BlpC-regulated bacteriocin production in Streptococcus thermophilus

Streptococcus thermophilus B59671 produces a bacteriocin with anti-pediococcal activity, but genes required for its production are not characterized. Genome sequencing of S. thermophilus has identified a genetic locus encoding a quorum sensing (QS) system that regulates production of class II bacteriocins. However, in strains possessing this gene cluster, production of bacteriocin like peptides (Blp) was only observed when excess pheromone was provided. PCR analysis revealed this strain possessed blpC, which encodes the 30-mer QS pheromone. To investigate if BlpC regulates bacteriocin production in S. thermophilus B59671, an integrative vector was used to replace blpC with a gene encoding for kanamycin resistance and the resulting mutant did not inhibit the growth of Pediococcus acidilactici. Constitutive expression of blpC from a shuttle vector restored the bacteriocin production, confirming the blp gene cluster is essential for bacteriocin activity in S. thermophilus B59671.     

Microbial Communication,Cooperation and Cheating: Quorum Sensing Drives the Evolution of Cooperation in Bacteria

An increasing body of empirical evidence suggests that cooperation among clone-mates is common in bacteria. Bacterial cooperation may take the form of the excretion of “public goods”: exoproducts such as virulence factors, exoenzymes or components of the matrix in biofilms, to yield significant benefit for individuals joining in the common effort of producing them. Supposedly in order to spare unnecessary costs when the population is too sparse to supply the sufficient exoproduct level, many bacteria have evolved a simple chemical communication system called quorum sensing (QS), to “measure” the population density of clone-mates in their close neighborhood. Cooperation genes are expressed only above a threshold rate of QS signal molecule re-capture, i.e., above the local quorum of cooperators. The cooperative population is exposed to exploitation by cheaters, i.e., mutants who contribute less or nil to the effort but fully enjoy the benefits of cooperation. The communication system is also vulnerable to a different type of cheaters (“Liars”) who may produce the QS signal but not the exoproduct, thus ruining the reliability of the signal. Since there is no reason to assume that such cheaters cannot evolve and invade the populations of honestly signaling cooperators, the empirical fact of the existence of both bacterial cooperation and the associated QS communication system seems puzzling. Using a stochastic cellular automaton approach and allowing mutations in an initially non-cooperating, non-communicating strain we show that both cooperation and the associated communication system can evolve, spread and remain persistent. The QS genes help cooperative behavior to invade the population, and vice versa; cooperation and communication might have evolved synergistically in bacteria. Moreover, in good agreement with the empirical data recently available, this synergism opens up a remarkably rich repertoire of social interactions in which cheating and exploitation are commonplace.     

The Alternative Sigma Factor SigX Controls Bacteriocin Synthesis and Competence,the Two Quorum Sensing Regulated Traits in Streptococcus mutans

Two small quorum sensing (QS) peptides regulate competence in S. mutans in a cell density dependent manner: XIP (sigX inducing peptide) and CSP (competence stimulating peptide). Depending on the environmental conditions isogenic S. mutans cells can split into a competent and non-competent subpopulation. The origin of this population heterogeneity has not been experimentally determined and it is unknown how the two QS systems are connected. We developed a toolbox of single and dual fluorescent reporter strains and systematically knocked out key genes of the competence signaling cascade in the reporter strain backgrounds. By following signal propagation on the single cell level we discovered that the master regulator of competence, the alternative sigma factor SigX, directly controls expression of the response regulator for bacteriocin synthesis ComE. Consequently, a SigX binding motif (cin-box) was identified in the promoter region of comE. Overexpressing the genetic components involved in competence development demonstrated that ComRS represents the origin of bimodality and determines the modality of the downstream regulators SigX and ComE. Moreover these analysis showed that there is no direct regulatory link between the two QS signaling cascades. Competence is induced through a hierarchical XIP signaling cascade, which has no regulatory input from the CSP cascade. CSP exclusively regulates bacteriocin synthesis. We suggest renaming it mutacin inducing peptide (MIP). Finally, using phosphomimetic comE mutants we show that unimodal bacteriocin production is controlled posttranslationally, thus solving the puzzling observation that in complex media competence is observed in a subpopulation only, while at the same time all cells produce bacteriocins. The control of both bacteriocin synthesis and competence through the alternative sigma-factor SigX suggests that S. mutans increases its genetic repertoire via QS controlled predation on neighboring species in its natural habitat.     

Linking Molecular and Population Processes in Mathematical Models of Quorum Sensing

Many bacteria alter their behaviors as a function of population density, via a process known as quorum sensing (QS). QS is achieved by the synthesis and detection of diffusible signal molecules, often involving complex signal transduction pathways and regulatory networks. Mathematical models have been developed to investigate a number of aspects of QS, resulting in a wide range of model structures; many have focused on either the molecular or the population scale. In this paper, I show that many published models fail to satisfy physical constraints (such as conservation of matter) or rely on a priori assumptions that may not be valid. I present new, simple models of canonical Gram-negative and Gram-positive QS systems, in both well-mixed and biofilm populations, focusing on the interaction between molecular and population processes. I show that this interaction may be crucial for several important features of QS, including bistability and the localization of QS in space. The results highlight the need to link molecular and population processes carefully in QS models, provide a general framework for understanding the behavior of complex system-specific models, and suggest new directions for both theoretical and experimental work.     

Gluconacetobacter diazotrophicus PAL5 possesses an active quorum sensing regulatory system

The endophytic bacterium Gluconacetobacter diazotrophicus colonizes a broad range of host plants. Its plant growth-promoting capability is related to the capacity to perform biological nitrogen fixation, the biosynthesis of siderophores, antimicrobial substances and the solubilization of mineral nutrients. Colonization of and survival in these endophytic niche requires a complex regulatory network. Among these, quorum sensing systems (QS) are signaling mechanisms involved in the control of several genes related to microbial interactions, host colonization and stress survival. G. diazotrophicus PAL5 possesses a QS composed of a luxR and a luxI homolog, and produces eight molecules from the AHL family as QS signals. In this report data are provided showing that glucose concentration modifies the relative levels of these signal molecules. The activity of G. diazotrophicus PAL5 QS is also altered in presence of other carbon sources and under saline stress conditions. Inactivation of the QS system of G. diazotrophicus PAL5 by means of a quorum quenching strategy allowed the identification of extracellular and intracellular proteins under the control of this regulatory mechanism.     

Bacterial quorum sensing: circuits and applications

Bacterial quorum sensing (QS) systems are cell density—dependent regulatory networks that coordinate bacterial behavioural changes from single cellular organisms at low cell densities to multicellular types when their population density reaches a threshold level. At this stage, bacteria produce and perceive small diffusible signal molecules, termed autoinducers in order to mediate gene expression. This often results in phenotypic shifts, like planktonic to biofilm or non-virulent to virulent. In this way, they regulate varied physiological processes by adjusting gene expression in concert with their population size. In this review we give a synopsis of QS mediated cell–cell communication in bacteria. The first part focuses on QS circuits of some Gram-negative and Gram-positive bacteria. Thereafter, attention is drawn on the recent applications of QS in development of synthetic biology modules, for studying the principles of pattern formation, engineering bi-directional communication system and building artificial communication networks. Further, the role of QS in solving the problem of biofouling is also discussed.     

Role of N-acyl homoserine lactone (AHL)-based quorum sensing (QS) in aerobic sludge granulation

N-acyl homoserine lactone (AHL)-based quorum sensing (QS) has been recognized to play an important role in the formation of biofilm. However, aerobic granular sludge is considered as a special biofilm, and its biological implication and role of AHL-based QS still remain unclear. This study investigated the role of AHL-based QS in aerobic granulation. Results showed that AHLs were necessary to the typical aerobic granulation, and AHL-associated coordination of bacteria in sludge aggregation was sludge density dependent only when it reached a threshold of 1.010 g/mL; AHL-based QS was activated to regulate aerobic granulation. Furthermore, a quorum quenching method was firstly adopted to investigate the role of AHLs in aerobic granules. Results showed inhibition of AHL by acylase that reduced the AHL content in aerobic granules and further weakened its attachment potential, which proved that AHLs play an important role in the formation of aerobic granules. Additionally, the assay of quorum quenching not only proved that AHL-based QS could regulate EPS production but also provided additional evidence for the role of AHLs in aerobic granulation by regulating EPS content and its component proportion.     

Three-Dimensional Numerical Simulations of Biofilm Dynamics with Quorum Sensing in a Flow Cell

We develop a multiphasic hydrodynamic theory for biofilms taking into account interactions among various bacterial phenotypes, extracellular polymeric substance (EPS), quorum sensing (QS) molecules, solvent, and antibiotics. In the model, bacteria are classified into down-regulated QS, up-regulated QS, and non-QS cells based on their QS ability. The model is first benchmarked against an experiment yielding an excellent fit to experimental measurements on the concentration of QS molecules and the cell density during biofilm development. It is then applied to study development of heterogeneous structures in biofilms due to interactions of QS regulation, hydrodynamics, and antimicrobial treatment. Our 3D numerical simulations have confirmed that (i). QS is beneficial for biofilm development in a long run by building a robust EPS population to protect the biofilm; (ii). biofilms located upstream can induce QS downstream when the colonies are close enough spatially; (iii). QS induction may not be fully operational and can even be compromised in strong laminar flows; (v). the hydrodynamic stress alters the biofilm morphology. Through further numerical investigations, our model suggests that (i). QS-regulated EPS production contributes to the structural formation of heterogeneous biofilms; (ii) QS down-regulated cells tend to grow at the surface of the biofilm while QS up-regulated ones tend to grow in the bulk; (iii) when nutrient supply is sufficient, QS induction might be more effective upstream than downstream; (iv) QS may be of little benefit in a short timescale in term of fighting against invading strain/species; (v) the material properties of biomass (bacteria and EPS) have strong impact on the dilution of QS molecules under strong shear flow. In addition, with this modeling framework, hydrodynamic details and rheological quantities associated with biofilm formation under QS regulation can be resolved.     

Cross-Strain Quorum Sensing Inhibition by Staphylococcus Aureus. Part 2: A Spatially Inhomogeneous Model

Staphylococcus aureus uses quorum sensing (QS) to enhance its pathogenicity. An intriguing aspect of this is that different strains are capable of inactivating the QS systems of opposing strains. In Part 1 of this study, we presented a model of this phenomenon in a well-mixed environment; here, we incorporate spatial structure. Two competitive strains occupying adjacent habitats with freely diffusing QS signal molecules (QSSMs) are considered. We investigate the effect of the QSSM diffusion coefficient and the relative size of the two populations on the ability of one population to dominate the other. Regarding population size, a larger population is generally at an advantage (initial conditions permitting), while the implications of different diffusivities are more complex and depend upon the sizes of the populations.     

A Tangled Web: Regulatory Connections between Quorum Sensing and Cyclic Di-GMP

Bacteria sense and respond to environmental cues to control important developmental processes. Two widely conserved and important strategies that bacteria employ to sense changes in population density and local environmental conditions are quorum sensing (QS) and cyclic di-GMP (c-di-GMP) signaling, respectively. The importance of these pathways in controlling a broad variety of functions, including virulence, biofilm formation, and motility, has been recognized in many species. Recent research has shown that these pathways are intricately intertwined. Here we review the regulatory connections between QS and c-di-GMP signaling. We propose that the integration of QS with c-di-GMP allows bacteria to assimilate information about the local bacterial population density with other physicochemical environmental signals within the broader c-di-GMP signaling network.     

Mini-review: quorum sensing in the marine environment and its relationship to biofouling

Bacterial quorum sensing (QS) is a cell–cell communication and gene regulatory mechanism that allows bacteria to coordinate swarming, biofilm formation, stress resistance, and production of toxins and secondary metabolites in response to threshold concentrations of QS signals that accumulate within a diffusion-limited environment. This review focuses on the role of QS signaling and QS inhibition in marine bacteria by compounds derived from marine organisms. Since the formation of a biofilm is considered to be an initial step in the development of fouling, direct and indirect effects of QS signals and inhibitors on the process of marine biofouling are discussed. Directions for future investigations and QS-related biotechnological applications are highlighted.     

Evolutionary theory of bacterial quorum sensing: when is a signal not a signal?

The term quorum sensing (QS) is used to describe the communication between bacterial cells, whereby a coordinated population response is controlled by diffusible molecules produced by individuals. QS has not only been described between cells of the same species (intraspecies), but also between species (interspecies) and between bacteria and higher organisms (inter-kingdom). The fact that QS-based communication appears to be widespread among microbes is strange, considering that explaining both cooperation and communication are two of the greatest problems in evolutionary biology. From an evolutionary perspective, intraspecies signalling can be explained using models such as kin selection, but when communication is described between species, it is more difficult to explain. It is probable that in many cases this involves QS molecules being used as 'cues' by other species as a guide to future action or as manipulating molecules whereby one species will 'coerce' a response from another. In these cases, the usage of QS molecules cannot be described as signalling. This review seeks to integrate the evolutionary literature on animal signalling with the microbiological literature on QS, and asks whether QS within bacteria is true signalling or whether these molecules are also used as cues or for the coercion of other cells.     

Mathematical modelling of the agr operon in Staphylococcus aureus

Staphylococcus aureus is a pathogenic bacterium that utilises quorum sensing (QS), a cell-to-cell signalling mechanism, to enhance its ability to cause disease. QS allows the bacteria to monitor their surroundings and the size of their population, and S. aureus makes use of this to regulate the production of virulence factors. Here we describe a mathematical model of this QS system and perform a detailed time-dependent asymptotic analysis in order to clarify the roles of the distinct interactions that make up the QS process, demonstrating which reactions dominate the behaviour of the system at various timepoints. We couple this analysis with numerical simulations and are thus able to gain insight into how a large population of S. aureus shifts from a relatively harmless state to a highly virulent one, focussing on the need for the three distinct phases which form the feedback loop of this particular QS system.     

革兰氏阴性细菌LuxR/I型群体感应抑制剂研究进展

摘要：细菌群体感应（Quorum sensing, QS）被视为对抗细菌感染与解决细菌耐药性问题的新靶点。以AHLs为信号分子的LuxR/I型群体感应系统广泛存在于革兰氏阴性菌包括多种临床致病菌中,因此寻找LuxR/I型群体感应抑制剂（Quorum sensing inhibitors, QSIs）是研发抗革兰氏阴性致病菌药物的重要途径。迄今为止,已知的LuxR/I型小分子QSIs来源包括化学合成、天然产物与已知药物库的化合物,大分子则包括群体感应淬灭酶与群体感应淬灭抗体。本文总结了近年来LuxR/I型QSIs研究进展,为新型抗菌药物研发提供理论依据。     

Quorum sensing regulation of gene expression: A promising target for drugs against bacterial pathogenicity

Bacteria are sensitive to an increase in population density and respond quickly and coordinately by induction of certain sets of genes. This mode of regulation, known as quorum sensing (QS), is based on the effect of low-molecular-weight signal molecules, autoinducers (AIs). When the population density is high, AIs accumulate in the medium and interact with regulatory receptor proteins. QS systems are global regulators of bacterial gene expression and play a key role in controlling many metabolic processes in the cell, including bacterial virulence. The review considers the molecular mechanisms of QS in different taxonomic groups of bacteria and discusses QS regulation as a possible target in treating bacterial infections. This is a new, alternative strategy of antibacterial therapy, which includes the construction of drugs acting directly against bacterial pathogenicity by suppressing QS (antipathogenicity drugs). This strategy makes it possible to avoid a wide distribution of antibiotic-resistant pathogenic bacteria and the formation of biofilms, which dramatically increase drug resistance.     

Quorum Sensing Regulation in Aeromonas hydrophila

We present detailed results on the C4-HSL-mediated quorum sensing (QS) regulatory system of the opportunistic Gram-negative bacterium Aeromonas hydrophila. This bacterium contains a particularly simple QS system that allows for a detailed modeling of kinetics. In a model system (i.e., the Escherichia coli monitor strain MH205), the C4-HSL production of A. hydrophila is interrupted by fusion of gfp(ASV). In the present in vitro study, we measure the response of the QS regulatory ahyRI locus in the monitor strain to predetermined concentrations of C4-HSL signal molecules. A minimal kinetic model describes the data well. It can be solved analytically, providing substantial insight into the QS mechanism: at high concentrations of signal molecules, a slow decay of the activated regulator sets the timescale for the QS regulation loop. Slow saturation ensures that, in an A. hydrophila cell, the QS system is activated only by signal molecules produced by other A. hydrophila cells. Separate information on the ahyR and ahyI loci can be extracted, thus allowing the probe to be used in identifying the target when testing QS inhibitors.