PepO,a CovRS‐controlled endopeptidase,disrupts Streptococcus pyogenes quorum sensing

Group A Streptococcus (GAS, Streptococcus pyogenes) is a human‐restricted pathogen with a capacity to both colonize asymptomatically and cause illnesses ranging from pharyngitis to necrotizing fasciitis. An understanding of how and when GAS switches between genetic programs governing these different lifestyles has remained an enduring mystery and likely requires carefully tuned environmental sensors to activate and silence genetic schemes when appropriate. Herein, we describe the relationship between the Control of Virulence (CovRS, CsrRS) two‐component system and the Rgg2/3 quorum‐sensing pathway. We demonstrate that responses of CovRS to the stress signals Mg²⁺ and a fragment of the antimicrobial peptide LL‐37 result in modulated activity of pheromone signaling of the Rgg2/3 pathway through a means of proteolysis of SHP peptide pheromones. This degradation is mediated by the cytoplasmic endopeptidase PepO, which is the first identified enzymatic silencer of an RRNPP‐type quorum‐sensing pathway. These results suggest that under conditions in which the virulence potential of GAS is elevated (i.e. enhanced virulence gene expression), cellular responses mediated by the Rgg2/3 pathway are abrogated and allow individuals to escape from group behavior. These results also indicate that Rgg2/3 signaling is instead functional during non‐virulent GAS lifestyles.     

A PTS EII mutant library in Group A Streptococcus identifies a promiscuous man‐family PTS transporter influencing SLS‐mediated hemolysis

The Group A Streptococcus (GAS, Streptococcus pyogenes) is a Gram‐positive human pathogen that must adapt to unique host environments in order to survive. Links between sugar metabolism and virulence have been demonstrated in GAS, where mutants in the phosphoenolpyruvate‐dependent phosphotransferase system (PTS) exhibited Streptolysin S (SLS)‐mediated hemolysis during exponential growth. This early onset hemolysis correlated with an increased lesion size and severity in a murine soft tissue infection model when compared with parental M1T1 MGAS5005. To identify the PTS components responsible for this phenotype, we insertionally inactivated the 14 annotated PTS EIIC‐encoding genes in the GAS MGAS5005 genome and subjected this library to metabolic and hemolysis assays to functionally characterize each EIIC. It was found that a few EIIs had a very limited influence on PTS sugar metabolism, whereas others were fairly promiscuous. The mannose‐specific EII locus, encoded by manLMN, was expressed as a mannose‐inducible operon that exhibited the most influence on PTS sugar metabolism, including mannose. Importantly, components of the mannose‐specific EII also acted to prevent the early onset of SLS‐mediated hemolysis. Interestingly, these roles were not identical in two different M1T1 GAS strains, highlighting the possible versatility of the PTS to adapt to strain‐specific needs.     

Virulence factor regulation and regulatory networks in Streptococcus pyogenes and their impact on pathogen-host interactions

Streptococcus pyogenes (group A streptococcus, GAS) is a very important human pathogen with remarkable adaptation capabilities. Survival within the harsh host surroundings requires sensing potential on the bacterial side, which leads in particular to coordinately regulated virulence factor expression. GAS 'stand-alone' response regulators (RRs) and two-component signal transduction systems (TCSs) link the signals from the host environment with adaptive responses of the bacterial cell. Numerous putative regulatory systems emerged from GAS genome sequences. Only three RRs [Mga, RofA-like protein (RALP) and Rgg/RopB] and three TCSs (CsrRS/CovRS, FasBCAX and Ihk/Irr) have been studied in some detail with respect to their growth-phase-dependent activity and their influence on GAS-host cell interaction. In particular, the Mga-, RALP- and Rgg/RopB-regulated pathways display interconnected activities that appear to influence GAS colonization, persistence and spreading mechanisms, in a growth-phase-related fashion. Here, we have summarized our current knowledge about these RRs and TCSs to highlight the questions that should be addressed in future research on GAS pathogenicity.     

Silencing the mob: disrupting quorum sensing as a means to fight plant disease

Bacteria are able to sense their population's density through a cell–cell communication system, termed ‘quorum sensing’ (QS). This system regulates gene expression in response to cell density through the constant production and detection of signalling molecules. These molecules commonly act as auto‐inducers through the up‐regulation of their own synthesis. Many pathogenic bacteria, including those of plants, rely on this communication system for infection of their hosts. The finding that the countering of QS‐disrupting mechanisms exists in many prokaryotic and eukaryotic organisms offers a promising novel method to fight disease. During the last decade, several approaches have been proposed to disrupt QS pathways of phytopathogens, and hence to reduce their virulence. Such studies have had varied success in vivo, but most lend promising support to the idea that QS manipulation could be a potentially effective method to reduce bacterial‐mediated plant disease. This review discusses the various QS‐disrupting mechanisms found in both bacteria and plants, as well as the different approaches applied artificially to interfere with QS pathways and thus protect plant health.     

The influence of developmental environment on courtship song in cactophilic Drosophila

Closely related species often differ in the signals involved in sexual communication and mate recognition. Determining the factors influencing signal quality (i.e. signal's content and conspicuousness) provides an important insight into the potential pathways by which these interspecific differences evolve. Host specificity could bias the direction of the evolution of sexual communication and the mate recognition system, favouring sensory channels that work best in the different host conditions. In this study, we focus on the cactophilic sibling species Drosophila buzzatii and D. koepferae that have diverged not only in the sensory channel used for sexual communication and mate recognition but also in the cactus species that use as primary hosts. We evaluate the role of the developmental environment in generating courtship song variation using an isofemale line design. Our results show that host environment during development induces changes in the courtship song of D. koepferae males, but not in D. buzzatii males. Moreover, we report for the first time that host rearing environment affects the conspicuousness of courtship song (i.e. song volume). Our results are mainly discussed in the context of the sensory drive hypothesis.     

Disruption of Bacterial Cell-to-Cell Communication by Marine Organisms and its Relevance to Aquaculture

Bacterial disease is one of the most critical problems in commercial aquaculture. Although various methods and treatments have been developed to curb the problem, yet they still have significant drawbacks. A novel and environmental-friendly approach in solving this problem is through the disruption of bacterial communication or quorum sensing (QS). In this communication scheme, bacteria regulate their own gene expression by producing, releasing, and sensing chemical signals from the environment. There seems to be a link between QS and diseases through the regulation of certain phenotypes and the induction of virulence factors responsible for pathogen–host association. Several findings have reported that numerous aquatic organisms such as micro-algae, macro-algae, invertebrates, or even other bacteria have the potential to disrupt QS. The mechanism of action varies from degradation of signals through enzymatic or chemical inactivation to antagonistic as well as agonistic activities. This review focuses on the existing marine organisms that are able to interfere with QS with potential application for aquaculture as bacterial control.     

CagA of Helicobacter pylori interacts with and inhibits the serine‐threonine kinase PRK2

CagA is a multifunctional toxin of Helicobacter pylori that is secreted into host epithelial cells by a type IV secretion system. Following host cell translocation, CagA interferes with various host–cell signalling pathways. Most notably this toxin is involved in the disruption of apical–basolateral cell polarity and cell adhesion, as well as in the induction of cell proliferation, migration and cell morphological changes. These are processes that also play an important role in epithelial‐to‐mesenchymal transition and cancer cell invasion. In fact, CagA is considered as the only known bacterial oncoprotein. The cellular effects are triggered by a variety of CagA activities including the inhibition of serine–threonine kinase Par1b/MARK2 and the activation of tyrosine phosphatase SHP‐2. Additionally, CagA was described to affect the activity of Src family kinases and C‐terminal Src kinase (Csk) suggesting that interference with multiple cellular kinase‐ and phosphatase‐associated signalling pathways is a major function of CagA. Here, we describe the effect of CagA on protein kinase C‐related kinase 2 (PRK2), which acts downstream of Rho GTPases and is known to affect cytoskeletal rearrangements and cell polarity. CagA interacts with PRK2 and inhibits its kinase activity. Because PRK2 has been linked to cytoskeletal rearrangements and establishment of cell polarity, we suggest that CagA may hijack PRK2 to further manipulate cancer‐related signalling pathways.     

Intercellular and intracellular signalling systems that globally control the expression of virulence genes in plant pathogenic bacteria

Plant pathogenic bacteria utilize complex signalling systems to control the expression of virulence genes at the cellular level and within populations. Quorum sensing (QS), an important intercellular communication mechanism, is mediated by different types of small molecules, including N‐acyl homoserine lactones (AHLs), fatty acids and small proteins. AHL‐mediated signalling systems dependent on the LuxI and LuxR family proteins play critical roles in the virulence of a wide range of Gram‐negative plant pathogenic bacteria belonging to the Alphaproteobacteria, Betaproteobacteria and Gammaproteobacteria. Xanthomonas spp. and Xylella fastidiosa, members of the Gammaproteobacteria, however, possess QS systems that are mediated by fatty acid‐type diffusible signal factors (DSFs). Recent studies have demonstrated that Ax21, a 194‐amino‐acid protein in Xanthomonas oryzae pv. oryzae, plays dual functions in activating a rice innate immune pathway through binding to the rice XA21 pattern recognition receptor and in regulating bacterial virulence and biofilm formation as a QS signal molecule. In xanthomonads, DSF‐mediated QS systems are connected with the signalling pathways mediated by cyclic diguanosine monophosphate (c‐di‐GMP), which functions as a second messenger for the control of virulence gene expression in these bacterial pathogens.     

Pathogenic mechanisms of invasive group A Streptococcus infections by influenza virus–group A Streptococcus superinfection

Group A Streptococcus (GAS) are pathogenic bacteria of the genus Streptococcus and cause severe invasive infections that comprise a wide range of diverse diseases, including acute respiratory distress syndrome, renal failure, toxic shock‐like syndrome, sepsis, cellulitis and necrotizing fasciitis. The essential virulence, infected host and external environmental factors required for invasive GAS infections have not yet been determined. Superinfection with influenza virus and GAS induced invasive GAS infections was demonstrated by our team in a mouse model, after which clinical cases of invasive GAS infections secondary to influenza virus infection were reported by other investigators in Japan, USA, Canada, UK China, and other countries. However, the pathogenic mechanisms underlying influenza virus‐GAS superinfection are not yet fully understood. The present review describes the current knowledge about invasive GAS infections by superinfection. Topics addressed include the bacteriological, virological and immunological mechanisms impacting invasion upon superinfection on top of underlying influenza virus infection by GAS and other bacteria (i.e., Streptococcus pneumoniae and Staphylococcus aureus). Future prospects are also discussed.

     

Contribution of quorum‐sensing system to hexadecane degradation and biofilm formation in Acinetobacter sp. strain DR1

Aims: To investigate roles of quorum‐sensing (QS) system in Acinetobacter sp. strain DR1 and rifampicin‐resistant variant (hereinafter DR1R). Methods and Results: The DR1 strain generated three putative acyl homoserine lactones (AHLs), while the DR1R produced only one signal and QS signal production was abrogated in the aqsI (LuxI homolog) mutant. The hexadecane‐degradation and biofilm‐formation capabilities of DR1, DR1R, and aqsI mutants were compared, along with their proteomic data. Proteomics analysis revealed that the AHL lactonase responsible for degrading QS signal was highly upregulated in both DR1R and aqsI mutant, also showed that several proteins, including ppGpp synthase, histidine kinase sensors, might be under the control of QS signalling. Interestingly, biofilm‐formation and hexadecane‐biodegradation abilities were reduced more profoundly in the aqsI mutant. These altered phenotypes of the aqsI mutant were restored via the addition of free wild‐type cell supernatant and exogenous C₁₂‐AHL. Conclusions: The QS system in strain DR1 contributes to hexadecane degradation and biofilm formation. Significance and Impact of the Study: This is the first report to demonstrate that a specific QS signal appears to be a critical factor for hexadecane degradation and biofilm formation in Acinetobacter sp. strain DR1.     

Bacterial quorum‐sensing signal IQS induces host cell apoptosis by targeting POT1–p53 signalling pathway

Pseudomonas aeruginosa, an opportunistic life‐threatening human bacterial pathogen, employs quorum‐sensing (QS) signal molecules to modulate virulence gene expression. 2‐(2‐hydroxyphenyl)‐thiazole‐4‐carbaldehyde (IQS) is a recently identified QS signal that integrates the canonical lasR‐type QS of P. aeruginosa and host phosphate stress response to fine‐tune its virulence production for a successful infection. To address the role of IQS in pathogen–host interaction, we here present that IQS inhibits host cell growth and stimulates apoptosis in a dosage‐dependent manner. By downregulating the telomere‐protecting protein POT1 in host cells, IQS activates CHK1, CHK2, and p53 in an Ataxia telangiectasia mutated (ATM)/ATM and RAD3‐related (ATR)‐dependent manner and induces DNA damage response. Overexpression of POT1 in host cells presents a resistance to IQS treatment. These results suggest a pivotal role of IQS in host apoptosis, highlighting the complexity of pathogenesis mechanisms developed by P. aeruginosa during infection.     

Interplay of classic Exp and specific Vfm quorum sensing systems on the phenotypic features of Dickeya solani strains exhibiting different virulence levels

Bacteria from the genus Dickeya cause severe symptoms on numerous economically important plants. Dickeya solani is the Dickeya species most frequently found on infected potato plants in Europe. D. solani strains from different countries show high genetic homogeneity, but significant differences in their virulence level. Dickeya species possess two quorum sensing (QS) mechanisms: the Exp system based on classic N‐acyl‐homoserine lactone (AHL) signals and a specific system depending on the production and perception of a molecule of unknown structure, Virulence Factor Modulating (VFM). To study the interplay between these two QS systems, five D. solani strains exhibiting different virulence levels were selected. Mutants were constructed by inactivating genes coding for each QS system. Double mutants were obtained by simultaneous inactivation of genes coding for both QS systems. Most of the D. solani mutants showed an attenuation of chicory maceration and a decreased production of plant cell wall‐degrading enzymes (PCWDEs) and motility, but to different degrees depending on the strain. The VFM‐QS system seems to regulate virulence in both D. solani and Dickeya dadantii, but the AHL‐QS system has greater effects in D. solani than in D. dadantii. The inactivation of both QS systems in D. solani did not reveal any additive effect on the tested features. The inactivation of vfm genes generally has a more dominant effect relative to that of exp genes. Thus, VFM‐ and AHL‐QS systems do not work in synergy to modulate the production of diverse virulence factors and the ability to macerate plant tissue.     

丁香假单胞菌中的III型分泌系统及其调控机制研究进展

丁香假单胞菌(Pseudomonas syringae)是引起许多作物病害的一种革兰氏阴性病原细菌。该细菌入侵寄主植物细胞主要通过其III型分泌系统(type III secretion system,T3SS)将效应蛋白转入到寄主真核细胞内,抑制寄主免疫功能,以达到成功侵染和定殖的目的。III型分泌系统的主调控因子RhpR/S通过感受环境信号的变化直接调控hrpR/S及其他毒力相关通路。同时III型分泌系统基因的表达也受到其他调控因子的影响,包括σ因子HrpL、双组分系统GacA/S、Lon蛋白酶、第二信使分子和环境信号等。本文在简要介绍丁香假单胞菌III型分泌系统组成和功能的基础上,综述丁香假单胞菌III型分泌系统调控机制的最新研究进展,以期为深入探究病原菌的致病机制提供参考和思路。     

Exploring ComQXPA quorum‐sensing diversity and biocontrol potential of Bacillus spp. isolates from tomato rhizoplane

Bacillus subtilis is a widespread and diverse bacterium t exhibits a remarkable intraspecific diversity of the ComQXPA quorum‐sensing (QS) system. This manifests in the existence of distinct communication groups (pherotypes) that can efficiently communicate within a group, but not between groups. Similar QS diversity was also found in other bacterial species, and its ecological and evolutionary meaning is still being explored. Here we further address the ComQXPA QS diversity among isolates from the tomato rhizoplane, a natural habitat of B. subtilis, where these bacteria likely exist in their vegetative form. Because this QS system regulates production of anti‐pathogenic and biofilm‐inducing substances such as surfactins, knowledge on cell–cell communication of this bacterium within rhizoplane is also important from the biocontrol perspective. We confirm the presence of pherotype diversity within B. subtilis strains isolated from a rhizoplane of a single plant. We also show that B. subtilis rhizoplane isolates show a remarkable diversity of surfactin production and potential plant growth promoting traits. Finally, we discover that effects of surfactin deletion on biofilm formation can be strain specific and unexpected in the light of current knowledge on its role it this process.