrrolnitrin, and siderophores for iron competition. Moreover, it also produced the plant growth hormone indole-3-acetic acid, suggesting that multiple mechanisms and their synergistic effects may be involved in biocontrol of plant diseases. Additionally, strain G3 can produce at least ten N-acyl homoserine lactones (AHLs) signal molecules for cell to cell communication, including unsubstituted, 3-oxo, and 3-hydroxy at the C3 position through liquid chromatography-tandem mass spectrometry (LC-MS/MS), which is different from the previously reported Serraia species. For the first time, N-3-oxo-heptanoyl-homoserine lactone, one of the main molecules was reported in the genus Serratia. The role of AHL-dependent quorum sensing system in the interactions between the endophytic strain G3 and host plants and its potential application in improving biocontrol efficacy will be further explored.

Aeromonads are inhabitants of aquatic ecosystems and are described as being involved in intestinal disturbances and other infections. The purpose of this study was to investigate the production of N-acyl-homoserine lactone (AHL) signal molecules and some virulence factors, including hemolysins, proteases, extracellular nucleases production and cytotoxicity by waterborne Aeromonas hydrophila. A total of 24 strains isolated from fresh-water or diseased fish were used in the study. The majority A.hydrophila strains produce two AHL molecules (21/24), one is N-butanoyl homoserine lactone (BHL), and the other is N-hexanoyl homoserine lactone (HHL) according to thin-layer chromatography analysis. Among the virulence factors tested, more than 83 % of the isolates produced β haemolysin when inoculated on sheep blood agar, only 50 % of the isolates displayed DNase activity, 75 % of the isolates shown proteolytic activity on skimmed milk plate, and cytotoxic activity was detected in 20 of 24 of the isolates. The strains producing AHLs possessed one or more virulence factors. In conclusion, the production of quorum sensing signal molecules is common among the strains that we examined, and there seems to some relationships between quorum sensing signal production and virulence factors in A. hydrophila.

The production of quorum-sensing-related signal molecules (QSRMs) among culturable bacteria comprising the community on wheat heads was investigated. The taxonomic position of 186 bacterial isolates obtained from ten heads was inferred based on 16S rRNA gene sequences, and their QSRM production was determined using two bioreporter strains of N-acylhomoserine lactones. Approximately 33% of isolates produced QSRMs, though the proportion of QSRM-producing isolates on a wheat head was significantly negatively correlated with population size. Most of the producing isolates were Pantoea species, most commonly Pantoea ananatis. Furthermore, the proportion of Pantoea ananatis that produced QSRMs was significantly negatively correlated with the number of bacterial genera (community richness) on each head. Finally, community richness was positively correlated with population size. Qualitative analysis using thin-layer-chromatography revealed that the QSRMs of Pantoea isolates were composed of at least two compounds. This is the first report indicating that Pantoea ananatis isolates inhabiting wheat heads are capable of producing QSRMs. QSRM production by Pantoea spp. may contribute to the predominance of this genus on wheat heads, particularly at relatively low population densities and community diversity.Key words: quorum sensing, signal molecule, epiphyte, wheat head, Pantoea spp.

Quorum sensing (QS) is a cell—cell communication system that controls gene expression in many bacterial species, mediated by diffusible signal molecules. Although the intracellular regulatory mechanisms of QS are often well-understood, the functional roles of QS remain controversial. In particular, the use of multiple signals by many bacterial species poses a serious challenge to current functional theories. Here, we address this challenge by showing that bacteria can use multiple QS signals to infer both their social (density) and physical (mass-transfer) environment. Analytical and evolutionary simulation models show that the detection of, and response to, complex social/physical contrasts requires multiple signals with distinct half-lives and combinatorial (nonadditive) responses to signal concentrations. We test these predictions using the opportunistic pathogen Pseudomonas aeruginosa and demonstrate significant differences in signal decay between its two primary signal molecules, as well as diverse combinatorial responses to dual-signal inputs. QS is associated with the control of secreted factors, and we show that secretome genes are preferentially controlled by synergistic \"AND-gate\" responses to multiple signal inputs, ensuring the effective expression of secreted factors in high-density and low mass-transfer environments. Our results support a new functional hypothesis for the use of multiple signals and, more generally, show that bacteria are capable of combinatorial communication.

The emergence of superbugs like methicillin-resistant Staphylococcus aureus exposed the limitations of treating microbial infections using antibiotics. At present, the discovery of novel and convincing therapeutic methods are being executed increasingly as possible substitutes to conventional antibiotic therapies. The quorum sensing helps Staphylococcus aureus become more viable through their signaling mechanisms. In recent years, targeting the prominent factors of quorum sensing has obtained remarkable attention as a futuristic approach to dealing with bacterial pathogenicity. The standard antibiotic therapy intends to inhibit the organism by targeting specific molecules and afford a chance for the evolution of antibiotic resistance. This prompts the development of novel therapeutic strategies like inhibiting quorum sensing that can limit bacterial virulence by decreasing the selective pressure, thereby restricting antibiotic resistance evolution. This review furnishes new insights into the accessory gene regulator quorum sensing in Staphylococcus aureus and its inhibition by targeting the genes that regulate the operon. Further, this review comprehensively explores the inhibitors reported up to date and their specific targets and discusses their potentially ineffective alternative therapy against methicillin-resistant Staphylococcus aureus . Graphical abstract

It is well recognized that bacteria communicate via small diffusible molecules, a process termed quorum sensing. The best understood quorum sensing systems are those that use acylated homoserine lactones (AHLs) for communication. The prototype of those systems consists of a LuxI-like AHL synthase and a cognate LuxR receptor that detects the signal. However, many proteobacteria possess LuxR receptors, yet lack any LuxI-type synthase, and thus these receptors are referred to as LuxR orphans or solos. In addition to the well-known AHLs, little is known about the signaling molecules that are sensed by LuxR solos. Here, we describe a novel cell–cell communication system in the insect and human pathogen Photorhabdus asymbiotica . We identified the LuxR homolog PauR to sense dialkylresorcinols (DARs) and cyclohexanediones (CHDs) instead of AHLs as signals. The DarABC synthesis pathway produces the molecules, and the entire system emerged as important for virulence. Moreover, we have analyzed more than 90 different Photorhabdus strains by HPLC/MS and showed that these DARs and CHDs are specific to the human pathogen P. asymbiotica . On the basis of genomic evidence, 116 other bacterial species are putative DAR producers, among them many human pathogens. Therefore, we discuss the possibility of DARs as novel and widespread bacterial signaling molecules and show that bacterial cell–cell communication goes far beyond AHL signaling in nature. Significance Bacteria can communicate with each other by small diffusible molecules, a process termed quorum sensing. Many bacteria use acylated homoserine lactones (AHLs) as signals, which are sensed by so-called LuxR-type receptors. With the photopyrones from the insect pathogenic bacterium Photorhabdus luminescens , we recently identified the first quorum sensing molecules different from AHLs that are sensed by a LuxR-type receptor. Here we describe the second novel quorum sensing molecule sensed by a LuxR-type receptor of Photorhabdus species, PauR of the human pathogen Photorhabdus asymbiotica . We demonstrate that P. asymbiotica communicates via dialkylresorcinols (DARs) and cyclohexanediones (CHDs). As the synthesis pathway is widespread, and often present in human pathogens, we discuss DARs and CHDs as novel and widespread signaling molecules.