Protease IV, a quorum sensing-dependent protease of Pseudomonas aeruginosa modulates insect innate immunity

Multifaceted strategies for alleviating Pseudomonas aeruginosa infection by targeting protease activity: Natural and synthetic molecules

Pseudomonas aeruginosa has become a top-priority pathogen in the health sector because it is ubiquitous, has high metabolic/genetic versatility, and is identified as an opportunistic pathogen. The production of numerous virulence factors by P. aeruginosa was reported to act individually or cooperatively to make them robots invasion, adherences, persistence, proliferation, and protection against host immune systems. P. aeruginosa produces various kinds of extracellular proteases such as alkaline protease, protease IV, elastase A, elastase B, large protease A, Pseudomonas small protease, P. aeruginosa aminopeptidase, and MucD. These proteases effectively allow the cells to invade and destroy host cells. Thus, inhibiting these protease activities has been recognized as a promising approach to controlling the infection caused by P. aeruginosa. The present review discussed in detail the characteristics of these proteases and their role in infection to the host system. The second part of the review discussed the recent updates on the multiple strategies for attenuating or inhibiting protease activity. These strategies include the application of natural and synthetic molecules, as well as metallic/polymeric nanomaterials. It has also been reported that a propeptide present in the middle domain of protease IV also attenuates the virulence properties and infection ability of P. aeruginosa.

Subversion of the Complement System by Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic pathogen heavily implicated in chronic diseases. Immunocompromised patients that become infected with P. aeruginosa usually are afflicted with a lifelong chronic infection, leading to worsened patient outcomes. The complement system is an integral piece of the first line of defense against invading microorganisms. Gram-negative bacteria are thought to be generally susceptible to attack from complement; however, P. aeruginosa can be an exception, with certain strains being serum resistant. Various molecular mechanisms have been described that confer P. aeruginosa unique resistance to numerous aspects of the complement response. In this review, we summarize the current published literature regarding the interactions of P. aeruginosa and complement, as well as the mechanisms used by P. aeruginosa to exploit various complement deficiencies and the strategies used to disrupt or hijack normal complement activities.

Anthranilate Acts as a Signal to Modulate Biofilm Formation, Virulence, and Antibiotic Tolerance of Pseudomonas aeruginosa and Surrounding Bacteria

Anthranilate is a diffusible molecule produced by Pseudomonas aeruginosa and accumulates as P. aeruginosa grows. Anthranilate is an important intermediate for the synthesis of tryptophan and the Pseudomonas quinolone signal (PQS), as well as metabolized by the anthranilate dioxygenase complex (antABC operon products). Here we demonstrate that anthranilate is a key factor that modulates the pathogenicity-related phenotypes of P. aeruginosa and other surrounding bacteria in the environment, such as biofilm formation, antibiotic tolerance, and virulence. We found that the anthranilate levels in P. aeruginosa cultures rapidly increased in the stationary phase and then decreased again, forming an anthranilate peak. Biofilm formation, antibiotic susceptibility, and virulence of P. aeruginosa were significantly altered before and after this anthranilate peak. In addition, these phenotypes were all modified by the mutation of antABC and exogenous addition of anthranilate. Anthranilate also increased the antibiotic susceptibility of other species of bacteria, such as Escherichia coli, Salmonella enterica, Bacillus subtilis, and Staphylococcus aureus. Before the anthranilate peak, the low intracellular anthranilate level was maintained through degradation from the antABC function, in which induction of antABC was also limited to a small extent. The premature degradation of anthranilate, due to its high levels, and antABC expression early in the growth phase, appears to be toxic to the cells. From these results, we propose that by generating an anthranilate peak as a signal, P. aeruginosa may induce some sort of physiological change in surrounding cells. IMPORTANCE Pseudomonas aeruginosa is a notorious pathogen with high antibiotic resistance, strong virulence, and ability to cause biofilm-mediated chronic infection. We found that these characteristics change profoundly before and after the time when anthranilate is produced as an "anthranilate peak". This peak acts as a signal that induces physiological changes in surrounding cells, decreasing their antibiotic tolerance and biofilm formation. This study is important in that it provides a new insight into how microbial signaling substances can induce changes in the pathogenicity-related phenotypes of cells in the environment. In addition, this study shows that anthranilate can be used as an adjuvant to antibiotics.

Alleviation of Pseudomonas aeruginosa Infection by Propeptide-Mediated Inhibition of Protease IV

Pseudomonas aeruginosa, an opportunistic human pathogen, expresses protease IV (PIV) for infection. Since the PIV activity can be inhibited by its propeptide, we tried to alleviate the severity of P. aeruginosa infection using the purified PIV propeptide (PIV_pp). The PIV_pp treatment of P. aeruginosa could significantly inhibit the PIV activity and reduce the virulence of P. aeruginosa in multiple invertebrate infection models, such as nematodes, brine shrimp, and mealworms. The effectiveness of PIV_pp was further confirmed using mouse skin infection and acute/chronic lung infection models. The amount of PIV_pp that inhibited the PIV activity of P. aeruginosa by 65% could alleviate the severity of infection significantly in all of the skin and acute/chronic lung infections. In addition, the PIV_pp treatment of P. aeruginosa facilitated the healing of the skin wound infections and repressed the growth of P. aeruginosa in the infected lung. The PIV_pp itself did not cause the induction of inflammatory cytokines or have any harmful effects on host tissues and did not affect bacterial growth. Taken together, P. aeruginosa infections can be alleviated by PIV_pp treatment. IMPORTANCE Pseudomonas aeruginosa is a highly antibiotic-resistant pathogen and is extremely difficult to treat. Instead of using conventional antibiotics, we attempted to alleviate P. aeruginosa infection using factors that P. aeruginosa itself produces naturally. Extracellular proteases are powerful virulence factors and important targets to control the P. aeruginosa infections. Propeptides are originally expressed as part of extracellular proteases, inhibiting their activity until they go out of the cell, preventing them from becoming toxic to the cells themselves. We confirmed, from multiple animal experiments, that treating P. aeruginosa with the purified propeptide can alleviate its infectivity. Propeptides specifically inhibit only their cognate protease without inhibiting other essential proteases of the host. The development of resistance can be avoided because the propeptide-mediated inhibition is an inherent mechanism of P. aeruginosa; hence, it will be difficult for P. aeruginosa to alter this mechanism. Since propeptides do not affect bacterial growth, there is no selective pressure to develop resistant cells.

Antipathogenic Compounds That Are Effective at Very Low Concentrations and Have Both Antibiofilm and Antivirulence Effects against Pseudomonas aeruginosa

Pseudomonas aeruginosa, a human pathogen, causes both acute and chronic infections that are mediated by virulence factor production and biofilm formation. Since both characteristics of P. aeruginosa are regulated by quorum sensing (QS), we screened 126 synthetic chemicals for anti-QS activity and finally selected the compounds that have both antivirulence and antibiofilm activities. To efficiently screen the chemical library, the following reporter-based bioassay systems were used: the QS- or biofilm-specific promoter-lacZ fusions (lasI_p- or PA1897_p-lacZ for the QS activity and cdrA_p-lacZ for measuring the intracellular c-di-GMP levels). We also measured the production of virulence factors and biofilm formation in P. aeruginosa. A small-animal infection model using mealworms was also used for virulence analysis. From this screening, MHY1383 and MHY1387 were found to have both antivirulence and antibiofilm activities in P. aeruginosa. Most importantly, MHY1383 and MHY1387 exhibited these activities at very low concentrations, showing a significant anti-QS effect at 100 pM and an antibiofilm effect at 1 to 10 pM. By treating P. aeruginosa with these compounds, the virulence factor production and biofilm formation of P. aeruginosa were significantly reduced. These compounds can be developed as promising antipathogenic and antibiofilm drugs that can be applied in situations where such compounds must be used in an extremely low concentration. Our findings also offer a significant advantage for developing therapeutic agents with few adverse side effects.

IMPORTANCE Many antibiotics are increasingly losing their efficacy due to antibiotic resistance mediated by biofilm formation. In this study, we screened a synthetic chemical library and discovered several compounds that have both antivirulence and antibiofilm effects against Pseudomonas aeruginosa, a notorious human pathogen. Two of them had these effects at extremely low concentrations and are expected not to develop resistance, unlike conventional antibiotics, because they have no effect on the growth of bacteria. Our results strongly suggest that these compounds act on the target in a noncompetitive manner, indicating that they are distinct from other previously known quorum sensing inhibitors or biofilm inhibitors. Our findings offer a significant advantage for developing therapeutic agents with few adverse side effects.

A Rapid and Sensitive Detection Method for Pseudomonas aeruginosa Using Visualized Recombinase Polymerase Amplification and Lateral Flow Strip Technology

Pseudomonas aeruginosa is a common opportunistic pathogen that causes acute nosocomial necrotizing pneumonia and is the predominant source of chronic lung infections in patients with the genetic disorder cystic fibrosis. Early diagnosis in infected patients and monitoring P. aeruginosa contamination is therefore of great importance in controlling disease spread and development with timely drugs intervention treatment and cut off infection source. Traditional culture-biochemical methods are time consuming and highly dependent on technicians and expensive instruments. To address these challenges, the present study aimed to develop a rapid, sensitive, and specific, on-site detection method for P. aeruginosa based on recombinase polymerase amplification (RPA) combined with lateral flow strip (LFS) technology. The experimental process included screening and modification of primer and probe sets targeting the unique virulence gene elastase B (lasB); specificity detection in 29 strains of P. aeruginosa and 23 closely-related pathogenic bacteria; sensitivity measurements with gradient-diluted P. aeruginosa genomic DNA and probit regression analysis; and clinical application evaluation using 574 patients samples and calculating coincidence rate and kappa index value in comparison with the culture-biochemical method. The P. aeruginosa RPA-LFS assay could complete the amplification process at 37°C constant temperature within 30 min and results could be visualized by the naked eye within 10 min on LFS. The assay displayed high sensitivity with a limit of detection of 3.05 CFU/reaction. It also demonstrated high specificity by showing no cross reaction with other pathogenic bacteria, and rapidness by being completed in less than an hour. Furthermore, when used with clinical samples, the assay had a coincidence rate of 98.26% with the culture-biochemical method and a kappa index value of 0.9433. These data indicate that the RPA-LFS assay represents a major improvement for P. aeruginosa detection, especially in resource-limited areas.

The Bactericidal Tandem Drug, AB569: How to Eradicate Antibiotic-Resistant Biofilm Pseudomonas aeruginosa in Multiple Disease Settings Including Cystic Fibrosis, Burns/Wounds and Urinary Tract Infections

The life-threatening pandemic concerning multi-drug resistant (MDR) bacteria is an evolving problem involving increased hospitalizations, billions of dollars in medical costs and a remarkably high number of deaths. Bacterial pathogens have demonstrated the capacity for spontaneous or acquired antibiotic resistance and there is virtually no pool of organisms that have not evolved such potentially clinically catastrophic properties. Although many diseases are linked to such organisms, three include cystic fibrosis (CF), burn/blast wounds and urinary tract infections (UTIs), respectively. Thus, there is a critical need to develop novel, effective antimicrobials for the prevention and treatment of such problematic infections. One of the most formidable, naturally MDR bacterial pathogens is Pseudomonas aeruginosa (PA) that is particularly susceptible to nitric oxide (NO), a component of our innate immune response. This susceptibility sets the translational stage for the use of NO-based therapeutics during the aforementioned human infections. First, we discuss how such NO therapeutics may be able to target problematic infections in each of the aforementioned infectious scenarios. Second, we describe a recent discovery based on years of foundational information, a novel drug known as AB569. AB569 is capable of forming a "time release" of NO from S-nitrosothiols (RSNO). AB569, a bactericidal tandem consisting of acidified NaNO₂ (A-NO₂ ^-) and Na₂-EDTA, is capable of killing all pathogens that are associated with the aforementioned disorders. Third, we described each disease state in brief, the known or predicted effects of AB569 on the viability of PA, its potential toxicity and highly remote possibility for resistance to develop. Finally, we conclude that AB569 can be a viable alternative or addition to conventional antibiotic regimens to treat such highly problematic MDR bacterial infections for civilian and military populations, as well as the economical burden that such organisms pose.

Pseudomonas aeruginosa: An Audacious Pathogen with an Adaptable Arsenal of Virulence Factors

Pseudomonas aeruginosa is a dominant pathogen in people with cystic fibrosis (CF) contributing to morbidity and mortality. Its tremendous ability to adapt greatly facilitates its capacity to cause chronic infections. The adaptability and flexibility of the pathogen are afforded by the extensive number of virulence factors it has at its disposal, providing P. aeruginosa with the facility to tailor its response against the different stressors in the environment. A deep understanding of these virulence mechanisms is crucial for the design of therapeutic strategies and vaccines against this multi-resistant pathogen. Therefore, this review describes the main virulence factors of P. aeruginosa and the adaptations it undergoes to persist in hostile environments such as the CF respiratory tract. The very large P. aeruginosa genome (5 to 7 MB) contributes considerably to its adaptive capacity; consequently, genomic studies have provided significant insights into elucidating P. aeruginosa evolution and its interactions with the host throughout the course of infection.

LasR-deficient Pseudomonas aeruginosa variants increase airway epithelial mICAM-1 expression and enhance neutrophilic lung inflammation

Pseudomonas aeruginosa causes chronic airway infections, a major determinant of lung inflammation and damage in cystic fibrosis (CF). Loss-of-function lasR mutants commonly arise during chronic CF infections, are associated with accelerated lung function decline in CF patients and induce exaggerated neutrophilic inflammation in model systems. In this study, we investigated how lasR mutants modulate airway epithelial membrane bound ICAM-1 (mICAM-1), a surface adhesion molecule, and determined its impact on neutrophilic inflammation in vitro and in vivo. We demonstrated that LasR-deficient strains induce increased mICAM-1 levels in airway epithelial cells compared to wild-type strains, an effect attributable to the loss of mICAM-1 degradation by LasR-regulated proteases and associated with enhanced neutrophil adhesion. In a subacute airway infection model, we also observed that lasR mutant-infected mice displayed greater airway epithelial ICAM-1 expression and increased neutrophilic pulmonary inflammation. Our findings provide new insights into the intricate interplay between lasR mutants, LasR-regulated proteases and airway epithelial ICAM-1 expression, and reveal a new mechanism involved in the exaggerated inflammatory response induced by lasR mutants.

Multifaceted interactions between the pseudomonads and insects: mechanisms and prospects

Insects and bacteria are the most widespread groups of organisms found in nearly all habitats on earth, establishing diverse interactions that encompass the entire range of possible symbiotic associations from strict parasitism to obligate mutualism. The complexity of their interactions is instrumental in shaping the roles of insects in the environment, meanwhile ensuring the survival and persistence of the associated bacteria. This review aims to provide detailed insight on the multifaceted symbiosis between one of the most versatile bacterial genera, Pseudomonas (Gammaproteobacteria: Pseudomonadaceae) and a diverse group of insect species. The Pseudomonas engages with varied interactions with insects, being either a pathogen or beneficial endosymbiont, as well as using insects as vectors. In addition, this review also provides updates on existing and potential applications of Pseudomonas and their numerous insecticidal metabolites as biocontrol agents against pest insects for the improvement of integrated pest management strategies. Here, we have summarized several known modes of action and the virulence factors of entomopathogenic Pseudomonas strains essential for their pathogenicity against insects. Meanwhile, the beneficial interactions between pseudomonads and insects are currently limited to a few known insect taxa, despite numerous studies reporting identification of pseudomonads in the guts and haemocoel of various insect species. The vector-symbiont association between pseudomonads and insects can be diverse from strict phoresy to a role switch from commensalism to parasitism following a dose-dependent response. Overall, the pseudomonads appeared to have evolved independently to be either exclusively pathogenic or beneficial towards insects.

Iron facilitates the RetS-Gac-Rsm cascade to inversely regulate protease IV (piv) expression via the sigma factor PvdS in Pseudomonas aeruginosa

Pseudomonas aeruginosa produces several proteases, such as an elastase (LasB protease), a LasA protease, and protease IV (PIV), which are thought as significant virulence factors during infection. Regulators of LasA and LasB expression have been identified and well characterized; however, the molecular details of this regulation of protease IV (PIV) remained largely unknown. Here, we describe the interaction between protease IV and the RetS/Rsm signalling pathway, which plays a central role in controlling the production of multiple virulence factors and the switch from planktonic to biofilm lifestyle. We show that the expression of piv was reduced in ΔretS or ΔrsmA strain grown under restrictive conditions but was induced in ΔretS or ΔrsmA mutant grown under rich conditions as compared with wild-type parent. We compare the expression of piv under various conditions and found that iron facilitates RetS/Rsm system to lead this inverse regulation. Moreover, we reveal that the RetS/Rsm pathway regulates PIV production dependent on the alternative sigma factor PvdS. Collectively, this study extends the understanding of the RetS/Rsm regulatory cascade in response to environmental signals and provides insights into how P. aeruginosa adapts to the complex conditions.

Mechanisms for Induction of Microbial Extracellular Proteases in Response to Exterior Proteins

Proteins are a main organic nitrogen source for microorganisms. Many heterotrophic microorganisms secrete extracellular proteases (ex-proteases) to efficiently decompose proteins into oligopeptides and amino acids when exterior proteins are required for growth. These ex-proteases not only play important roles in microbial nutrient acquisition or host infection but also contribute greatly to the global recycling of carbon and nitrogen. Moreover, may microbial ex-proteases have important applications in industrial, medical, and biotechnological areas. Therefore, uncovering the mechanisms by which microorganisms initiate the expression of ex-protease genes in response to exterior proteins is of great significance. In this review, the progress made in understanding the induction mechanisms of microbial ex-proteases in response to exterior proteins is summarized, with a focus on the inducer molecules, membrane sensors, and downstream pathways. Problems to be solved for better understanding of the induction mechanisms of microbial ex-proteases are also discussed.

Discovery and Characterization of Pure RhlR Antagonists against Pseudomonas aeruginosa Infections

Pseudomonas aeruginosa (P. aeruginosa) is an opportunistic human pathogen that forms biofilms and produces virulence factors via quorum sensing (QS). Blocking the QS system in P. aeruginosa is an excellent strategy to reduce biofilm formation and the production of virulence factors. RhlR plays an essential role in the QS system of P. aeruginosa. We synthesized 55 analogues based on the chemical structure of 4-gingerol and evaluated their RhlR inhibitory activities using the cell-based reporter strain assay. Comprehensive structure-activity relationship studies identified the alkynyl ketone 30 as the most potent RhlR antagonist. This compound displayed selective RhlR antagonism over LasR and PqsR, strong inhibition of biofilm formation, and reduced production of virulence factors in P. aeruginosa. Furthermore, the survival rate of Tenebrio molitor larvae treated with 30 in vivo greatly improved. Therefore, compound 30, a pure RhlR antagonist, can be utilized for developing QS-modulating molecules in the control of P. aeruginosa infections.

Erwinia carotovora Quorum Sensing System Regulates Host-Specific Virulence Factors and Development Delay in Drosophila melanogaster

Integration of genetic networks allows bacteria to rapidly adapt to changing environments. This is particularly important in bacteria that interact with multiple hosts. Erwinia carotovora is a plant pathogen that uses Drosophila melanogaster as a vector. To interact with these two hosts, Ecc15 uses different sets of virulence factors: plant cell wall-degrading enzymes to infect plants and the Erwinia virulence factor (evf) to infect Drosophila. Our work shows that, despite the virulence factors being specific for each host, both sets are coactivated by homoserine lactone quorum sensing and by the two-component GacS/A system in infected plants. This regulation is essential for Ecc15 loads in the gut of Drosophila and minimizes the developmental delay caused by the bacteria with respect to the insect vector. Our findings provide evidence that coactivation of the host-specific factors in the plant may function as a predictive mechanism to maximize the probability of transit of the bacteria between hosts.

Multihost bacteria have to rapidly adapt to drastic environmental changes, relying on a fine integration of multiple stimuli for an optimal genetic response. Erwinia carotovora spp. are phytopathogens that cause soft-rot disease. Strain Ecc15 in particular is a model for bacterial oral-route infection in Drosophila melanogaster as it harbors a unique gene, evf, that encodes the Erwinia virulence factor (Evf), which is a major determinant for infection of the D. melanogaster gut. However, the factors involved in the regulation of evf expression are poorly understood. We investigated whether evf could be controlled by quorum sensing as, in the Erwinia genus, quorum sensing regulates pectolytic enzymes, the major virulence factors needed to infect plants. Here, we show that transcription of evf is positively regulated by quorum sensing in Ecc15 via acyl-homoserine lactone (AHL) signal synthase ExpI and AHL receptors ExpR1 and ExpR2. We also show that the load of Ecc15 in the gut depends upon the quorum sensing-mediated regulation of evf. Furthermore, we demonstrate that larvae infected with Ecc15 suffer a developmental delay as a direct consequence of the regulation of evf via quorum sensing. Finally, we demonstrate that evf is coexpressed with plant cell wall-degrading enzymes (PCWDE) during plant infection in a quorum sensing-dependent manner. Overall, our results show that Ecc15 relies on quorum sensing to control production of both pectolytic enzymes and Evf. This regulation influences the interaction of Ecc15 with its two known hosts, indicating that quorum sensing signaling may impact bacterial dissemination via insect vectors that feed on rotting plants.

Inhibition of biofilm and virulence properties of Pseudomonas aeruginosa by sub-inhibitory concentrations of aminoglycosides

Aminoglycosides are a commonly used class of antibiotics; however, their application has been discontinued due to the emergence of multi-drug resistance bacterial strains. In the present study, the subinhibitory concentrations (sub-MIC) of several aminoglycosides were determined and tested as an antibiofilm and for their anti-virulence properties against Pseudomonas aeruginosa PAO1, which is an opportunistic foodborne pathogen. P. aeruginosa PAO1 exhibits multiple mechanisms of resistance, including the formation of biofilm and production of several virulence factors, against aminoglycoside antibiotics. The sub-MIC of these antibiotics exhibited biofilm inhibition of P. aeruginosa in alkaline TSB (pH 7.9). Moreover, various concentrations of these aminoglycosides also eradicate the mature biofilm of P. aeruginosa. In the presence of sub-MIC of aminoglycosides, the morphological changes of P. aeruginosa were found to change from rod-shaped to the filamentous, elongated, and streptococcal forms. Similar growth conditions and sub-MIC of aminoglycosides were also found to attenuate several virulence properties of P. aeruginosa PAO1. Molecular docking studies demonstrate that these aminoglycosides possess strong binding properties with the LasR protein, which is a well-characterized quorum-sensing receptor of P. aeruginosa. The present study suggests a new approach to revitalize aminoglycosides as antibiofilm and antivirulence drugs to treat infections caused by pathogenic bacteria.

Cooperation and Cheating through a Secreted Aminopeptidase in the Pseudomonas aeruginosa RpoS Response

Bacterial stress responses are generally considered protective measures taken by individual cells. Enabled by an experimental evolution approach, we describe a contrasting property, collective nutrient acquisition, in the RpoS-dependent stress response of the opportunistic human pathogen P. aeruginosa. Specifically, we identify the secreted P. aeruginosa aminopeptidase (PaAP) as an essential RpoS-controlled function in extracellular proteolysis. As a secreted “public good,” PaAP permits cheating by rpoS mutants that save the metabolic costs of expressing RpoS-controlled genes dispensable under the given growth conditions. Proteolytic enzymes are important virulence factors in P. aeruginosa pathogenesis and constitute a potential target for antimicrobial therapy. More broadly, our work contributes to recent findings in higher organisms that stress affects not only individual fitness and competitiveness but also cooperative behavior.

The global stress response controlled by the alternative sigma factor RpoS protects enteric bacteria from a variety of environmental stressors. The role of RpoS in other, nonenteric bacteria, such as the opportunistic pathogen Pseudomonas aeruginosa, is less well understood. Here, we employed experimental social evolution to reveal that cooperative behavior via secreted public goods is an important function in the RpoS response of P. aeruginosa. Using whole-genome sequencing, we identified rpoS loss-of-function mutants among isolates evolved in a protein growth medium that requires extracellular proteolysis. We found that rpoS mutants comprise up to 25% of the evolved population and that they behave as social cheaters, with low fitness in isolation but high fitness in mixed culture with the cooperating wild type. We conclude that rpoS mutants cheat because they exploit an RpoS-controlled public good produced by the wild type, the secreted aminopeptidase PaAP, and because they do not carry the metabolic costs of expressing PaAP and many other gene products in the large RpoS regulon. Our results suggest that PaAP is an integral part of a proteolytic sequence in P. aeruginosa that permits the utilization of protein as a nutrient source. Our work broadens the scope of stress response functions in bacteria.

Products Derived from Buchenavia tetraphylla Leaves Have In Vitro Antioxidant Activity and Protect Tenebrio molitor Larvae against Escherichia coli-Induced Injury

The relevance of oxidative stress in the pathogenesis of several diseases (including inflammatory disorders) has traditionally led to the search for new sources of antioxidant compounds. In this work, we report the selection of fractions with high antioxidant action from B. tetraphylla (BT) leaf extracts. In vitro methods (DPPH and ABTS assays; determination of phenolic and flavonoid contents) were used to select products derived from B. tetraphylla with high antioxidant action. Then, the samples with the highest potentials were evaluated in a model of injury based on the inoculation of a lethal dose of heat-inactivated Escherichia coli in Tenebrio molitor larvae. Due to its higher antioxidant properties, the methanolic extract (BTME) was chosen to be fractionated using Sephadex LH-20 column-based chromatography. Two fractions from BTME (BTFC and BTFD) were the most active fractions. Pre-treatment with these fractions protected larvae of T. molitor from the stress induced by inoculation of heat-inactivated E. coli. Similarly, BTFC and BTFD increased the lifespan of larvae infected with a lethal dose of enteroaggregative E. coli 042. NMR data indicated the presence of aliphatic compounds (terpenes, fatty acids, carbohydrates) and aromatic compounds (phenolic compounds). These findings suggested that products derived from B. tetraphylla leaves are promising candidates for the development of antioxidant and anti-infective agents able to treat oxidative-related dysfunctions.

Pseudomonas aeruginosa Toxin ExoU as a Therapeutic Target in the Treatment of Bacterial Infections

The opportunistic pathogen Pseudomonas aeruginosa employs the type III secretion system (T3SS) and four effector proteins, ExoS, ExoT, ExoU, and ExoY, to disrupt cellular physiology and subvert the host’s innate immune response. Of the effector proteins delivered by the T3SS, ExoU is the most toxic. In P. aeruginosa infections, where the ExoU gene is expressed, disease severity is increased with poorer prognoses. This is considered to be due to the rapid and irreversible damage exerted by the phospholipase activity of ExoU, which cannot be halted before conventional antibiotics can successfully eliminate the pathogen. This review will discuss what is currently known about ExoU and explore its potential as a therapeutic target, highlighting some of the small molecule ExoU inhibitors that have been discovered from screening approaches.

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.

Pseudomonas aeruginosa Keratitis: Protease IV and PASP as Corneal Virulence Mediators

Pseudomonas aeruginosa is a leading cause of bacterial keratitis, especially in users of contact lenses. These infections are characterized by extensive degradation of the corneal tissue mediated by Pseudomonas protease activities, including both Pseudomonas protease IV (PIV) and the P. aeruginosa small protease (PASP). The virulence role of PIV was determined by the reduced virulence of a PIV-deficient mutant relative to its parent strain and the mutant after genetic complementation (rescue). Additionally, the non-ocular pathogen Pseudomonas putida acquired corneal virulence when it produced active PIV from a plasmid-borne piv gene. The virulence of PIV is not limited to the mammalian cornea, as evidenced by its destruction of respiratory surfactant proteins and the cytokine interleukin-22 (IL-22), the key inducer of anti-bacterial peptides. Furthermore, PIV contributes to the P. aeruginosa infection of both insects and plants. A possible limitation of PIV is its inefficient digestion of collagens; however, PASP, in addition to cleaving multiple soluble proteins, is able to efficiently cleave collagens. A PASP-deficient mutant lacks the corneal virulence of its parent or rescue strain evidencing its contribution to corneal damage, especially epithelial erosion. Pseudomonas-secreted proteases contribute importantly to infections of the cornea, mammalian lung, insects, and plants.

Catabolism of Nucleic Acids by a Cystic Fibrosis Pseudomonas aeruginosa Isolate: An Adaptive Pathway to Cystic Fibrosis Sputum Environment

Pseudomonas aeruginosa is a major cause of morbidity and mortality in patients with cystic fibrosis (CF). We undertook Biolog Phenotype Microarray testing of P. aeruginosa CF isolates to investigate their catabolic capabilities compared to P. aeruginosa laboratory strains PAO1 and PA14. One strain, PASS4, displayed an unusual phenotype, only showing strong respiration on adenosine and inosine. Further testing indicated that PASS4 could grow on DNA as a sole carbon source, with a higher biomass production than PAO1. This suggested that PASS4 was specifically adapted to metabolize extracellular DNA, a substrate present at high concentrations in the CF lung. Transcriptomic and proteomic profiling of PASS4 and PAO1 when grown with DNA as a sole carbon source identified a set of upregulated genes, including virulence and host-adaptation genes. PASS4 was unable to utilize N-Acetyl-D-glucosamine, and when we selected PASS4 mutants able to grow on this carbon source, they also displayed a gain in ability to catabolize a broad range of other carbon sources. Genome sequencing of the mutants revealed they all contained mutations within the purK gene, encoding a key protein in the de novo purine biosynthesis pathway. This suggested that PASS4 was a purine auxotroph. Growth assays in the presence of 2 mM adenosine and the complementation of PASS4 with an intact purK gene confirmed this conclusion. Purine auxotrophy may represent a viable microbial strategy for adaptation to DNA-rich environments such as the CF lung.

Social cheating in a Pseudomonas aeruginosa quorum-sensing variant

The opportunistic bacterial pathogen Pseudomonas aeruginosa has a layered acyl-homoserine lactone (AHL) quorum-sensing (QS) system, which controls production of a variety of extracellular metabolites and enzymes. The LasRI system activates genes including those coding for the extracellular protease elastase and for the second AHL QS system, RhlRI. Growth of P. aeruginosa on casein requires elastase production and LasR-mutant social cheats emerge in populations growing on casein. P. aeruginosa colonizes the lungs of individuals with the genetic disease cystic fibrosis (CF), and LasR mutants can be isolated from the colonized lungs; however, unlike laboratory-generated LasR mutants, many of these CF isolates have functioning RhlR-RhlI systems. We show that one such mutant can use the RhlR-RhlI system to activate expression of elastase and grow on casein. We carried out social-evolution experiments by growing this isolate on caseinate and, as with wild-type P. aeruginosa, elastase-negative mutants emerge as cheats, but these are not RhlR mutants; rather, they are mutants that do not produce the non-AHL Pseudomonas quinolone signal (PQS). Furthermore, we generated a RhlRI mutant and showed it had a fitness defect when growing together with the parent. Apparently, RhlR QS and PQS collude to support growth on caseinate in the absence of a functional LasR. Our findings provide a plausible explanation as to why P. aeruginosa LasR mutants, but not RhlR mutants, are common in CF lungs.

Diffusible signal factor (DSF)-mediated quorum sensing modulates expression of diverse traits in Xanthomonas citri and responses of citrus plants to promote disease

Background

The gram-negative Xanthomonas genus contains a large group of economically important plant pathogens, which cause severe diseases on many crops worldwide. The diffusible signal factor (DSF) - mediated quorum sensing (QS) system coordinates expression of virulence factors in plant pathogenic Xanthomonas spp. However, the regulatory effects of this system during the Xanthomonas- plant interactions remain unclear from both the pathogen and host aspects.

Results

In this study, we investigated the in planta DSF- mediated QS regulon of X. citri subsp. citri (Xac), the causal agent of citrus canker. We also characterized the transcriptional responses of citrus plants to DSF-mediated Xac infection via comparing the gene expression patterns of citrus trigged by wild type Xac strain 306 with those trigged by its DSF- deficient (∆rpfF) mutant using the dual RNA-seq approach. Comparative global transcript profiles of Xac strain 306 and the ∆rpfF mutant during host infection revealed that DSF- mediated QS specifically modulates bacterial adaptation, nutrition uptake and metabolisms, stress tolerance, virulence, and signal transduction to favor host infection. The transcriptional responses of citrus to DSF-mediated Xac infection are characterized by downregulation of photosynthesis genes and plant defense related genes, suggesting photosynthetically inactive reactions and repression of defense responses. Alterations of phytohormone metabolism and signaling pathways were also triggered by DSF-mediated Xac infection to benefit the pathogen.

Conclusions

Collectively, our findings provide new insight into the DSF- mediated QS regulation during plant-pathogen interactions and advance the understanding of traits used by Xanthomonas to promote infection on host plants.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5384-4) contains supplementary material, which is available to authorized users.

Manipulation of the silkworm immune system by a metalloprotease from the pathogenic bacterium Pseudomonas aeruginosa

Antimicrobial peptide (AMP) production and melanization are two key humoral immune responses in insects. Induced synthesis of AMPs results from Toll and IMD signal transduction whereas melanization depends on prophenoloxidase (PPO) activation system. During invasion, pathogens produce toxins and other virulent factors to counteract host immune responses. Here we show that the pathways leading to PPO activation and AMP synthesis in the silkworm Bombyx mori are affected by a metalloprotease, named elastase B, secreted by Pseudomonas aeruginosa (PAO1). The metalloprotease gene (lasB) was expressed shortly after PAO1 cells had been injected into the larval silkworm hemocoel, leading to an increase of elastase activity. Injection of the purified PAO1 elastase B into silkworm hemolymph compromised PPO activation. In contrast, the protease caused a level increase of gloverin, an AMP in the hemolymph. To verify our results obtained using the purified elastase B, we infected B. mori with PAO1 ΔlasB mutant and found that PO activity in hemolymph of the PAO1 ΔlasB-infected larvae was significantly higher than that in the wild type-infected. The mutant-inhabited hemolymph had lower levels of gloverin and antimicrobial activity. PAO1 ΔlasB showed a decreased viability in the silkworm hemolymph whereas the host had a lower mortality. In addition, the effects caused by the ΔlasB mutant were restored by a complementary strain. These data collectively indicated that the elastase B produced by PAO1 is an important virulent factor that manipulates the silkworm immune system during infection.

Bacterial ornithine lipid, a surrogate membrane lipid under phosphate-limiting conditions, plays important roles in bacterial persistence and interaction with host

Ornithine lipids (OLs) are bacteria-specific lipids that are found in the outer membrane of Gram (-) bacteria and increase as surrogates of phospholipids under phosphate-limited environmental conditions. We investigated the effects of OL increase in bacterial membranes on pathogen virulence and the host immune response. In Pseudomonas aeruginosa, we increased OL levels in membranes by overexpressing the OL-synthesizing operon (olsBA). These increases changed the bacterial surface charge and hydrophobicity, which reduced bacterial susceptibility to antibiotics and antimicrobial peptides (AMPs), interfered with the binding of macrophages to bacterial cells and enhanced bacterial biofilm formation. When grown under low phosphate conditions, P. aeruginosa became more persistent in the treatment of antibiotics and AMPs in an olsBA-dependent manner. While OLs increased persistence, they attenuated P. aeruginosa virulence; in host cells, they reduced the production of inflammatory factors (iNOS, COX-2, PGE₂ and nitric oxide) and increased intracellular Ca²⁺ release. Exogenously added OL had similar effects on P. aeruginosa and host cells. Our results suggest that bacterial OL plays important roles in bacteria-host interaction in a way that enhances bacterial persistence and develops chronic adaptation to infection.

A Metabolic Trade-Off Modulates Policing of Social Cheaters in Populations of Pseudomonas aeruginosa

Pseudomonas aeruginosa uses quorum sensing (QS) to regulate the production of public goods such as the secreted protease elastase. P. aeruginosa requires the LasI-LasR QS circuit to induce elastase and enable growth on casein as the sole carbon and energy source. The LasI-LasR system also induces a second QS circuit, the RhlI-RhlR system. During growth on casein, LasR-mutant social cheaters emerge, and this can lead to a population collapse. In a minimal medium containing ammonium sulfate as a nitrogen source, populations do not collapse, and cheaters and cooperators reach a stable equilibrium; however, without ammonium sulfate, cheaters overtake the cooperators and populations collapse. We show that ammonium sulfate enhances the activity of the RhlI-RhlR system in casein medium and this leads to increased production of cyanide, which serves to control levels of cheaters. This enhancement of cyanide production occurs because of a trade-off in the metabolism of glycine: exogenous ammonium ion inhibits the transformation of glycine to 5,10-methylenetetrahydrofolate through a reduction in the expression of the glycine cleavage genes gcvP1 and gcvP2, thereby increasing the availability of glycine as a substrate for RhlR-regulated hydrogen cyanide synthesis. Thus, environmental ammonia enhances cyanide production and stabilizes QS in populations of P. aeruginosa.

Pseudomonas aeruginosa quorum sensing modulates immune responses: An updated review article

Pseudomonas aeruginosa is an opportunistic bacterium which induces some complications in immunocompromised patients. Pseudomonas aeruginosa is a quorum-sensing using bacterium which regulates its genes expression. The bacterium uses two famous pathways for quorum sensing entitled LasI/LasR and RhlI/RhlR systems. It has been documented that the bacteria which use quorum sensing are able to overcome immune responses. This review article aims to present recent information regarding the effects of Pseudomonas aeruginosa quorum sensing systems on the host immune responses.

Post-secretional activation of Protease IV by quorum sensing in Pseudomonas aeruginosa

Protease IV (PIV), a key virulence factor of Pseudomonas aeruginosa is a secreted lysyl-endopeptidase whose expression is induced by quorum sensing (QS). We found that PIV expressed in QS mutant has severe reduction of activity in culture supernatant (CS), even though it is overexpressed to high level. PIV purified from the QS mutant (M-PIV) had much lower activity than the PIV purified from wild type (P-PIV). We found that the propeptide cleaved from prepro-PIV was co-purified with M-PIV, but never with P-PIV. Since the activity of M-PIV was restored by adding the CS of QS-positive and PIV-deficient strain, we hypothesized that the propeptide binds to and inhibits PIV, and is degraded to activate PIV by a QS-dependent factor. In fact, the CS of the QS-positive and PIV-deficient strain was able to degrade the propeptide. Since the responsible factor should be a QS-dependently expressed extracellular protease, we tested QS-dependent proteases of P. aeruginosa and found that LasB (elastase) can degrade the propeptide and activate M-PIV. We purified the propeptide of PIV and confirmed that the propeptide can bind to and inhibit PIV. We suggest that PIV is post-secretionally activated through the extracellular degradation of the propeptide by LasB, a QS-dependent protease.

Manipulating virulence factor availability can have complex consequences for infections

Given the rise of bacterial resistance against antibiotics, we urgently need alternative strategies to fight infections. Some propose we should disarm rather than kill bacteria, through targeted disruption of their virulence factors. It is assumed that this approach (i) induces weak selection for resistance because it should only minimally impact bacterial fitness, and (ii) is specific, only interfering with the virulence factor in question. Given that pathogenicity emerges from complex interactions between pathogens, hosts and their environment, such assumptions may be unrealistic. To address this issue in a test case, we conducted experiments with the opportunistic human pathogen Pseudomonas aeruginosa, where we manipulated the availability of a virulence factor, the iron‐scavenging pyoverdine, within the insect host Galleria mellonella. We observed that pyoverdine availability was not stringently predictive of virulence and affected bacterial fitness in nonlinear ways. We show that this complexity could partly arise because pyoverdine availability affects host responses and alters the expression of regulatorily linked virulence factors. Our results reveal that virulence factor manipulation feeds back on pathogen and host behaviour, which in turn affects virulence. Our findings highlight that realizing effective and evolutionarily robust antivirulence therapies will ultimately require deeper engagement with the intrinsic complexity of host–pathogen systems.

Possible mechanisms of Pseudomonas aeruginosa-associated lung disease

Pseudomonas aeruginosa is an opportunistic bacterium causing lung injury in immunocompromised patients correlated with high morbidity and mortality. Many bacteria, including P. aeruginosa, use extracellular signals to synchronize group behaviors, a process known as quorum sensing (QS). In the P. aeruginosa complex QS system controls expression of over 300 genes, including many involved in host colonization and disease. P. aeruginosa infection elicits a complex immune response due to a large number of immunogenic factors present in the bacteria or released during infection. Here, we focused on the mechanisms by which P. aeruginosa triggers lung injury and inflammation, debating the possible ways that P. aeruginosa evades the host immune system, which leads to immune suppression and resistance.

Campylobacter jejuni outer membrane vesicle-associated proteolytic activity promotes bacterial invasion by mediating cleavage of intestinal epithelial cell E-cadherin and occludin

Outer membrane vesicles (OMVs) play an important role in the pathogenicity of Gram-negative bacteria. Campylobacter jejuni produces OMVs that trigger IL-8, IL-6, hBD-3 and TNF-α responses from T84 intestinal epithelial cells and are cytotoxic to Caco-2 IECs and Galleria mellonella larvae. Proteomic analysis of 11168H OMVs identified the presence of three proteases, HtrA, Cj0511 and Cj1365c. In this study, 11168H OMVs were shown to possess proteolytic activity that was reduced by pretreatment with specific serine protease inhibitors. OMVs isolated from 11168H htrA, Cj0511 or Cj1365c mutants possess significantly reduced proteolytic activity. 11168H OMVs are able to cleave both E-cadherin and occludin, but this cleavage is reduced with OMVs pretreated with serine protease inhibitors and also with OMVs isolated from htrA or Cj1365c mutants. Co-incubation of T84 monolayers with 11168H OMVs results in a visible reduction in both E-cadherin and occludin. The addition of 11168H OMVs to the co-culture of live 11168H bacteria with T84 cells results in enhanced levels of bacterial adhesion and invasion in a time-dependent and dose-dependent manner. Further investigation of the cleavage of host cell structural proteins by C. jejuni OMVs should enhance our understanding of the interactions of this important pathogen with intestinal epithelial cells.

Quorum sensing and policing of Pseudomonas aeruginosa social cheaters

The bacterium Pseudomonas aeruginosa is an opportunistic human pathogen that uses a quorum sensing signal cascade to activate expression of dozens of genes when sufficient population densities have been reached. Quorum sensing controls production of several key virulence factors, including secreted proteases such as elastase. Cooperating groups of bacteria growing on protein are susceptible to social cheating by quorum-sensing defective mutants. A possible way to restrict cheater emergence is by policing where cooperators produce costly goods to sanction or punish cheats. The P. aeruginosa LasR-LasI quorum sensing system controls genes including those encoding proteases and also those encoding a second quorum-sensing system, the RhlR-RhlI system, which controls numerous genes including those for cyanide production. By using RhlR quorum sensing mutants and cyanide synthesis mutants, we show that cyanide production is costly and cyanide-producing cooperators use cyanide to punish LasR-null social cheaters. Cooperators are less susceptible to cyanide than are LasR mutants. These experiments demonstrate policing in P. aeruginosa, provide a mechanistic understanding of policing, and show policing involves the cascade organization of the two quorum sensing systems in this bacterium.