Monitoring of Vibrio harveyi quorum sensing activity in real time during infection of brine shrimp larvae

Total Synthesis and Antibacterial Investigations of 1-Hydroxyboivinianin A

Synthetic investigations of natural products has been instrumental in the development of novel antibacterial small molecules. 1-hydroxyboivinianin A, a lactone containing phenolic bisabolane isolated from marine sediment, has reported antibacterial activity against the aquatic pathogen Vibrio harveyi. The total synthesis of 1-hydroxyboivinianin A and its enantiomer was completed in a six-step sequence in 42 % overall yield. The synthesis leveraged a key diastereoselective nucleophilic addition with chiral imidazolidinone to establish the benzylic tertiary alcohol and intramolecular Horner-Wadsworth Emmons to furnish the lactone. Both enantiomers were found to have negligible antibacterial activity against a panel of gram-positive and negative bacteria and minimal antifungal activity against phytopathogens. Investigations of a possible in vitro lactone hydrolysis to produce an inactive linear acid led to the discovery of a spontaneous cyclization, suggesting the lactone is resistant to hydrolysis and the lactone is not degrading to produce an inactive species.

Computationally Designed Anti-LuxP DNA Aptamer Suppressed Flagellar Assembly- and Quorum Sensing-Related Gene Expression in Vibrio parahaemolyticus

(1) Background: Quorum sensing (QS) is the chemical communication between bacteria that sense chemical signals in the bacterial population to control phenotypic changes through the regulation of gene expression. The inhibition of QS has various potential applications, particularly in the prevention of bacterial infection. QS can be inhibited by targeting the LuxP, a periplasmic receptor protein that is involved in the sensing of the QS signaling molecule known as the autoinducer 2 (AI-2). The sensing of AI-2 by LuxP transduces the chemical information through the inner membrane sensor kinase LuxQ protein and activates the QS cascade. (2) Methods: An in silico approach was applied to design DNA aptamers against LuxP in this study. A method combining molecular docking and molecular dynamics simulations was used to select the oligonucleotides that bind to LuxP, which were then further characterized using isothermal titration calorimetry. Subsequently, the bioactivity of the selected aptamer was examined through comparative transcriptome analysis. (3) Results: Two aptamer candidates were identified from the ITC, which have the lowest dissociation constants (Kd) of 0.2 and 0.5 micromolar. The aptamer with the lowest Kd demonstrated QS suppression and down-regulated the flagellar-assembly-related gene expression. (4) Conclusions: This study developed an in silico approach to design an aptamer that possesses anti-QS properties.

Supplementation of ex situ produced bioflocs improves immune response against AHPND in Pacific whiteleg shrimp (Litopenaeus vannamei) postlarvae

The emergence of Vibrio diseases, including acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio spp., had resulted in heavy losses in global shrimp production. Biofloc technology is a closed aquaculture system developed as one of the sustainable solutions to increase system resilience in the shrimp industry. In this study, biofloc was formed externally (ex situ biofloc) with probiotics Bacillus sp. strain BME and Bacillus sp. strain BCE, diatom microalgae Chaetoceros calcitrans, and a consortium of nitrifying bacteria, in the ratio of 1:1:6:6 as a starter. The study showed that the ex situ biofloc supplementation in Pacific whiteleg shrimp (L. vannamei) postlarvae culture can increase the shrimp culture performance (shrimp survival and growth), reduce Vibrio counts in the water and shrimp body, and provide stimulation of the shrimp immune response through humoral immune responses, such as pattern recognition protein (C-type lectin) and melanization process (proPO). Overall, the results indicate that the supplementation of ex situ biofloc provided protection to shrimp under Vibrio infection, regardless of the timing of addition (before, simultaneously, or after addition of Vibrio sp. strain VPA). This suggests that the ex situ biofloc can be effective as a preventive and a supportive treatment against potential AHPND infection in L. vannamei postlarvae culture. Taken together, the ability of the ex situ biofloc to modulate immune-related gene expression and resistance of L. vannamei against potentially AHPND-causing Vibrio sp. strain makes it an effective aquaculture technology for infectious disease control in shrimp production with high-density and minimal water exchange culture. KEY POINTS: • Supplementation of ex situ produced biofloc in shrimp postlarvae culture. • Ex situ biofloc reduces Vibrio counts in the water and shrimp body. • Ex situ biofloc stimulates shrimp humoral immune responses and survival.

How Safe to Eat Are Raw Bivalves? Host Pathogenic and Public Health Concern Microbes within Mussels, Oysters, and Clams in Greek Markets

Raw-bivalves consumption is a wide trend in Mediterranean countries. Despite the unambiguous nutritional value of seafood, raw consumption of bivalves may involve risks that could pose a significant threat to consumers' health. Their filter-feeding behavior is responsible for the potential hosting of a wide variety of microorganisms, either pathogenic for the bivalves or public health threats. Under this prism, the current study was conducted in an effort to evaluate the risk of eating raw bivalves originating from the two biggest seafood markets in Thessaloniki, the largest production area of bivalves in Greece. Both microbiological and molecular methodologies were applied in order to assess the presence of various harmful microbes, including noroviruses, Bonamia, Marteilia, Esherichia coli, Salmonella, and Vibrio. Results indicated the presence of several Vibrio strains in the analyzed samples, of which the halophilic Vibrio harveyi was verified by 16S rRNA sequencing; other than this, no enteropathogenic Vibrio spp. was detected. Furthermore, although Esherichia coli was detected in several samples, it was mostly below the European Union (EU) legislation thresholds. Interestingly, the non-target Photobacterium damselae was also detected, which is associated with both wound infections in human and aquatic animals. Regarding host pathogenic microorganisms, apart from Vibrio harveyi, the protozoan parasite Marteilia refrigens was identified in oysters, highlighting the continuous infection of this bivalve in Greece. In conclusion, bivalves can be generally characterized as a safe-to-eat raw food, hosting more bivalve pathogenic microbes than those of public health concern.

Sodium Ascorbate as a Quorum-Sensing Inhibitor Leads to Decreased Virulence in Vibrio campbellii

Vibrio campbellii is one of the major bacterial pathogens for animals reared in aquaculture, affecting both vertebrates and invertebrates, and causes significant economic losses. It is now evident that the expressions of virulence factors in this pathogen are regulated by the density of the bacterial population. This type of regulation, termed quorum sensing (QS), is mediated by extracellular signal molecules called autoinducers. In this study, the impact of sodium ascorbate (NaAs) on the virulence of V. campbellii was investigated under both in vitro and in vivo conditions, to develop a natural anti-infective strategy to contain V. campbellii infection in aquacultured animals. Results showed that NaAs significantly decreased swimming motility, biofilm production, and the production of virulence enzymes, such as lipase, caseinase, phospholipase, and hemolysin in V. campbellii. Consistent with this, pretreatment of V. campbellii with NaAs before inoculation into the rearing water resulted in significantly increased survival of gnotobiotic brine shrimp larvae, when compared to larvae challenged with untreated V. campbellii. Furthermore, NaAs could interfere with QS-regulated bioluminescence in V. campbellii, suggesting the QS-inhibitory activity largely determines the protective effect of NaAs toward the brine shrimp. In essence, due to the potent anti-virulence effects observed in in vitro studies and the clinical brine shrimp-V. campbellii infection model, NaAs constitute a promising novel strategy for the control of V. campbellii infections in aquaculture.

Vibrio harveyi: A brief survey of general characteristics and recent epidemiological traits associated with climate change

Here we briefly review the major characteristics of the emerging pathogen Vibrio harveyi and discuss survival strategies and adaptation mechanisms underlying the capacity of this marine bacterium to thrive in natural and artificial aquatic settings. Recent studies suggest that some adaptation mechanisms can easily be acquired by V. harveyi and other members of the Vibrionaceae family owing to efficient horizontal gene transfer and elevated mutation rate. While discussing the main factors in charge of the expansion of Vibrio spp. habitats and concomitant spread of Vibrio-associated diseases under climate change, this review highlights the need for future studies able to address the joint impact of environmental and anthropogenic factors on the long-term dynamics and virulence of V. harveyi populations at the global scale.

Phloroglucinol Treatment Induces Transgenerational Epigenetic Inherited Resistance Against Vibrio Infections and Thermal Stress in a Brine Shrimp (Artemia franciscana) Model

Emerging, infectious diseases in shrimp like acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio parahaemolyticus and mortality caused by other Vibrio species such as Vibrio harveyi are worldwide related to huge economic losses in industrial shrimp production. As a strategy to prevent disease outbreaks, a plant-based phenolic compound could be used as a biocontrol agent. Here, using the brine shrimp (Artemia franciscana) as a model system, we showed that phloroglucinol treatment of the parental animals at early life stages resulted in transgenerational inherited increased resistance in their progeny against biotic stress, i.e., bacteria (V. parahaemolyticus AHPND strain and V. harveyi) and abiotic stress, i.e., lethal heat shock. Increased resistance was recorded in three subsequent generations. Innate immune-related gene expression profiles and potential epigenetic mechanisms were studied to discover the underlying protective mechanisms. Our results showed that phloroglucinol treatment of the brine shrimp parents significantly (P < 0.05) enhanced the expression of a core set of innate immune genes (DSCAM, proPO, PXN, HSP90, HSP70, and LGBP) in subsequent generations. We also demonstrated that epigenetic mechanisms such as DNA methylation, m6A RNA methylation, and histone acetylation and methylation (active chromatin marker i.e., H3K4Me3, H3K4me1, H3K27me1, H3 hyperacetylation, H3K14ac and repression marker, i.e., H3K27me3, H4 hypoacetylation) might play a role in regulation of gene expression leading toward the observed transgenerational inheritance of the resistant brine shrimp progenies. To our knowledge, this is the first report on transgenerational inheritance of a compound-induced robust protected phenotype in brine shrimp, particularly protected against AHPND caused by V. parahaemolyticus and vibriosis caused by V. harveyi. Results showed that epigenetic reprogramming is likely to play a role in the underlying mechanism.

Regulatory effects of Shewanella putrefaciens isolated from shrimp Penaeus orientalis on the virulence factors of Vibrio parahaemolyticus and evaluation of the role of quorum sensing in virulence factors regulation

As an aquatic pathogen widely present in aquatic food, Vibrio parahaemolyticus causes outbreaks of gastroenteritis across the globe. Virulence factors of V. parahaemolyticus increases with the amount of spoilage in aquatic organisms including shrimp, but mechanisms regulating its virulence factors are not well understood. In this study, five spoilage bacteria isolated from shrimp were investigated for their regulatory effects on the virulence factors including haemolysin and biofilm of V. parahaemolyticus. Among these isolates, Shewanella putrefaciens induced haemolytic activity in V. parahaemolyticus in a time-dose-temperature-dependent manner and we found the main component responsible for this effect to be the supernatant or cell-free extract of S. putrefaciens. Total haemolytic activity, expression of the thermostable direct haemolysin gene tdh and biofilm production of V. parahaemolyticus were significantly up-regulated by S. putrefaciens, but also by deletion of quorum-sensing luxM or luxS gene of V. parahaemolyticus. However, this regulation by S. putrefaciens was significantly impaired by deletion of the luxM gene, but not by deletion of the luxS gene. Further study showed that S. putrefaciens exhibited a strong degradation ability on the signalling molecule acylated homoserine lactone (AHL) synthesised by the LuxM enzyme. This study revealed a novel virulence regulatory mechanism that S. putrefaciens can significantly increase the virulence factors of V. parahaemolyticus via interfering with the luxM- type quorum-sensing signalling pathway through its AHL-degradation ability.

Quorum sensing is required for full virulence of Vibrio campbellii towards tiger grouper (Epinephelus fuscoguttatus) larvae

The link between quorum sensing in Vibrio campbellii and its virulence towards tiger grouper (Epinephelus fuscoguttatus) was investigated using V. campbellii wild type and quorum-sensing mutants with inactive quorum sensing or constitutively maximal quorum-sensing activity, and signal molecule synthase mutants. The results showed that wild-type V. campbellii is pathogenic to grouper larvae, causing more than 50% mortality after 4 days of challenge. Furthermore, the mortality of larvae challenged with the mutant with maximally active quorum sensing was significantly higher than that of larvae challenged with the wild type, whereas a higher survival was observed in the larvae challenged to the mutant with a completely inactive quorum-sensing system. Grouper larvae challenged with either the signal molecule synthase triple mutant, the harveyi autoinducer-1 (HAI-1) synthase mutant and the autoinducer-2 (AI-2) synthase mutant showed higher survival than larvae challenged with the wild type. In contrast, larvae challenged with the cholerae autoinducer-1 (CAI-1) synthase mutant showed high mortality. This indicates that HAI-1 and AI-2, but not CAI-1, are required for full virulence of V. campbellii towards grouper larvae. Our data suggest that quorum-sensing inhibition could be an effective strategy to control V. campbellii infections in tiger grouper.

Virulence-inhibitory activity of the degradation product 3-hydroxybutyrate explains the protective effect of poly-β-hydroxybutyrate against the major aquaculture pathogen Vibrio campbellii

The bacterial storage compound poly-β-hydroxybutyrate, a polymer of the short-chain fatty acid 3-hydroxybutyrate, has been reported to protect various aquatic animals from bacterial disease. In order to obtain a better mechanistic insight, we aimed to (1) investigate whether 3-hydroxybutyrate is released from poly-β-hydroxybutyrate within sterile brine shrimp larvae, (2) determine the impact of 3-hydroxybutyrate on the virulence of Vibrio campbellii to brine shrimp larvae and on its cell density in the shrimp, and (3) determine the impact of this compound on virulence factor production in the pathogen. We detected 3-hydroxybutyrate in poly-β-hydroxybutyrate-fed brine shrimp, resulting in 24 mM 3-hydroxybutyrate in the intestinal tract of shrimp reared in the presence of 1000 mg l-1 poly-β-hydroxybutyrate. We further demonstrate that this concentration of 3-hydroxybutyrate does not affect the growth of V. campbellii, whereas it decreases the production of different virulence factors, including hemolysin, phospholipase and protease activities, and swimming motility. We hypothesize that by affecting all these virulence factors at once, 3-hydroxybutyrate (and thus also poly-β-hydroxybutyrate) can exert a significant impact on the virulence of V. campbellii. This hypothesis was confirmed in a challenge test showing that 3-hydroxybutyrate protected gnotobiotic brine shrimp from pathogenic V. campbellii, without affecting the number of host-associated vibrios.

Inhibition of Steptococcus mutans biofilm formation by extracts of Tenacibaculum sp. 20J, a bacterium with wide-spectrum quorum quenching activity

Background: Previous studies have suggested the quorum sensing signal AI-2 as a potential target to prevent the biofilm formation by Streptococcus mutans, a pathogen involved in tooth decay.

Objective: To obtain inhibition of biofilm formation by S. mutans by extracts obtained from the marine bacterium Tenacibaculum sp. 20J interfering with the AI-2 quorum sensing system.

Design: The AI-2 inhibitory activity was tested with the biosensors Vibrio harveyi BB170 and JMH597. S. mutans ATCC25175 biofilm formation was monitored using impedance real-time measurements with the xCELLigence system®, confocal laser microscopy, and the crystal violet quantification method.

Results: The addition of the cell extract from Tenacibaculum sp. 20J reduced biofilm formation in S. mutans ATCC25175 by 40–50% compared to the control without significantly affecting growth. A decrease of almost 40% was also observed in S. oralis DSM20627 and S. dentisani 7747 biofilms.

Conclusions: The ability of Tenacibaculum sp. 20J to interfere with AI-2 and inhibit biofilm formation in S. mutans was demonstrated. The results indicate that the inhibition of quorum sensing processes may constitute a suitable strategy for inhibiting dental plaque formation, although additional experiments using mixed biofilm models would be required.

Quorum-Sensing Systems as Targets for Antivirulence Therapy

The development of novel therapies to control diseases caused by antibiotic-resistant pathogens is one of the major challenges we are currently facing. Many important plant, animal, and human pathogens regulate virulence by quorum sensing, bacterial cell-to-cell communication with small signal molecules. Consequently, a significant research effort is being undertaken to identify and use quorum-sensing-interfering agents in order to control diseases caused by these pathogens. In this review, an overview of our current knowledge of quorum-sensing systems of Gram-negative model pathogens is presented as well as the link with virulence of these pathogens, and recent advances and challenges in the development of quorum-sensing-interfering therapies are discussed.

Indole signalling and (micro)algal auxins decrease the virulence of Vibrio campbellii, a major pathogen of aquatic organisms

Vibrios belonging to the Harveyi clade are major pathogens of marine vertebrates and invertebrates, causing major losses in wild and cultured organisms. Despite their significant impact, the pathogenicity mechanisms of these bacteria are not yet completely understood. In this study, the impact of indole signalling on the virulence of Vibrio campbellii was investigated. Elevated indole levels significantly decreased motility, biofilm formation, exopolysaccharide production and virulence to crustacean hosts. Indole furthermore inhibited the three-channel quorum sensing system of V. campbellii, a regulatory mechanism that is required for full virulence of the pathogen. Further, indole signalling was found to interact with the stress sigma factor RpoS. Together with the observations that energy-consuming processes (motility and bioluminescence) are downregulated, and microarray-based transcriptomics demonstrating that indole decreases the expression of genes involved in energy and amino acid metabolism, the data suggest that indole is a starvation signal in V. campbellii. Finally, it was found that the auxins indole-3-acetic acid and indole-3-acetamide, which were produced by various (micro)algae sharing the aquatic environment with V. campbellii, have a similar effect as observed for indole. Auxins might, therefore, have a significant impact on the interactions between vibrios, (micro)algae and higher organisms, with major ecological and practical implications.

Nitric Oxide Regulation of H-NOX Signaling Pathways in Bacteria

Nitric oxide (NO) is a freely diffusible, radical gas that has now been established as an integral signaling molecule in eukaryotes and bacteria. It has been demonstrated that NO signaling is initiated upon ligation to the heme iron of an H-NOX domain in mammals and in some bacteria. Bacterial H-NOX proteins have been found to interact with enzymes that participate in signaling pathways and regulate bacterial processes such as quorum sensing, biofilm formation, and symbiosis. Here, we review the biochemical characterization of these signaling pathways and, where available, describe how ligation of NO to H-NOX specifically regulates the activity of these pathways and their associated bacterial phenotypes.

Identification and Initial Characterization of Prophages in Vibrio campbellii

Phages are bacteria targeting viruses and represent the most abundant biological entities on earth. Marine environments are exceptionally rich in bacteriophages, harboring a total of 4x10³⁰ viruses. Nevertheless, marine phages remain poorly characterized. Here we describe the identification of intact prophage sequences in the genome of the marine γ-proteobacterium Vibrio campbellii ATCC BAA-1116 (formerly known as V. harveyi ATCC BAA-1116), which presumably belong to the family of Myoviridae. One prophage was found on chromosome I and shows significant similarities to the previously identified phage ΦHAP-1. The second prophage region is located on chromosome II and is related to Vibrio phage kappa. Exposure of V. campbellii to mitomycin C induced the lytic cycle of two morphologically distinct phages and, as expected, extracellular DNA from induced cultures was found to be specifically enriched for the sequences previously identified as prophage regions. Heat stress (50°C, 30 min) was also found to induce phage release in V. campbellii. Notably, promoter activity of two representative phage genes indicated heterogeneous phage induction within the population.

Specific quorum sensing-disrupting activity (A QSI) of thiophenones and their therapeutic potential

Disease caused by antibiotic resistant pathogens is becoming a serious problem, both in human and veterinary medicine. The inhibition of quorum sensing, bacterial cell-to-cell communication, is a promising alternative strategy to control disease. In this study, we determined the quorum sensing-disrupting activity of 20 thiophenones towards the quorum sensing model bacterium V. harveyi. In order to exclude false positives, we propose a new parameter (A_QSI) to describe specific quorum sensing activity. A_QSI is defined as the ratio between inhibition of quorum sensing-regulated activity in a reporter strain and inhibition of the same activity when it is independent of quorum sensing. Calculation of A_QSI allowed to exclude five false positives, whereas the six most active thiophenones (TF203, TF307, TF319, TF339, TF342 and TF403) inhibited quorum sensing at 0.25 μM, with A_QSI higher than 10. Further, we determined the protective effect and toxicity of the thiophenones in a highly controlled gnotobiotic model system with brine shrimp larvae. There was a strong positive correlation between the specific quorum sensing-disrupting activity of the thiophenones and the protection of brine shrimp larvae against pathogenic V. harveyi. Four of the most active quorum sensing-disrupting thiophenones (TF 203, TF319, TF339 and TF342) were considered to be promising since they have a therapeutic potential of at least 10.

Expression and Quorum Sensing Regulation of Type III Secretion System Genes of Vibrio harveyi during Infection of Gnotobiotic Brine Shrimp

Type III secretion systems enable pathogens to inject their virulence factors directly into the cytoplasm of the host cells. The type III secretion system of Vibrio harveyi, a major pathogen of aquatic organisms and a model species in quorum sensing studies, is repressed by the quorum sensing master regulator LuxR. In this study, we found that during infection of gnotobiotic brine shrimp larvae, the expression levels of three type III secretion operons in V. harveyi increased within the first 12h after challenge and decreased again thereafter. The in vivo expression levels were highest in a mutant with a quorum sensing system that is locked in low cell density configuration (minimal LuxR levels) and lowest in a mutant with a quorum sensing system that is locked in the high cell density configuration (maximal LuxR levels), which is consistent with repression of type III secretion by LuxR. Remarkably, in vivo expression levels of the type III secretion system genes were much (> 1000 fold) higher than the in vitro expression levels, indicating that (currently unknown) host factors significantly induce the type III secretion system. Given the fact that type III secretion is energy-consuming, repression by the quorum sensing master regulators might be a mechanism to save energy under conditions where it does not provide an advantage to the cells.

Quorum sensing inhibitors from Leucetta chagosensis Dendy, 1863

Sponges are a rich source for investigation of bioactive small molecules. They have been mostly investigated for the search of new pharmacological models or therapeutic agents for the treatment of human diseases. Micro-organisms can also represent a virulent pathogen for marine invertebrates such as sponges, which need to protect themselves against these microbes. Sponges' self defence mechanisms involving dialogue molecules thus represent a pertinent research track for potent anti-infective and anti-biofilm activities such as quorum sensing inhibitors (QSIs). The investigation of the QSI crude extract of Leucetta chagosensis Dendy, 1863 led to the isolation of three new alkaloids, isonaamine D, di-isonaamidine A and leucettamine D, along with the known isonaamine A and isonaamidine A. Isonaamidine A and isonaamine D were identified as inhibitors of the three quorum sensing pathways of Vibrio harveyi (CAI-1, AI-2 and harveyi auto inducer), but isonaamidine A displayed the strongest activity on AI-2 biosensor. Both compounds are new examples of natural QSIs of V. harveyi. These results outline the importance of these secondary metabolites for their producing organisms themselves in their natural environment, as well as the potential of the marine resource for aquaculture needs.

SIGNIFICANCE AND IMPACT OF THE STUDY

A new type of quorum sensing inhibitors was isolated from the sponge Leucetta chagosensis. One of them inhibits strongly the AI-2 channel of Vibrio harveyi, a marine pathogen of special importance in aquaculture. The activity of five different related compounds, including three new natural products discovered there, was investigated leading to structure-activity relationships which are useful for the design of new quorum sensing inhibitors to control marine infectious pathogens.

The emergence of Vibrio pathogens in Europe: ecology, evolution, and pathogenesis (Paris, 11-12th March 2015)

Global change has caused a worldwide increase in reports of Vibrio-associated diseases with ecosystem-wide impacts on humans and marine animals. In Europe, higher prevalence of human infections followed regional climatic trends with outbreaks occurring during episodes of unusually warm weather. Similar patterns were also observed in Vibrio-associated diseases affecting marine organisms such as fish, bivalves and corals. Basic knowledge is still lacking on the ecology and evolutionary biology of these bacteria as well as on their virulence mechanisms. Current limitations in experimental systems to study infection and the lack of diagnostic tools still prevent a better understanding of Vibrio emergence. A major challenge is to foster cooperation between fundamental and applied research in order to investigate the consequences of pathogen emergence in natural Vibrio populations and answer federative questions that meet societal needs. Here we report the proceedings of the first European workshop dedicated to these specific goals of the Vibrio research community by connecting current knowledge to societal issues related to ocean health and food security.

Bacillus sp. LT3 improves the survival of gnotobiotic brine shrimp (Artemia franciscana) larvae challenged with Vibrio campbellii by enhancing the innate immune response and by decreasing the activity of shrimp-associated vibrios

Bacteria belonging to the genus Bacillus are amongst the most intensively studied group of bacteria for use as probiotics in aquaculture. However, the exact mechanism of action of these bacteria is often not well described, and the microbiota that are naturally present in cultures of test organisms often compromise the interpretation of the results. The present study aimed to evaluate the putative probiotic effect of Bacillus sp. LT3 in a model system with gnotobiotic brine shrimp Artemia franciscana larvae. The strain significantly increased the survival of brine shrimp larvae challenged with Vibrio campbellii when administered 6h before the challenge. Under these conditions, LT3 was able to colonize the brine shrimp gastrointestinal tract and to decrease the in vivo pathogen activity as indicated by the bioluminescence of the V. campbellii associated with brine shrimp larvae. In order to investigate the effect of the Bacillus strain on the innate immune system of the brine shrimp larvae, prophenoloxidase and transglutaminase mRNA levels were monitored, while heat shock protein 70 mRNA levels were measured as an indicator of physiological stress. Interestingly, 12h after challenge, the prophenoloxidase mRNA level in the larvae pre-treated with LT3 and challenged with V. campbellii was approximately 8-fold higher than in the other treatments. Further, a decreased mRNA level of transglutaminase gene and heat shock protein 70 gene suggested that pretreatment with LT3 results in less stress and tissue damage in the brine shrimp larvae upon V. campbellii challenge. These results indicated that Bacillus sp. LT3 could improve the survival of brine shrimp larvae when challenged with pathogenic V. campbellii, both by decreasing the in vivo activity of the pathogen and by priming the innate immune response through activating the prophenoloxidase system.

Exposure to static magnetic field stimulates quorum sensing circuit in luminescent Vibrio strains of the Harveyi clade

In this study, the evidence of electron-dense magnetic inclusions with polyhedral shape in the cytoplasm of Harveyi clade Vibrio strain PS1, a bioluminescent bacterium living in symbiosis with marine organisms, led us to investigate the behavior of this bacterium under exposure to static magnetic fields ranging between 20 and 2000 Gauss. When compared to sham-exposed, the light emission of magnetic field-exposed bacteria growing on solid medium at 18°C ±0.1°C was increased up to two-fold as a function of dose and growth phase. Stimulation of bioluminescence by magnetic field was more pronounced during the post-exponential growth and stationary phase, and was lost when bacteria were grown in the presence of the iron chelator deferoxamine, which caused disassembly of the magnetic inclusions suggesting their involvement in magnetic response. As in luminescent Vibrio spp. bioluminescence is regulated by quorum sensing, possible effects of magnetic field exposure on quorum sensing were investigated. Measurement of mRNA levels by reverse transcriptase real time-PCR demonstrated that luxR regulatory gene and luxCDABE operon coding for luciferase and fatty acid reductase complex were significantly up-regulated in magnetic field-exposed bacteria. In contrast, genes coding for a type III secretion system, whose expression was negatively affected by LuxR, were down-regulated. Up-regulation of luxR paralleled with down-regulation of small RNAs that mediate destabilization of luxR mRNA in quorum sensing signaling pathways. The results of experiments with the well-studied Vibrio campbellii strain BB120 (originally classified as Vibrio harveyi) and derivative mutants unable to synthesize autoinducers suggest that the effects of magnetic fields on quorum sensing may be mediated by AI-2, the interspecies quorum sensing signal molecule.

Quorum sensing positively regulates flagellar motility in pathogenic Vibrio harveyi

Vibrios belonging to the Harveyi clade are among the major pathogens of aquatic organisms. Quorum sensing (QS) is essential for virulence of V. harveyi towards different hosts. However, most virulence factors reported to be controlled by QS to date are negatively regulated by QS, therefore suggesting that their impact on virulence is limited. In this study, we report that QS positively regulates flagellar motility. We found that autoinducer synthase mutants showed significantly lower swimming motility than the wild type, and the swimming motility could be restored by adding synthetic signal molecules. Further, motility of a luxO mutant with inactive QS (LuxO D47E) was significantly lower than that of the wild type and of a luxO mutant with constitutively maximal QS activity (LuxO D47A). Furthermore, we found that the expression of flagellar genes (both early, middle and late genes) was significantly lower in the luxO mutant with inactive QS when compared with wild type and the luxO mutant with maximal QS activity. Motility assays and gene expression also revealed the involvement of the quorum-sensing master regulator LuxR in the QS regulation of motility. Finally, the motility inhibitor phenamil significantly decreased the virulence of V. harveyi towards gnotobiotic brine shrimp larvae.

The apparent quorum-sensing inhibitory activity of pyrogallol is a side effect of peroxide production

There currently is more and more interest in the use of natural products, such as tea polyphenols, as therapeutic agents. The polyphenol compound pyrogallol has been reported before to inhibit quorum-sensing-regulated bioluminescence in Vibrio harveyi. Here, we report that the addition of 10 mg · liter(-1) pyrogallol protects both brine shrimp (Artemia franciscana) and giant river prawn (Macrobrachium rosenbergii) larvae from pathogenic Vibrio harveyi, whereas the compound showed relatively low toxicity (therapeutic index of 10). We further demonstrate that the apparent quorum-sensing-disrupting activity is a side effect of the peroxide-producing activity of this compound rather than true quorum-sensing inhibition. Our results emphasize that verification of minor toxic effects by using sensitive methods and the use of appropriate controls are essential when characterizing compounds as being able to disrupt quorum sensing.