Identification of a chitin-binding protein secreted by Pseudomonas aeruginosa

The type II secretion system as an underappreciated and understudied mediator of interbacterial antagonism

Interbacterial antagonism involves all major phyla, occurs across the full range of ecological niches, and has great significance for the environment, clinical arena, and agricultural and industrial sectors. Though the earliest insight into interbacterial antagonism traces back to the discovery of antibiotics, a paradigm shift happened when it was learned that protein secretion systems (e.g., types VI and IV secretion systems) deliver toxic "effectors" against competitors. However, a link between interbacterial antagonism and the Gram-negative type II secretion system (T2SS), which exists in many pathogens and environmental species, is not evident in prior reviews on bacterial competition or T2SS function. A current examination of the literature revealed four examples of a T2SS or one of its known substrates having a bactericidal activity against a Gram-positive target or another Gram-negative. When further studied, the T2SS effectors proved to be peptidases that target the peptidoglycan of the competitor. There are also reports of various bacteriolytic enzymes occurring in the culture supernatants of some other Gram-negative species, and a link between these bactericidal activities and T2SS is suggested. Thus, a T2SS can be a mediator of interbacterial antagonism, and it is possible that many T2SSs have antibacterial outputs. Yet, at present, the T2SS remains relatively understudied for its role in interbacterial competition. Arguably, there is a need to analyze the T2SSs of a broader range of species for their role in interbacterial antagonism. Such investigation offers, among other things, a possible pathway toward developing new antimicrobials for treating disease.

Pangenomic Analysis of Nucleo-Cytoplasmic Large DNA Viruses. I: The Phylogenetic Distribution of Conserved Oxygen-Dependent Enzymes Reveals a Capture-Gene Process

The Nucleo-Cytoplasmic Large DNA Viruses (NCLDVs) infect a wide range of eukaryotic species, including amoeba, algae, fish, amphibia, arthropods, birds, and mammals. This group of viruses has linear or circular double-stranded DNA genomes whose size spans approximately one order of magnitude, from 100 to 2500 kbp. The ultimate origin of this peculiar group of viruses remains an open issue. Some have argued that NCLDVs' origin may lie in a bacteriophage ancestor that increased its genome size by subsequent recruitment of eukaryotic and bacterial genes. Others have suggested that NCLDVs families originated from cells that underwent an irreversible process of genome reduction. However, the hypothesis that a number of NCLDVs sequences have been recruited from the host genomes has been largely ignored. In the present work, we have performed pangenomic analyses of each of the seven known NCLDVs families. We show that these families' core- and shell genes have cellular homologs, supporting possible escaping-gene events as part of its evolution. Furthermore, the detection of sequences that belong to two protein families (small chain ribonucleotide reductase and Erv1/Air) and to one superfamily [2OG-Fe(II) oxygenases] that are for distribution in all NCLDVs core and shell clusters encoding for oxygen-dependent enzymes suggests that the highly conserved core these viruses originated after the Proterozoic Great Oxidation Event that transformed the terrestrial atmosphere 2.4-2.3 Ga ago.

Assessment of the Glycan-Binding Profile of Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is an opportunistic pathogen that can establish acute and chronic infections in individuals who lack fully functional innate immunity. In particular, phagocytosis by neutrophils and macrophages is a key mechanism that modulates host control and clearance of P. aeruginosa. Individuals with neutropenia or cystic fibrosis are highly susceptible to P. aeruginosa infection, thus underscoring the importance of the host innate immune response. Cell-to-cell contact between host innate immune cells and the pathogen, a first step in phagocytic uptake, is facilitated by simple and complex glycan structures present at the host cell surface. We have previously shown that endogenous polyanionic N-linked glycans localized to the cell surface of phagocytes mediate the binding and subsequent phagocytosis of P. aeruginosa cells. However, the suite of glycans that P. aeruginosa cells bind to on host phagocytic cells remains poorly characterized. Here, we demonstrate, with the use of exogenous N-linked glycans and a glycan array, that P. aeruginosa PAO1 cells preferentially attach to a subset of glycans, including a bias toward monosaccharide versus more complex glycan structures. Consistent with these findings, we were able to competitively inhibit bacterial adherence and uptake by the addition of exogenous N-linked mono- and disaccharide glycans. We discuss our findings in the context of previous reports of P. aeruginosa glycan binding. IMPORTANCE P. aeruginosa cells bind to a variety of glycans as part of their interaction with host cells, and a number of P. aeruginosa-encoded receptors and target ligands have been described that allow this microbe to bind to such glycans. Here, we extend this work by studying the glycans used by P. aeruginosa PAO1 cells to bind to phagocytic cells and by using a glycan array to characterize the suite of such molecules that can facilitate host cell binding by this microbe. This study provides an increased understanding of the glycans bound by P. aeruginosa and furthermore provides a useful data set for future studies of P. aeruginosa-glycan interactions.

Immunization with lytic polysaccharide monooxygenase CbpD induces protective immunity against Pseudomonas aeruginosa pneumonia

Pseudomonas aeruginosa (PA) CbpD belongs to the lytic polysaccharide monooxygenases (LPMOs), a family of enzymes that cleave chitin or related polysaccharides. Here, we demonstrate a virulence role of CbpD in PA pneumonia linked to impairment of host complement function and opsonophagocytic clearance. Following intratracheal challenge, a PA ΔCbpD mutant was more easily cleared and produced less mortality than the wild-type parent strain. The x-ray crystal structure of the CbpD LPMO domain was solved to subatomic resolution (0.75Å) and its two additional domains modeled by small-angle X-ray scattering and Alphafold2 machine-learning algorithms, allowing structure-based immune epitope mapping. Immunization of naive mice with recombinant CbpD generated high IgG antibody titers that promoted human neutrophil opsonophagocytic killing, neutralized enzymatic activity, and protected against lethal PA pneumonia and sepsis. IgG antibodies generated against full-length CbpD or its noncatalytic M2+CBM73 domains were opsonic and protective, even in previously PA-exposed mice, while antibodies targeting the AA10 domain were not. Preexisting antibodies in PA-colonized cystic fibrosis patients primarily target the CbpD AA10 catalytic domain. Further exploration of LPMO family proteins, present across many clinically important and antibiotic-resistant human pathogens, may yield novel and effective vaccine antigens.

Bacterial Chitinases and Their Role in Human Infection

It has been widely appreciated that numerous bacterial species express chitinases for the purpose of degrading environmental chitin. However, chitinases and chitin-binding proteins are also expressed by pathogenic bacterial species during infection even though mammals do not produce chitin. Alternative molecular targets are therefore likely present within the host. Here, we will describe our current understanding of chitinase/chitin-binding proteins as virulence factors that promote bacterial colonization and infection. The targets of these chitinases in the host have been shown to include immune system components, mucins, and surface glycans. Bacterial chitinases have also been shown to interact with other microorganisms, targeting the peptidoglycan or chitin in the bacterial and fungal cell wall, respectively. This review highlights that even though the name "chitinase" implies activity toward chitin, chitinases can have a wide diversity of targets, including ones relevant to host infection. Chitinases may therefore be useful as a target of future anti-infective therapeutics.

Assessment of the Glycan-Binding Profile of Pseudomonas aeruginosa PAO1

Pseudomonas aeruginosa is an opportunistic pathogen that can establish acute and chronic infections in individuals that lack fully functional innate immunity. In particular, phagocytosis by neutrophils and macrophages is a key mechanism that modulates host control and clearance of P. aeruginosa . Individuals with neutropenia or cystic fibrosis are highly susceptible to P. aeruginosa infection thus underscoring the importance of the host innate immune response. Cell-to-cell contact between host innate immune cells and the pathogen, a first step in phagocytic uptake, is facilitated by simple and complex glycan structures present at the host cell surface. We have previously shown that endogenous polyanionic N-linked glycans localized to the cell surface of phagocytes mediate binding and subsequent phagocytosis of P. aeruginosa . However, the suite of glycans that P. aeruginosa binds to on host phagocytic cells remains poorly characterized. Here we demonstrate, with the use of exogenous N-linked glycans and a glycan array, that P. aeruginosa PAO1 preferentially attaches to a subset of glycans, including a bias towards monosaccharide versus more complex glycan structures. Consistent with these findings, we were able to competitively inhibit bacterial adherence and uptake by the addition of exogenous N-linked mono- and di-saccharide glycans. We discuss of findings in the context of previous reports of P. aeruginosa glycan binding.

IMPORTANCE

P. aeruginosa binds to a variety of glycans as part of its interaction with host cells, and a number of P. aeruginosa- encoded receptors and target ligands have been described that allow this microbe to bind to such glycans. Here we extend this work by studying the glycans used by P. aeruginosa PAO1 to bind to phagocytic cells and by using a glycan array to characterize the suite of such molecules that could facilitate host cell-binding by this microbe. This study provides an increased understanding of the glycans bound by P. aeruginosa , and furthermore, provides a useful dataset for future studies of P. aeruginosa- glycan interactions.

The crystal structure of CbpD clarifies substrate-specificity motifs in chitin-active lytic polysaccharide monooxygenases

Pseudomonas aeruginosa secretes diverse proteins via its type 2 secretion system, including a 39 kDa chitin-binding protein, CbpD. CbpD has recently been shown to be a lytic polysaccharide monooxygenase active on chitin and to contribute substantially to virulence. To date, no structure of this virulence factor has been reported. Its first two domains are homologous to those found in the crystal structure of Vibrio cholerae GbpA, while the third domain is homologous to the NMR structure of the CBM73 domain of Cellvibrio japonicus CjLPMO10A. Here, the 3.0 Å resolution crystal structure of CbpD solved by molecular replacement is reported, which required ab initio models of each CbpD domain generated by the artificial intelligence deep-learning structure-prediction algorithm RoseTTAFold. The structure of CbpD confirms some previously reported substrate-specificity motifs among LPMOAA10s, while challenging the predictive power of others. Additionally, the structure of CbpD shows that post-translational modifications occur on the chitin-binding surface. Moreover, the structure raises interesting possibilities about how type 2 secretion-system substrates may interact with the secretion machinery and demonstrates the utility of new artificial intelligence protein structure-prediction algorithms in making challenging structural targets tractable.

Contribution of Nε-lysine Acetylation towards Regulation of Bacterial Pathogenesis

N^ε-lysine acetylation is an important, dynamic regulatory posttranslational modification (PTM) that is common in bacteria. Protein acetylomes have been characterized for more than 30 different species, and it is known that acetylation plays important regulatory roles in many essential biological processes. The levels of acetylation are enzymatically controlled by the opposing actions of lysine acetyltransferases and deacetylases. In bacteria, a second mechanism of acetylation exists and occurs via an enzyme-independent manner using the secondary metabolite acetyl-phosphate. Nonenzymatic acetylation accounts for global low levels of acetylation. Recently, studies concerning the role of protein acetylation in bacterial virulence have begun. Acetylated virulence factors have been identified and further characterized. The roles of the enzymes that acetylate and deacetylate proteins in the establishment of infection and biofilm formation have also been investigated. In this review, we discuss the acetylomes of human bacterial pathogens. We highlight examples of known acetylated virulence proteins and examine how they affect survival in the host. Finally, we discuss how acetylation might influence host-pathogen interactions and look at the contribution of acetylation to antimicrobial resistance.

Oxidative Power: Tools for Assessing LPMO Activity on Cellulose

Lytic polysaccharide monooxygenases (LPMOs) have sparked a lot of research regarding their fascinating mode-of-action. Particularly, their boosting effect on top of the well-known cellulolytic enzymes in lignocellulosic hydrolysis makes them industrially relevant targets. As more characteristics of LPMO and its key role have been elucidated, the need for fast and reliable methods to assess its activity have become clear. Several aspects such as its co-substrates, electron donors, inhibiting factors, and the inhomogeneity of lignocellulose had to be considered during experimental design and data interpretation, as they can impact and often hamper outcomes. This review provides an overview of the currently available methods to measure LPMO activity, including their potential and limitations, and it is illustrated with practical examples.

Proteomic profiling of clinical and environmental strains of Pseudomonas aeruginosa

Pseudomonas aeruginosa is a ubiquitous bacterium, which is able to change its physiological characteristics in response to different habitats. Environmental strains are presumably less pathogenic than clinical strains and whether or not the clinical strains originate from the environment or through inter-host transmission remains poorly understood. To minimize the risk of infection, a better understanding of proteomic profiling of P. aeruginosa is necessary for elucidating the correlation between environmental and clinical strains. Based on antimicrobial susceptibility and patterns of virulence, we selected 12 clinical and environmental strains: (i) environmental, (ii) multidrug resistant (MDR) clinical and (iii) susceptible clinical strains. Whole-cell protein was extracted from each strain and subjected to two-dimensional differential gel electrophoresis (2-D DIGE) and liquid chromatography tandem mass spectrometry quadrupole time-of-flight (LC-MS QTOF). All 12 strains were clustered into 3 distinct groups based on their variance in protein expression. A total of 526 matched spots were detected and four differentially expressed protein spots (p < 0.05) were identified and all differential spots were downregulated in MDR strain J3. Upregulation of chitin binding and BON domain proteins was present in the environmental and some MDR strains, whereas the clinical strains exhibited distinct proteomic profiles with increased expression of serine protein kinase and arginine/ornithine transport ATP-binding proteins. Significant difference in expression was observed between susceptible clinical and MDR strains, as well as susceptible clinical and environmental strains. Transition from an environmental saprophyte to a clinical strain could alter its physiological characteristics to further increase its adaptation.

Pseudomonas aeruginosa elastase (LasB) as a therapeutic target

Why is P. aeruginosa LasB elastase an attractive target for antivirulence therapy and what is the state-of-the art in LasB inhibitor design and development?

The lytic polysaccharide monooxygenase CbpD promotes Pseudomonas aeruginosa virulence in systemic infection

The recently discovered lytic polysaccharide monooxygenases (LPMOs), which cleave polysaccharides by oxidation, have been associated with bacterial virulence, but supporting functional data is scarce. Here we show that CbpD, the LPMO of Pseudomonas aeruginosa, is a chitin-oxidizing virulence factor that promotes survival of the bacterium in human blood. The catalytic activity of CbpD was promoted by azurin and pyocyanin, two redox-active virulence factors also secreted by P. aeruginosa. Homology modeling, molecular dynamics simulations, and small angle X-ray scattering indicated that CbpD is a monomeric tri-modular enzyme with flexible linkers. Deletion of cbpD rendered P. aeruginosa unable to establish a lethal systemic infection, associated with enhanced bacterial clearance in vivo. CbpD-dependent survival of the wild-type bacterium was not attributable to dampening of pro-inflammatory responses by CbpD ex vivo or in vivo. Rather, we found that CbpD attenuates the terminal complement cascade in human serum. Studies with an active site mutant of CbpD indicated that catalytic activity is crucial for virulence function. Finally, profiling of the bacterial and splenic proteomes showed that the lack of this single enzyme resulted in substantial re-organization of the bacterial and host proteomes. LPMOs similar to CbpD occur in other pathogens and may have similar immune evasive functions.

Arming the troops: Post-translational modification of extracellular bacterial proteins

Protein secretion is almost universally employed by bacteria. Some proteins are retained on the cell surface, whereas others are released into the extracellular milieu, often playing a key role in virulence. In this review, we discuss the diverse types and potential functions of post-translational modifications (PTMs) occurring to extracellular bacterial proteins.

Quorum sensing-dependent post-secretional activation of extracellular proteases in Pseudomonas aeruginosa

Pseudomonas aeruginosa secretes multiple proteases that are implicated in its pathogenesis, and most of them are regulated by quorum sensing (QS). In this study, we found that the activities of three major extracellular proteases, protease IV (PIV), elastase A (LasA), and elastase B (LasB), are reduced considerably when expressed in a QS mutant (MW1). PIV and LasA expressed in MW1 exhibited little activity, even when purified, and their activities were inhibited by noncleavage or binding of their propeptides. LasB was activated by a QS-dependent factor, indicating that, unlike what has been proposed previously, LasB is not autoactivated. When LasB was relieved from inhibition, it activated PIV, which then sequentially processed pro-LasA to mature LasA. When activated, LasB was not inhibited by exogenous addition of its propeptide, but LasA and PIV were inhibited by their propeptides, even after prior activation. These differences may be explained by the fact that LasB can degrade its own propeptide but PIV and LasA cannot. We also found that, although PIV is the preferred LasA-activating factor, LasB can also partially activate LasA. Overall, LasB, PIV, and LasA were activated postsecretionally in a cascading manner in which the initial activation of LasB was controlled tightly by QS at the protein level in addition to the well-known transcriptional control of these proteases by QS. Interestingly, human elastase also activated LasA, indicating that the activation cascade is triggered by host factors during infection. In summary, a QS-induced proteolytic cascade activates secreted proteases from P. aeruginosa.

Unusually High Prevalence of Cosecretion of Ambler Class A and B Carbapenemases and Nonenzymatic Mechanisms in Multidrug-Resistant Clinical Isolates of Pseudomonas aeruginosa in Lebanon

The opportunistic pathogen, Pseudomonas aeruginosa, is a main cause of nosocomial infections in Lebanese hospitals. This pathogen is highly threatening due to its ability to develop multiresistance toward a large variety of antibiotics, including the carbapenem subgroup of β-lactams. In this study, we surveyed the enzymatic and nonenzymatic mechanisms of carbapenem resistance in several multidrug-resistant (MDR) strains of P. aeruginosa isolated from patients suffering from nosocomial urinary tract infections in a Lebanese hospital. The occurrence of β-lactamase-encoding genes notably GES, KPC, IMP, VIM, NDM, and OXA, which are characterized by a carbapenemase activity was checked by genomic analyses. Our results provide a first evidence of the occurrence of GES in clinical P. aeruginosa isolates resistant to carbapenems in Lebanon. More interestingly, we showed that almost 40% of the analyzed strains have acquired a dual-carbapenemase secretion of GES-6 and VIM-2 or IMP-15, this being a rare phenomenon among this type of multidrug resistance. Moreover, LC-MS/MS analyses revealed a high prevalence of another enzymatic mechanism of resistance; this is the coexistence of AmpC and Pdc-13 as well as a number of virulence proteins, for instance pilin, lytic transglycosylase, ecotin, chitin-binding protein (Cbp), and TolB-dependent receptor. It is to be noted that a mutation of the oprD2 gene encoding a porin selective for carbapenems has been detected in almost 66% of our strains. All in all, our study reveals by the use of different methods, unusual simultaneous enzymatic (GES, IMP, VIM, pdc13, and AmpC) and nonenzymatic mechanisms of resistance (reduction of OprD2 expression) for MDR Pseudomonas aeruginosa.

LasB and CbpD Virulence Factors of Pseudomonas aeruginosa Carry Multiple Post-Translational Modifications on Their Lysine Residues

Pseudomonas aeruginosa is a multi-drug resistant human pathogen largely involved in nosocomial infections. Today, effective antibacterial agents are lacking. Exploring the bacterial physiology at the post-translational modifications (PTM) level may contribute to the renewal of fighting strategies. Indeed, some correlations between PTMs and the bacterial virulence, adaptation, and resistance have been shown. In a previous study performed in P. aeruginosa, we reported that many virulence factors like chitin-binding protein CbpD and elastase LasB were multiphosphorylated. Besides phosphorylation, other PTMs, like those occurring on lysine, seem to play key roles in bacteria. In the present study, we investigated for the first time the lysine succinylome and acetylome of the extracellular compartment of P. aeruginosa by using a two-dimensional immunoaffinity approach. Some virulence factors were identified as multimodified on lysine residues, among them, LasB and CbpD. Lysine can be modified by a wide range of chemical groups. In order to check the presence of other chemical groups on modified lysines identified on LasB and CbpD, we used 1- and 2- dimensional gel electrophoresis approaches to target lysine modified by 7 other modifications: butyrylation, crotonylation, dimethylation, malonylation, methylation, propionylation, and trimethylation. We showed that some lysines of these two virulence factors were modified by these 9 different PTMs. Interestingly, we found that the PTMs recovered on these two virulence factors were different than those previously reported in the intracellular compartment.

Biocontrol of Orchid-pathogenic Mold, Phytophthora palmivora, by Antifungal Proteins from Pseudomonas aeruginosa RS1

Black rot disease in orchids is caused by the water mold Phytophthora palmivora. To gain better biocontrol performance, several factors affecting growth and antifungal substance production by Pseudomonas aeruginosa RS1 were verified. These factors include type and pH of media, temperature, and time for antifungal production. The results showed that the best conditions for P. aeruginosa RS1 to produce the active compounds was cultivating the bacteria in Luria-Bertani medium at pH 7.0 for 21 h at 37 °C. The culture filtrate was subjected to stepwise ammonium sulfate precipitation. The precipitated proteins from the 40% to 80% fraction showed antifungal activity and were further purified by column chromatography. The eluted proteins from fractions 9-10 and 33-34 had the highest antifungal activity at about 75% and 82% inhibition, respectively. SDS-PAGE revealed that the 9-10 fraction contained mixed proteins with molecular weights of 54 kDa, 32 kDa, and 20 kDa, while the 33-34 fraction contained mixed proteins with molecular weights of 40 kDa, 32 kDa, and 29 kDa. Each band of the proteins was analyzed by LC/MS to identify the protein. The result from Spectrum Modeler indicated that these proteins were closed similarly to three groups of the following proteins; catalase, chitin binding protein, and protease. Morphological study under scanning electron microscopy demonstrated that the partially purified proteins from P. aeruginosa RS1 caused abnormal growth and hypha elongation in P. palmivora. The bacteria and/or these proteins may be useful for controlling black rot disease caused by P. palmivora in orchid orchards.

Physiological and Molecular Understanding of Bacterial Polysaccharide Monooxygenases

Bacteria have long been known to secrete enzymes that degrade cellulose and chitin. The degradation of these two polymers predominantly involves two enzyme families that work synergistically with one another: glycoside hydrolases (GHs) and polysaccharide monooxygenases (PMOs). Although bacterial PMOs are a relatively recent addition to the known biopolymer degradation machinery, there is an extensive amount of literature implicating PMO in numerous physiological roles. This review focuses on these diverse and physiological aspects of bacterial PMOs, including facilitating endosymbiosis, conferring a nutritional advantage, and enhancing virulence in pathogenic organisms. We also discuss the correlation between the presence of PMOs and bacterial lifestyle and speculate on the advantages conferred by PMOs under these conditions. In addition, the molecular aspects of bacterial PMOs, as well as the mechanisms regulating PMO expression and the function of additional domains associated with PMOs, are described. We anticipate that increasing research efforts in this field will continue to expand our understanding of the molecular and physiological roles of bacterial PMOs.

Elucidating biochemical features and biological roles of Streptomyces proteins recognizing crystalline chitin- and cellulose-types and their soluble derivatives

Pioneering biochemical, immunological, physiological and microscopic studies in combination with gene cloning allowed uncovering previously unknown genes encoding proteins of streptomycetes to target crystalline chitin and cellulose as well as their soluble degradation-compounds via binding protein dependent transporters. Complementary analyses provoked an understanding of novel regulators governing transcription of selected genes. These discoveries induced detecting close and distant homologues of former orphan proteins encoded by genes from different bacteria. Grounded on structure-function-relationships, several researchers identified a few of these proteins as novel members of the growing family for lytic polysaccharides monooxygenases. Exemplary, the ecological significance of the characterized proteins including their role to promote interactions among organisms is outlined and discussed.

Protein Acetylation and Its Role in Bacterial Virulence

Protein acetylation is a universal post-translational modification which is found in both eukaryotes and prokaryotes. This process is achieved enzymatically by the protein acetyltransferase Pat, and nonenzymatically by metabolic intermediates (e.g., acetyl phosphate) in bacteria. Protein acetylation plays a role in bacterial chemotaxis, metabolism, DNA replication, and other cellular processes. Recently, accumulating evidence has suggested that protein acetylation might be involved in bacterial virulence because a number of bacterial virulence factors are acetylated. In this review, we summarize the progress in understanding bacterial protein acetylation and discuss how it mediates bacterial virulence.

The Role of Bacterial Secretion Systems in the Virulence of Gram-Negative Airway Pathogens Associated with Cystic Fibrosis

Cystic fibrosis (CF) is the most common lethal inherited disorder in Caucasians. It is caused by mutation of the CF transmembrane conductance regulator (CFTR) gene. A defect in the CFTR ion channel causes a dramatic change in the composition of the airway surface fluid, leading to a highly viscous mucus layer. In healthy individuals, the majority of bacteria trapped in the mucus layer are removed and destroyed by mucociliary clearance. However, in the lungs of patients with CF, the mucociliary clearance is impaired due to dehydration of the airway surface fluid. As a consequence, patients with CF are highly susceptible to chronic or intermittent pulmonary infections, often causing extensive lung inflammation and damage, accompanied by a decreased life expectancy. This mini review will focus on the different secretion mechanisms used by the major bacterial CF pathogens to release virulence factors, their role in resistance and discusses the potential for therapeutically targeting secretion systems.

Secretory locations of SIPC in Amphibalanus amphitrite cyprids and a novel function of SIPC in biomineralization

Settlement-inducing protein complex (SIPC) is a pheromone that triggers conspecific larval settlement in the barnacle Amphibalanus amphitrite. In the present study, immunostaining and scanning electron microscopy of SIPC revealed signals in the frontal horn pores and the secretions from carapace pores, suggesting that SIPC might be directly secreted from these organs in A. amphitrite cyprids. Further observations showed that the frontal horn pores could contact surfaces while cyprids were "walking". Immunostaining for SIPC on the contacted surfaces displayed SIPC signals. These signals were similar to the frontal horn pores in size and morphology, suggesting that frontal horn pores might deposit SIPC. Besides, full-length SIPC was expressed and subsequent assays indicated that recombinant SIPC was able to bind to chitins and induce the precipitation of CaCO3. Furthermore, recombinant SIPC inhibited the formation of vaterites and regulated the morphology of calcite crystals. The crystals that formed with recombinant SIPC were more stable against water erosion. Overall, these results reported a novel function of recombinant SIPC that regulates crystal formation in barnacle shells.

Contribution of the Twin Arginine Translocation system to the exoproteome of Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa uses secretion systems to deliver exoproteins into the environment. These exoproteins contribute to bacterial survival, adaptation, and virulence. The Twin arginine translocation (Tat) export system enables the export of folded proteins into the periplasm, some of which can then be further secreted outside the cell. However, the full range of proteins that are conveyed by Tat is unknown, despite the importance of Tat for the adaptability and full virulence of P. aeruginosa. In this work, we explored the P. aeruginosa Tat-dependent exoproteome under phosphate starvation by two-dimensional gel analysis. We identified the major secreted proteins and new Tat-dependent exoproteins. These exoproteins were further analyzed by a combination of in silico analysis, regulation studies, and protein localization. Altogether we reveal that the absence of the Tat system significantly affects the composition of the exoproteome by impairing protein export and affecting gene expression. Notably we discovered three new Tat exoproteins and one novel type II secretion substrate. Our data also allowed the identification of two new start codons highlighting the importance of protein annotation for subcellular predictions. The new exoproteins that we identify may play a significant role in P. aeruginosa pathogenesis, host interaction and niche adaptation.

Metabolism and Pathogenicity of Pseudomonas aeruginosa Infections in the Lungs of Individuals with Cystic Fibrosis

Individuals with the genetic disease cystic fibrosis (CF) accumulate mucus or sputum in their lungs. This sputum is a potent growth substrate for a range of potential pathogens, and the opportunistic bacterium Pseudomonas aeruginosa is generally most difficult of these to eradicate. As a result, P. aeruginosa infections are frequently maintained in the CF lung throughout life, and are the leading cause of death for these individuals. While great effort has been expended to better understand and treat these devastating infections, only recently have researchers begun to rigorously examine the roles played by specific nutrients in CF sputum to cue P. aeruginosa pathogenicity. This chapter summarizes the current state of knowledge regarding how P. aeruginosa metabolism in CF sputum affects initiation and maintenance of these infections. It contains an overview of CF lung disease and the mechanisms of P. aeruginosa pathogenicity. Several model systems used to study these infections are described with emphasis on the challenge of replicating the chronic infections observed in humans with CF. Nutrients present in CF sputum are surveyed, and the impacts of these nutrients on the infection are discussed. The chapter concludes by addressing the future of this line of research including the use of next-generation technologies and the potential for metabolism-based therapeutics.

Post-translational modifications in Pseudomonas aeruginosa revolutionized by proteomic analysis

Pseudomonas aeruginosa is an opportunistic pathogen that causes severe infections in vulnerable individuals. It is known that post-translational modifications (PTMs) play a key role in bacterial physiology. Their characterization is still challenging and the recent advances in proteomics allow large-scale and high-throughput analyses of PTMs. Here, we provide an overview of proteomic data about the modified proteins in P. aeruginosa. We emphasize the significant contribution of proteomics in knowledge enhancement of PTMs (phosphorylation, N-acetylation and glycosylation) and we discuss their importance in P. aeruginosa physiology.

The Role of CHI3L1 (Chitinase-3-Like-1) in the Pathogenesis of Infections in Burns in a Mouse Model

In severe burn injury the unique setting of a depleted, dysfunctional immune system along with a loss of barrier function commonly results in opportunistic infections that eventually proof fatal. Unfortunately, the dynamic sequence of bacterial contamination, colonization and eventually septic invasion with bacteria such as Pseudomonas species is still poorly understood although a limiting factor in clinical decision making. Increasing evidence supports the notion that inhibition of bacterial translocation into the wound site may be an effective alternative to prevent infection. In this context we investigated the role of the mammalian Chitinase-3-Like-1 (CHI3L1) non-enyzmatic protein predominately expressed on epithelial as well as innate immune cells as a potential bacterial-translocation-mediating factor. We show a strong trend that a modulation of chitinase expression is likely to be effective in reducing mortality rates in a mouse model of burn injury with superinfection with the opportunistic PA14 Pseudomonas strain, thus demonstrating possible clinical leverage.

Tracking the Dynamic Relationship between Cellular Systems and Extracellular Subproteomes in Pseudomonas aeruginosa Biofilms

The transition of the opportunistic pathogen Pseudomonas aeruginosa from free-living bacteria into surface-associated biofilm communities represents a viable target for the prevention and treatment of chronic infectious disease. We have established a proteomics platform that identified 2443 and 1142 high-confidence proteins in P. aeruginosa whole cells and outer-membrane vesicles (OMVs), respectively, at three time points during biofilm development (ProteomeXchange identifier PXD002605). The analysis of cellular systems, specifically the phenazine biosynthetic pathway, demonstrates that whole-cell protein abundance correlates to end product (i.e., pyocyanin) concentrations in biofilm but not in planktonic cultures. Furthermore, increased cellular protein abundance in this pathway results in quantifiable pyocyanin in early biofilm OMVs and OMVs from both growth modes isolated at later time points. Overall, our data indicate that the OMVs being released from the surface of the biofilm whole cells have unique proteomes in comparison to their planktonic counterparts. The relative abundance of OMV proteins from various subcellular sources showed considerable differences between the two growth modes over time, supporting the existence and preferential activation of multiple OMV biogenesis mechanisms under different conditions. The consistent detection of cytoplasmic proteins in all of the OMV subproteomes challenges the notion that OMVs are composed of outer membrane and periplasmic proteins alone. Direct comparisons of outer-membrane protein abundance levels between OMVs and whole cells shows ratios that vary greatly from 1:1 and supports previous studies that advocate the specific inclusion, or "packaging", of proteins into OMVs. The quantitative analysis of packaged protein groups suggests biogenesis mechanisms that involve untethered, rather than absent, peptidoglycan-binding proteins. Collectively, individual protein and biological system analyses of biofilm OMVs show that drug-binding cytoplasmic proteins and porins are potentially shuttled from the whole cell into the OMVs and may contribute to the antibiotic resistance of P. aeruginosa whole cells within biofilms.

Quantitative proteomics unravels that the post-transcriptional regulator Crc modulates the generation of vesicles and secreted virulence determinants of Pseudomonas aeruginosa

Recent research indicates that the post-transcriptional regulator Crc modulates susceptibility to antibiotics and virulence in Pseudomonas aeruginosa. Several P. aeruginosa virulence factors are secreted or engulfed in vesicles. To decipher the Crc modulation of P. aeruginosa virulence, we constructed a crc deficient mutant and measure the proteome associated extracellular vesicles and the vesicle-free secretome using iTRAQ. Fifty vesicle-associated proteins were more abundant and 14 less abundant in the crc-defective strain, whereas 37 were more abundant and 17 less abundant in the vesicle-free secretome. Among them, virulence determinants, such as ToxA, protease IV, azurin, chitin-binding protein, PlcB and Hcp1, were less abundant in the crc-defective mutant. Transcriptomic analysis revealed that some of the observed changes were post-transcriptional and, thus, could be attributed to a direct Crc regulatory role; whereas, for other differentially secreted proteins, the regulatory role was likely indirect. We also observed that the crc mutant presented an impaired vesicle-associated secretion of quorum sensing signal molecules and less cytotoxicity than its wild-type strain. Our results offer new insights into the mechanisms by which Crc regulates P. aeruginosa virulence, through the modulation of vesicle formation and secretion of both virulence determinants and quorum sensing signals. This article is part of a Special Issue entitled: HUPO 2014.

Proteomic profiling of lysine acetylation in Pseudomonas aeruginosa reveals the diversity of acetylated proteins

Protein lysine acetylation is a reversible and highly regulated post-translational modification with the well demonstrated physiological relevance in eukaryotes. Recently, its important role in the regulation of metabolic processes in bacteria was highlighted. Here, we reported the lysine acetylproteome of Pseudomonas aeruginosa using a proteomic approach. We identified 430 unique peptides corresponding to 320 acetylated proteins. In addition to the proteins involved in various metabolic pathways, several enzymes contributing to the lipopolysaccharides biosynthesis were characterized as acetylated. This data set illustrated the abundance and the diversity of acetylated lysine proteins in P. aeruginosa and opens opportunities to explore the role of the acetylation in the bacterial physiology.

Global Regulator MorA Affects Virulence-Associated Protease Secretion in Pseudomonas aeruginosa PAO1

Bacterial invasion plays a critical role in the establishment of Pseudomonas aeruginosa infection and is aided by two major virulence factors--surface appendages and secreted proteases. The second messenger cyclic diguanylate (c-di-GMP) is known to affect bacterial attachment to surfaces, biofilm formation and related virulence phenomena. Here we report that MorA, a global regulator with GGDEF and EAL domains that was previously reported to affect virulence factors, negatively regulates protease secretion via the type II secretion system (T2SS) in P. aeruginosa PAO1. Infection assays with mutant strains carrying gene deletion and domain mutants show that host cell invasion is dependent on the active domain function of MorA. Further investigations suggest that the MorA-mediated c-di-GMP signaling affects protease secretion largely at a post-translational level. We thus report c-di-GMP second messenger system as a novel regulator of T2SS function in P. aeruginosa. Given that T2SS is a central and constitutive pump, and the secreted proteases are involved in interactions with the microbial surroundings, our data broadens the significance of c-di-GMP signaling in P. aeruginosa pathogenesis and ecological fitness.

A metaproteomics approach to elucidate host and pathogen protein expression during catheter-associated urinary tract infections (CAUTIs)

Long-term catheterization inevitably leads to a catheter-associated bacteriuria caused by multispecies bacterial biofilms growing on and in the catheters. The overall goal of the presented study was (1) to unravel bacterial community structure and function of such a uropathogenic biofilm and (2) to elucidate the interplay between bacterial virulence and the human immune system within the urine. To this end, a metaproteomics approach combined with in vitro proteomics analyses was employed to investigate both, the pro- and eukaryotic protein inventory. Our proteome analyses demonstrated that the biofilm of the investigated catheter is dominated by three bacterial species, that is, Pseudomonas aeruginosa, Morganella morganii, and Bacteroides sp., and identified iron limitation as one of the major challenges in the bladder environment. In vitro proteome analysis of P. aeruginosa and M. morganii isolated from the biofilm revealed that these opportunistic pathogens are able to overcome iron restriction via the production of siderophores and high expression of corresponding receptors. Notably, a comparison of in vivo and in vitro protein profiles of P. aeruginosa and M. morganii also indicated that the bacteria employ different strategies to adapt to the urinary tract. Although P. aeruginosa seems to express secreted and surface-exposed proteases to escape the human innate immune system and metabolizes amino acids, M. morganii is able to take up sugars and to degrade urea. Most interestingly, a comparison of urine protein profiles of three long-term catheterized patients and three healthy control persons demonstrated the elevated level of proteins associated with neutrophils, macrophages, and the complement system in the patient's urine, which might point to a specific activation of the innate immune system in response to biofilm-associated urinary tract infections. We thus hypothesize that the often asymptomatic nature of catheter-associated urinary tract infections might be based on a fine-tuned balance between the expression of bacterial virulence factors and the human immune system.

Mep72, a metzincin protease that is preferentially secreted by biofilms of Pseudomonas aeruginosa

In this work, we compared the profile of proteins secreted by planktonic and biofilm cultures of Pseudomonas aeruginosa using two-dimensional difference gel electrophoresis (2D-DiGE). This revealed that a novel metzincin protease, Mep72, was secreted during biofilm growth. Subsequent Western blotting and reverse transcription-PCR (RT-PCR) analyses demonstrated that Mep72 was expressed only during biofilm growth. Mep72 has a tridomain structure comprised of a metzincin protease-like domain and two tandem carbohydrate-binding domains. Unlike the only other metzincin (alkaline protease; AprA) in P. aeruginosa, Mep72 is secreted through the type II pathway and undergoes processing during export. During this processing, the metzincin domain is liberated from the carbohydrate-binding domains. This processing may be self-catalyzed, since purified Mep72 autodegraded in vitro. This autodegradation was retarded in the presence of alginate (an extracellular matrix component of many P. aeruginosa biofilms). The expression of full-length mep72 in Escherichia coli was toxic. However, this toxicity could be alleviated by coexpression of mep72 with the adjacent gene, bamI. Mep72 and BamI were found to form a protein-protein complex in vitro. 2D-DiGE revealed that the electrophoretic mobility of several discrete protein spots was altered in the biofilm secretome of an mep72 mutant, including type III secretion proteins (PopD, PcrV, and ExoS) and a flagellum-associated protein (FliD). Mep72 was found to bind directly to ExoS and PcrV and to affect the processing of these proteins in the biofilm secretome. We conclude that Mep72 is a secreted biofilm-specific regulator that affects the processing of a very specific subset of virulence factors.

Txc, a new type II secretion system of Pseudomonas aeruginosa strain PA7, is regulated by the TtsS/TtsR two-component system and directs specific secretion of the CbpE chitin-binding protein

We present here the functional characterization of a third complete type II secretion system (T2SS) found in newly sequenced Pseudomonas aeruginosa strain PA7. We call this system Txc (third Xcp homolog). This system is encoded by the RGP69 region of genome plasticity found uniquely in strain PA7. In addition to the 11 txc genes, RGP69 contains two additional genes encoding a possible T2SS substrate and a predicted unorthodox sensor protein, TtsS (type II secretion sensor). We also identified a gene encoding a two-component response regulator called TtsR (type II secretion regulator), which is located upstream of the ttsS gene and just outside RGP69. We show that TtsS and TtsR constitute a new and functional two-component system that controls the production and secretion of the RGP69-encoded T2SS substrate in a Txc-dependent manner. Finally, we demonstrate that this Txc-secreted substrate binds chitin, and we therefore name it CbpE (chitin-binding protein E).

Secretome of transmissible Pseudomonas aeruginosa AES-1R grown in a cystic fibrosis lung-like environment

Pseudomonas aeruginosa is the predominant cause of mortality in patients with cystic fibrosis (CF). We examined the secretome of an acute, transmissible CF P. aeruginosa (Australian epidemic strain 1-R; AES-1R) compared with laboratory-adapted PAO1. Culture supernatant proteins from rich (LB) and minimal (M9) media were compared using 2-DE and 2DLC-MS/MS, which revealed elevated abundance of PasP protease and absence of AprA protease in AES-1R. CF lung-like artificial sputum medium (ASMDM) contains serum and mucin that generally preclude proteomics of secreted proteins. ASMDM culture supernatants were subjected to 2DLC-MS/MS, which allowed the identification of 57 P. aeruginosa proteins, and qualitative spectral counting was used to estimate relative abundance. AES-1R-specific AES_7139 and PasP were more abundant in AES-1R ASMDM culture supernatants, while AprA could only be identified in PAO1. Relative quantitation was performed using selected reaction monitoring. Significantly elevated levels of PasP, LasB, chitin-binding protein (CbpD), and PA4495 were identified in AES-1R ASMDM supernatants. Quantitative PCR showed elevated pasP in AES-1R during early (18 h) ASMDM growth, while no evidence of aprA expression could be observed. Genomic screening of CF isolates revealed aes_7139 was present in all AES-1 and one pair of sequential nonepidemic isolates. Secreted proteins may be crucial in aiding CF-associated P. aeruginosa to establish infection and for adaptation to the CF lung.

Interaction between extracellular lipase LipA and the polysaccharide alginate of Pseudomonas aeruginosa

Background

As an opportunistic human pathogen Pseudomonas aeruginosa is able to cause acute and chronic infections. The biofilm mode of life significantly contributes to the growth and persistence of P. aeruginosa during an infection process and mediates the pathogenicity of the bacterium. Within a biofilm mucoid strains of P. aeruginosa simultaneously produce and secrete several hydrolytic enzymes and the extracellular polysaccharide alginate. The focus of the current study was the interaction between extracellular lipase LipA and alginate, which may be physiologically relevant in biofilms of mucoid P. aeruginosa.

Results

Fluorescence microscopy of mucoid P. aeruginosa biofilms were performed using fluorogenic lipase substrates. It showed a localization of the extracellular enzyme near the cells. A microtiter plate-based binding assay revealed that the polyanion alginate is able to bind LipA. A molecular modeling approach showed that this binding is structurally based on electrostatic interactions between negatively charged residues of alginate and positively charged amino acids of the protein localized opposite of the catalytic centre. Moreover, we showed that the presence of alginate protected the lipase activity by protection from heat inactivation and from degradation by the endogenous, extracellular protease elastase LasB. This effect was influenced by the chemical properties of the alginate molecules and was enhanced by the presence of O-acetyl groups in the alginate chain.

Conclusion

We demonstrate that the extracellular lipase LipA from P. aeruginosa interacts with the polysaccharide alginate in the self-produced extracellular biofilm matrix of P. aeruginosa via electrostatic interactions suggesting a role of this interaction for enzyme immobilization and accumulation within biofilms. This represents a physiological advantage for the cells. Especially in the biofilm lifestyle, the enzyme is retained near the cell surface, with the catalytic centre exposed towards the substrate and is protected from denaturation and proteolytic degradation.

Bacterial chitinases and chitin-binding proteins as virulence factors

Bacterial chitinases (EC 3.2.1.14) and chitin-binding proteins (CBPs) play a fundamental role in the degradation of the ubiquitous biopolymer chitin, and the degradation products serve as an important nutrient source for marine- and soil-dwelling bacteria. However, it has recently become clear that representatives of both Gram-positive and Gram-negative bacterial pathogens encode chitinases and CBPs that support infection of non-chitinous mammalian hosts. This review addresses this biological role of bacterial chitinases and CBPs in terms of substrate specificities, regulation, secretion and involvement in cellular and animal infection.

Surface thermodynamic and adhesion force evaluation of the role of chitin-binding protein in the physical interaction between Pseudomonas aeruginosa and Candida albicans

Candida albicans and Pseudomonas aeruginosa are able to form pathogenic polymicrobial communities. P. aeruginosa colonizes and kills hyphae but is unable to attach to yeast. It is unknown why the interaction of P. aeruginosa is different with yeast than with hyphae. Here we aim to evaluate the role of P. aeruginosa chitin-binding protein (CbpD) in its physical interaction with C. albicans hyphae or yeast, based on surface thermodynamic and atomic force microscopic analyses. A P. aeruginosa mutant lacking CbpD was unable to express strong adhesion forces with hyphae (-2.9 nN) as compared with the parent strain P. aeruginosa PAO1 (-4.8 nN) and showed less adhesion to hyphae. Also blocking of CbpD using N-acetyl-glucosamine yielded a lower adhesion force (-4.3 nN) with hyphae. Strong adhesion forces were restored after complementing the expression of CbpD in P. aeruginosa PAO1 ΔcbpD yielding an adhesion force of -5.1 nN. These observations were confirmed with microscopic evaluation of adhesion tests. Regardless of the absence or presence of CbpD on the bacterial cell surfaces, or their blocking, P. aeruginosa experienced favorable thermodynamic conditions for adhesion with hyphae, which were absent with yeast. In addition, adhesion forces with yeast were less than 0.5 nN in all cases. Concluding, CbpD in P. aeruginosa is responsible for strong physical interactions with C. albicans hyphae. The development of this interaction requires time due to the fact that CbpDs have to invade the outermost mannoprotein layer on the hyphal cell surfaces. In order to do this, thermodynamic conditions at the outermost cell surfaces have to be favorable.

Type II secretion in Yersinia-a secretion system for pathogenicity and environmental fitness

In Yersinia species, type III secretion (T3S) is the most prominent and best studied secretion system and a hallmark for the infection process of pathogenic Yersinia species. Type II secretion (T2S), on the other hand, is less well-characterized, although all Yersinia species, pathogenic as well as non-pathogenic, possess one or even two T2S systems. The only Yersinia strain in which T2S has so far been studied is the human pathogenic strain Y. enterocolitica 1b. Mouse infection experiments showed that at least one of the two T2S systems of Y. enterocolitica 1b, termed Yts1, is involved in dissemination and colonization of deeper tissues like liver and spleen. Interestingly, in vitro studies revealed a complex regulation of the Yts1 system, which is mainly active at low temperatures and high Mg²⁺-levels. Furthermore, the functional characterization of the proteins secreted in vitro indicates a role of the Yts1 machinery in survival of the bacteria in an environmental habitat. In silico analyses identified Yts1 homologous systems in bacteria that are known as plant symbionts or plant pathogens. Thus, the recent studies point to a dual function of the Yts1 T2S systems, playing a role in virulence of humans and animals, as well as in the survival of the bacteria outside of the mammalian host. In contrast, the role of the second T2S system, Yts2, remains ill defined. Whereas the T3S system and its virulence-mediating role has been intensively studied, it might now be time to also focus on the T2S system and its role in the Yersinia lifestyle, especially considering that most of the Yersinia isolates are not found in infected humans but have been gathered from various environmental samples.

Production of a Thermostable and Alkaline Chitinase by Bacillus thuringiensis subsp. kurstaki Strain HBK-51

This paper reports the isolation and identification of chitinase-producing Bacillus from chitin-containing wastes, production of a thermostable and alkaline chitinasese, and enzyme characterization. Bacillus thuringiensis subsp. kurstaki HBK-51 was isolated from soil and was identified. Chitinase was obtained from supernatant of B. thuringiensis HBK-51 strain and showed its optimum activity at 110°C and at pH 9.0. Following 3 hours of incubation period, the enzyme showed a high level of activity at 110°C (96% remaining activity) and between pH 9.0 and 12.0 (98% remaining activity). Considering these characteristics, the enzyme was described as hyperthermophile-thermostable and highly alkaline. Two bands of the enzyme weighing 50 and 125 kDa were obtained following 12% SDS-PAGE analyses. Among the metal ions and chemicals used, Ni(2+) (32%), K(+) (44%), and Cu(2+) (56%) increased the enzyme activity while EDTA (7%), SDS (7%), Hg(2+) (11%), and ethyl-acetimidate (20%) decreased the activity of the enzyme. Bacillus thuringiensis subsp. kurstaki HBK-51 is an important strain which can be used in several biotechnological applications as a chitinase producer.

Chitin binding proteins act synergistically with chitinases in Serratia proteamaculans 568

Genome sequence of Serratia proteamaculans 568 revealed the presence of three family 33 chitin binding proteins (CBPs). The three Sp CBPs (Sp CBP21, Sp CBP28 and Sp CBP50) were heterologously expressed and purified. Sp CBP21 and Sp CBP50 showed binding preference to β-chitin, while Sp CBP28 did not bind to chitin and cellulose substrates. Both Sp CBP21 and Sp CBP50 were synergistic with four chitinases from S. proteamaculans 568 (Sp ChiA, Sp ChiB, Sp ChiC and Sp ChiD) in degradation of α- and β-chitin, especially in the presence of external electron donor (reduced glutathione). Sp ChiD benefited most from Sp CBP21 or Sp CBP50 on α-chitin, while Sp ChiB and Sp ChiD had major advantage with these Sp CBPs on β-chitin. Dose responsive studies indicated that both the Sp CBPs exhibit synergism ≥0.2 µM. The addition of both Sp CBP21 and Sp CBP50 in different ratios to a synergistic mixture did not significantly increase the activity. Highly conserved polar residues, important in binding and activity of CBP21 from S. marcescens (Sm CBP21), were present in Sp CBP21 and Sp CBP50, while Sp CBP28 had only one such polar residue. The inability of Sp CBP28 to bind to the test substrates could be attributed to the absence of important polar residues.

Ajoene, a sulfur-rich molecule from garlic, inhibits genes controlled by quorum sensing

In relation to emerging multiresistant bacteria, development of antimicrobials and new treatment strategies of infections should be expected to become a high-priority research area. Quorum sensing (QS), a communication system used by pathogenic bacteria like Pseudomonas aeruginosa to synchronize the expression of specific genes involved in pathogenicity, is a possible drug target. Previous in vitro and in vivo studies revealed a significant inhibition of P. aeruginosa QS by crude garlic extract. By bioassay-guided fractionation of garlic extracts, we determined the primary QS inhibitor present in garlic to be ajoene, a sulfur-containing compound with potential as an antipathogenic drug. By comprehensive in vitro and in vivo studies, the effect of synthetic ajoene toward P. aeruginosa was elucidated. DNA microarray studies of ajoene-treated P. aeruginosa cultures revealed a concentration-dependent attenuation of a few but central QS-controlled virulence factors, including rhamnolipid. Furthermore, ajoene treatment of in vitro biofilms demonstrated a clear synergistic, antimicrobial effect with tobramycin on biofilm killing and a cease in lytic necrosis of polymorphonuclear leukocytes. Furthermore, in a mouse model of pulmonary infection, a significant clearing of infecting P. aeruginosa was detected in ajoene-treated mice compared to a nontreated control group. This study adds to the list of examples demonstrating the potential of QS-interfering compounds in the treatment of bacterial infections.

Potential role of chitinases and chitin-binding proteins in host-microbial interactions during the development of intestinal inflammation

The small and large intestines contain an abundance of luminal antigens derived from food products and enteric microorganisms. The function of intestinal epithelial cells is tightly regulated by several factors produced by enteric bacteria and the epithelial cells themselves. Epithelial cells actively participate in regulating the homeostasis of intestine, and failure of this function leads to abnormal and host-microbial interactions resulting in the development of intestinal inflammation. Major determinants of host susceptibility against luminal commensal bacteria include genes regulating mucosal immune responses, intestinal barrier function and microbial defense. Of note, it has been postulated that commensal bacterial adhesion and invasion on/into host cells may be strongly involved in the pathogenesis of inflammatory bowel disease (IBD). During the intestinal inflammation, the composition of the commensal flora is altered, with increased population of aggressive and detrimental bacteria and decreased populations of protective bacteria. In fact, some pathogenic bacteria, including Adherent-Invasive Escherichia coli, Listeria monocytogenes and Vibrio cholerae are likely to initiate their adhesion to the host cells by expressing accessory molecules such as chitinases and/or chitin-binding proteins on themselves. In addition, several inducible molecules (e.g., chitinase 3-like 1, CEACAM6) are also induced on the host cells (e.g. epithelial cells, lamina proprial macrophages) under inflammatory conditions, and are actively participated in the host-microbial interactions. In this review, we will summarize and discuss the potential roles of these important molecules during the development of acute and chronic inflammatory conditions.

Protein Secretion Systems in Pseudomonas aeruginosa: An Essay on Diversity, Evolution, and Function

Protein secretion systems are molecular nanomachines used by Gram-negative bacteria to thrive within their environment. They are used to release enzymes that hydrolyze complex carbon sources into usable compounds, or to release proteins that capture essential ions such as iron. They are also used to colonize and survive within eukaryotic hosts, causing acute or chronic infections, subverting the host cell response and escaping the immune system. In this article, the opportunistic human pathogen Pseudomonas aeruginosa is used as a model to review the diversity of secretion systems that bacteria have evolved to achieve these goals. This diversity may result from a progressive transformation of cell envelope complexes that initially may not have been dedicated to secretion. The striking similarities between secretion systems and type IV pili, flagella, bacteriophage tail, or efflux pumps is a nice illustration of this evolution. Differences are also needed since various secretion configurations call for diversity. For example, some proteins are released in the extracellular medium while others are directly injected into the cytosol of eukaryotic cells. Some proteins are folded before being released and transit into the periplasm. Other proteins cross the whole cell envelope at once in an unfolded state. However, the secretion system requires conserved basic elements or features. For example, there is a need for an energy source or for an outer membrane channel. The structure of this review is thus quite unconventional. Instead of listing secretion types one after each other, it presents a melting pot of concepts indicating that secretion types are in constant evolution and use basic principles. In other words, emergence of new secretion systems could be predicted the way Mendeleïev had anticipated characteristics of yet unknown elements.

Parasitic growth of Pseudomonas aeruginosa in co-culture with the chitinolytic bacterium Aeromonas hydrophila

Polymer-degrading bacteria face exploitation by opportunistic bacteria that grow with the degradation products without investing energy into production of extracellular hydrolytic enzymes. This scenario was investigated with a co-culture of Aeromonas hydrophila and Pseudomonas aeruginosa with chitin as carbon, nitrogen and energy source. In single cultures, A. hydrophila could grow with chitin, while P. aeruginosa could not. Co-cultures with both strains had a biphasic course. In the first phase, P. aeruginosa grew along with A. hydrophila without affecting it. The second phase was initiated by a rapid inactivation of and a massive acetate release by A. hydrophila. Both processes coincided and were dependent on quorum sensing-regulated production of secondary metabolites by P. aeruginosa. Among these the redox-active phenazine compound pyocyanin caused the release of acetate by A. hydrophila by blocking the citric acid cycle through inhibition of aconitase. Thus, A. hydrophila was forced into an incomplete oxidation of chitin with acetate as end-product, which supported substantial growth of P. aeruginosa in the second phase of the co-culture. In conclusion, P. aeruginosa could profit from a substrate that was originally not bioavailable to it by influencing the metabolism and viability of A. hydrophila in a parasitic way.