Membrane fluidity homeostasis is required for tobramycin-enhanced biofilm in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen, which causes chronic infections, especially in cystic fibrosis (CF) patients where it colonizes the lungs via the build-up of biofilms. Tobramycin, an aminoglycoside, is often used to treat P. aeruginosa infections in CF patients. Tobramycin at sub-minimal inhibitory concentrations enhances both biofilm biomass and thickness in vitro; however, the mechanism(s) involved are still unknown. Herein, we show that tobramycin increases the expression and activity of SigX, an extracytoplasmic sigma factor known to be involved in the biosynthesis of membrane lipids and membrane fluidity homeostasis. The biofilm enhancement by tobramycin is not observed in a sigX mutant, and the sigX mutant displays increased membrane stiffness. Remarkably, the addition of polysorbate 80 increases membrane fluidity of sigX-mutant cells in biofilm, restoring the tobramycin-enhanced biofilm formation. Our results suggest the involvement of membrane fluidity homeostasis in biofilm development upon tobramycin exposure.IMPORTANCEPrevious studies have shown that sub-lethal concentrations of tobramycin led to an increase biofilm formation in the case of infections with the opportunistic pathogen Pseudomonas aeruginosa. We show that the mechanism involved in this phenotype relies on the cell envelope stress response, triggered by the extracytoplasmic sigma factor SigX. This phenotype was abolished in a sigX-mutant strain. Remarkably, we show that increasing the membrane fluidity of the mutant strain is sufficient to restore the effect of tobramycin. Altogether, our data suggest the involvement of membrane fluidity homeostasis in biofilm development upon tobramycin exposure.

The natriuretic peptide receptor agonist osteocrin disperses Pseudomonas aeruginosa biofilm

Biofilms are highly tolerant to antimicrobials and host immune defense, enabling pathogens to thrive in hostile environments. The diversity of microbial biofilm infections requires alternative and complex treatment strategies. In a previous work we demonstrated that the human Atrial Natriuretic Peptide (hANP) displays a strong anti-biofilm activity toward Pseudomonas aeruginosa and that the binding of hANP by the AmiC protein supports this effect. This AmiC sensor has been identified as an analog of the human natriuretic peptide receptor subtype C (h-NPRC). In the present study, we evaluated the anti-biofilm activity of the h-NPRC agonist, osteocrin (OSTN), a hormone that displays a strong affinity for the AmiC sensor at least in vitro. Using molecular docking, we identified a pocket in the AmiC sensor that OSTN reproducibly docks into, suggesting that OSTN might possess an anti-biofilm activity as well as hANP. This hypothesis was validated since we observed that OSTN dispersed established biofilm of P. aeruginosa PA14 strain at the same concentrations as hANP. However, the OSTN dispersal effect is less marked than that observed for the hANP (-61% versus -73%). We demonstrated that the co-exposure of P. aeruginosa preformed biofilm to hANP and OSTN induced a biofilm dispersion with a similar effect to that observed with hANP alone suggesting a similar mechanism of action of these two peptides. This was confirmed by the observation that OSTN anti-biofilm activity requires the activation of the complex composed by the sensor AmiC and the regulator AmiR of the ami pathway. Using a panel of both P. aeruginosa laboratory reference strains and clinical isolates, we observed that the OSTN capacity to disperse established biofilms is highly variable from one strain to another. Taken together, these results show that similarly to the hANP hormone, OSTN has a strong potential to be used as a tool to disperse P. aeruginosa biofilms.

High affinity iron uptake by pyoverdine in Pseudomonas aeruginosa involves multiple regulators besides Fur, PvdS, and FpvI

Pseudomonas aeruginosa is a Gram-negative bacterium which can cause serious infections among immune-depressed people including cystic fibrosis patients where it can colonize the lungs causing chronic infections. Iron is essential for P. aeruginosa and can be provided via three sources under aerobic conditions: its own siderophores pyochelin (PCH) and pyoverdine (PVD), xenosiderophores, or heme, respectively. Pyoverdine is the high affinity siderophore and its synthesis and uptake involve more than 30 genes organized in different operons. Its synthesis and uptake are triggered by iron scarcity via the Fur regulator and involves two extra cytoplasmic sigma factors (ECF), PvdS for the biosynthesis of PVD and FpvI for the uptake via the TonB-dependent FpvA outer membrane transporter and other periplasmic and inner membrane proteins. It appeared recently that the regulation of PVD biosynthesis and uptake involves other regulators, including other ECF factors, and LysR regulators. This is the case especially for the genes coding for periplasmic and inner membrane proteins involved in the reduction of Fe3+ to Fe2+ and the transport of ferrous iron to the cytoplasm that appears to represent a crucial step in the uptake process.

Effect of Phthalates and Their Substitutes on the Physiology of Pseudomonas aeruginosa

Phthalates are used in a variety of applications—for example, as plasticizers in polyvinylchloride products to improve their flexibility—and can be easily released into the environment. In addition to being major persistent organic environmental pollutants, some phthalates are responsible for the carcinogenicity, teratogenicity, and endocrine disruption that are notably affecting steroidogenesis in mammals. Numerous studies have thus focused on deciphering their effects on mammals and eukaryotic cells. While multicellular organisms such as humans are known to display various microbiota, including all of the microorganisms that may be commensal, symbiotic, or pathogenic, few studies have aimed at investigating the relationships between phthalates and bacteria, notably regarding their effects on opportunistic pathogens and the severity of the associated pathologies. Herein, the effects of phthalates and their substitutes were investigated on the human pathogen, Pseudomonas aeruginosa, in terms of physiology, virulence, susceptibility to antibiotics, and ability to form biofilms. We show in particular that most of these compounds increased biofilm formation, while some of them enhanced the bacterial membrane fluidity and altered the bacterial morphology.

Impact of Carbon Source Supplementations on Pseudomonas aeruginosa Physiology

Pseudomonas aeruginosa is an opportunistic pathogen highly resistant to a wide range of antimicrobial agents, making its infections very difficult to treat. Since microorganisms need to perpetually adapt to their surrounding environment, understanding the effect of carbon sources on P. aeruginosa physiology is therefore essential to avoid increasing drug-resistance and better fight this pathogen. By a global proteomic approach and phenotypic assays, we investigated the impact of various carbon source supplementations (glucose, glutamate, succinate, and citrate) on the physiology of the P. aeruginosa PA14 strain. A total of 581 proteins were identified as differentially expressed in the 4 conditions. Most of them were more abundant in citrate supplementation and were involved in virulence, motility, biofilm development, and antibiotic resistance. Phenotypic assays were performed to check these hypotheses. By coupling all this data, we highlight the importance of the environment in which the bacterium evolves on its metabolism, and thus the necessity to better understand the metabolic pathways implied in its adaptative response according to the nutrient availability.

Pseudomonas aeruginosa Biofilm Dispersion by the Human Atrial Natriuretic Peptide

Pseudomonas aeruginosa biofilms cause chronic, antibiotic tolerant infections in wounds and lungs. Numerous recent studies demonstrate that bacteria can detect human communication compounds through specific sensor/receptor tools that modulate bacterial physiology. Consequently, interfering with these mechanisms offers an exciting opportunity to directly affect the infection process. It is shown that the human hormone Atrial Natriuretic Peptide (hANP) both prevents the formation of P. aeruginosa biofilms and strongly disperses established P. aeruginosa biofilms. This hANP action is dose-dependent with a strong effect at low nanomolar concentrations and takes effect in 30-120 min. Furthermore, although hANP has no antimicrobial effect, it acts as an antibiotic adjuvant. hANP enhances the antibiofilm action of antibiotics with diverse modes of action, allowing almost full biofilm eradication. The hANP effect requires the presence of the P. aeruginosa sensor AmiC and the AmiR antiterminator regulator, indicating a specific mode of action. These data establish the activation of the ami pathway as a potential mechanism for P. aeruginosa biofilm dispersion. hANP appears to be devoid of toxicity, does not enhance bacterial pathogenicity, and acts synergistically with antibiotics. These data show that hANP is a promising powerful antibiofilm weapon against established P. aeruginosa biofilms in chronic infections.

Tackling Pseudomonas aeruginosa Virulence by Mulinane-Like Diterpenoids from Azorella atacamensis

Pseudomonas aeruginosa is an important multidrug-resistant human pathogen by dint of its high intrinsic, acquired, and adaptive resistance mechanisms, causing great concern for immune-compromised individuals and public health. Additionally, P. aeruginosa resilience lies in the production of a myriad of virulence factors, which are known to be tightly regulated by the quorum sensing (QS) system. Anti-virulence therapy has been adopted as an innovative alternative approach to circumvent bacterial antibiotic resistance. Since plants are known repositories of natural phytochemicals, herein, we explored the anti-virulence potential of Azorella atacamensis, a medicinal plant from the Taira Atacama community (Calama, Chile), against P. aeruginosa. Interestingly, A. atacamensis extract (AaE) conferred a significant protection for human lung cells and Caenorhabditis elegans nematodes towards P. aeruginosa pathogenicity. The production of key virulence factors was decreased upon AaE exposure without affecting P. aeruginosa growth. In addition, AaE was able to decrease QS-molecules production. Furthermore, metabolite profiling of AaE and its derived fractions achieved by combination of a molecular network and in silico annotation allowed the putative identification of fourteen diterpenoids bearing a mulinane-like skeleton. Remarkably, this unique interesting group of diterpenoids seems to be responsible for the interference with virulence factors as well as on the perturbation of membrane homeostasis of P. aeruginosa. Hence, there was a significant increase in membrane stiffness, which appears to be modulated by the cell wall stress response ECFσ SigX, an extracytoplasmic function sigma factor involved in membrane homeostasis as well as P. aeruginosa virulence.

The Temperature-Regulation of Pseudomonas aeruginosa cmaX-cfrX-cmpX Operon Reveals an Intriguing Molecular Network Involving the Sigma Factors AlgU and SigX

Pseudomonas aeruginosa is a highly adaptable Gram-negative opportunistic pathogen, notably due to its large number of transcription regulators. The extracytoplasmic sigma factor (ECFσ) AlgU, responsible for alginate biosynthesis, is also involved in responses to cell wall stress and heat shock via the RpoH alternative σ factor. The SigX ECFσ emerged as a major regulator involved in the envelope stress response via membrane remodeling, virulence and biofilm formation. However, their functional interactions to coordinate the envelope homeostasis in response to environmental variations remain to be determined. The regulation of the putative cmaX-cfrX-cmpX operon located directly upstream sigX was investigated by applying sudden temperature shifts from 37°C. We identified a SigX- and an AlgU- dependent promoter region upstream of cfrX and cmaX, respectively. We show that cmaX expression is increased upon heat shock through an AlgU-dependent but RpoH independent mechanism. In addition, the ECFσ SigX is activated in response to valinomycin, an agent altering the membrane structure, and up-regulates cfrX-cmpX transcription in response to cold shock. Altogether, these data provide new insights into the regulation exerted by SigX and networks that are involved in maintaining envelope homeostasis.

Membrane-Interactive Compounds From Pistacia lentiscus L. Thwart Pseudomonas aeruginosa Virulence

Pseudomonas aeruginosa is capable to deploy a collection of virulence factors that are not only essential for host infection and persistence, but also to escape from the host immune system and to become more resistant to drug therapies. Thus, developing anti-virulence agents that may directly counteract with specific virulence factors or disturb higher regulatory pathways controlling the production of virulence armories are urgently needed. In this regard, this study reports that Pistacia lentiscus L. fruit cyclohexane extract (PLFE1) thwarts P. aeruginosa virulence by targeting mainly the pyocyanin pigment production by interfering with 4-hydroxy-2-alkylquinolines molecules production. Importantly, the anti-virulence activity of PLFE1 appears to be associated with membrane homeostasis alteration through the modulation of SigX, an extracytoplasmic function sigma factor involved in cell wall stress response. A thorough chemical analysis of PLFE1 allowed us to identify the ginkgolic acid (C17:1) and hydroginkgolic acid (C15:0) as the main bioactive membrane-interactive compounds responsible for the observed increased membrane stiffness and anti-virulence activity against P. aeruginosa. This study delivers a promising perspective for the potential future use of PLFE1 or ginkgolic acid molecules as an adjuvant therapy to fight against P. aeruginosa infections.

Understanding Ciprofloxacin Failure in Pseudomonas aeruginosa Biofilm: Persister Cells Survive Matrix Disruption

Biofilms are commonly recalcitrant to antibiotics, through incompletely elucidated mechanisms such as tolerance and persistence. We aimed at investigating how a Pseudomonas aeruginosa biofilm escapes ciprofloxacin treatment. P. aeruginosa PA14 in vitro mature biofilms were challenged with supra-MIC ciprofloxacin concentrations. Cell viability was quantified by fluorescein diacetate assay. Population dynamics were determined by counts of surviving culturable cells. Biofilms were analyzed using confocal laser scanning microscopy (CLSM), and the expression of genes involved in stringent response, toxin-antitoxin HigB/HigA, and type 3 secretion system (T3SS) was quantified by RT-qPCR in untreated and treated biofilms. Ciprofloxacin exposure resulted in an initial reduction of bacterial counts following a biphasic time-kill curve. After 24 h of treatment, the overall cell activity and the density of culturable cells significantly decreased as compared to untreated biofilm. No resistant mutant was isolated among the <1% surviving cells. Phenotypic adaptation toward persistence appeared to start after only 1 h of antibiotic exposure, by an overexpression of the genes involved in stringent response and in the toxin-antitoxin system, whereas the expression of genes encoding for the T3SS remained unchanged. After 4 h of ciprofloxacin exposure, stringent response genes returned to their basal level of expression. After a prolonged ciprofloxacin exposure, a deep alteration in the matrix structure that became thinner and lost mushroom-like aggregates was observed, in relation with reduced biovolumes of exopolysaccharides and extracellular DNA. These results support that ciprofloxacin might first induce the bacterial killing of most bacterial cells, but simultaneously activate stringent response mechanisms contributing to the switch of a subpopulation toward a persister phenotype. Once the persister phenotype is expressed, and despite an unexpected alteration of the biofilm matrix, ciprofloxacin fails to eradicate biofilm.

The absence of SigX results in impaired carbon metabolism and membrane fluidity in Pseudomonas aeruginosa

In Pseudomonas aeruginosa, SigX is an extra-cytoplasmic function σ factor that belongs to the cell wall stress response network. In previous studies, we made the puzzling observation that sigX mutant growth was severely affected in rich lysogeny broth (LB) but not in minimal medium. Here, through comparative transcriptomic and proteomic analysis, we show that the absence of SigX results in dysregulation of genes, whose products are mainly involved in transport, carbon and energy metabolisms. Production of most of these genes is controlled by carbon catabolite repression (CCR), a key regulatory system than ensures preferential carbon source uptake and utilization, substrate prioritization and metabolism. The strong CCR response elicited in LB was lowered in a sigX mutant, suggesting altered nutrient uptake. Since the absence of SigX affects membrane composition and fluidity, we suspected membrane changes to cause such phenotype. The detergent polysorbate 80 (PS80) can moderately destabilize the envelope resulting in non-specific increased nutrient intake. Remarkably, growth, membrane fluidity and expression of dysregulated genes in the sigX mutant strain were restored in LB supplemented with PS80. Altogether, these data suggest that SigX is indirectly involved in CCR regulation, possibly via its effects on membrane integrity and fluidity.

In vitro Assessment of the Probiotic Properties and Bacteriocinogenic Potential of Pediococcus pentosaceus MZF16 Isolated From Artisanal Tunisian Meat "Dried Ossban"

Pediococcus pentosaceus MZF16 has been isolated from artisanal Tunisian meat so called "Dried Ossban," an original ecological niche, and identified by MALDI-TOF mass spectrometry and 16S rDNA sequencing. This bacterium showed a high tolerance to gastric stress conditions, and toward bile salts. P. pentosaceus MZF16 also demonstrated a hydrophobic surface profile (high adhesion to xylene), autoaggregation, and adhesive abilities to the human intestinal Caco-2/TC7 cell line. These properties may help the bacterium colonizing the gut. Furthermore, MZF16 was found to be resistant to gentamycin and chloramphenicol but did not harbor any transferable resistance determinants and/or virulence genes. The data also demonstrated absence of cytotoxicity of this strain. Conversely, P. pentosaceus MZF16 can slightly stimulate the immune system and enhance the intestinal epithelial barrier function. Moreover, this bacterium has been shown to be highly active against Listeria spp. due to bacteriocin production. Characterization of the bacteriocin by PCR amplification, sequencing and bioinformatic analyses revealed that MZF16 produces a bacteriocin 100% identical to coagulin, a pediocin-like inhibitory substance produced by Bacillus coagulans. To our knowledge, this is the first report that highlights the production of a pediocin 100% identical to coagulin in a Pediococcus strain. As coagulin, pediocin MZF16 has the consensus sequence YYGNGVXCXXXXCXVXXXXA (X denotes any amino acid), which confirms its belonging to class IIa bacteriocins, and its suitability to preserve foods from Listeria monocytogenes development. According to these results, P. pentosaceus MZF16 can be proposed as a probiotic and bioprotective agent for fermented foods, including Tunisian dry meat and sausages. Further investigations will aim to study the behavior of this strain in meat products as a component of functional food.

Host Peptidic Hormones Affecting Bacterial Biofilm Formation and Virulence

Bacterial biofilms constitute a critical problem in hospitals, especially in resuscitation units or for immunocompromised patients, since bacteria embedded in their own matrix are not only protected against antibiotics but also develop resistant variant strains. In the last decade, an original approach to prevent biofilm formation has consisted of studying the antibacterial potential of host communication molecules. Thus, some of these compounds have been identified for their ability to modify the biofilm formation of both Gram-negative and Gram-positive bacteria. In addition to their effect on biofilm production, a detailed study of the mechanism of action of these human hormones on bacterial physiology has allowed the identification of new bacterial pathways involved in biofilm formation. In this review, we focus on the impact of neuropeptidic hormones on bacteria, address some future therapeutic issues, and provide a new view of inter-kingdom communication.

Extracytoplasmic function sigma factors in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa, like all members of the genus Pseudomonas, has the capacity to thrive in very different environments, ranging from water, plant roots, to animals, including humans to whom it can cause severe infections. This remarkable adaptability is reflected in the number of transcriptional regulators, including sigma factors in this bacterium. Among those, the 19 to 21 extracytoplasmic sigma factors (ECFσ) are endowed with different regulons and functions, including the iron starvation σ (PvdS, FpvI, HasI, FecI, FecI2 and others), the cell wall stress ECFσ AlgU, SigX and SbrI, and the unorthodox σ^VreI involved in the expression of virulence. Recently published data show that these ECFσ have separate regulons although presenting some cross-talk. We will present evidence that these different ECFσ are involved in the expression of different phenotypes, ranging from cell-wall stress response, production of extracellular polysaccharides, formation of biofilms, to iron acquisition.

Different Dose-Dependent Modes of Action of C-Type Natriuretic Peptide on Pseudomonas aeruginosa Biofilm Formation

We have previously shown that the C-type Natriuretic Peptide (CNP), a peptide produced by lungs, is able to impact Pseudomonasaeruginosa physiology. In the present work, the effect of CNP at different concentrations on P. aeruginosa biofilm formation was studied and the mechanisms of action of this human hormone on P. aeruginosa were deciphered. CNP was shown to inhibit dynamic biofilm formation in a dose-dependent manner without affecting the bacterial growth at any tested concentrations. The most effective concentrations were 1 and 0.1 µM. At 0.1 µM, the biofilm formation inhibition was fully dependent on the CNP sensor protein AmiC, whereas it was only partially AmiC-dependent at 1 µM, revealing the existence of a second AmiC-independent mode of action of CNP on P. aeruginosa. At 1 µM, CNP reduced both P. aeruginosa adhesion on glass and di-rhamnolipid production and also increased the bacterial membrane fluidity. The various effects of CNP at 1 µM and 0.1 µM on P. aeruginosa shown here should have major consequences to design drugs for biofilm treatment or prevention.

Structure, function and regulation of Pseudomonas aeruginosa porins

Pseudomonas aeruginosa is a Gram-negative bacterium belonging to the γ-proteobacteria. Like other members of the Pseudomonas genus, it is known for its metabolic versatility and its ability to colonize a wide range of ecological niches, such as rhizosphere, water environments and animal hosts, including humans where it can cause severe infections. Another particularity of P. aeruginosa is its high intrinsic resistance to antiseptics and antibiotics, which is partly due to its low outer membrane permeability. In contrast to Enterobacteria, pseudomonads do not possess general diffusion porins in their outer membrane, but rather express specific channel proteins for the uptake of different nutrients. The major outer membrane 'porin', OprF, has been extensively investigated, and displays structural, adhesion and signaling functions while its role in the diffusion of nutrients is still under discussion. Other porins include OprB and OprB2 for the diffusion of glucose, the two small outer membrane proteins OprG and OprH, and the two porins involved in phosphate/pyrophosphate uptake, OprP and OprO. The remaining nineteen porins belong to the so-called OprD (Occ) family, which is further split into two subfamilies termed OccD (8 members) and OccK (11 members). In the past years, a large amount of information concerning the structure, function and regulation of these porins has been published, justifying why an updated review is timely.

Proteomic Response of Pseudomonas aeruginosa PAO1 Adhering to Solid Surfaces

Pseudomonas aeruginosa is a pathogenic micro-organism responsible for many hospital-acquired infections. It is able to adhere to solid surfaces and develop an immobilized community or so-called biofilm. Many studies have been focusing on the use of specific materials to prevent the formation of these biofilms, but the reactivity of the bacteria in contact to surfaces remains unknown. The aim of this study was to evaluate the impact of the abiotic surface on the physiology of adherent bacteria. Three different materials, stainless steel (SS), glass (G), and polystyrene (PS) that were relevant to industrial or medical environments were characterized at the physicochemical level in terms of their hydrophobicity and roughness. We showed that SS was moderately hydrophilic and rough, potentially containing crevices, G was hydrophilic and smooth while PS was hydrophobic and smooth. We further showed that P. aeruginosa cells were more likely able to adhere to SS and G rather than PS surfaces under our experimental conditions. The physiological response of P. aeruginosa when adhering to each of these materials was then evaluated by global proteomic analysis. The abundance of 70 proteins was shown to differ between the materials suggesting that their abundance was modified as a function of the material to which bacteria adhered. Our data lead to enabling the identification of abundance patterns that appeared to be specific to a given surface. Taken together, our data showed that P. aeruginosa is capable of sensing and responding to a surface probably via specific programmes to adapt its physiological response accordingly.

The outer membrane porin OmpW of Acinetobacter baumannii is involved in iron uptake and colistin binding

This study was undertaken to characterize functions of the outer membrane protein OmpW, which potentially contributes to the development of colistin- and imipenem-resistance in Acinetobacter baumannii. Reconstitution of OmpW in artificial lipid bilayers showed that it forms small channels (23 pS in 1 m KCl) and markedly interacts with iron and colistin, but not with imipenem. In vivo, (55) Fe uptake assays comparing the behaviours of ΔompW mutant and wild-type strains confirmed a role for OmpW in A. baumannii iron homeostasis. However, the loss of OmpW expression did not have an impact on A. baumannii susceptibilities to colistin or imipenem.

Expression of the translocator protein (TSPO) from Pseudomonas fluorescens Pf0-1 requires the stress regulatory sigma factors AlgU and RpoH

The translocator protein (TSPO), previously designated as peripheral-type benzodiazepine receptor, is an evolutionary conserved protein that is found in many Eukarya, Archae, and Bacteria, in which it plays several important functions including for example membrane biogenesis, signaling, and stress response. A tspo homolog gene has been identified in several members of the Pseudomonas genus, among which the soil bacterium P. fluorescens Pf0-1. In this bacterium, the tspo gene is located in the vicinity of a putative hybrid histidine kinase-encoding gene. Since tspo has been involved in water stress related response in plants, we explored the effects of hyperosmolarity and temperature on P. fluorescens Pf0-1 tspo expression using a strategy based on lux-reporter fusions. We show that the two genes Pfl01_2810 and tspo are co-transcribed forming a transcription unit. The expression of this operon is growth phase-dependent and is increased in response to high concentrations of NaCl, sucrose and to a D-cycloserine treatment, which are conditions leading to activity of the major cell wall stress responsive extracytoplasmic sigma factor AlgU. Interestingly, the promoter region activity is strongly lowered in a P. aeruginosa algU mutant, suggesting that AlgU may be involved at least partly in the molecular mechanism leading to Pfl01_2810-tspo expression. In silico analysis of this promoter region failed to detect an AlgU consensus binding site; however, a putative binding site for the heat shock response RpoH sigma factor was detected. Accordingly, the promoter activity of the region containing this sequence is increased in response to high growth temperature and slightly lowered in a P. aeruginosa rpoH mutant strain. Taken together, our data suggest that P. fluorescens tspo gene may belong at least partly to the cell wall stress response.

The absence of the Pseudomonas aeruginosa OprF protein leads to increased biofilm formation through variation in c-di-GMP level

OprF is the major outer membrane porin in bacteria belonging to the Pseudomonas genus. In previous studies, we have shown that OprF is required for full virulence expression of the opportunistic pathogen Pseudomonas aeruginosa. Here, we describe molecular insights on the nature of this relationship and report that the absence of OprF leads to increased biofilm formation and production of the Pel exopolysaccharide. Accordingly, the level of c-di-GMP, a key second messenger in biofilm control, is elevated in an oprF mutant. By decreasing c-di-GMP levels in this mutant, both biofilm formation and pel gene expression phenotypes were restored to wild-type levels. We further investigated the impact on two small RNAs, which are associated with the biofilm lifestyle, and found that expression of rsmZ but not of rsmY was increased in the oprF mutant and this occurs in a c-di-GMP-dependent manner. Finally, the extracytoplasmic function (ECF) sigma factors AlgU and SigX displayed higher activity levels in the oprF mutant. Two genes of the SigX regulon involved in c-di-GMP metabolism, PA1181 and adcA (PA4843), were up-regulated in the oprF mutant, partly explaining the increased c-di-GMP level. We hypothesized that the absence of OprF leads to a cell envelope stress that activates SigX and results in a c-di-GMP elevated level due to higher expression of adcA and PA1181. The c-di-GMP level can in turn stimulate Pel synthesis via increased rsmZ sRNA levels and pel mRNA, thus affecting Pel-dependent phenotypes such as cell aggregation and biofilm formation. This work highlights the connection between OprF and c-di-GMP regulatory networks, likely via SigX (ECF), on the regulation of biofilm phenotypes.

Structure-to-function relationships of bacterial translocator protein (TSPO): a focus on Pseudomonas

The translocator protein (TSPO), which was previously designated as the peripheral-type benzodiazepine receptor, is a 3.5 billion year-old evolutionarily conserved protein expressed by most Eukarya, Archae and Bacteria, but its organization and functions differ remarkably. By taking advantage of the genomic data available on TSPO, we focused on bacterial TSPO and attempted to define functions of TSPO in Pseudomonas via in silico approaches. A tspo ortholog has been identified in several fluorescent Pseudomonas. This protein presents putative binding motifs for cholesterol and PK 11195, which is a specific drug ligand of mitochondrial TSPO. While it is a common surface distribution, the sense of insertion and membrane localization differ between α- and γ-proteobacteria. Experimental published data and STRING analysis of common TSPO partners in fluorescent Pseudomonas indicate a potential role of TSPO in the oxidative stress response, iron homeostasis and virulence expression. In these bacteria, TSPO could also take part in signal transduction and in the preservation of membrane integrity.

Sucrose favors Pseudomonas aeruginosa pellicle production through the extracytoplasmic function sigma factor SigX

Pseudomonas aeruginosa biofilm formation was increased by addition of sucrose to Luria-Bertani medium, whereas addition of NaCl to a final similar osmolarity and use of maltose instead of sucrose, were ineffective. In a previous study, we showed that the extracytoplasmic sigma factor SigX is activated in the presence of sucrose. The sucrose-mediated pellicle increase was abolished in a sigX mutant strain. Sucrose addition led to an increase in pel expression and cyclic-diguanylate (c-di-GMP) pool level production. Interestingly, these two phenotypes were strongly decreased in a sigX mutant. Since pel is not known as a SigX-target, we suspect SigX to be involved in the c-di-GMP production. We found that expression of the diguanylate cyclase PA4843 gene was increased in the presence of sucrose at least partly through SigX activity. Our study shows that sucrose itself rather than osmolarity favours the biofilm mode of P. aeruginosa through the activation of SigX.

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.

A proteomic approach of SigX function in Pseudomonas aeruginosa outer membrane composition

SigX is one of the 19 extracytoplasmic function sigma factors that have been predicted in the human opportunistic pathogen Pseudomonas aeruginosa genome. SigX is involved in the transcription of oprF, encoding the major outer membrane protein OprF, a pleiotropic porin that contributes to the maintaining of the wall structure, and is essential to P. aeruginosa virulence. This study aimed to get further insights into the functions of SigX. We performed here an outer membrane subproteome of a sigX mutant. Proteomic investigations revealed lower production of 8 porins among which 4 gated channels involved in iron or hem uptake, OprF, and the three substrate-specific proteins OprD, OprQ and OprE. On the other side, the glucose-specific porin OprB and the lipid A 3-O-deacylase that is involved in LPS modification were up-regulated. Our results indicate that SigX may be involved in the control and/or regulation of the outer membrane composition.

BIOLOGICAL SIGNIFICANCE

A proteomic approach was used herein to get further insights into SigX functions in P. aeruginosa. The data presented here suggest that SigX is involved in the outer membrane protein composition, and could be linked to a regulatory network involved in OM homeostasis.

Cytotoxicity and inflammatory potential of two Pseudomonas mosselii strains isolated from clinical samples of hospitalized patients

BACKGROUND

The genus Pseudomonas includes a heterogeneous set of microorganisms that can be isolated from many different niches and nearly 100 different strains have been described. The best characterized bacterium is Pseudomonas aeruginosa which is the primary agent of opportunistic infection in humans, causing both acute and chronic infections. Other species like fluorescens, putida or mosselii have been sporadically isolated from hospitalized patients but their association with the pathology often remains unclear.

RESULTS

This study focuses on the cytotoxicity and inflammatory potential of two strains of Pseudomonas mosselii (ATCC BAA-99 and MFY161) that were recently isolated from clinical samples of hospitalized patients. The behavior of these bacteria was compared to that of the well-known opportunistic pathogen P. aeruginosa PAO1. We found that P. mosselii ATCC BAA-99 and MFY161 are cytotoxic towards Caco-2/TC7 cells, have low invasive capacity, induce secretion of human β-defensin 2 (HBD-2), alter the epithelial permeability of differentiated cells and damage the F-actin cytoskeleton.

CONCLUSIONS

These data bring new insights into P. mosselii virulence, since this bacterium has often been neglected due to its rare occurrence in hospital.

Effects of a pulsed light-induced stress on Enterococcus faecalis

AIMS

Pulsed light (PL) technology is a surface decontamination process that can be used on food, packaging or water. PL efficiency may be limited by its low degree of penetration or because of a shadow effect. In these cases, surviving bacteria will be able to perceive PL as a stress. Such a stress was mimicked using low transmitted energy conditions, and its effects were investigated on the highly environmental adaptable bacterium Enterococcus faecalis V583.

METHODS AND RESULTS

In these laboratory conditions, a complete decontamination of the artificially inoculated medium was performed using energy doses as low as 1.8 J cm(-2) , while a treatment of 0.5, 1 and 1.2 J cm(-2) led to a 2.2, 6 and 7-log(10) CFU ml(-1) reduction in the initial bacterial population, respectively. Application of a 0.5 J cm(-2) pretreatment allowed the bacteria to resist more efficiently a 1.2 J cm(-2) subsequent PL dose. This 0.5 J cm(-2) treatment increased the bacterial mutation frequency and affected the abundance of 19 proteins as revealed by a global proteome analysis.

CONCLUSIONS

Enterococcus faecalis is able to adapt to a PL treatment, providing a molecular response to low-energy PL dose, leading to enhanced resistance to a subsequent treatment and increasing the mutation frequency.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study gives further insights on Ent. faecalis capacities to adapt and to resist to stress.

Deciphering the function of the outer membrane protein OprD homologue of Acinetobacter baumannii

The increasing number of carbapenem-resistant Acinetobacter baumannii isolates is a major cause for concern which restricts therapeutic options to treat severe infections caused by this emerging pathogen. To identify the molecular mechanisms involved in carbapenem resistance, we studied the contribution of an outer membrane protein homologue of the Pseudomonas aeruginosa OprD porin. Suspected to be the preferred pathway of carbapenems in A. baumannii, the oprD homologue gene was inactivated in strain ATCC 17978. Comparison of wild-type and mutant strains did not confirm the expected increased resistance to any antibiotic tested. OprD homologue sequence analysis revealed that this protein actually belongs to an OprD subgroup but is closer to the P. aeruginosa OprQ protein, with which it could share some functions, e.g., allowing bacterial survival under low-iron or -magnesium growth conditions or under poor oxygenation. We thus overexpressed and purified a recombinant OprD homologue protein to further examine its functional properties. As a specific channel, this porin presented rather low single-channel conductance, i.e., 28 pS in 1 M KCl, and was partially closed by micro- and millimolar concentrations of Fe(3+) and Mg(2+), respectively, but not by imipenem and meropenem or basic amino acids. The A. baumannii OprD homologue is likely not involved in the carbapenem resistance mechanism, but as an OprQ-like protein, it could contribute to the adaptation of this bacterium to magnesium- and/or iron-depleted environments.

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.

High-affinity DNA binding sites for H-NS provide a molecular basis for selective silencing within proteobacterial genomes

The global transcriptional regulator H-NS selectively silences bacterial genes associated with pathogenicity and responses to environmental insults. Although there is ample evidence that H-NS binds preferentially to DNA containing curved regions, we show here that a major basis for this selectivity is the presence of a conserved sequence motif in H-NS target transcriptons. We further show that there is a strong tendency for the H-NS binding sites to be clustered, both within operons and in genes contained in the pathogenicity-associated islands. In accordance with previously published findings, we show that these motifs occur in AT-rich regions of DNA. On the basis of these observations, we propose that H-NS silences extensive regions of the bacterial chromosome by binding first to nucleating high-affinity sites and then spreading along AT-rich DNA. This spreading would be reinforced by the frequent occurrence of the motif in such regions. Our findings suggest that such an organization enables the silencing of extensive regions of the genetic material, thereby providing a coherent framework that unifies studies on the H-NS protein and a concrete molecular basis for the genetic control of H-NS transcriptional silencing.

H-NS cooperative binding to high-affinity sites in a regulatory element results in transcriptional silencing

H-NS is a protein of the bacterial nucleoid involved in DNA compaction and transcription regulation. In vivo, H-NS selectively silences specific genes of the bacterial chromosome. However, many studies have concluded that H-NS binds sequence-independently to DNA, leaving the molecular basis for its selectivity unexplained. We show that the negative regulatory element (NRE) of the supercoiling-sensitive Escherichia coliproU gene contains two identical high-affinity binding sites for H-NS. Cooperative binding of H-NS is abrogated by changes in DNA superhelical density and temperature. We further demonstrate that the high-affinity sites nucleate cooperative binding and establish a nucleoprotein structure required for silencing. Mutations in these sites result in loss of repression by H-NS. In this model, silencing at proU, and by inference at other genes directly regulated by H-NS, is tightly controlled by the cooperativity between bound H-NS molecules.

Rapid coupling of Surface Plasmon Resonance (SPR and SPRi) and ProteinChip based mass spectrometry for the identification of proteins in nucleoprotein interactions

We compared coupling approaches of SPR to LC-MS and ProteinChip™-based mass spectrometry (SELDI™) as a means of identifying proteins captured on DNA surfaces. The approach we outline has the potential to allow multiple, quantitative analysis of macromolecular interactions followed by rapid mass spectrometry identification of retained material.

The degree of oligomerization of the H-NS nucleoid structuring protein is related to specific binding to DNA

At several E. coli promoters, initiation of transcription is repressed by a tight nucleoprotein complex formed by the assembly of the H-NS protein. In order to characterize the relationship between the structure of H-NS oligomers in solution and on relevant DNA fragments, we have compared wild-type H-NS and several transdominant H-NS mutants using gel shift assays, DNase I footprinting, analytical ultracentrifugation, and reactivity toward a cross-linking reagent. In solution, oligomerization occurs through two protein interfaces, one necessary to construct a dimeric core (and involving residues 1-64) and the other required for subsequent assembly of these dimers. We show that, as well as region 64-95, residues present in the NH(2)-terminal coiled coil domain also participate in this second interface. Our results support the view that the same interacting interfaces are also involved on the DNA. We propose that the dimeric core recognizes specific motifs, with the second interface being critical for their correct head to tail assembly. The COOH-terminal domain of the protein contains the DNA binding motif essential for the discrimination of this specific functional assembly over competitive nonspecific H-NS polymers.