Interplay between two quorum sensing-regulated pathways, violacein biosynthesis and VacJ/Yrb, dictates outer membrane vesicle biogenesis in Chromobacterium violaceum

A cytochrome bd repressed by a MarR family regulator confers resistance to metals, nitric oxide, sulfide, and cyanide in Chromobacterium violaceum

Chromobacterium violaceum is a ubiquitous environmental pathogen. Despite its remarkable adaptability, little is known about the mechanisms of stress resistance in this bacterium. Here, in a screen for iron-susceptible transposon mutants, we identified a cytochrome bd that protects C. violaceum against multiple stresses. The two subunits of this cytochrome bd (CioAB) are encoded by the cioRAB operon, which also encodes a GbsR-type MarR family transcription factor (CioR). A ∆cioAB mutant strain was sensitive to iron and the iron-requiring antibiotic streptonigrin and showed a decrease in siderophore production. Growth curves and survival assays revealed that the ∆cioAB strain was also sensitive to zinc, hydrogen peroxide, nitric oxide, sulfide, and cyanide. Expression analysis showed that the promoter activity of the cioRAB operon and the transcript levels of the cioAB genes were increased in a ∆cioR mutant. CioR bound the promoter region of the cio operon in vitro, indicating that CioR is a direct repressor of its own operon. Expression of the cio operon increased at high cell density and was dependent on the quorum-sensing regulator CviR. As cyanide is also a signal for cio expression, and production of endogenous cyanide is known to be a quorum sensing-regulated trait in C. violaceum, we suggest that CioAB is a cyanide-insensitive terminal oxidase that allows respiration under cyanogenic growth conditions. Our findings indicate that the cytochrome bd CioAB protects C. violaceum against multiple stress agents that are potentially produced endogenously or during interactions with a host.

IMPORTANCE

The terminal oxidases of bacterial respiratory chains rely on heme-copper (heme-copper oxidases) or heme (cytochrome bd) to catalyze the reduction of molecular oxygen to water. Chromobacterium violaceum is a facultative anaerobic bacterium that uses oxygen and other electron acceptors for respiration under conditions of varying oxygen availability. The C. violaceum genome encodes multiple respiratory terminal oxidases, but their role and regulation remain unexplored. Here, we demonstrate that CioAB, the single cytochrome bd from C. violaceum, protects this bacterium against multiple stressors that are inhibitors of heme-copper oxidases, including nitric oxide, sulfide, and cyanide. CioAB also confers C. violaceum resistance to iron, zinc, and hydrogen peroxide. This cytochrome bd is encoded by the cioRAB operon, which is under direct repression by the MarR-type regulator CioR. In addition, the cioRAB operon responds to quorum sensing and to cyanide, suggesting a protective mechanism of increasing CioAB in the setting of high endogenous cyanide production.

A review of quorum-sensing and its role in mediating interkingdom interactions in the ocean

Quorum sensing, first described in marine systems five decades ago, is a well-characterized chemical communication system used to coordinate bacterial gene expression and behavior; however, the impact of quorum sensing on interkingdom interactions has been vastly understudied. In this review, we examine how these molecules mediate communication between bacteria and marine eukaryotes; influencing processes such as development, disease pathogenesis, and microbiome regulation within marine ecosystems. We describe the varied mechanisms eukaryotes have evolved to interfere with bacterial quorum sensing signaling, the crucial role these signals play in host-virus interactions, and how their exchange may be governed by outer membrane vesicles, prevalent in marine systems. Here, we present a dynamic portrayal of the impact of quorum sensing signals beyond bacterial communication, laying the groundwork for future investigations on their roles in shaping marine ecosystem structure and function.

Quorum quenching effects of linoleic and stearic acids on outer membrane vesicle-mediated virulence in Chromobacterium violaceum

Chromobacterium violaceum is a pathogenic bacterium that can infect humans and animals, yet the role of its outer membrane vesicles (OMVs) in mediating pathogenicity remains underexplored. This study evaluated the effects of linoleic acid (LA) and stearic acid (SA) on quorum sensing (QS)-mediated violacein production, biofilm formation, and OMV biogenesis in C. violaceum. Our findings revealed that 2 mM LA and 1 mM SA effectively quench QS, leading to a significant reduction in violacein production, biofilm formation, and OMV biogenesis. Gene expression analysis confirmed the downregulation of QS-related genes, including cviI, cviR, vioA, vioB, and vioC, in fatty acid-treated C. violaceum. Additionally, we assessed the antimicrobial activity of C. violaceum-derived OMVs on Rhizobium sp., a PGPR and observed a marked reduction in bactericidal activity in the treated OMVs. This study suggests that LA and SA have potential as anti-infective agents to mitigate OMV-mediated virulence and combat antibiotic resistance in pathogens.

Extracellular vesicles of Janthinobacterium lividum as violacein carriers in melanoma cell treatment

Violacein is a natural indole-derived purple pigment of microbial origin that has attracted attention for its remarkable biological properties. Due to its poor solubility in aqueous media, most studies of this pigment use extracts of the compound obtained with common solvents. Violacein is also transported in bacterial extracellular vesicles (EVs) and transferred via this type of carrier remains stable in an aqueous environment. This paper is the first to present an in-depth study of Janthinobacterium lividum EVs as violacein carriers. J. lividum EVs were studied for their contribution to violacein translocation, size, morphology and protein composition. The production of violacein encapsulated in EVs was more efficient than the intracellular production of this compound. The average size of the violacein-containing EVs was 124.07 ± 3.74 nm. Liquid chromatography-tandem mass spectrometry analysis (LC-MS/MS) revealed 932 proteins common to three independent EVs isolations. The high proportion of proteins with intracellular localisation, which are involved in many fundamental cellular processes, suggests that J. lividum EVs could be generated in a cell lysis model, additionally stimulated by violacein production. Using human keratinocytes and melanoma cell lines, it was confirmed that J. lividum EVs are able to react with and deliver their cargo to mammalian cells. The EVs-delivered violacein was shown to retain its activity against melanoma cells, and the dose and timing of treatment can be selected to target only cancer cells. The characterisation of J. lividum EVs, described in the following paper, represents a milestone for their future potential anticancer application. KEY POINTS: • This report focuses on the investigation of Janthinobacterium lividum EVs as a new delivery vehicle for violacein, a compound with a previously demonstrated broad spectrum of activity. • EVs were characterised for size, morphology and protein composition. • Studies on human keratinocytes and a melanoma cell model confirmed that the activity of violacein applied in the encapsulated form of EVs is similar to that of its organic solvent extract, but their production is much more environmentally friendly.

Insight into the outer membrane asymmetry of P. aeruginosa and the role of MlaA in modulating the lipidic composition, mechanical, biophysical, and functional membrane properties of the cell envelope

In Gram-negative bacteria, the outer membrane (OM) is asymmetric, with lipopolysaccharides (LPS) in the outer leaflet and glycerophospholipids (GPLs) in the inner leaflet. The asymmetry is maintained by the Mla system (MlaA-MlaBCDEF), which contributes to lipid homeostasis by removing mislocalized GPLs from the outer leaflet of the OM. Here, we ascribed how Pseudomonas aeruginosa ATCC 27853 coordinately regulates pathways to provide defense against the threats posed by the deletion of mlaA. Especially, we explored (i) the effects on membrane lipid composition including LPS, GPLs, and lysophospholipids, (ii) the biophysical properties of the OM such as stiffness and fluidity, and (iii) the impact of these changes on permeability, antibiotic susceptibility, and membrane vesicles (MVs) generation. Deletion of mlaA induced an increase in total GPLs and a decrease in LPS level while also triggering alterations in lipid A structures (arabinosylation and palmitoylation), likely to be induced by a two-component system (PhoPQ-PmrAB). Altered lipid composition may serve a physiological purpose in regulating the mechanobiological and functional properties of P. aeruginosa. We demonstrated an increase in cell stiffness without alteration of turgor pressure and inner membrane (IM) fluidity in ∆mlaA. In addition, membrane vesiculation increased without any change in OM/IM permeability. An amphiphilic aminoglycoside derivative (3',6-dinonyl neamine) that targets P. aeruginosa membranes induced an opposite effect on ∆mlaA strain with a trend toward a return to the situation observed for the WT strain. Efforts dedicated to understanding the crosstalk between the OM lipid composition, and the mechanical behavior of bacterial envelope, is one needed step for designing new targets or new drugs to fight P. aeruginosa infections.IMPORTANCEPseudomonas aeruginosa is a Gram-negative bacterium responsible for severe hospital-acquired infections. The outer membrane (OM) of Gram-negative bacteria acts as an effective barrier against toxic compounds, and therefore, compromising this structure could increase sensitivity to antibiotics. The OM is asymmetric with the highly packed lipopolysaccharide monolayer at the outer leaflet and glycerophospholipids at the inner leaflet. OM asymmetry is maintained by the Mla pathway resulting in the retrograde transport of glycerophospholipids from the OM to the inner membrane. In this study, we show that deleting mlaA, the membrane component of Mla system located at the OM, affects the mechanical and functional properties of P. aeruginosa cell envelope. Our results provide insights into the role of MlaA, involved in the Mla transport pathway in P. aeruginosa.

A cytochrome bd repressed by a MarR family regulator confers resistance to metals, nitric oxide, sulfide, and cyanide in Chromobacterium violaceum

Chromobacterium violaceum is a ubiquitous environmental pathogen. Despite its remarkable adaptability, little is known about the mechanisms of stress resistance in this bacterium. Here, in a screen for iron-susceptible transposon mutants, we identified a cytochrome bd that protects C. violaceum against multiple stresses. The two subunits of this cytochrome bd (CioAB) are encoded by the cioRAB operon, which also encodes a GbsR-type MarR family transcription factor (CioR). A Δ cioAB mutant strain was sensitive to iron and the iron-requiring antibiotic streptonigrin and showed a decrease in siderophore production. Growth curves and survival assays revealed that the Δ cioAB strain was also sensitive to zinc, hydrogen peroxide, nitric oxide, sulfide, and cyanide. Expression analysis showed that the promoter activity of the cioRAB operon and the transcript levels of the cioAB genes were increased in a Δ cioR mutant. CioR bound the promoter region of the cio operon in vitro , indicating that CioR is a direct repressor of its own operon. Expression of the cio operon increased at high cell density and was dependent on the quorum-sensing regulator CviR. As cyanide is also a signal for cio expression, and production of endogenous cyanide is known to be a quorum sensing-regulated trait in C. violaceum , we suggest that CioAB is a cyanide-insensitive terminal oxidase that allow respiration under cyanogenic growth conditions. Our findings indicate that the cytochrome bd CioAB protects C. violaceum against multiple stress agents that are potentially produced endogenously or during interactions with a host.

IMPORTANCE

The terminal oxidases of bacterial respiratory chains rely on heme-copper (heme-copper oxidases) or heme (cytochrome bd ) to catalyze reduction of molecular oxygen to water. Chromobacterium violaceum is a facultative anaerobic bacterium that uses oxygen and other electron acceptors for respiration under conditions of varying oxygen availability. The C. violaceum genome encodes multiple respiratory terminal oxidases, but their role and regulation remain unexplored. Here, we demonstrate that CioAB, the single cytochrome bd from C. violaceum , protects this bacterium against multiple stressors that are inhibitors of heme-copper oxidases, including nitric oxide, sulfide, and cyanide. CioAB also confers C. violaceum resistance to iron, zinc, and hydrogen peroxide. This cytochrome bd is encoded by the cioRAB operon, which is under direct repression by the MarR-type regulator CioR. In addition, the cioRAB operon responds to quorum sensing and to cyanide, suggesting a protective mechanism of increasing CioAB in the setting of high endogenous cyanide production.

Maintenance of bacterial outer membrane lipid asymmetry: insight into MlaA

The outer membrane (OM) of Gram-negative bacteria acts as an effective barrier to protect against toxic compounds. By nature, the OM is asymmetric with the highly packed lipopolysaccharide (LPS) at the outer leaflet and glycerophospholipids at the inner leaflet. OM asymmetry is maintained by the Mla system, in which is responsible for the retrograde transport of glycerophospholipids from the OM to the inner membrane. This system is comprised of six Mla proteins, including MlaA, an OM lipoprotein involved in the removal of glycerophospholipids that are mis-localized at the outer leaflet of the OM. Interestingly, MlaA was initially identified - and called VacJ - based on its role in the intracellular spreading of Shigella flexneri.Many open questions remain with respect to the Mla system and the mechanism involved in the translocation of mislocated glycerophospholipids at the outer leaflet of the OM, by MlaA. After summarizing the current knowledge on MlaA, we focus on the impact of mlaA deletion on OM lipid composition and biophysical properties of the OM. How changes in OM lipid composition and biophysical properties can impact the generation of membrane vesicles and membrane permeability is discussed. Finally, we explore whether and how MlaA might be a candidate for improving the activity of antibiotics and as a vaccine candidate.Efforts dedicated to understanding the relationship between the OM lipid composition and the mechanical strength of the bacterial envelope and, in turn, how such properties act against external stress, are needed for the design of new targets or drugs for Gram-negative infections.

Arginine-linked HPV-associated E7 displaying bacteria-derived outer membrane vesicles as a potent antigen-specific cancer vaccine

BACKGROUND

Bacteria-based cancer therapy have demonstrated innovative strategies to combat tumors. Recent studies have focused on gram-negative bacterial outer membrane vesicles (OMVs) as a novel cancer immunotherapy strategy due to its intrinsic properties as a versatile carrier.

METHOD

Here, we developed an Human Papillomavirus (HPV)-associated E7 antigen displaying Salmonella-derived OMV vaccine, utilizing a Poly(L-arginine) cell penetrating peptide (CPP) to enhance HPV16 E7 (aa49-67) H-2 Db and OMV affinity, termed SOMV-9RE7.

RESULTS

Due to OMV's intrinsic immunogenic properties, SOMV-9RE7 effectively activates adaptive immunity through antigen-presenting cell uptake and antigen cross-presentation. Vaccination of engineered OMVs shows immediate tumor suppression and recruitment of infiltrating tumor-reactive immune cells.

CONCLUSION

The simplicity of the arginine coating strategy boasts the versatility of immuno-stimulating OMVs that can be broadly implemented to personalized bacterial immunotherapeutic applications.

Regulation of virulence in Chromobacterium violaceum and strategies to combat it

Chromobacterium is a rod-shaped, Gram-negative, facultatively anaerobic bacteria with a cosmopolitan distribution. Just about 160 Chromobacterium violaceum incidents have been reported globally, but then once infected, it has the ability to cause deadly septicemia, and infections in the lungs, liver, brain, spleen, and lymphatic systems that might lead to death. C. violaceum produces and utilizes violacein to kill bacteria that compete with it in an ecological niche. Violacein is a hydrophobic bisindole that is delivered through an efficient transport route termed outer membrane vesicles (OMVs) through the aqueous environment. OMVs are small, spherical segments detached from the outer membrane of Gram-negative bacteria. C. violaceum OMV secretions are controlled by a mechanism called the quorum sensing system CviI/CviR, which enables cell-to-cell communication between them and regulation of various virulence factors such as biofilm formation, and violacein biosynthesis. Another virulence factor bacterial type 3 secretion system (T3SS) is divided into two types: Cpi-1 and Cpi-2. Cpi-1's needle and rod effector proteins are perhaps recognized by NAIP receptors in humans and mice, activating the NLRC4 inflammasome cascade, effectively clearing spleen infections via pyroptosis, and cytotoxicity mediated by IL-18-driven Natural killer (NK) cells in the liver. In this paper, we attempt to interrelate quorum-controlled biofilm formation, violacein production, violacein delivery by OMVs and T3SS effector protein production and host mediated immunological effects against the Cpi1 of T3SS. We suggest a research path with natural bioactive molecule like palmitic acid that can act as an anti-quorum agent by reducing the expression of virulence factors as well as an immunomodulatory agent that can augment innate immune defense by hyperactivation of NLRC4 inflammasome hence dramatically purge C. violaceum infections.

Exploring engineered vesiculation by Pseudomonas putida KT2440 for natural product biosynthesis

Pseudomonas species have become promising cell factories for the production of natural products due to their inherent robustness. Although these bacteria have naturally evolved strategies to cope with different kinds of stress, many biotechnological applications benefit from engineering of optimised chassis strains with specially adapted tolerance traits. Here, we explored the formation of outer membrane vesicles (OMV) of Pseudomonas putida KT2440. We found OMV production to correlate with the recombinant production of a natural compound with versatile beneficial properties, the tripyrrole prodigiosin. Further, several P. putida genes were identified, whose up- or down-regulated expression allowed controlling OMV formation. Finally, genetically triggering vesiculation in production strains of the different alkaloids prodigiosin, violacein, and phenazine-1-carboxylic acid, as well as the carotenoid zeaxanthin, resulted in up to three-fold increased product yields. Consequently, our findings suggest that the construction of robust strains by genetic manipulation of OMV formation might be developed into a useful tool which may contribute to improving limited biotechnological applications.

Salmonella enterica and outer membrane vesicles are current and future options for cancer treatment

Conventional cancer therapies have many limitations. In the last decade, it has been suggested that bacteria-mediated immunotherapy may circumvent the restrictions of traditional treatments. For example, Salmonella enterica is the most promising bacteria for treating cancer due to its intrinsic abilities, such as killing tumor cells, targeting, penetrating, and proliferating into the tumor. S. enterica has been genetically modified to ensure safety and increase its intrinsic antitumor efficacy. This bacterium has been used as a vector for delivering anticancer agents and as a combination therapy with chemotherapy, radiotherapy, or photothermic. Recent studies have reported the antitumor efficacy of outer membrane vesicles (OMVs) derived from S. enterica. OMVs are considered safer than attenuated bacteria and can stimulate the immune system as they comprise most of the immunogens found on the surface of their parent bacteria. Furthermore, OMVs can also be used as nanocarriers for antitumor agents. This review describes the advances in S. enterica as immunotherapy against cancer and the mechanisms by which Salmonella fights cancer. We also highlight the use of OMVs as immunotherapy and nanocarriers of anticancer agents. OMVs derived from S. enterica are innovative and promising strategies requiring further investigation.

Biotransformation Of l-Tryptophan To Produce Arcyriaflavin A With Pseudomonas putida KT2440

Natural products such as indolocarbazoles are a valuable source of highly bioactive compounds with numerous potential applications in the pharmaceutical industry. Arcyriaflavin A, isolated from marine invertebrates and slime molds, is one representative of this group and acts as a cyclin D1-cyclin-dependent kinase 4 inhibitor. To date, access to this compound has mostly relied on multi-step total synthesis. In this study, biosynthetic access to arcyriaflavin A was explored using recombinant Pseudomonas putida KT2440 based on a previously generated producer strain. We used a Design of Experiment approach to analyze four key parameters, which led to the optimization of the bioprocess. By engineering the formation of outer membrane vesicles and using an adsorbent in the culture broth, we succeeded to increase the yield of arcyriaflavin A in the cell-free supernatant, resulting in a nearly eight-fold increase in the overall production titers. Finally, we managed to scale up the bioprocess leading to a final yield of 4.7 mg arcyriaflavin A product isolated from 1 L of bacterial culture. Thus, this study showcases an integrative approach to improve biotransformation and moreover also provides starting points for further optimization of indolocarbazole production in P. putida.

Mini-Review of Biofilm Interactions with Surface Materials in Industrial Piping System

The growth of biofilm, which is caused by microorganism accumulation and growth on wetted surfaces, may damage industrial piping systems, increase maintenance and cleaning costs for the system sterilization, and even divulge the immune system into high risk. This article systematically analyzes the biofilm interactions with piping surface materials from the perspectives of physical convection, and biological and chemical adhesion. The thermodynamics of the flow, bacterial surface sensing, and bio-communication are the most critical factors for biofilm attachment. Furthermore, experimental analysis methods as well as biofilm control and removal approaches, are also included in this study. Finally, the resistance and growth of biofilm, as well as the practical and advanced methodology to control the biofilm and challenges associated with technology, are also discussed. Moreover, this paper may also offer a significant reference for the practice and strategic applications to address the biofilm resistance issues in industrial piping.

A Quorum Sensing-Regulated Type VI Secretion System Containing Multiple Nonredundant VgrG Proteins Is Required for Interbacterial Competition in Chromobacterium violaceum

The environmental pathogenic bacterium Chromobacterium violaceum kills Gram-positive bacteria by delivering violacein packed into outer membrane vesicles, but nothing is known about its contact-dependent competition mechanisms. In this work, we demonstrate that C. violaceum utilizes a type VI secretion system (T6SS) containing multiple VgrG proteins primarily for interbacterial competition. The single T6SS of C. violaceum contains six vgrG genes, which are located in the main T6SS cluster and four vgrG islands. Using T6SS core component-null mutant strains, Western blotting, fluorescence microscopy, and competition assays, we showed that the C. violaceum T6SS is active and required for competition against Gram-negative bacteria such as Pseudomonas aeruginosa but dispensable for C. violaceum infection in mice. Characterization of single and multiple vgrG mutants revealed that, despite having high sequence similarity, the six VgrGs show little functional redundancy, with VgrG3 showing a major role in T6SS function. Our coimmunoprecipitation data support a model of VgrG3 interacting directly with the other VgrGs. Moreover, we determined that the promoter activities of T6SS genes increased at high cell density, but the produced Hcp protein was not secreted under such condition. This T6SS growth phase-dependent regulation was dependent on CviR but not on CviI, the components of a C. violaceum quorum sensing (QS) system. Indeed, a ΔcviR but not a ΔcviI mutant was completely defective in Hcp secretion, T6SS activity, and interbacterial competition. Overall, our data reveal that C. violaceum relies on a QS-regulated T6SS to outcompete other bacteria and expand our knowledge about the redundancy of multiple VgrGs.

IMPORTANCE The type VI secretion system (T6SS) is a contractile nanomachine used by many Gram-negative bacteria to inject toxic effectors into adjacent cells. The delivered effectors are bound to the components of a puncturing apparatus containing the protein VgrG. The T6SS has been implicated in pathogenesis and, more commonly, in competition among bacteria. Chromobacterium violaceum is an environmental bacterium that causes deadly infections in humans. In this work, we characterized the single T6SS of C. violaceum ATCC 12472, including its six VgrG proteins, regarding its function and regulation. This previously undescribed C. violaceum T6SS is active, regulated by QS, and required for interbacterial competition instead of acute infection in mice. Among the VgrGs, VgrG3, encoded outside the main T6SS cluster, showed a major contribution to T6SS function. These results shed light on a key contact-dependent killing mechanism used by C. violaceum to antagonize other bacteria.

Comparative Genomics Reveals Insights into Induction of Violacein Biosynthesis and Adaptive Evolution in Janthinobacterium

Violacein has different bioactive properties conferring distinct selective advantages, such as defense from predation and interspecific competition. Adaptation of Janthinobacterium to diverse habitats likely leads to variation in violacein production among phylogenetically closely related species inhabiting different environments, yet genomic mechanisms and the influence of adaptive evolution underpinning violacein biosynthesis in Janthinobacterium are not clear. In this study, we performed genome sequencing, comparative genomic analysis, and phenotypic characterization to investigate genomic factors regulating violacein production in nine Janthinobacterium strains, including a type strain from soil and eight strains we isolated from terrestrial subsurface sediment and groundwater. Results show that although all nine Janthinobacterium strains are phylogenetically closely related and contain genes essential for violacein biosynthesis, they vary in carbon usage and violacein production. Sediment and groundwater strains are weak violacein producers and possess far fewer secondary metabolite biosynthesis genes, indicating genome adaptation compared to soil strains. Further examination suggests that quorum sensing (QS) may play an important role in regulating violacein in Janthinobacterium: the strains exhibiting strong potential in violacein production possess both N-acyl-homoserine lactone (AHL) QS and Janthinobacterium QS (JQS) systems in their genomes, while weaker violacein-producing strains harbor only the JQS system. Preliminary tests of spent media of two Janthinobacterium strains possessing both AHL QS and JQS systems support the potential role of AHLs in inducing violacein production in Janthinobacterium. Overall, results from this study reveal potential genomic mechanisms involved in violacein biosynthesis in Janthinobacterium and provide insights into evolution of Janthinobacterium for adaptation to oligotrophic terrestrial subsurface environment.

IMPORTANCE Phylogenetically closely related bacteria can thrive in diverse environmental habitats due to adaptive evolution. Genomic changes resulting from adaptive evolution lead to variations in cellular function, metabolism, and secondary metabolite biosynthesis. The most well-known secondary metabolite produced by Janthinobacterium is the purple-violet pigment violacein. To date, the mechanisms of induction of violacein biosynthesis in Janthinobacterium is not clear. Comparative genome analysis of closely related Janthinobacterium strains isolated from different environmental habitats not only reveals potential mechanisms involved in induction of violacein production by Janthinobacterium but also provides insights into the survival strategy of Janthinobacterium for adaptation to oligotrophic terrestrial subsurface environment.

Chemical probe of AHL modulators on quorum sensing in Gram-Negative Bacteria and as antiproliferative agents: A review

Pathogenic bacteria use an intercellular chemical communication system called quorum sensing (QS) to control the expression of cellular functions such as virulence factors, biofilm formation, toxin production, and antibiotic resistance in a manner that is highly dependent on population density. Hence, since the emergence of QS, there has been a great interest in exploiting the QS mechanism as a new drug target. Therefore, blocking the QS mechanism can be an effective strategy to control infection and solve the problem of drug resistance. So far, there is no clinically approved anti-QS drug that can disable the circuits of QS systems. This review discusses the quorum-sensing network systems and novel anti-QS inhibitors in some Gram-negative bacteria.

Multi-target drug with potential applications: violacein in the spotlight

The aim of the current review is to address updated research on a natural pigment called violacein, with emphasis on its production, biological activity and applications. New information about violacein's action mechanisms as antitumor agent and about its synergistic action in drug delivery systems has brought new alternatives for anticancer therapy. Thus, violacein is introduced as reliable drug capable of overcoming at least three cancer hallmarks, namely: proliferative signaling, cell death resistance and metastasis. In addition, antimicrobial effects on several microorganisms affecting humans and other animals turn violacein into an attractive drug to combat resistant pathogens. Emphasis is given to effects of violacein combined with different agents, such as antibiotics, anticancer agents and nanoparticles. Although violacein is well-known for many decades, it remains an attractive compound. Thus, research groups have been making continuous effort to help improving its production in recent years, which can surely enable its pharmaceutical and chemical application as multi-task compound, even in the cosmetics and food industries.

Bacterial Outer Membrane Vesicles: From Discovery to Applications

Secretion of cellular components across the plasma membrane is an essential process that enables organisms to interact with their environments. Production of extracellular vesicles in bacteria is a well-documented but poorly understood process. Outer membrane vesicles (OMVs) are produced in gram-negative bacteria by blebbing of the outer membrane. In addition to their roles in pathogenesis, cell-to-cell communication, and stress responses, OMVs play important roles in immunomodulation and the establishment and balance of the gut microbiota. In this review, we discuss the multiple roles of OMVs and the current knowledge of OMV biogenesis. We also discuss the growing and promising biotechnological applications of OMV. Expected final online publication date for the Annual Review of Microbiology, Volume 75 is October 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Biological and Physiochemical Methods of Biofilm Adhesion Resistance Control of Medical-Context Surface

The formation of biofilms on medical-context surfaces gives the EPS embedded bacterial community protection and additional advantages that planktonic cells would not have such as increased antibiotic resistance and horizontal gene transfer. Bacterial cells tend to attach to a conditioning layer after overcoming possible electrical barriers and go through two phases of attachments: reversible and irreversible. In the first, bacterial attachment to the surface is reversible and occurs quickly whilst the latter is permanent and takes place over a longer period of time. Upon reaching a certain density in the bacterial community, quorum sensing causes phenotypical changes leading to a loss in motility and the production of EPS. This position paper seeks to address the problem of bacterial adhesion and biofilm formation for the medical surfaces by comparing inhabiting physicochemical interactions and biological mechanisms. Several physiochemical methodologies (e.g. ultrasonication, alternating magnetic field and chemical surface coating) and utilizing biological mechanisms (e.g. quorum quenching and EPS degrading enzymes) were suggested. The possible strategical applications of each category were suggested and evaluated to a balanced position to possibly eliminate the adhesion and formation of biofilms on medical-context surfaces.

Outer Membrane Vesicles (OMVs) Produced by Gram-Negative Bacteria: Structure, Functions, Biogenesis, and Vaccine Application

Gram-negative bacteria produce outer membrane vesicles (OMVs) with 10 to 300 nm of diameter. The contribution of OMVs to bacterial pathogenesis is a topic of great interest, and their capacity to be combined with antigens impact in the future to the development of vaccines.

Direct RBS Engineering of the biosynthetic gene cluster for efficient productivity of violaceins in E. coli

Background

Violaceins have attracted much attention as potential targets used in medicines, food additives, insecticides, cosmetics and textiles, but low productivity was the key factor to limit their large-scale applications. This work put forward a direct RBS engineering strategy to engineer the violacein biosynthetic gene cluster cloned from Chromobacterium violaceum ATCC 12,472 to efficiently improve the fermentation titers.

Results

Through four-rounds of engineering of the native RBSs within the violaceins biosynthetic operon vioABCDE, this work apparently broke through the rate-limiting steps of intermediates conversion, resulting in 2.41-fold improvement of violaceins production compared to the titers of the starting strain Escherichia coli BL21(DE3) (Vio12472). Furthermore, by optimizing the batch-fermentation parameters including temperature, concentration of IPTG inducer and fermentation time, the maximum yield of violaceins from (BCDE)m (tnaA⁻) reached 3269.7 µM at 2 mM tryptophan in the medium. Interestingly, rather than previous reported low temperature (20 ℃), we for the first time found the RBS engineered Escherichia coli strain (BCDE)m worked better at higher temperature (30 ℃ and 37 ℃), leading to a higher-level production of violaceins.

Conclusions

To our knowledge, this is the first time that a direct RBS engineering strategy is used for the biosynthesis of natural products, having the potential for a greater improvement of the product yields within tryptophan hyperproducers and simultaneously avoiding the costly low temperature cultivation for large-scale industrial production of violaciens. This direct RBS engineering strategy could also be easily and helpfully used in engineering the native RBSs of other larger and value-added natural product biosynthetic gene clusters by widely used site-specific mutagenesis methods represented by inverse PCR or CRISPR-Cas9 techniques to increase their fermentation titers in the future.

Integrated into Environmental Biofilm Chromobacterium vaccinii Survives Winter with Support of Bacterial Community

Chromobacterium species are common in tropical and subtropical zones in environmental samples according to numerous studies. Here, we describe an environmental case of resident Chromobacterium vaccinii in biofilms associated with Carex spp. roots in Moscow region, Russia (warm-summer humid continental climate zone). We performed broad characterization of individual properties as well as surrounding context for better understanding of the premise of C. vaccinii survival during the winter season. Genome properties of isolated strains propose some insights into adaptation to habit and biofilm mode of life, including social cheaters carrying ΔluxR mutation. Isolated C. vaccinii differs from previously described strains in some biochemical properties and some basic characteristics like fatty acid composition as well as unique genome features. Despite potential to modulate membrane fluidity and presence of several genes responsible for cold shock response, isolated C. vaccinii did not survive during exposure to 4 °C, while in the complex biofilm sample, it was safely preserved for at least half a year in vitro at 4 °C. The surrounding bacterial community within the same biofilm with C. vaccinii represented a series of psychrophilic bacterial species, which may share resistance to low temperatures with other species within biofilm and provide C. vaccinii an opportunity to survive during the cold winter season.