Pseudomonas aeruginosa ExlA and Serratia marcescens ShlA trigger cadherin cleavage by promoting calcium influx and ADAM10 activation

The Balance between Protealysin and Its Substrate, the Outer Membrane Protein OmpX, Regulates Serratia proteamaculans Invasion

Serratia are opportunistic bacteria, causing infections in plants, insects, animals and humans under certain conditions. The development of bacterial infection in the human body involves several stages of host-pathogen interaction, including entry into non-phagocytic cells to evade host immune cells. The facultative pathogen Serratia proteamaculans is capable of penetrating eukaryotic cells. These bacteria synthesize an actin-specific metalloprotease named protealysin. After transformation with a plasmid carrying the protealysin gene, noninvasive E. coli penetrate eukaryotic cells. This suggests that protealysin may play a key role in S. proteamaculans invasion. This review addresses the mechanisms underlying protealysin's involvement in bacterial invasion, highlighting the main findings as follows. Protealysin can be delivered into the eukaryotic cell by the type VI secretion system and/or by bacterial outer membrane vesicles. By cleaving actin in the host cell, protealysin can mediate the reversible actin rearrangements required for bacterial invasion. However, inactivation of the protealysin gene leads to an increase, rather than decrease, in the intensity of S. proteamaculans invasion. This indicates the presence of virulence factors among bacterial protealysin substrates. Indeed, protealysin cleaves the virulence factors, including the bacterial surface protein OmpX. OmpX increases the expression of the EGFR and β1 integrin, which are involved in S. proteamaculans invasion. It has been shown that an increase in the invasion of genetically modified S. proteamaculans may be the result of the accumulation of full-length OmpX on the bacterial surface, which is not cleaved by protealysin. Thus, the intensity of the S. proteamaculans invasion is determined by the balance between the active protealysin and its substrate OmpX.

G protein-coupled estrogen receptor agonist G-1 decreases ADAM10 levels and NLRP3-inflammasome component activation in response to Staphylococcus aureus alpha-hemolysin

The G protein-coupled estrogen receptor, also known as GPER1 or originally GPR30, is found in various tissues, indicating its diverse functions. It is typically present in immune cells, suggesting its role in regulating immune responses to infectious diseases. Our previous studies have shown that G-1, a selective GPER agonist, can limit the pathogenesis mediated by Staphylococcus aureus alpha-hemolysin (Hla). It aids in clearing bacteria in a mouse skin infection model and restricts the surface display of the Hla receptor, ADAM10 (a disintegrin and metalloprotease 10) in HaCaT keratinocytes. In this report, we delve into the modulation of GPER in human immune cells in relation to the NLRP3 inflammasome. We used macrophage-like differentiated THP-1 cells for our study. We found that treating these cells with G-1 reduces ATP release, decreases the activity of the caspase-1 enzyme, and lessens cell death following Hla intoxication. This is likely due to the reduced levels of ADAM10 and NLRP3 proteins, as well as the decreased display of the ADAM10 receptor in the G-1-treated THP-1 cells. Our studies, along with our previous work, suggest the potential therapeutic use of G-1 in reducing Hla susceptibility in humans. This highlights the importance of GPER in immune regulation and its potential as a therapeutic target.

Porcine circovirus type 2 and Glaesserella parasuis serotype 4 co-infection activates Snail1 to disrupt the intercellular junctions and facilitate bacteria translocation across the tracheal epithelium

Clinically, Porcine circovirus type 2 (PCV2) often causes disease through coinfection with other bacterial pathogens, including Glaesserella parasuis (G. parasuis), which causes high morbidity and mortality. However, the mechanism of PCV2 and G. parasuis serotype 4 (GPS4) co-infection is still not fully understood. In this study, swine tracheal epithelial cells (STEC) were used as a barrier model, and our results showed that PCV2 infection increased the adhesion of GPS4 to STEC, while decreasing the levels of ZO-1, Occludin and increasing tracheal epithelial permeability, and ultimately facilitated GPS4 translocation. Snail1 is a transcriptional repressor, and has been known to induce epithelial-to-mesenchymal transition (EMT) during development or in cancer metastasis. Importantly, we found that Snail1, as a transcriptional repressor, was crucial in destroying the tracheal epithelial barrier induced by PCV2, GPS4, PCV2 and GPS4 coinfection. For the first time, we found that PCV2, GPS4, PCV2 and GPS4 coinfection cross-activates TGF-β and p38/MAPK signaling pathways to upregulate the expression of Snail1, down-regulate the levels of ZO-1 and Occludin, and thus disrupt the integrity of tracheal epithelial barrier then promoting GPS4 translocation. Finally, PCV2 and GPS4 co-infection also can activate TGF-β and p38/MAPK signaling pathways in vivo and upregulate Snail1, ultimately down-regulating the expression of ZO-1 and Occludin. Our study elucidates how PCV2 infection promotes GPS4 to breach the tracheal epithelial barrier and aggravate clinical manifestations.

Host-Cell-Dependent Roles of E-Cadherin in Serratia Invasion

Bacteria use cell surface proteins to mediate host-pathogen interactions. Proteins responsible for cell adhesion, including E-cadherin, serve as receptors for entry into the host cell. We have previously shown that an increase in eukaryotic cell sensitivity to Serratia grimesii correlates with an increase in E-cadherin expression. On the other hand, Serratia proteamaculans invasion involves the EGFR, which can interact with E-cadherin on the surface of host cells. Therefore, we investigated the role of E-cadherin in Serratia invasion into M-HeLa and Caco-2 cells. Bacterial infection increased E-cadherin expression in both cell lines. Moreover, E-cadherin was detected in the Caco-2 cells in a full-length form and in the M-HeLa cells in only a truncated form in response to incubation with bacteria. Transfection with siRNA targeting E-cadherin inhibited S. proteamaculans invasion only into the Caco-2 cells. Thus, only full-length E-cadherin is involved in S. proteamaculans invasion. On the other hand, transfection with siRNA targeting E-cadherin inhibited S. grimesii invasion into both cell lines. Thus, not only may full-length E-cadherin but also truncated E-cadherin be involved in S. grimesii invasion. Truncated E-cadherin can be formed as a result of cleavage by bacterial proteases or the Ca2+-activated cellular protease ADAM10. The rate of Ca2+ accumulation in the host cells depends on the number of bacteria per cell upon infection. During incubation, Ca2+ accumulates only when more than 500 S. grimesii bacteria are infected per eukaryotic cell, and only under these conditions does the ADAM10 inhibitor reduce the sensitivity of the cells to bacteria. An EGFR inhibitor has the same quantitative effect on S. grimesii invasion. Apparently, as a result of infection with S. grimesii, Ca2+ accumulates in the host cells and may activate the ADAM10 sheddase, which can promote invasion by cleaving E-cadherin and, as a result, triggering EGFR signaling. Thus, the invasion of S. proteamaculans can only be promoted by full-length E-cadherin, and S. grimesii invasion can be promoted by both full-length and truncated E-cadherin.

Endothelial ADAM10 utilization defines a molecular pathway of vascular injury in mice with bacterial sepsis

The endothelium plays a critical role in the host response to infection and has been a focus of investigation in sepsis. While it is appreciated that intravascular thrombus formation, severe inflammation, and loss of endothelial integrity impair tissue oxygenation during sepsis, the precise molecular mechanisms that lead to endothelial injury remain poorly understood. We demonstrate here that endothelial ADAM10 was essential for the pathogenesis of Staphylococcus aureus sepsis, contributing to α-toxin-mediated (Hla-mediated) microvascular thrombus formation and lethality. As ADAM10 is essential for endothelial development and homeostasis, we examined whether other major human sepsis pathogens also rely on ADAM10-dependent pathways in pathogenesis. Mice harboring an endothelium-specific knockout of ADAM10 were protected against lethal Pseudomonas aeruginosa and Streptococcus pneumoniae sepsis, yet remained fully susceptible to group B streptococci and Candida albicans sepsis. These studies illustrate a previously unknown role for ADAM10 in sepsis-associated endothelial injury and suggest that understanding pathogen-specific divergent host pathways in sepsis may enable more precise targeting of disease.

The Anti-Virulence Activities of the Antihypertensive Drug Propranolol in Light of Its Anti-Quorum Sensing Effects against Pseudomonas aeruginosa and Serratia marcescens

The development of bacterial resistance is an increasing global concern that requires discovering new antibacterial agents and strategies. Bacterial quorum sensing (QS) systems play important roles in controlling bacterial virulence, and their targeting could lead to diminishing bacterial pathogenesis. In this context, targeting QS systems without significant influence on bacterial growth is assumed as a promising strategy to overcome resistance development. This study aimed at evaluating the anti-QS and anti-virulence activities of the β-adrenoreceptor antagonist propranolol at sub-minimal inhibitory concentrations (sub-MIC) against two Gram-negative bacterial models Pseudomonas aeruginosa and Serratia marcescens. The effect of propranolol on the expression of QS-encoding genes was evaluated. Additionally, the affinity of propranolol to QS receptors was virtually attested. The influence of propranolol at sub-MIC on biofilm formation, motility, and production of virulent factors was conducted. The outcomes of the propranolol combination with different antibiotics were assessed. Finally, the in vivo protection assay in mice was performed to assess propranolol's effect on lessening the bacterial pathogenesis. The current findings emphasized the significant ability of propranolol at sub-MIC to reduce the formation of biofilms, motility, and production of virulence factors. In addition, propranolol at sub-MIC decreased the capacity of tested bacteria to induce pathogenesis in mice. Furthermore, propranolol significantly downregulated the QS-encoding genes and showed significant affinity to QS receptors. Finally, propranolol at sub-MIC synergistically decreased the MICs of different antibiotics against tested bacteria. In conclusion, propranolol might serve as a plausible adjuvant therapy with antibiotics for the treatment of serious bacterial infections after further pharmacological and pharmaceutical studies.

Pore-forming activity of S. pneumoniae pneumolysin disrupts the paracellular localization of the epithelial adherens junction protein E-cadherin

Streptococcus pneumoniae, a common cause of community-acquired bacterial pneumonia, can cross the respiratory epithelial barrier to cause lethal septicemia and meningitis. S. pneumoniae pore-forming toxin pneumolysin (PLY) triggers robust neutrophil (PMN) infiltration that promotes bacterial transepithelial migration in vitro and disseminated disease in mice. Apical infection of polarized respiratory epithelial monolayers by S. pneumoniae at a multiplicity of infection (MOI) of 20 resulted in recruitment of PMNs, loss of 50% of the monolayer, and PMN-dependent bacterial translocation. Reducing the MOI to 2 decreased PMN recruitment two-fold and preserved the monolayer, but apical-to-basolateral translocation of S. pneumoniae remained relatively efficient. At both MOI of 2 and 20, PLY was required for maximal PMN recruitment and bacterial translocation. Co-infection by wild-type S. pneumoniae restored translocation by a PLY-deficient mutant, indicating that PLY can act in trans. Investigating the contribution of S. pneumoniae infection on apical junction complexes in the absence of PMN transmigration, we found that S. pneumoniae infection triggered the cleavage and mislocalization of the adherens junction (AJ) protein E-cadherin. This disruption was PLY-dependent at MOI of 2 and was recapitulated by purified PLY, requiring its pore-forming activity. In contrast, at MOI of 20, E-cadherin disruption was independent of PLY, indicating that S. pneumoniae encodes multiple means to disrupt epithelial integrity. This disruption was insufficient to promote bacterial translocation in the absence of PMNs. Thus, S. pneumoniae triggers cleavage and mislocalization of E-cadherin through PLY-dependent and -independent mechanisms, but maximal bacterial translocation across epithelial monolayers requires PLY-dependent neutrophil transmigration.

ADAM10-a "multitasker" in sepsis: focus on its posttranslational target

BACKGROUND

Sepsis has a complex pathogenesis in which the uncontrolled systemic inflammatory response triggered by infection leads to vascular barrier disruption, microcirculation dysfunction and multiple organ dysfunction syndrome. Numerous recent studies reveal that a disintegrin and metalloproteinase 10 (ADAM10) acts as a "molecular scissor" playing a pivotal role in the inflammatory response during sepsis by regulating proteolysis by cleaving various membrane protein substrates, including proinflammatory cytokines, cadherins and Notch, which are involved in intercellular communication. ADAM10 can also act as the cellular receptor for Staphylococcus aureus α-toxin, leading to lethal sepsis. However, its substrate-specific modulation and precise targets in sepsis have not yet to be elucidated.

METHODS

We performed a computer-based online search using PubMed and Google Scholar for published articles concerning ADAM10 and sepsis.

CONCLUSIONS

In this review, we focus on the functions of ADAM10 in sepsis-related complex endothelium-immune cell interactions and microcirculation dysfunction through the diversity of its substrates and its enzymatic activity. In addition, we highlight the posttranslational mechanisms of ADAM10 at specific subcellular sites, or in multimolecular complexes, which will provide the insight to intervene in the pathophysiological process of sepsis caused by ADAM10 dysregulation.

Alpha-1 antitrypsin limits neutrophil extracellular trap disruption of airway epithelial barrier function

Neutrophil extracellular traps contribute to lung injury in cystic fibrosis and asthma, but the mechanisms are poorly understood. We sought to understand the impact of human NETs on barrier function in primary human bronchial epithelial and a human airway epithelial cell line. We demonstrate that NETs disrupt airway epithelial barrier function by decreasing transepithelial electrical resistance and increasing paracellular flux, partially by NET-induced airway cell apoptosis. NETs selectively impact the expression of tight junction genes claudins 4, 8 and 11. Bronchial epithelia exposed to NETs demonstrate visible gaps in E-cadherin staining, a decrease in full-length E-cadherin protein and the appearance of cleaved E-cadherin peptides. Pretreatment of NETs with alpha-1 antitrypsin (A1AT) inhibits NET serine protease activity, limits E-cadherin cleavage, decreases bronchial cell apoptosis and preserves epithelial integrity. In conclusion, NETs disrupt human airway epithelial barrier function through bronchial cell death and degradation of E-cadherin, which are limited by exogenous A1AT.

Genomic erosion and horizontal gene transfer shape functional differences of the ExlA toxin in Pseudomonas spp

Two-partner secretion (TPS) is widespread in the bacterial world. The pore-forming TPS toxin ExlA of Pseudomonas aeruginosa is conserved in pathogenic and environmental Pseudomonas. While P. chlororaphis and P. entomophila displayed ExlA-dependent killing, P. putida did not cause damage to eukaryotic cells. ExlA proteins interacted with epithelial cell membranes; however, only ExlA^Pch induced the cleavage of the adhesive molecule E-cadherin. ExlA proteins participated in insecticidal activity toward the larvae of Galleria mellonella and the fly Drosophila melanogaster. Evolutionary analyses demonstrated that the differences in the C-terminal domains are partly due to horizontal movements of the operon within the genus Pseudomonas. Reconstruction of the evolutionary history revealed the complex horizontal acquisitions. Together, our results provide evidence that conserved TPS toxins in environmental Pseudomonas play a role in bacteria-insect interactions and discrete differences in CTDs may determine their specificity and mode of action toward eukaryotic cells.

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ExlA is a two-partner secreted toxin conserved across Pseudomonas spp.

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Environmental Pseudomonas strains encode ExlA with different cytotoxic activities

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ExlA of environmental Pseudomonas strains play a role in bacteria-insect interactions

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ExlBA operon shows a complex evolutionary history of horizontal gene transfer

ExlA: A New Contributor to Pseudomonas aeruginosa Virulence

ExlA (also called exolysin) is a recently discovered virulence factor secreted by a subset of Pseudomonas aeruginosa strains in which a type 3 secretion system is lacking. exlA-positive strains were identified worldwide in the clinic, causing several types of infectious diseases, and were detected in various locations in the environment. ExlA possesses pore-forming activity and is cytolytic for most human cell types. It belongs to a class of poorly characterized bacterial toxins, sharing a similar protein domain organization and a common secretion pathway. This review summarizes the recent findings regarding ExlA synthesis, its secretion pathway, and its toxic behavior for host cells.

Dynamic Python-Based Method Provides Quantitative Analysis of Intercellular Junction Organization During S. pneumoniae Infection of the Respiratory Epithelium

Many respiratory pathogens compromise epithelial barrier function during lung infection by disrupting intercellular junctions, such as adherens junctions and tight junctions, that maintain intercellular integrity. This includes Streptococcus pneumoniae, a leading cause of pneumonia, which can successfully breach the epithelial barrier and cause severe infections such as septicemia and meningitis. Fluorescence microscopy analysis on intercellular junction protein manipulation by respiratory pathogens has yielded major advances in our understanding of their pathogenesis. Unfortunately, a lack of automated image analysis tools that can tolerate variability in sample-sample staining has limited the accuracy in evaluating intercellular junction organization quantitatively. We have created an open source, automated Python computer script called "Intercellular Junction Organization Quantification" or IJOQ that can handle a high degree of sample-sample staining variability and robustly measure intercellular junction integrity. In silico validation of IJOQ was successful in analyzing computer generated images containing varying degrees of simulated intercellular junction disruption. Accurate IJOQ analysis was further confirmed using images generated from in vitro and in vivo bacterial infection models. When compared in parallel to a previously published, semi-automated script used to measure intercellular junction organization, IJOQ demonstrated superior analysis for all in vitro and in vivo experiments described herein. These data indicate that IJOQ is an unbiased, easy-to-use tool for fluorescence microscopy analysis and will serve as a valuable, automated resource to rapidly quantify intercellular junction disruption under diverse experimental conditions.

Bacterial pore-forming toxins

Pore-forming toxins (PFTs) are widely distributed in both Gram-negative and Gram-positive bacteria. PFTs can act as virulence factors that bacteria utilise in dissemination and host colonisation or, alternatively, they can be employed to compete with rival microbes in polymicrobial niches. PFTs transition from a soluble form to become membrane-embedded by undergoing large conformational changes. Once inserted, they perforate the membrane, causing uncontrolled efflux of ions and/or nutrients and dissipating the protonmotive force (PMF). In some instances, target cells intoxicated by PFTs display additional effects as part of the cellular response to pore formation. Significant progress has been made in the mechanistic description of pore formation for the different PFTs families, but in several cases a complete understanding of pore structure remains lacking. PFTs have evolved recognition mechanisms to bind specific receptors that define their host tropism, although this can be remarkably diverse even within the same family. Here we summarise the salient features of PFTs and highlight where additional research is necessary to fully understand the mechanism of pore formation by members of this diverse group of protein toxins.

Secnidazole Is a Promising Imidazole Mitigator of Serratia marcescens Virulence

Serratia marcescens is an opportunistic pathogen that causes diverse nosocomial infections. S. marcescens has developed considerable resistance to different antibiotics and is equipped with an armory of virulence factors. These virulence factors are regulated in S. marcescens by an intercellular communication system termed quorum sensing (QS). Targeting bacterial virulence and QS is an interesting approach to mitigating bacterial pathogenesis and overcoming the development of resistance to antimicrobials. In this study, we aimed to evaluate the anti-virulence activities of secnidazole on a clinical isolate of S. marcescens. The effects of secnidazole at sub-inhibitory concentrations (sub-MICs) on virulence factors, swarming motility, biofilm formation, proteases, hemolysin activity, and prodigiosin production were evaluated in vitro. Secnidazole's protective activity against S. marcescens pathogenesis was assessed in vivo in mice. Furthermore, a molecular docking study was conducted to evaluate the binding ability of secnidazole to the S. marcescens SmaR QS receptor. Our findings showed that secnidazole at sub-MICs significantly reduced S. marcescens virulence factor production in vitro and diminished its pathogenesis in mice. The insilico docking study revealed a great ability of secnidazole to competitively hinder the binding of the autoinducer to the SmaR QS receptor. In conclusion, secnidazole is a promising anti-virulence agent that may be used to control infections caused by S. marcescens.

ExlA Pore-Forming Toxin: Localization at the Bacterial Membrane, Regulation of Secretion by Cyclic-Di-GMP, and Detection In Vivo

ExlA is a highly virulent pore-forming toxin that has been recently discovered in outlier strains from Pseudomonas aeruginosa. ExlA is part of a two-partner secretion system, in which ExlA is the secreted passenger protein and ExlB the transporter embedded in the bacterial outer membrane. In previous work, we observed that ExlA toxicity in a host cell was contact-dependent. Here, we show that ExlA accumulates at specific points of the outer membrane, is likely entrapped within ExlB pore, and is pointing outside. We further demonstrate that ExlA is maintained at the membrane in conditions where the intracellular content of second messenger cyclic-di-GMP is high; lowering c-di-GMP levels enhances ExlB-dependent ExlA secretion. In addition, we set up an ELISA to detect ExlA, and we show that ExlA is poorly secreted in liquid culture, while it is highly detectable in broncho-alveolar lavage fluids of mice infected with an exlA+ strain. We conclude that ExlA translocation is halted at mid-length in the outer membrane and its secretion is regulated by c-di-GMP. In addition, we developed an immunological test able to quantify ExlA in biological samples.

Phylogenetically closely related pseudomonads isolated from arthropods exhibit differential insect-killing abilities and genetic variations in insecticidal factors

Strains belonging to the Pseudomonas protegens and Pseudomonas chlororaphis species are able to control soilborne plant pathogens and to kill pest insects by producing virulence factors such as toxins, chitinases, antimicrobials or two‐partner secretion systems. Most insecticidal Pseudomonas described so far were isolated from roots or soil. It is unknown whether these bacteria naturally occur in arthropods and how they interact with them. Therefore, we isolated P. protegens and P. chlororaphis from various healthy insects and myriapods, roots and soil collected in an agricultural field and a neighbouring grassland. The isolates were compared for insect killing, pathogen suppression and host colonization abilities. Our results indicate that neither the origin of isolation nor the phylogenetic position mirror the degree of insecticidal activity. Pseudomonas protegens strains appeared homogeneous regarding phylogeny, biocontrol and insecticidal capabilities, whereas P. chlororaphis strains were phylogenetically and phenotypically more heterogenous. A phenotypic and genomic analysis of five closely related P. chlororaphis isolates displaying varying levels of insecticidal activity revealed variations in genes encoding insecticidal factors that may account for the reduced insecticidal activity of certain isolates. Our findings point towards an adaption to insects within closely related pseudomonads and contribute to understand the ecology of insecticidal Pseudomonas.

Xylitol Inhibits Growth and Blocks Virulence in Serratia marcescens

Serratia marcescens is an opportunistic nosocomial pathogen and causes wound and burn infections. It shows high resistance to antibiotics and its pathogenicity is mediated by an arsenal of virulence factors. Another therapeutic option to such infections is targeting quorum sensing (QS), which controls the expression of different S. marcescens virulence factors. Prevention of QS can deprive S. marcescens from its bacterial virulence without applying stress on the bacterial growth and facilitates the eradication of the bacteria by immunity. The objective of the current study is to explore the antimicrobial and antivirulence activities of xylitol against S. marcescens. Xylitol could inhibit the growth of S. marcescens. Sub-inhibitory concentrations of xylitol could inhibit biofilm formation, reduce prodigiosin production, and completely block protease activity. Moreover, xylitol decreased swimming motility, swarming motility and increased the sensitivity to hydrogen peroxide. The expression of rsmA, pigP, flhC, flhD fimA, fimC, shlA bsmB, and rssB genes that regulate virulence factor production was significantly downregulated by xylitol. In silico study showed that xylitol could bind with the SmaR receptor by hydrophobic interaction and hydrogen bonding, and interfere with the binding of the natural ligand with SmaR receptor. An in vivo mice survival test confirmed the ability of xylitol to protect mice against the virulence of S. marcescens. In conclusion, xylitol is a growth and virulence inhibitor in S. marcescens and can be employed for the treatment of S. marcescens wound and burn infections.

Repurposing of antidiabetics as Serratia marcescens virulence inhibitors

BACKGROUND

Serratia marcescens becomes an apparent nosocomial pathogen and causes a variety of infections. S. marcescens possess various virulence factors that are regulated by intercellular communication system quorum sensing (QS). Targeting bacterial virulence is a proposed strategy to overcome bacterial resistance. Sitagliptin anti-QS activity has been demonstrated previously and we aimed in this study to investigate the effects of antidiabetic drugs vildagliptin and metformin compared to sitagliptin on S. marcescens pathogenesis.

METHODS

We assessed the effects of tested drugs in subinhibitory concentrations phenotypically on the virulence factors and genotypically on the virulence encoding genes' expressions. The protection of tested drugs on S. marcescens pathogenesis was performed in vivo. Molecular docking study has been conducted to evaluate the interference capabilities of tested drugs to the SmaR QS receptor.

RESULTS

Vildagliptin reduced the expression of virulence encoding genes but did not show in vitro or in vivo anti-virulence activities. Metformin reduced the expression of virulence encoding genes and inhibited bacterial virulence in vitro but did not show in vivo protection. Sitagliptin significantly inhibited virulence factors in vitro, reduced the expression of virulence factors and protected mice from S. marcescens. Docking study revealed that sitagliptin is more active than metformin and fully binds to SmaR receptor, whereas vildagliptin had single interaction to SmaR.

CONCLUSION

The downregulation of virulence genes was not enough to show anti-virulence activities. Hindering of QS receptors may play a crucial role in diminishing bacterial virulence.

Cadherin-mediated host-pathogen interactions

Cell adhesion molecules mediate cell-to-cell and cell-to-matrix adhesions and play an immense role in a myriad of physiological processes during the growth and development of a multicellular organism. Cadherins belong to a major group of membrane-bound cell surface proteins that, in coordination with nectins, drive the formation and maintenance of adherens junctions for mediating cell to cell adhesion, cellular communication and signalling. Alongside adhesive function, the involvement of cadherins in mediating host-pathogen interactions has been extensively explored in recent years. In this review, we provide an in-depth understanding of microbial pathogens and their virulence factors that exploit cadherins for their strategical invasion into the host cell. Furthermore, macromolecular interactions involving cadherins and various microbial factors such as secretory toxins and adhesins lead to the disintegration of host cell junctions followed by the entry of the pathogen or triggering downstream signalling pathways responsible for successful invasion of the pathogenic microbes are discussed. Besides providing a comprehensive insight into some of the structural complexes involving cadherins and microbial factors to offer the mechanistic details of host-pathogen interactions, the current review also highlights novel constituents of various cell signalling events such as endocytosis machinery elicited upon microbial infections.

Getting Your Neutrophil: Neutrophil Transepithelial Migration in the Lung

Neutrophil transepithelial migration is a fundamental process that facilitates the rapid trafficking of neutrophils to inflammatory foci and occurs across a diverse range of tissues. For decades there has been widespread interest in understanding the mechanisms that drive this migratory process in response to different pathogens and organ systems. This has led to the successful integration of key findings on neutrophil transepithelial migration from the intestines, lungs, liver, genitourinary tract, and other tissues into a single, cohesive model. However, recent studies have identified organ specific differences in neutrophil transepithelial migration. These findings support a model where the tissue in concert with the pro-inflammatory stimuli dictate a unique collection of signals that drive neutrophil trafficking. This review focuses on the mechanisms that drive neutrophil transepithelial migration in response to microbial infection of a single organ, the lung. Herein we provide a detailed analysis of the adhesion molecules and chemoattractants that contribute to the recruitment of neutrophil into the airways. We also highlight important advances in experimental models for studying neutrophil transepithelial migration in the lung over the last decade.

A Disintegrin and Metalloproteinase-Control Elements in Infectious Diseases

Despite recent advances in treatment strategies, infectious diseases are still under the leading causes of death worldwide. Although the activation of the inflammatory cascade is one prerequisite of defense, persistent and exuberant immune response, however, may lead to chronicity of inflammation predisposing to a temporal or permanent tissue damage not only of the site of infection but also among different body organs. The initial response to invading pathogens is mediated by the recognition through various pattern-recognition receptors along with cellular engulfment resulting in a coordinated release of soluble effector molecules and cytokines aiming to terminate the external stimuli. Members of the ‘a disintegrin and metalloproteinase’ (ADAM) family have the capability to proteolytically cleave transmembrane molecules close to the plasma membrane, a process called ectodomain shedding. In fact, in infectious diseases dysregulation of numerous ADAM substrates such as junction molecules (e.g., E-cadherin, VE-cadherin, JAM-A), adhesion molecules (e.g., ICAM-1, VCAM-1, L-selectin), and chemokines and cytokines (e.g., CXCL16, TNF-α) has been observed. The alpha-cleavage by ADAM proteases represents a rate limiting step for downstream regulated intramembrane proteolysis (RIPing) of several substrates, which influence cellular differentiation, cell signaling pathways and immune modulation. Both the substrates mentioned above and RIPing crucially contribute to a systematic damage in cardiovascular, endocrine, and/or gastrointestinal systems. This review will summarize the current knowledge of ADAM function and the subsequent RIPing in infectious diseases (e.g., pathogen recognition and clearance) and discuss the potential long-term effect on pathophysiological changes such as cardiovascular diseases.

Transcriptome plasticity underlying plant root colonization and insect invasion by Pseudomonas protegens

Pseudomonas protegens shows a high degree of lifestyle plasticity since it can establish both plant-beneficial and insect-pathogenic interactions. While P. protegens protects plants against soilborne pathogens, it can also invade insects when orally ingested leading to the death of susceptible pest insects. The mechanism whereby pseudomonads effectively switch between lifestyles, plant-beneficial or insecticidal, and the specific factors enabling plant or insect colonization are poorly understood. We generated a large-scale transcriptomics dataset of the model P. protegens strain CHA0 which includes data from the colonization of wheat roots, the gut of Plutella xylostella after oral uptake and the Galleria mellonella hemolymph after injection. We identified extensive plasticity in transcriptomic profiles depending on the environment and specific factors associated to different hosts or different stages of insect infection. Specifically, motor-activity and Reb toxin-related genes were highly expressed on wheat roots but showed low expression within insects, while certain antimicrobial compounds (pyoluteorin), exoenzymes (a chitinase and a polyphosphate kinase), and a transposase exhibited insect-specific expression. We further identified two-partner secretion systems as novel factors contributing to pest insect invasion. Finally, we use genus-wide comparative genomics to retrace the evolutionary origins of cross-kingdom colonization.

Mitochondrial Targeting of the Enteropathogenic Escherichia coli Map Triggers Calcium Mobilization, ADAM10-MAP Kinase Signaling, and Host Cell Apoptosis

Enteropathogenic E. coli (EPEC) is an important human diarrhea-causing bacterium. The pathogenic effects of EPEC largely depend upon its ability to inject a series of proteins, termed effectors, into the host cells. One such effector is the mitochondrion-associated protein (Map). Map has been shown to induce actin-rich projections (i.e., filopodia) on the infected cell surface and activate a Rho GTPase enzyme termed Cdc42. Nonetheless, although most injected Map localizes to host mitochondria, its functions in the mitochondria remain unknown. Here, we show that Map targeting of mitochondria stimulates the disruption of mitochondrial membrane potential to induce Ca²⁺ efflux into the host cytoplasm. The efflux stimulates the activity of a protein termed ADAM10, which induces activation of a mitogen-activated protein kinase cascade leading to host cell apoptosis. As apoptosis plays a central role in host-pathogen interactions, our findings provide novel insights into the functions of mitochondrial Map in promoting the EPEC disease.

The ability of diarrheagenic bacterial pathogens, such as enteropathogenic Escherichia coli (EPEC), to modulate the activity of mitogen-activated protein kinases (MAPKs) and cell survival has been suggested to benefit bacterial colonization and infection. However, our understanding of the mechanisms by which EPEC modulate these functions is incomplete. In this study, we show that the EPEC type III secreted effector Map stimulates the sheddase activity of the disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) and the ERK and p38 MAPK signaling cascades. Remarkably, all these activities were dependent upon the ability of Map to target host mitochondria, mainly via its mitochondrial toxicity region (MTR). Map targeting of mitochondria disrupted the mitochondrial membrane potential, causing extrusion of mitochondrial Ca²⁺ into the host cell cytoplasm. We also found that Map targeting of mitochondria is essential for triggering host cell apoptosis. Based on these findings, we propose a model whereby Map imported into mitochondria causes mitochondrial dysfunction and Ca²⁺ efflux into the host cytoplasm. Since Ca²⁺ has been reported to promote ADAM10 activation, the acute elevation of Ca²⁺ in the cytoplasm may stimulate the ADAM10 sheddase activity, resulting in the release of epidermal growth factors that stimulate the ERK signaling cascade. As p38 activity is also Ca²⁺ sensitive, elevation in cytoplasmic Ca²⁺ may independently also activate p38. We hypothesize that Map-dependent MAPK activation, combined with Map-mediated mitochondrial dysfunction, evokes mitochondrial host cell apoptosis, potentially contributing to EPEC colonization and infection of the gut.

Repurposing anti-diabetic drug "Sitagliptin" as a novel virulence attenuating agent in Serratia marcescens

Background

Serratia marcescens is an emerging pathogen that causes a variety of health care associated infections. S. marcescens is equipped with an arsenal of virulence factors such as biofilm formation, swimming and swarming motilities, prodigiosin, protease and others which enable it to initiate and cause the infection. These virulence factors are orchestrated under the umbrella of an intercellular communication system named Quorum sensing (QS). QS allows bacterial population to synchronize the expression of virulence genes upon detection of a chemical signaling molecule. Targeting bacterial virulence is a promising approach to attenuate bacteria and enhances the ability of immune system to eradicate the bacterial infection. Drug repurposing is an advantageous strategy that confers new applications for drugs outside the scope of their original medical use. This promising strategy offers the use of safe approved compounds, which potentially lowers the costs and shortens the time than that needed for development of new drugs. Sitagliptin is dipeptidyl peptidase-4 (DPP-4) inhibitor, is used to treat diabetes mellitus type II as it increases the production of insulin and decreasing the production of glucagon by the pancreas. We aimed in this study to repurpose sitagliptin, investigating the anti-virulence activities of sitagliptin on S. marcescens.

Methods

The effect of sub-inhibitory concentrations of sitagliptin on virulence factors; protease, prodigiosin, biofilm formation, swimming and swarming motilities was estimated phenotypically. The qRT-PCR was used to show the effect of sitagliptin on the expression of QS-regulated virulence genes. The in-vivo protective activity of sitagliptin on S. marcescens pathogenesis was evaluated on mice.

Results

Sitagliptin (1 mg/ml) significantly reduced the biofilm formation, swimming and swarming motilities, prodigiosin and protease. The qRT-PCR confirmed the effect on virulence as shown by down regulating the expression of fimA, fimC, flhC, flhD, bsmB, rssB, rsmA, pigP, and shlA genes. Moreover, the in-vivo findings showed the efficient ability of sitagliptin to weaken S. marcescens pathogenesis.

Conclusion

Sitagliptin is a promising anti-virulence agent against S. marcescens that may be beneficial in the control of healthcare associated infections caused by S. marcescens.

Impact of Bacterial Toxins in the Lungs

Bacterial toxins play a key role in the pathogenesis of lung disease. Based on their structural and functional properties, they employ various strategies to modulate lung barrier function and to impair host defense in order to promote infection. Although in general, these toxins target common cellular signaling pathways and host compartments, toxin- and cell-specific effects have also been reported. Toxins can affect resident pulmonary cells involved in alveolar fluid clearance (AFC) and barrier function through impairing vectorial Na⁺ transport and through cytoskeletal collapse, as such, destroying cell-cell adhesions. The resulting loss of alveolar-capillary barrier integrity and fluid clearance capacity will induce capillary leak and foster edema formation, which will in turn impair gas exchange and endanger the survival of the host. Toxins modulate or neutralize protective host cell mechanisms of both the innate and adaptive immunity response during chronic infection. In particular, toxins can either recruit or kill central players of the lung's innate immune responses to pathogenic attacks, i.e., alveolar macrophages (AMs) and neutrophils. Pulmonary disorders resulting from these toxin actions include, e.g., acute lung injury (ALI), the acute respiratory syndrome (ARDS), and severe pneumonia. When acute infection converts to persistence, i.e., colonization and chronic infection, lung diseases, such as bronchitis, chronic obstructive pulmonary disease (COPD), and cystic fibrosis (CF) can arise. The aim of this review is to discuss the impact of bacterial toxins in the lungs and the resulting outcomes for pathogenesis, their roles in promoting bacterial dissemination, and bacterial survival in disease progression.

Staphylococcus aureus α-toxin: small pore, large consequences

The small β-pore-forming α-toxin, also termed α-hemolysin or Hla is considered to be an important virulence factor of Staphylococcus aureus. Perforation of the plasma membrane (PM) by Hla leads to uncontrolled flux of ions and water. Already a small number of toxin pores seems to be sufficient to induce complex cellular responses, many of which depend on the efflux of potassium. In this article, we discuss the implications of secondary membrane lesions, for example, by endogenous channels, for Hla-mediated toxicity, for calcium-influx and membrane repair. Activation of purinergic receptors has been proposed to be a major contributor to the lytic effects of various pore forming proteins, but new findings raise doubts that this holds true for Hla. However, the recently discovered cellular pore forming proteins gasdermin D and Mixed lineage kinase domain-like pseudokinase (MLKL) which perforate the PM from the cytosolic side might contribute to both calcium-influx-dependent damage and membrane repair. Activation of endogenous pore forming proteins by Hla above a threshold concentration could explain the apparent dependence of pore characteristics on toxin concentrations. If secondary membrane damage in the aftermath of Hla-attack contributes significantly to overall PM permeability, it might be an interesting target for new therapeutic approaches.

Neuronal Wiskott-Aldrich syndrome protein regulates Pseudomonas aeruginosa-induced lung vascular permeability through the modulation of actin cytoskeletal dynamics

Pulmonary edema associated with increased vascular permeability is a severe complication of Pseudomonas (P.) aeruginosa-induced acute lung injury. The mechanisms underlying P aeruginosa-induced vascular permeability are not well understood. In the present study, we investigated the role of neuronal Wiskott Aldrich syndrome protein (N-WASP) in modulating P aeruginosa-induced vascular permeability. Using lung microvascular endothelial and alveolar epithelial cells, we demonstrated that N-WASP downregulation attenuated P aeruginosa-induced actin stress fiber formation and prevented paracellular permeability. P aeruginosa-induced dissociation between VE-cadherin and β-catenin, but increased association between N-WASP and VE-cadherin, suggesting a role for N-WASP in promoting P aeruginosa-induced adherens junction rupture. P aeruginosa increased N-WASP-Y256 phosphorylation, which required the activation of Rho GTPase and focal adhesion kinase. Increased N-WASP-Y256 phosphorylation promotes N-WASP and integrin αVβ6 association as well as TGF-β-mediated permeability across alveolar epithelial cells. Inhibition of N-WASP-Y256 phosphorylation by N-WASP-Y256F overexpression blocked N-WASP effects in P aeruginosa-induced actin stress fiber formation and increased paracellular permeability. In vivo, N-WASP knockdown attenuated the development of pulmonary edema and improved survival in a mouse model of P aeruginosa pneumonia. Together, our data demonstrate that N-WASP plays an essential role in P aeruginosa-induced vascular permeability and pulmonary edema through the modulation of actin cytoskeleton dynamics.

Phobalysin: Fisheye View of Membrane Perforation, Repair, Chemotaxis and Adhesion

Phobalysin P (PhlyP, for photobacterial lysin encoded on a plasmid) is a recently described small β-pore forming toxin of Photobacterium damselae subsp. damselae (Pdd). This organism, belonging to the family of Vibrionaceae, is an emerging pathogen of fish and various marine animals, which occasionally causes life-threatening soft tissue infections and septicemia in humans. By using genetically modified Pdd strains, PhlyP was found to be an important virulence factor. More recently, in vitro studies with purified PhlyP elucidated some basic consequences of pore formation. Being the first bacterial small β-pore forming toxin shown to trigger calcium-influx dependent membrane repair, PhlyP has advanced to a revealing model toxin to study this important cellular function. Further, results from co-culture experiments employing various Pdd strains and epithelial cells together with data on other bacterial toxins indicate that limited membrane damage may generally enhance the association of bacteria with target cells. Thereby, remodeling of plasma membrane and cytoskeleton during membrane repair could be involved. In addition, a chemotaxis-dependent attack-and track mechanism influenced by environmental factors like salinity may contribute to PhlyP-dependent association of Pdd with cells. Obviously, a synoptic approach is required to capture the regulatory links governing the interaction of Pdd with target cells. The characterization of Pdd’s secretome may hold additional clues because it may lead to the identification of proteases activating PhlyP’s pro-form. Current findings on PhlyP support the notion that pore forming toxins are not just killer proteins but serve bacteria to fulfill more subtle functions, like accessing their host.

Blowing epithelial cell bubbles with GumB: ShlA-family pore-forming toxins induce blebbing and rapid cellular death in corneal epithelial cells

Medical devices, such as contact lenses, bring bacteria in direct contact with human cells. Consequences of these host-pathogen interactions include the alteration of mammalian cell surface architecture and induction of cellular death that renders tissues more susceptible to infection. Gram-negative bacteria known to induce cellular blebbing by mammalian cells, Pseudomonas and Vibrio species, do so through a type III secretion system-dependent mechanism. This study demonstrates that a subset of bacteria from the Enterobacteriaceae bacterial family induce cellular death and membrane blebs in a variety of cell types via a type V secretion-system dependent mechanism. Here, we report that ShlA-family cytolysins from Proteus mirabilis and Serratia marcescens were required to induce membrane blebbling and cell death. Blebbing and cellular death were blocked by an antioxidant and RIP-1 and MLKL inhibitors, implicating necroptosis in the observed phenotypes. Additional genetic studies determined that an IgaA family stress-response protein, GumB, was necessary to induce blebs. Data supported a model where GumB and shlBA are in a regulatory circuit through the Rcs stress response phosphorelay system required for bleb formation and pathogenesis in an invertebrate model of infection and proliferation in a phagocytic cell line. This study introduces GumB as a regulator of S. marcescens host-pathogen interactions and demonstrates a common type V secretion system-dependent mechanism by which bacteria elicit surface morphological changes on mammalian cells. This type V secretion-system mechanism likely contributes bacterial damage to the corneal epithelial layer, and enables access to deeper parts of the tissue that are more susceptible to infection.

Type V Secretion Systems: An Overview of Passenger Domain Functions

Bacteria secrete proteins for different purposes such as communication, virulence functions, adhesion to surfaces, nutrient acquisition, or growth inhibition of competing bacteria. For secretion of proteins, Gram-negative bacteria have evolved different secretion systems, classified as secretion systems I through IX to date. While some of these systems consist of multiple proteins building a complex spanning the cell envelope, the type V secretion system, the subject of this review, is rather minimal. Proteins of the Type V secretion system are often called autotransporters (ATs). In the simplest case, a type V secretion system consists of only one polypeptide chain with a β-barrel translocator domain in the membrane, and an extracellular passenger or effector region. Depending on the exact domain architecture of the protein, type V secretion systems can be further separated into sub-groups termed type Va through e, and possibly another recently identified subtype termed Vf. While this classification works well when it comes to the architecture of the proteins, this is not the case for the function(s) of the secreted passenger. In this review, we will give an overview of the functions of the passengers of the different AT classes, shedding more light on the variety of functions carried out by type V secretion systems.

Functional Consequences of Calcium Influx Promoted by Bacterial Pore-Forming Toxins

Bacterial pore-forming toxins induce a rapid and massive increase in cytosolic Ca²⁺ concentration due to the formation of pores in the plasma membrane and/or activation of Ca²⁺-channels. As Ca²⁺ is an essential messenger in cellular signaling, a sustained increase in Ca²⁺ concentration has dramatic consequences on cellular behavior, eventually leading to cell death. However, host cells have adapted mechanisms to protect against Ca²⁺ intoxication, such as Ca²⁺ efflux and membrane repair. The final outcome depends upon the nature and concentration of the toxin and on the cell type. This review highlights the repercussions of Ca²⁺ overload on the induction of cell death, repair mechanisms, cellular adhesive properties, and the inflammatory response.

Pseudomonas aeruginosa Takes a Multi-Target Approach to Achieve Junction Breach

Pseudomonas aeruginosa is an opportunistic pathogen which uses a number of strategies to cross epithelial and endothelial barriers at cell–cell junctions. In this review, we describe how the coordinated actions of P. aeruginosa's virulence factors trigger various molecular mechanisms to disarm the junctional gate responsible for tissue integrity.

Exolysin Shapes the Virulence of Pseudomonas aeruginosa Clonal Outliers

Bacterial toxins are important weapons of toxicogenic pathogens. Depending on their origin, structure and targets, they show diverse mechanisms of action and effects on eukaryotic cells. Exolysin is a secreted 170 kDa pore-forming toxin employed by clonal outliers of Pseudomonas aeruginosa providing to some strains a hyper-virulent behaviour. This group of strains lacks the major virulence factor used by classical strains, the Type III secretion system. Here, we review the structural features of the toxin, the mechanism of its secretion and the effects of the pore formation on eukaryotic cells.