Cytotoxic effects of the lipopolysaccharide from Pseudomonas fluorescens on neurons and glial cells

Impact of Growth Conditions on Pseudomonas fluorescens Morphology Characterized by Atomic Force Microscopy

This work is dedicated to the characterization by Atomic Force Microscopy (AFM) of Pseudomonas fluorescens, bacteria having high potential in biotechnology. They were first studied first in optimal conditions in terms of culture medium and temperature. AFM revealed a more-or-less elongated morphology with typical dimensions in the micrometer range, and an organization of the outer membrane characterized by the presence of long and randomly distributed ripples, which are likely related to the organization of lipopolysaccharides (LPS). The outer membrane also presents invaginations, some of them showing a reorganization of ripples, which could be the first sign of a bacterial stress response. In a second step, bacteria grown under unfavorable conditions were characterized. The choice of the medium appeared to be more critical in the case of the second generation of cells, the less adapted medium inducing not only changes in the membrane organization but also larger damages in bacteria. An increased growth temperature affected both the usual “swollen” morphology and the organization of the outer membrane. Here also, LPS likely contribute to membrane remodelling, which makes them potential markers to track cell state changes.

Cotton and Flax Textiles Leachables Impact Differently Cutaneous Staphylococcus aureus and Staphylococcus epidermidis Biofilm Formation and Cytotoxicity

Bacteria can bind on clothes, but the impacts of textiles leachables on cutaneous bacteria remain unknown. Here, we studied for the first time the effects of cotton and flax obtained through classical and soft ecological agriculture on the representatives S. aureus and S. epidermidis bacteria of the cutaneous microbiota. Crude flax showed an inhibitory potential on S. epidermidis bacterial lawns whereas cotton had no effect. Textile fiber leachables were produced in bacterial culture media, and these extracts were tested on S. aureus and S. epidermidis. Bacterial growth was not impacted, but investigation by the crystal violet technique and confocal microscopy showed that all extracts affected biofilm formation by the two staphylococci species. An influence of cotton and flax culture conditions was clearly observed. Flax extracts had strong inhibitory impacts and induced the formation of mushroom-like defense structures by S. aureus. Conversely, production of biosurfactant by bacteria and their surface properties were not modified. Resistance to antibiotics also remained unchanged. All textile extracts, and particularly soft organic flax, showed strong inhibitory effects on S. aureus and S. epidermidis cytotoxicity on HaCaT keratinocytes. Analysis of flax leachables showed the presence of benzyl alcohol that could partly explain the effects of flax extracts.

Adaptation of acneic and non acneic strains of Cutibacterium acnes to sebum-like environment

Cutibacterium acnes, former Proprionibacterium acnes, is a heterogeneous species including acneic bacteria such as the RT4 strain, and commensal bacteria such as the RT6 strain. These strains have been characterized by metagenomic analysis but their physiology was not investigated until now. Bacteria were grown in different media, brain heart infusion medium (BHI), reinforced clostridial medium (RCM), and in sebum like medium (SLM) specifically designed to reproduce the lipid rich environment of the sebaceous gland. Whereas the RT4 acneic strain showed maximal growth in SLM and lower growth in RCM and BHI, the RT6 non acneic strain was growing preferentially in RCM and marginally in SLM. These differences were correlated with the lipophilic surface of the RT4 strain and to the more polar surface of the RT6 strain. Both strains also showed marked differences in biofilm formation activity which was maximal for the RT4 strain in BHI and for the RT6 strain in SLM. However, cytotoxicity of both strains on HaCaT keratinocytes remained identical and limited. The RT4 acneic strain showed higher inflammatory potential than the RT6 non acneic strain, but the growth medium was without significant influence. Both bacteria were also capable to stimulate β‐defensine 2 secretion by keratinocytes but no influence of the bacterial growth conditions was observed. Comparative proteomics analysis was performed by nano LC‐MS/MS and revealed that whereas the RT4 strain only expressed triacylglycerol lipase, the principal C. acnes virulence factor, when it was grown in SLM, the RT6 strain expressed another virulence factor, the CAMP factor, exclusively when it was grown in BHI and RCM. This study demonstrates the key influence of growth conditions on virulence expression by C. acnesand suggest that acneic and non acneic strains are related to different environmental niches.

Skin-bacteria communication: Involvement of the neurohormone Calcitonin Gene Related Peptide (CGRP) in the regulation of Staphylococcus epidermidis virulence

Staphylococci can sense Substance P (SP) in skin, but this molecule is generally released by nerve terminals along with another neuropeptide, Calcitonin Gene Related Peptide (CGRP). In this study, we investigated the effects of αCGRP on Staphylococci. CGRP induced a strong stimulation of Staphylococcus epidermidis virulence with a low threshold (<10⁻¹²M) whereas Staphylococcus aureus was insensitive to CGRP. We observed that CGRP-treated S. epidermidis induced interleukin 8 release by keratinocytes. This effect was associated with an increase in cathelicidin LL37 secretion. S. epidermidis displayed no change in virulence factors secretion but showed marked differences in surface properties. After exposure to CGRP, the adherence of S. epidermidis to keratinocytes increased, whereas its internalization and biofilm formation activity were reduced. These effects were correlated with an increase in surface hydrophobicity. The DnaK chaperone was identified as the S. epidermidis CGRP-binding protein. We further showed that the effects of CGRP were blocked by gadolinium chloride (GdCl₃), an inhibitor of MscL mechanosensitive channels. In addition, GdCl₃ inhibited the membrane translocation of EfTu, the Substance P sensor. This work reveals that through interaction with specific sensors S. epidermidis integrates different skin signals and consequently adapts its virulence.

Nav Channels in Damaged Membranes

Sick excitable cells (ie, Nav channel-expressing cells injured by trauma, ischemia, inflammatory, and other conditions) typically exhibit "acquired sodium channelopathies" which, we argue, reflect bleb-damaged membranes rendering their Nav channels "leaky." The situation is excitotoxic because untreated Nav leak exacerbates bleb damage. Fast Nav inactivation (a voltage-independent process) is so tightly coupled, kinetically speaking, to the inherently voltage-dependent process of fast activation that when bleb damage accelerates and thus left-shifts macroscopic fast activation, fast inactivation accelerates to the same extent. The coupled g(V) and availability(V) processes and their window conductance regions consequently left-shift by the same number of millivolts. These damage-induced hyperpolarizing shifts, whose magnitude increases with damage intensity, are called coupled left shift (CLS). Based on past work and modeling, we discuss how to test for Nav-CLS, emphasizing the virtue of sawtooth ramp clamp. We explain that it is the inherent mechanosensitivity of Nav activation that underlies Nav-CLS. Using modeling of excitability, we show the known process of Nav-CLS is sufficient to predict a wide variety of "sick excitable cell" phenomena, from hyperexcitability through to depolarizing block. When living cells are mimicked by inclusion of pumps, mild Nav-CLS produces a wide array of burst phenomena and subthreshold oscillations. Dynamical analysis of mild damage scenarios shows how these phenomena reflect changes in spike thresholds as the pumps try to counteract the leaky Nav channels. Smart Nav inhibitors designed for sick excitable cells would target bleb-damaged membrane, buying time for cell-mediated removal or repair of Nav-bearing membrane that has become bleb-damaged (ie, detached from the cytoskeleton).

Effect of Substance P in Staphylococcus aureus and Staphylococcus epidermidis Virulence: Implication for Skin Homeostasis

Staphylococcus aureus and Staphylococcus epidermidis are two major skin associated bacteria, and Substance P (SP) is a major skin neuropeptide. Since bacteria are known to sense and response to many human hormones, we investigated the effects of SP on Staphylococci virulence in reconstructed human epidermis model and HaCaT keratinocytes. We show that SP is stimulating the virulence of S. aureus and S. epidermidis in a reconstructed human epidermis model. qRT-PCR array analysis of 64 genes expressed by keratinocytes in the response to bacterial infection revealed a potential link between the action of SP on Staphylococci and skin physiopathology. qRT-PCR and direct assay of cathelicidin and human β-defensin 2 secretion also provided that demonstration that the action of SP on bacteria is independent of antimicrobial peptide expression by keratinocytes. Considering an effect of SP on S. aureus and S. epidermidis, we observed that SP increases the adhesion potential of both bacteria on keratinocytes. However, SP modulates the virulence of S. aureus and S. epidermidis through different mechanisms. The response of S. aureus is associated with an increase in Staphylococcal Enterotoxin C2 (SEC2) production and a reduction of exolipase processing whereas in S. epidermidis the effect of SP appears mediated by a rise in biofilm formation activity. The Thermo unstable ribosomal Elongation factor Ef-Tu was identified as the SP-interacting protein in S. aureus and S. epidermidis. SP appears as an inter-kingdom communication factor involved in the regulation of bacterial virulence and essential for skin microflora homeostasis.

Possible link between Toxoplasma gondii and the anosmia associated with neurodegenerative diseases

Toxoplasma gondii is an intracellular protozoan infecting 30% to 50% of global human population. Recently, it was suggested that chronic latent neuroinflammation caused by the parasite may be responsible for the development of several neurodegenerative diseases manifesting with the loss of smell. Studies in animals inoculated with the parasite revealed cysts in various regions of the brain, including olfactory bulb. Development of behavioral changes was paralleled by the preferential persistence of cysts in defined anatomic structures of the brain, depending on the host, strain of the parasite, its virulence, and route of inoculation. Olfactory dysfunction reported in Alzheimer's disease, multiple sclerosis, and schizophrenia was frequently associated with the significantly increased serum anti-T gondii immunoglobulin G antibody levels. Damage of the olfactory system may be also at least in part responsible for the development of depression because T gondii infection worsened mood in such patients, and the olfactory bulbectomized rat serves as a model of depression.

A type VI secretion system is involved in Pseudomonas fluorescens bacterial competition

Protein secretion systems are crucial mediators of bacterial interactions with other organisms. Among them, the type VI secretion system (T6SS) is widespread in Gram-negative bacteria and appears to inject toxins into competitor bacteria and/or eukaryotic cells. Major human pathogens, such as Vibrio cholerae, Burkholderia and Pseudomonas aeruginosa, express T6SSs. Bacteria prevent self-intoxication by their own T6SS toxins by producing immunity proteins, which interact with the cognate toxins. We describe here an environmental P. fluorescens strain, MFE01, displaying an uncommon oversecretion of Hcp (hemolysin-coregulated protein) and VgrG (valine-glycine repeat protein G) into the culture medium. These proteins are characteristic components of a functional T6SS. The aim of this study was to attribute a role to this energy-consuming overexpression of the T6SS. The genome of MFE01 contains at least two hcp genes (hcp1 and hcp2), suggesting that there may be two putative T6SS clusters. Phenotypic studies have shown that MFE01 is avirulent against various eukaryotic cell models (amebas, plant or animal cell models), but has antibacterial activity against a wide range of competitor bacteria, including rhizobacteria and clinical bacteria. Depending on the prey cell, mutagenesis of the hcp2 gene in MFE01 abolishes or reduces this antibacterial killing activity. Moreover, the introduction of T6SS immunity proteins from S. marcescens, which is not killed by MFE01, protects E. coli against MFE01 killing. These findings suggest that the protein encoded by hcp2 is involved in the killing activity of MFE01 mediated by effectors of the T6SS targeting the peptidoglycan of Gram-negative bacteria. Our results indicate that MFE01 can protect potato tubers against Pectobacterium atrosepticum, which causes tuber soft rot. Pseudomonas fluorescens is often described as a major PGPR (plant growth-promoting rhizobacterium), and our results suggest that there may be a connection between the T6SS and the PGPR properties of this bacterium.

The extra-cytoplasmic function sigma factor sigX modulates biofilm and virulence-related properties in Pseudomonas aeruginosa

SigX, one of the 19 extra-cytoplasmic function sigma factors of P. aeruginosa, was only known to be involved in transcription of the gene encoding the major outer membrane protein OprF. We conducted a comparative transcriptomic study between the wildtype H103 strain and its sigX mutant PAOSX, which revealed a total of 307 differentially expressed genes that differed by more than 2 fold. Most dysregulated genes belonged to six functional classes, including the “chaperones and heat shock proteins”, “antibiotic resistance and susceptibility”, “energy metabolism”, “protein secretion/export apparatus”, and “secreted factors”, and “motility and attachment” classes. In this latter class, the large majority of the affected genes were down-regulated in the sigX mutant. In agreement with the array data, the sigX mutant was shown to demonstrate substantially reduced motility, attachment to biotic and abiotic surfaces, and biofilm formation. In addition, virulence towards the nematode Caenorhabditis elegans was reduced in the sigX mutant, suggesting that SigX is involved in virulence-related phenotypes.

Effect of GABA, a bacterial metabolite, on Pseudomonas fluorescens surface properties and cytotoxicity

Different bacterial species and, particularly Pseudomonas fluorescens, can produce gamma-aminobutyric acid (GABA) and express GABA-binding proteins. In this study, we investigated the effect of GABA on the virulence and biofilm formation activity of different strains of P. fluorescens. Exposure of a psychotropic strain of P. fluorescens (MF37) to GABA (10-5 M) increased its necrotic-like activity on eukaryotic (glial) cells, but reduced its apoptotic effect. Conversely, muscimol and bicuculline, the selective agonist and antagonist of eukaryote GABAA receptors, respectively, were ineffective. P. fluorescens MF37 did not produce biosurfactants, and its caseinase, esterase, amylase, hemolytic activity or pyoverdine productions were unchanged. In contrast, the effect of GABA was associated to rearrangements of the lipopolysaccharide (LPS) structure, particularly in the lipid A region. The surface hydrophobicity of MF37 was marginally modified, and GABA reduced its biofilm formation activity on PVC, but not on glass, although the initial adhesion was increased. Five other P. fluorescens strains were studied, and only one, MFP05, a strain isolated from human skin, showed structural differences of biofilm maturation after exposure to GABA. These results reveal that GABA can regulate the LPS structure and cytotoxicity of P. fluorescens, but that this property is specific to some strains.

Paradoxical Role of 3-Methyladenine in Pyocyanin-Induced Toxicity in 1321N1 Astrocytoma and SH-SY5Y Neuroblastoma Cells

The role of autophagy in pyocyanin (PCN)-induced toxicity in the central nervous system (CNS) remains unclear, with only evidence from our group identifying it as a mechanism underlying toxicity in 1321N1 astrocytoma cells. Therefore, the aim of this study was to further examine the role of autophagy in PCN-induced toxicity in the CNS. To achieve this, we exposed 1321N1 astrocytoma and SH-SY5Y neuroblastoma cells to PCN (0-100 μmol/L) and tested the contribution of autophagy by measuring the impact of the autophagy inhibitor 3-methyladenine (3-MA) using a series of biochemical and molecular markers. Pretreatment of 1321N1 astrocytoma cells with 3-MA (5 mmol/L) decreased the PCN-induced acidic vesicular organelle and autophagosome formation as measured using acridine orange and green fluorescent protein-LC3 -LC3 fluorescence, respectively. Furthermore, 3-MA (5 mmol/L) significantly protected 1321N1 astrocytoma cells against PCN-induced toxicity. In contrast pretreatment with 3-MA (5 mmol/L) increased PCN-induced toxicity in SH-SY5Y neuroblastoma cells. Given the influence of autophagy in inflammatory responses, we investigated whether the observed effects in this study involved inflammatory mediators. The PCN (100 μmol/L) significantly increased the production of interleukin-8 (IL-8), prostaglandin E2 (PGE2), and leukotriene B4 (LTB4) in both cell lines. Consistent with its paradoxical role in modulating PCN-induced toxicity, 3-MA (5 mmol/L) significantly reduced the PCN-induced production of IL-8, PGE2, and LTB4 in 1321N1 astrocytoma cells but augmented their production in SH-SY5Y neuroblastoma cells. In conclusion, we show here for the first time the paradoxical role of autophagy in mediating PCN-induced toxicity in 1321N1 astrocytoma and SH-SY5Y neuroblastoma cells and provide novel evidence that these actions may be mediated by effects on IL-8, PGE2, and LTB4 production.

Gamma-aminobutyric acid acts as a specific virulence regulator in Pseudomonas aeruginosa

Gamma-aminobutyric acid (GABA) is widespread in the environment and can be used by animal and plants as a communication molecule. Pseudomonas species, in particular fluorescent ones, synthesize GABA and express GABA-binding proteins. In this study, we investigated the effects of GABA on the virulence of Pseudomonas aeruginosa. While exposure to GABA (10 µM) did not modify either the growth kinetics or the motility of the bacterium, its cytotoxicity and virulence were strongly increased. The Caenorhabditis elegans 'fast killing test' model revealed that GABA acts essentially through an increase in diffusible toxin(s). GABA also modulates the biofilm formation activity and adhesion properties of PAO1. GABA has no effect on cell surface polarity, biosurfactant secretion or on the lipopolysaccharide structure. The production of several exo-enzymes, pyoverdin and exotoxin A is not modified by GABA but we observed an increase in cyanogenesis which, by itself, could explain the effect of GABA on P. aeruginosa virulence. This mechanism appears to be regulated by quorum sensing. A proteomic analysis revealed that the effect of GABA on cyanogenesis is correlated with a reduction of oxygen accessibility and an over-expression of oxygen-scavenging proteins. GABA also promotes specific changes in the expression of thermostable and unstable elongation factors Tuf/Ts involved in the interaction of the bacterium with the host proteins. Taken together, these results suggest that GABA is a physiological regulator of P. aeruginosa virulence.

Full virulence of Pseudomonas aeruginosa requires OprF

OprF is a general outer membrane porin of Pseudomonas aeruginosa, a well-known human opportunistic pathogen associated with severe hospital-acquired sepsis and chronic lung infections of cystic fibrosis patients. A multiphenotypic approach, based on the comparative study of a wild-type strain of P. aeruginosa, its isogenic oprF mutant, and an oprF-complemented strain, showed that OprF is required for P. aeruginosa virulence. The absence of OprF results in impaired adhesion to animal cells, secretion of ExoT and ExoS toxins through the type III secretion system (T3SS), and production of the quorum-sensing-dependent virulence factors pyocyanin, elastase, lectin PA-1L, and exotoxin A. Accordingly, in the oprF mutant, production of the signal molecules N-(3-oxododecanoyl)-l-homoserine lactone and N-butanoyl-l-homoserine lactone was found to be reduced and delayed, respectively. Pseudomonas quinolone signal (PQS) production was decreased, while its precursor, 4-hydroxy-2-heptylquinoline (HHQ), accumulated in the cells. Taken together, these results show the involvement of OprF in P. aeruginosa virulence, at least partly through modulation of the quorum-sensing network. This is the first study showing a link between OprF, PQS synthesis, T3SS, and virulence factor production, providing novel insights into virulence expression.

Mammalian cell-based sensor system

Use of living cells or cellular components in biosensors is receiving increased attention and opens a whole new area of functional diagnostics. The term "mammalian cell-based biosensor" is designated to biosensors utilizing mammalian cells as the biorecognition element. Cell-based assays, such as high-throughput screening (HTS) or cytotoxicity testing, have already emerged as dependable and promising approaches to measure the functionality or toxicity of a compound (in case of HTS); or to probe the presence of pathogenic or toxigenic entities in clinical, environmental, or food samples. External stimuli or changes in cellular microenvironment sometimes perturb the "normal" physiological activities of mammalian cells, thus allowing CBBs to screen, monitor, and measure the analyte-induced changes. The advantage of CBBs is that they can report the presence or absence of active components, such as live pathogens or active toxins. In some cases, mammalian cells or plasma membranes are used as electrical capacitors and cell-cell and cell-substrate contact is measured via conductivity or electrical impedance. In addition, cytopathogenicity or cytotoxicity induced by pathogens or toxins resulting in apoptosis or necrosis could be measured via optical devices using fluorescence or luminescence. This chapter focuses mainly on the type and applications of different mammalian cell-based sensor systems.

Bacterial ortholog of mammalian translocator protein (TSPO) with virulence regulating activity

The translocator protein (TSPO), previously designated as peripheral-type benzodiazepine receptor, is a protein mainly located in the outer mitochondrial membrane of eukaryotic cells. TSPO is implicated in major physiological functions and functionally associated with other proteins such as the voltage-dependent anionic channel, also designated as mitochondrial porin. Surprisingly, a TSPO-related protein was identified in the photosynthetic bacterium Rhodobacter sphaeroides but it was initially considered as a relict of evolution. In the present study we cloned a tspO gene in Pseudomonas fluorescens MF37, a non-photosynthetic eubacterium and we used bioinformatics tools to identify TSPO in the genome of 97 other bacteria. P. fluorescens TSPO was recognized by antibodies against mouse protein and by PK 11195, an artificial ligand of mitochondrial TSPO. As in eukaryotes, bacterial TSPO appears functionally organized as a dimer and the apparent Kd for PK 11195 is in the same range than for its eukaryotic counterpart. When P. fluorescens MF37 was treated with PK 11195 (10(-5) M) adhesion to living or artificial surfaces and biofilm formation activity were increased. Conversely, the apoptotic potential of bacteria on eukaryotic cells was significantly reduced. This effect of PK11195 was abolished in a mutant of P. fluorescens MF37 deficient for its major outer membrane porin, OprF. The present results demonstrate the existence of a bacterial TSPO that shares common structural and functional characteristics with its mammalian counterpart. This protein, apparently involved in adhesion and virulence, reveals the existence of a possible new inter kingdom signalling system and suggests that the human microbiome should be involuntarily exposed to the evolutionary pressure of benzodiazepines and related molecules. This discovery also represents a promising opportunity for the development of alternative antibacterial strategies.

Natriuretic peptides modify Pseudomonas fluorescens cytotoxicity by regulating cyclic nucleotides and modifying LPS structure

Background

Nervous tissues express various communication molecules including natriuretic peptides, i.e. Brain Natriuretic Peptide (BNP) and C-type Natriuretic Peptide (CNP). These molecules share structural similarities with cyclic antibacterial peptides. CNP and to a lesser extent BNP can modify the cytotoxicity of the opportunistic pathogen Pseudomonas aeruginosa. The psychrotrophic environmental species Pseudomonas fluorescens also binds to and kills neurons and glial cells, cell types that both produce natriuretic peptides. In the present study, we investigated the sensitivity of Pseudomonas fluorescens to natriuretic peptides and evaluated the distribution and variability of putative natriuretic peptide-dependent sensor systems in the Pseudomonas genus.

Results

Neither BNP nor CNP modified P. fluorescens MF37 growth or cultivability. However, pre-treatment of P. fluorescens MF37 with BNP or CNP provoked a decrease of the apoptotic effect of the bacterium on glial cells and an increase of its necrotic activity. By homology with eukaryotes, where natriuretic peptides act through receptors coupled to cyclases, we observed that cell-permeable stable analogues of cyclic AMP (dbcAMP) and cyclic GMP (8BcGMP) mimicked the effect of BNP and CNP on bacteria. Intra-bacterial concentrations of cAMP and cGMP were measured to study the involvement of bacterial cyclases in the regulation of P. fluorescens cytotoxicity by BNP or CNP. BNP provoked an increase (+49%) of the cAMP concentration in P. fluorescens, and CNP increased the intra-bacterial concentrations of cGMP (+136%). The effect of BNP and CNP on the virulence of P. fluorescens was independent of the potential of the bacteria to bind to glial cells. Conversely, LPS extracted from MF37 pre-treated with dbcAMP showed a higher necrotic activity than the LPS from untreated or 8BcGMP-pre-treated bacteria. Capillary electrophoresis analysis suggests that these different effects of the LPS may be due, at least in part, to variations in the structure of the macromolecule.

Conclusion

These observations support the hypothesis that P. fluorescens responds to natriuretic peptides through a putative sensor system coupled to a cyclase that could interfere with LPS synthesis and thereby modify the overall virulence of the micro-organism.

Comparative study of 7 fluorescent pseudomonad clinical isolates

There is some debate about the potential survival of Pseudomonas fluorescens at temperatures above 37 °C and its consequences for infectious potential, owing to the heterogeneity of clinical strains. Seven clinical strains growing at 37 °C or more were submitted for polyphasic identification; 2 were identified as Pseudomonas mosselii and 4 were precisely characterized as P. fluorescens bv. I or II. The binding indexes on glial cells of the strains identified as P. fluorescens bv. I and P. mosselii were compared with that of a reference psychrotrophic strain, P. fluorescens MF37 (bv. V). Clinical P. fluorescens had a similar adherence potential range than strain MF37. Conversely, the binding indexes for P. mosselii strains were 3 times greater than that for strain MF37. These data, and those obtained by comparing the cytotoxic activities of P. fluorescens clinical strains, suggest the existence of different virulence mechanisms, leading either to a low infectious form or to a microorganism with cytotoxic activity in the same range as that of P. mosselii or even Pseudomonas aeruginosa .

Natriuretic peptides affect Pseudomonas aeruginosa and specifically modify lipopolysaccharide biosynthesis

Natriuretic peptides of various forms are present in animals and plants, and display structural similarities to cyclic antibacterial peptides. Pretreatment of Pseudomonas aeruginosa PAO1 with brain natriuretic peptide (BNP) or C-type natriuretic peptide (CNP) increases bacterium-induced glial cell necrosis. In eukaryotes, natriuretic peptides act through receptors coupled to cyclases. We observed that stable analogs of cAMP (dibutyryl cAMP) and cGMP (8-bromo-cGMP) mimicked the effect of brain natriuretic peptide and CNP on bacteria. Further evidence for the involvement of bacterial cyclases in the regulation of P. aeruginosa PAO1 cytotoxicity by natriuretic peptides is provided by the observed doubling of intrabacterial cAMP concentration after exposure to CNP. Lipopolysaccharide (LPS) extracted from P. aeruginosa PAO1 treated with both dibutyryl cAMP and 8-bromo-cGMP induces higher levels of necrosis than LPS extracted from untreated bacteria. Capillary electrophoresis and MALDI-TOF MS analysis have shown that differences in LPS toxicity are due to specific differences in the structure of the macromolecule. Using a strain deleted in the vfr gene, we showed that the Vfr protein is essential for the effect of natriuretic peptides on P. aeruginosa PAO1 virulence. These data support the hypothesis that P. aeruginosa has a cyclic nucleotide-dependent natriuretic peptide sensor system that may affect virulence by activating the expression of Vfr and LPS biosynthesis.

Gliding arc discharge in the potato pathogen Erwinia carotovora subsp. atroseptica: mechanism of lethal action and effect on membrane-associated molecules

Gliding arc (glidarc) discharge is a physicochemical technique for decontamination at atmospheric pressure and ambient temperature. It leads to the destruction of bacterial phytopathogens responsible for important losses in industrial agriculture, namely, Erwinia spp., without the formation of resistant forms. We investigated the effect of a novel optimized prototype allowing bacterial killing without lag time. This prototype also decreases the required duration of treatment by 50%. The study of the time course effect of the process on bacterial morphology suggests that glidarc induces major alterations of the bacterial membrane. We showed that glidarc causes the release of bacterial genomic DNA. By contrast, an apparent decrease in the level of extractible lipopolysaccharide was observed; however, no changes in the electrophoretic pattern and cytotoxic activity of the macromolecule were noted. Analysis of extractible proteins from the outer membrane of the bacteria revealed that glidarc discharge induces the release of these proteins from the lipid environment, but may also be responsible for protein dimerization and/or aggregation. This effect was not observed in secreted enzymatic proteins, such as pectate lyase. Analysis of the data supports the hypothesis that the plasma generated by glidarc discharge is acting essentially through oxidative mechanisms. Furthermore, these results indicate that, in addition to effectively destroying bacteria, glidarc discharge should be used to improve the extraction of bacterial molecules.

Effect of root canal filling/sealer systems on apical endotoxin penetration: a coronal leakage evaluation

Endotoxin, elaborated by gram-negative organisms, is an important factor in apical periodontitis. The objective of this study was to evaluate the magnitude of endotoxin penetration through root canal treated teeth using a dual chamber model system. Forty-four maxillary anterior teeth were prepared endodontically and canals filled either by lateral condensation or a warm thermoplasticized technique in combination with either Roth's 801 or AH 26 sealer. Teeth were suspended in the model system with a mixed anaerobic bacterial suspension in the upper chamber and HBSS in the lower chamber. The QCL-1000 LAL assay was used to measure endotoxin at 0, 1, 7, 14, and 21 days. Response feature analysis using trapezoidal area under the curve was performed; the four treatment groups were compared using nonparametric methods. Groups differed (p = 0.028), with thermoplasticized root canal filling/Roth's 801 sealer permitting the least apical endotoxin penetration. Results suggest that Roth's 801 sealer may have a role in inhibiting endotoxin penetration.

Lipopolysaccharide induces cell death in cultured porcine myenteric neurons

Enteric bacteria execute, via lipopolysaccharide (LPS), a pathogenic role in intestinal inflammation. The effects of LPS on survival and neurotransmitter expression in cultured porcine myenteric neurons were investigated. Myenteric neurons were isolated and cultured for 6 days in medium, in LPS (100 ng/ml) with or without alpha-ketoglutarate or the nitric oxide synthase (NOS) inhibitor L-NAME, in alpha-ketoglutarate or in the NO donor SNAP. Neuronal survival and expression of vasoactive intestinal peptide (VIP) and NOS were evaluated by immunocytochemistry. Addition of LPS significantly decreased neuronal survival; only 40% survived, compared to controls run in parallel. The LPS-induced neurotoxic effect was not counteracted by the simultaneous presence of alpha-ketoglutarate or L-NAME. Either SNAP or alpha-ketoglutarate influenced neuronal survival. Culturing, particularly in the presence of LPS, markedly increased the proportion of VIP-immunoreactive neurons; NOS-immunoreactive neurons were unchanged. The reported LPS-induced neurotoxicity indicates loss of enteric neurons as a consequence of intestinal inflammation.

Bioinformatics correctly identifies many type III secretion substrates in the plant pathogen Pseudomonas syringae and the biocontrol isolate P. fluorescens SBW25

The plant pathogen Pseudomonas syringae causes disease by secreting a potentially large set of virulence proteins called effectors directly into host cells, their environment, or both, using a type III secretion system (T3SS). Most P. syringae effectors have a common upstream element called the hrp box, and their N-terminal regions have amino acids biases, features that permit their bioinformatic prediction. One of the most prominent biases is a positive serine bias. We previously used the truncated AvrRpt2(81-255) effector containing a serine-rich stretch from amino acids 81 to 100 as a T3SS reporter. Region 81 to 100 of this reporter does not contribute to the secretion or translocation of AvrRpt2 or to putative effector protein chimeras. Rather, the serine-rich region from the N-terminus of AvrRpt2 is important for protein accumulation in bacteria. Most of the N-terminal region (amino acids 15 to 100) is not essential for secretion in culture or delivery to plants. However, portions of this sequence may increase the efficiency of AvrRpt2 secretion, delivery to plants, or both. Two effectors previously identified with the AvrRpt2(81-255) reporter were secreted in culture independently of AvrRpt2, validating the use of the C terminus of AvrRpt2 as a T3SS reporter. Finally, using the reduced AvrRpt2(101-255) reporter, we confirmed seven predicted effectors from P. syringae pv. tomato DC3000, four from P. syringae pv. syringae B728a, and two from P. fluorescens SBW25.

Sequential activation of constitutive and inducible nitric oxide synthase (NOS) in rat cerebellar granule neurons by Pseudomonas fluorescens and invasive behaviour of the bacteria

Previous studies have shown that Pseudomonas fluorescens and its lipopolysaccharide (LPS) exert dose-related cytotoxic effects on neurons and glial cells. In the present work, we investigated the time course effect of P. fluorescens MF37 and its LPS on cultured rat cerebellar granule neurons. The kinetics of binding of P. fluorescens to cerebellar granule neurons is rapid and reaches a mean of 3 bacteria/cell after 5 h. As demonstrated by measurement of the concentration of nitrite in the culture medium, P. fluorescens induces a rapid stimulation (3 h) of the nitric oxide synthase (NOS) activity of the cells. In contrast, LPS extracted from P. fluorescens requires a long lag phase (24 h) before observation of an activation of NOS. Measurement of the membrane resting potential of granule neurons showed that within 3 h of incubation there was no difference of effect between the action of P. fluorescens and that of its endotoxin. Two complementary approaches allowed to demonstrate that P. fluorescens MF37 presents a rapid invasive behaviour suggesting a mobilisation of calcium in its early steps of action. The present study reveals that P. fluorescens induces the sequential activation of a constitutive calcium-dependent NOS and that of an inducible NOS activated by LPS. Our results also suggest that in P. fluorescens cytotoxicity and invasion are not mutually exclusive events.

Neuroprotective actions of eicosapentaenoic acid on lipopolysaccharide-induced dysfunction in rat hippocampus

Eicosapentaenoic acid (EPA) protects hippocampus from age-related and irradiation-induced changes that lead to impairment in synaptic function; the evidence suggests that this is due to its anti-inflammatory effects, specifically preventing changes induced by the proinflammatory cytokine, interleukin-1beta (IL-1beta). In this study, we have investigated the possibility that EPA may prevent the effects of lipopolysaccharide (LPS) administration, which have been shown to lead to deterioration of synaptic function in rat hippocampus. The data indicate that treatment of hippocampal neurones with EPA abrogated the LPS-induced increases in phosphorylation of the mitogen-activated protein kinase, c-Jun N-terminal kinase (JNK), the transcription factor, c-Jun and the mitochondrial protein, Bcl-2. In parallel, we report that intraperitoneal administration of LPS to adult rats increases phosphorylation of JNK, c-Jun and Bcl-2 in hippocampal tissue and that these changes are coupled with increased IL-1beta concentration. Treatment of rats with EPA abrogates these effects and also blocks the LPS-induced impairment in long-term potentiation in perforant path-granule cell synapses that accompanies these changes. We propose that the neuroprotective effect of EPA may be dependent on its ability to inhibit the downstream consequences of JNK activation.

Regulation of the cytotoxic effects of Pseudomonas fluorescens by growth temperature

We had previously shown that the psychrotrophic bacterium Pseudomonas fluorescens can act as a pathogen, inducing apoptosis and necrosis in neurons and glial cells. In the present study, we investigated the influence of the growth temperature of P. fluorescens on its infectious potential. Adherence of P. fluorescens to glial cells was found to be maximal with bacteria grown at a low temperature (8 degrees C). At that temperature the swimming behaviour was markedly reduced. An increase in the growth temperature to 19, 28 or 32 degrees C strongly diminished the binding of bacteria to host cells. Thus, the adhesion phenotype of P. fluorescens appears to be independent of the motility of the bacteria. The apoptotic effect of P. fluorescens, determined by morphological (nuclear condensation) and biochemical (induction of nitric oxide synthase activity) indicators, correlated well with its binding activity on glial cells. In contrast, there was a clear dissociation between maximum binding and maximal necrotic action (measured by the release of lactate dehydrogenase) observed with bacteria grown at 19 degrees C. As suggested by capillary electrophoresis analysis, the differences in apoptotic effects may be related to variations in the molecular structure of LPS originating from bacteria grown at low and high temperatures, whereas the necrotic effect, which was maximal at the optimum temperature for the secretion of exoenzymes, could reflect variations in the metabolic activity of bacteria.