Activation of the Pseudomonas aeruginosa AlgU regulon through mucA mutation inhibits cyclic AMP/Vfr signaling

Amoebae as training grounds for microbial pathogens

Grazing of amoebae on microorganisms represents one of the oldest predator-prey dynamic relationships in nature. It represents a genetic "melting pot" for an ancient and continuous multi-directional inter- and intra-kingdom horizontal gene transfer between amoebae and its preys, intracellular microbial residents, endosymbionts, and giant viruses, which has shaped the evolution, selection, and adaptation of microbes that evade degradation by predatory amoeba. Unicellular phagocytic amoebae are thought to be the ancient ancestors of macrophages with highly conserved eukaryotic processes. Selection and evolution of microbes within amoeba through their evolution to target highly conserved eukaryotic processes have facilitated the expansion of their host range to mammals, causing various infectious diseases. Legionella and environmental Chlamydia harbor an immense number of eukaryotic-like proteins that are involved in ubiquitin-related processes or are tandem repeats-containing proteins involved in protein-protein and protein-chromatin interactions. Some of these eukaryotic-like proteins exhibit novel domain architecture and novel enzymatic functions absent in mammalian cells, such as ubiquitin ligases, likely acquired from amoebae. Mammalian cells and amoebae may respond similarly to microbial factors that target highly conserved eukaryotic processes, but mammalian cells may undergo an accidental response to amoeba-adapted microbial factors. We discuss specific examples of microbes that have evolved to evade amoeba predation, including the bacterial pathogens- Legionella, Chlamydia, Coxiella, Rickettssia, Francisella, Mycobacteria, Salmonella, Bartonella, Rhodococcus, Pseudomonas, Vibrio, Helicobacter, Campylobacter, and Aliarcobacter. We also discuss the fungi Cryptococcus, and Asperigillus, as well as amoebae mimiviruses/giant viruses. We propose that amoeba-microbe interactions will continue to be a major "training ground" for the evolution, selection, adaptation, and emergence of microbial pathogens equipped with unique pathogenic tools to infect mammalian hosts. However, our progress will continue to be highly dependent on additional genomic, biochemical, and cellular data of unicellular eukaryotes.

Transcriptional Regulators Controlling Virulence in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a pathogen capable of colonizing virtually every human tissue. The host colonization competence and versatility of this pathogen are powered by a wide array of virulence factors necessary in different steps of the infection process. This includes factors involved in bacterial motility and attachment, biofilm formation, the production and secretion of extracellular invasive enzymes and exotoxins, the production of toxic secondary metabolites, and the acquisition of iron. Expression of these virulence factors during infection is tightly regulated, which allows their production only when they are needed. This process optimizes host colonization and virulence. In this work, we review the intricate network of transcriptional regulators that control the expression of virulence factors in P. aeruginosa, including one- and two-component systems and σ factors. Because inhibition of virulence holds promise as a target for new antimicrobials, blocking the regulators that trigger the production of virulence determinants in P. aeruginosa is a promising strategy to fight this clinically relevant pathogen.

AlgU controls environmental stress adaptation, biofilm formation, motility, pyochelin synthesis and antagonism potential in Pseudomonas protegens SN15-2

Pseudomonas protegens is a typical plant-growth-promoting rhizobacterium that can serve as an agricultural biocontrol agent. The extracytoplasmic function (ECF) sigma factor AlgU is a global transcription regulator controlling stress adaption and virulence in Pseudomonas aeruginosa and Pseudomonas syringae. Meanwhile, the regulatory role of AlgU in the biocontrol ability of P.protegens has been poorly studied. In this study, deletion mutations of algU and its antagonist coding gene mucA were constructed to investigate the function of AlgU in P.protegens SN15-2 via phenotypic experiment and transcriptome sequencing analysis. On the basis of phenotypic analyses, it was concluded that the AlgU whose transcription was induced by osmotic stress and oxidative stress positively regulated biofilm formation and tolerance towards osmotic, heat, and oxidation stresses, while it negatively regulated motility, pyochelin synthesis, and the ability to inhibit pathogens. On the basis of the RNA-seq analysis, compared to the wild-type strain, 12 genes were significantly upregulated and 77 genes were significantly downregulated in ΔalgU, while 407 genes were significantly upregulated and 279 genes were significantly downregulated in ΔmucA, indicating the involvement of AlgU in several cellular processes, mainly related to resistance, carbohydrate metabolism, membrane formation, alginate production, the type VI secretion system, flagella motility and pyochelin production. Our findings provide insights into the important role of AlgU of P.protegens in biocontrol, which is of value in improving the biocontrol ability of P.protegens.

A truncated mutation of MucA in Pseudomonas aeruginosa from a bronchiectasis patient affects T3SS expression and inflammasome activation

Pseudomonas aeruginosa is an opportunistic pathogen that causes chronic airway infection in bronchiectasis patients and is closely associated with poor prognosis. Strains isolated from chronically infected patients typically have a mucoid phenotype due to the overproduction of alginate. In this study, we isolate a P. aeruginosa strain from the sputum of a patient with bronchiectasis and find that a truncated mutation occurred in mucA, which is named mucA117. mucA117 causes the strain to transform into a mucoid phenotype, downregulates the expression of T3SS and inflammasome ligands such as fliC and allows it to avoid inflammasome activation. The truncated mutation of the MucA protein may help P. aeruginosa escape clearance by the immune system, enabling long-term colonization.

Pf4 Phage Variant Infection Reduces Virulence-Associated Traits in Pseudomonas aeruginosa

Pf4 is a filamentous bacteriophage integrated as a prophage into the genome of Pseudomonas aeruginosa PAO1. Pf4 virions can be produced without killing P. aeruginosa. However, cell lysis can occur during superinfection when Pf virions successfully infect a host lysogenized by a Pf superinfective variant. We have previously shown that infection of P. aeruginosa PAO1 with a superinfective Pf4 variant abolished twitching motility and altered biofilm architecture. More precisely, most of the cells embedded into the biofilm were showing a filamentous morphology, suggesting the activation of the cell envelope stress response involving both AlgU and SigX extracytoplasmic function sigma factors. Here, we show that Pf4 variant infection results in a drastic dysregulation of 3,360 genes representing about 58% of P. aeruginosa genome; of these, 70% of the virulence factors encoding genes show a dysregulation. Accordingly, Pf4 variant infection (termed Pf4*) causes in vivo reduction of P. aeruginosa virulence and decreased production of N-acyl-homoserine lactones and 2-alkyl-4-quinolones quorum-sensing molecules and related virulence factors, such as pyocyanin, elastase, and pyoverdine. In addition, the expression of genes involved in metabolism, including energy generation and iron homeostasis, was affected, suggesting further relationships between virulence and central metabolism. Altogether, these data show that Pf4 phage variant infection results in complex network dysregulation, leading to reducing acute virulence in P. aeruginosa. This study contributes to the comprehension of the bacterial response to filamentous phage infection. IMPORTANCE Filamentous bacteriophages can become superinfective and infect P. aeruginosa, even though they are inserted in the genome as lysogens. Despite this productive infection, growth of the host is only mildly affected, allowing the study of the interaction between the phage and the host, which is not possible in the case of lytic phages killing rapidly their host. Here, we demonstrate by transcriptome and phenotypic analysis that the infection by a superinfective filamentous phage variant causes a massive disruption in gene expression, including those coding for virulence factors and metabolic pathways.

Global Regulatory Pathways Converge To Control Expression of Pseudomonas aeruginosa Type IV Pili

The opportunistic pathogen Pseudomonas aeruginosa relies upon type IV pili (Tfp) for host colonization and virulence. Tfp are retractile surface appendages that promote adherence to host tissue and mediate twitching motility, a form of surface-associated translocation. Tfp are composed of a major structural pilin protein (PilA), several less abundant, fiber-associated pilin-like proteins (FimU, PilV, PilW, PilX, and PilE), and a pilus-associated tip adhesin and surface sensor (PilY1). Several proteins critical for Tfp biogenesis and surface sensing are encoded by the fimU-pilVWXY1Y2E operon. Tfp biogenesis is regulated by the global transcription factor Vfr and its allosteric effector, cyclic AMP (cAMP). Our investigation into the basis for reduced Tfp production in cAMP/vfr mutants revealed a defect in the expression of the fimU operon. We found that cAMP/Vfr activation of the fimU operon occurs via direct binding of Vfr to a specific fimU promoter sequence. We also refined the role of the AlgZ/AlgR two-component system in fimU regulation by demonstrating that phosphorylation of the response regulator AlgR is required for maximal binding to the fimU promoter region in vitro. Vfr also regulates expression of the algZR operon, revealing an indirect regulatory loop affecting fimU operon transcription. Overall, these results demonstrate that two linked but independent regulatory systems couple the expression of Tfp biogenesis and surface sensing genes and highlight the regulatory complexity governing expression of P. aeruginosa virulence factors. IMPORTANCE Pseudomonas aeruginosa is an opportunistic pathogen responsible for a wide range of infections. An extensive repertoire of virulence factors aid in P. aeruginosa pathogenesis. Type IV pili (Tfp) play a critical role in host colonization and infection by promoting adherence to host tissue, facilitating twitching motility and mediating surface-associated behaviors. The fimU operon encodes several pilus-associated proteins that are essential for proper Tfp function and surface sensing. In this study, we report that linked but independent regulatory systems dictate Tfp biogenesis. We also demonstrated the importance of different phosphorylation states of the AlgZ/AlgR two-component system and its role in Tfp biogenesis. Overall, this study furthers our understanding of the complex regulatory mechanisms that govern the production of a critical and multifaceted virulence factor.

Genome evolution drives transcriptomic and phenotypic adaptation in Pseudomonas aeruginosa during 20 years of infection

The opportunistic pathogen Pseudomonas aeruginosa chronically infects the lungs of patients with cystic fibrosis (CF). During infection the bacteria evolve and adapt to the lung environment. Here we use genomic, transcriptomic and phenotypic approaches to compare multiple isolates of P. aeruginosa collected more than 20 years apart during a chronic infection in a CF patient. Complete genome sequencing of the isolates, using short- and long-read technologies, showed that a genetic bottleneck occurred during infection and was followed by diversification of the bacteria. A 125 kb deletion, an 0.9 Mb inversion and hundreds of smaller mutations occurred during evolution of the bacteria in the lung, with an average rate of 17 mutations per year. Many of the mutated genes are associated with infection or antibiotic resistance. RNA sequencing was used to compare the transcriptomes of an earlier and a later isolate. Substantial reprogramming of the transcriptional network had occurred, affecting multiple genes that contribute to continuing infection. Changes included greatly reduced expression of flagellar machinery and increased expression of genes for nutrient acquisition and biofilm formation, as well as altered expression of a large number of genes of unknown function. Phenotypic studies showed that most later isolates had increased cell adherence and antibiotic resistance, reduced motility, and reduced production of pyoverdine (an iron-scavenging siderophore), consistent with genomic and transcriptomic data. The approach of integrating genomic, transcriptomic and phenotypic analyses reveals, and helps to explain, the plethora of changes that P. aeruginosa undergoes to enable it to adapt to the environment of the CF lung during a chronic infection.

Cautionary Notes on the Use of Arabinose- and Rhamnose-Inducible Expression Vectors in Pseudomonas aeruginosa

The Pseudomonas aeruginosa virulence factor regulator (Vfr) is a cyclic AMP (cAMP)-responsive transcription factor homologous to the Escherichia coli cAMP receptor protein (CRP). Unlike CRP, which plays a central role in E. coli energy metabolism and catabolite repression, Vfr is primarily involved in the control of P. aeruginosa virulence factor expression. Expression of the Vfr regulon is controlled at the level of vfr transcription, Vfr translation, cAMP synthesis, and cAMP degradation. While investigating mechanisms that regulate Vfr translation, we placed vfr transcription under the control of the rhaBp rhamnose-inducible promoter system (designated P_Rha) and found that P_Rha promoter activity was highly dependent upon vfr. Vfr dependence was also observed for the araBp arabinose-inducible promoter (designated P_BAD). The observation of Vfr dependence was not entirely unexpected. Both promoters are derived from E. coli, where maximal promoter activity is dependent upon CRP. Like CRP, we found that Vfr directly binds to promoter probes derived from the P_Rha and P_BAD promoters in vitro. Because Vfr-cAMP activity is highly integrated into numerous global regulatory systems, including c-di-GMP signaling, the Gac/Rsm system, MucA/AlgU/AlgZR signaling, and Hfq/sRNAs, the potential exists for significant variability in P_Rha and P_BAD promoter activity in a variety of genetic backgrounds, and use of these promoter systems in P. aeruginosa should be employed with caution. IMPORTANCE Heterologous gene expression and complementation constitute a valuable and widely utilized tool in bacterial genetics. The arabinose-inducible P_araBAD (P_BAD) and rhamnose-inducible P_rhaBAD (P_Rha) promoter systems are commonly used in P. aeruginosa genetics and prized for the tight control and dynamic expression ranges that can be achieved. In this study, we demonstrate that the activity of both promoters is dependent upon the cAMP-dependent transcription factor Vfr. While this poses an obvious problem for use in a vfr mutant background, the issue is more pervasive, considering that vfr transcription/synthesis and cAMP homeostasis are highly integrated into the cellular physiology of the organism and influenced by numerous global regulatory systems. Fortunately, the synthetic P_Tac promoter is not subject to Vfr regulatory control.

The anti-sigma factor MucA is required for viability in Pseudomonas aeruginosa

During decades-long infections in the cystic fibrosis (CF) airway, Pseudomonas aeruginosa undergoes selection. One bacterial genetic adaptation often observed in CF isolates is mucA mutations. MucA inhibits the sigma factor AlgU. Mutations in mucA lead to AlgU misregulation, resulting in a mucoid phenotype that is associated with poor CF disease outcomes. Due to its ability to be mutated, mucA is assumed to be dispensable for bacterial viability. Here we show that, paradoxically, a portion of mucA is essential in P. aeruginosa. We demonstrate that mucA is no longer required in a strain lacking algU, that mucA alleles encoding for proteins that do not bind to AlgU are insufficient for viability, and that mucA is no longer essential in mutant strains containing AlgU variants with reduced sigma factor activity. Furthermore, we found that overexpression of algU prevents cell growth in the absence of MucA, and that this phenotype can be rescued by the overproduction of RpoD, the housekeeping sigma factor. Together, these results suggest that in the absence of MucA, the inability to regulate AlgU activity results in the loss of bacterial viability. Finally, we speculate that the essentiality of anti-sigma factors that regulate envelope function may be a widespread phenomenon in bacteria.

DNA alternate polymerase PolB mediates inhibition of type III secretion in Pseudomonas aeruginosa

Opportunistic pathogen Pseudomonas aeruginosa uses a variety of virulence factors to cause acute and chronic infections. We previously found that alternate DNA polymerase gene polB inhibits P. aeruginosa pyocyanin production. We investigated whether polB also affects T3SS expression. polB overexpression significantly reduced T3SS transcription and repressed translation of the master T3SS regulator ExsA, while not affecting exsA mRNA transcript abundance. Further, polB does not act through previously described genetic pathways that post-transcriptionally regulate ExsA. Our results show a novel T3SS regulatory component which may lead to development of future drugs to target this mechanism.

Sex Steroids Induce Membrane Stress Responses and Virulence Properties in Pseudomonas aeruginosa

Molecular mechanisms by which sex steroids interact with P. aeruginosa to modulate its virulence have yet to be reported. Our work provides the first characterization of a steroid-induced membrane stress mechanism promoting P. aeruginosa virulence, which includes the release of proinflammatory outer membrane vesicles, resulting in inflammation, host tissue damage, and reduced bacterial clearance. We further demonstrate that at nanomolar (physiological) concentrations, male and female sex steroids promote virulence in clinical strains of P. aeruginosa based on their dynamic membrane fluidic properties. This work provides, for the first-time, mechanistic insight to better understand and predict the P. aeruginosa related response to sex steroids and explain the interindividual patient variability observed in respiratory diseases such as cystic fibrosis that are complicated by gender differences and chronic P. aeruginosa infection.

Estrogen, a major female sex steroid hormone, has been shown to promote the selection of mucoid Pseudomonas aeruginosa in the airways of patients with chronic respiratory diseases, including cystic fibrosis. This results in long-term persistence, poorer clinical outcomes, and limited therapeutic options. In this study, we demonstrate that at physiological concentrations, sex steroids, including testosterone and estriol, induce membrane stress responses in P. aeruginosa. This is characterized by increased virulence and consequent inflammation and release of proinflammatory outer membrane vesicles promoting in vivo persistence of the bacteria. The steroid-induced P. aeruginosa response correlates with the molecular polarity of the hormones and membrane fluidic properties of the bacteria. This novel mechanism of interaction between sex steroids and P. aeruginosa explicates the reported increased disease severity observed in females with cystic fibrosis and provides evidence for the therapeutic potential of the modulation of sex steroids to achieve better clinical outcomes in patients with hormone-responsive strains.

Hfq and sRNA 179 Inhibit Expression of the Pseudomonas aeruginosa cAMP-Vfr and Type III Secretion Regulons

Pseudomonas aeruginosa is a Gram-negative opportunistic pathogen causing skin and soft tissue, respiratory, and bloodstream infections. The type III secretion system (T3SS) is one important virulence factor. Production of the T3SS is controlled by ExsA, a transcription factor that activates expression of the entire T3SS regulon. Global regulators including Vfr, RsmA, and Hfq also contribute to regulation of the T3SS. Vfr is a cAMP-responsive transcription factor that activates exsA transcription. RsmA, an RNA-binding protein, inversely controls expression of the T3SS and the type VI secretion system (T6SS). Hfq is an RNA chaperone that functions by stabilizing small noncoding RNAs (sRNAs) and/or facilitating base pairing between sRNAs and mRNA targets. A previous study identified sRNA 1061, which directly targets the exsA mRNA and likely inhibits ExsA synthesis. In this study, we screened an sRNA expression library and identified sRNA 179 as an Hfq-dependent inhibitor of T3SS gene expression. Further characterization revealed that sRNA 179 inhibits the synthesis of both ExsA and Vfr. The previous finding that RsmA stimulates ExsA and Vfr synthesis suggested that sRNA 179 impacts the Gac/Rsm system. Consistent with that idea, the inhibitory activity of sRNA 179 is suppressed in a mutant lacking rsmY and rsmZ, and sRNA 179 expression stimulates rsmY transcription. RsmY and RsmZ are small noncoding RNAs that sequester RsmA from target mRNAs. Our combined findings show that Hfq and sRNA 179 indirectly regulate ExsA and Vfr synthesis by reducing the available pool of RsmA, leading to reduced expression of the T3SS and cAMP-Vfr regulons.IMPORTANCE Control of gene expression by small noncoding RNA (sRNA) is well documented but underappreciated. Deep sequencing of mRNA preparations from Pseudomonas aeruginosa suggests that >500 sRNAs are generated. Few of those sRNAs have defined roles in gene expression. To address that knowledge gap, we constructed an sRNA expression library and identified sRNA 179 as a regulator of the type III secretion system (T3SS) and the cAMP-Vfr regulons. The T3SS- and cAMP-Vfr-controlled genes are critical virulence factors. Increased understanding of the signals and regulatory mechanisms that control these important factors will enhance our understanding of disease progression and reveal potential approaches for therapeutic intervention.

A molecular mechanism for how sigma factor AlgT and transcriptional regulator AmrZ inhibit twitching motility in Pseudomonas aeruginosa

Pseudomonas aeruginosa isolates from cystic fibrosis patients are often mucoid (due to the overexpression of exopolysaccharide alginate) yet lost motility. It remains unclear about how P. aeruginosa coordinately regulates alginate production and the type IV pili-driven twitching motility. Here we showed that sigma 22 factor (AlgT/U), an activator of alginate biosynthesis, repressed twitching motility by inhibiting the expression of pilin (PilA) through the intermediate transcriptional regulator AmrZ, which directly bound to the promoter region of pilA in both mucoid strain FRD1 and non-mucoid strain PAO1. Four conserved AmrZ-binding sites were found in pilA promoters among 10 P. aeruginosa strains although their entire pilA promoters had low identity. AmrZ has been reported to be essential for twitching in PAO1. We found that AmrZ was also required for twitching in mucoid FRD1, yet a high level of AmrZ inhibited twitching motility. This result was consistent with the phenomenon that twitching is frequently repressed in mucoid strains, in which the expression of AmrZ was highly activated by AlgT. Additionally, AlgT also inhibited the transcription of pilMNOP operon, which is involved in efficient pilus assembly. Our data elucidated a mechanism for how AlgT and AmrZ coordinately controlled twitching motility in P. aeruginosa.

Cooperativity between Stenotrophomonas maltophilia and Pseudomonas aeruginosa during Polymicrobial Airway Infections

Stenotrophomonas maltophilia is a Gram-negative bacterium found ubiquitously in the environment that has historically been regarded as nonpathogenic. S. maltophilia is increasingly observed in patient sputa in cystic fibrosis (CF), and while existing epidemiology indicates that patients with S. maltophilia have poorer diagnoses, its clinical significance remains unclear. Moreover, as multidrug resistance is common among S. maltophilia isolates, treatment options for these infections may be limited. Here, we investigated the pathogenicity of S. maltophilia alone and during polymicrobial infection with Pseudomonas aeruginosa Colonization, persistence, and virulence of S. maltophilia were assessed in experimental respiratory infections of mice. The results of this study indicate that S. maltophilia transiently colonizes the lung accompanied by significant weight loss and immune cell infiltration and the expression of early inflammatory markers, including interleukin 6 (IL-6), IL-1α, and tumor necrosis factor alpha (TNF-α). Importantly, polymicrobial infection with P. aeruginosa elicited significantly higher S. maltophilia counts in bronchoalveolar lavages and lung tissue homogenates. This increase in bacterial load was directly correlated with the density of the P. aeruginosa population and required viable P. aeruginosa bacteria. Microscopic analysis of biofilms formed in vitro revealed that S. maltophilia formed well-integrated biofilms with P. aeruginosa, and these organisms colocalize in the lung during dual-species infection. Based on these results, we conclude that active cellular processes by P. aeruginosa afford a significant benefit to S. maltophilia during polymicrobial infections. Furthermore, these results indicate that S. maltophilia may have clinical significance in respiratory infections.

Bacterial alginate regulators and phage homologs repress CRISPR-Cas immunity

CRISPR-Cas systems are adaptive immune systems that protect bacteria from bacteriophage (phage) infection¹. To provide immunity, RNA-guided protein surveillance complexes recognize foreign nucleic acids, triggering their destruction by Cas nucleases². While the essential requirements for immune activity are well understood, the physiological cues that regulate CRISPR-Cas expression are not. Here, a forward genetic screen identifies a two-component system (KinB/AlgB), previously characterized in regulating Pseudomonas aeruginosa alginate biosynthesis^3,4, as a regulator of the expression and activity of the P. aeruginosa Type I-F CRISPR-Cas system. Downstream of KinB/AlgB, activators of alginate production AlgU (a σ^E orthologue) and AlgR, repress CRISPR-Cas activity during planktonic and surface-associated growth⁵. AmrZ, another alginate regulator⁶, is triggered to repress CRISPR-Cas immunity during surface-association. Pseudomonas phages and plasmids have taken advantage of this regulatory scheme, and carry hijacked homologs of AmrZ that repress CRISPR-Cas expression and activity. This suggests that while CRISPR-Cas regulation may be important to limit self-toxicity, endogenous repressive pathways represent a vulnerability for parasite manipulation.

The impact of CFTR modulator therapies on CF airway microbiology

Major historical advances in cystic fibrosis (CF) respiratory clinical care, including mechanical airway clearance and inhaled medications, have aimed to address the consequences of cystic fibrosis transmembrane conductance regulator (CFTR) dysfunction. In contrast, CFTR modulator therapies instead target the underlying protein defect that leads to CF lung disease. The extent to which these therapies might reduce susceptibility to chronic lung infections remains to be seen. However, by improving airway clearance, reducing the requirement for antibiotics, and in some cases, through direct antimicrobial effects, CFTR modulators are likely to result in substantial changes in CF airway microbiology. These changes could contribute substantially to the clinical benefit associated with modulator therapies, as well as providing an important indicator of treatment efficacy and residual pathophysiology. Indeed, the widespread introduction of modulator therapies might require us to re-consider our models of CF airway microbiology.

Cystic Fibrosis and Pseudomonas aeruginosa: the Host-Microbe Interface

In human pathophysiology, the clash between microbial infection and host immunity contributes to multiple diseases. Cystic fibrosis (CF) is a classical example of this phenomenon, wherein a dysfunctional, hyperinflammatory immune response combined with chronic pulmonary infections wreak havoc upon the airway, leading to a disease course of substantial morbidity and shortened life span. Pseudomonas aeruginosa is an opportunistic pathogen that commonly infects the CF lung, promoting an accelerated decline of pulmonary function. Importantly, P. aeruginosa exhibits significant resistance to innate immune effectors and to antibiotics, in part, by expressing specific virulence factors (e.g., antioxidants and exopolysaccharides) and by acquiring adaptive mutations during chronic infection. In an effort to review our current understanding of the host-pathogen interface driving CF pulmonary disease, we discuss (i) the progression of disease within the primitive CF lung, specifically focusing on the role of host versus bacterial factors; (ii) critical, neutrophil-derived innate immune effectors that are implicated in CF pulmonary disease, including reactive oxygen species (ROS) and antimicrobial peptides (e.g., LL-37); (iii) P. aeruginosa virulence factors and adaptive mutations that enable evasion of the host response; and (iv) ongoing work examining the distribution and colocalization of host and bacterial factors within distinct anatomical niches of the CF lung.

Fitting Pieces into the Puzzle of Pseudomonas aeruginosa Type III Secretion System Gene Expression

Type III secretion systems (T3SS) are widely distributed in Gram-negative microorganisms and critical for host-pathogen and host-symbiont interactions with plants and animals. Central features of the T3SS are a highly conserved set of secretion and translocation genes and contact dependence wherein host-pathogen interactions trigger effector protein delivery and serve as an inducing signal for T3SS gene expression. In addition to these conserved features, there are pathogen-specific properties that include a unique repertoire of effector genes and mechanisms to control T3SS gene expression. The Pseudomonas aeruginosa T3SS serves as a model system to understand transcriptional and posttranscriptional mechanisms involved in the control of T3SS gene expression. The central regulatory feature is a partner-switching system that controls the DNA-binding activity of ExsA, the primary regulator of T3SS gene expression. Superimposed upon the partner-switching mechanism are cyclic AMP and cyclic di-GMP signaling systems, two-component systems, global regulators, and RNA-binding proteins that have positive and negative effects on ExsA transcription and/or synthesis. In the present review, we discuss advances in our understanding of how these regulatory systems orchestrate the activation of T3SS gene expression in the context of acute infections and repression of the T3SS as P. aeruginosa adapts to and colonizes the cystic fibrosis airways.

Pseudomonas aeruginosa type IV minor pilins and PilY1 regulate virulence by modulating FimS-AlgR activity

Type IV pili are expressed by a wide range of prokaryotes, including the opportunistic pathogen Pseudomonas aeruginosa. These flexible fibres mediate twitching motility, biofilm maturation, surface adhesion, and virulence. The pilus is composed mainly of major pilin subunits while the low abundance minor pilins FimU-PilVWXE and the putative adhesin PilY1 prime pilus assembly and are proposed to form the pilus tip. The minor pilins and PilY1 are encoded in an operon that is positively regulated by the FimS-AlgR two-component system. Independent of pilus assembly, PilY1 was proposed to be a mechanosensory component that—in conjunction with minor pilins—triggers up-regulation of acute virulence phenotypes upon surface attachment. Here, we investigated the link between the minor pilins/PilY1 and virulence. pilW, pilX, and pilY1 mutants had reduced virulence towards Caenorhabditis elegans relative to wild type or a major pilin mutant, implying a role in pathogenicity that is independent of pilus assembly. We hypothesized that loss of specific minor pilins relieves feedback inhibition on FimS-AlgR, increasing transcription of the AlgR regulon and delaying C. elegans killing. Reporter assays confirmed that FimS-AlgR were required for increased expression of the minor pilin operon upon loss of select minor pilins. Overexpression of AlgR or its hyperactivation via a phosphomimetic mutation reduced virulence, and the virulence defects of pilW, pilX, and pilY1 mutants required FimS-AlgR expression and activation. We propose that PilY1 and the minor pilins inhibit their own expression, and that loss of these proteins leads to FimS-mediated activation of AlgR that suppresses expression of acute-phase virulence factors and delays killing. This mechanism could contribute to adaptation of P. aeruginosa in chronic lung infections, as mutations in the minor pilin operon result in the loss of piliation and increased expression of AlgR-dependent virulence factors–such as alginate–that are characteristic of such infections.

Pseudomonas aeruginosa causes dangerous infections, including chronic lung infections in cystic fibrosis patients. It uses many strategies to infect its hosts, including deployment of grappling hook-like fibres called type IV pili. Among the components involved in assembly and function of the pilus are five proteins called minor pilins that—along with a larger protein called PilY1—may help the pilus attach to surfaces. In a roundworm infection model, loss of PilY1 and specific minor pilins delayed killing, while loss of other pilus components did not. We traced this effect to increased activation of the FimS-AlgR regulatory system that inhibits the expression of virulence factors used early in infection, while positively regulating chronic infection traits such as alginate production, a phenotype called mucoidy. A disruption in the appropriate timing of FimS-AlgR-dependent virulence factor expression when select minor pilins or PilY1 are missing may explain why those pilus-deficient mutants have reduced virulence compared with others whose products are not under FimS-AlgR control. Increased FimS-AlgR activity upon loss of PilY1 and specific minor pilins could help to explain the frequent co-occurrence of the non-piliated and mucoid phenotypes that are hallmarks of chronic P. aeruginosa lung infections.

Extracytoplasmic function sigma factors in Pseudomonas aeruginosa

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

Mixed Communities of Mucoid and Nonmucoid Pseudomonas aeruginosa Exhibit Enhanced Resistance to Host Antimicrobials

Pseudomonas aeruginosa causes chronic pulmonary infections in patients with cystic fibrosis (CF). P. aeruginosa mucoid conversion, defined by overproduction of the exopolysaccharide alginate, correlates with accelerated decline in CF patient lung function. Recalcitrance of the mucoid phenotype to clearance by antibiotics and the immune response is well documented. However, despite advantages conferred by mucoidy, mucoid variants often revert to a nonmucoid phenotype both in vitro and in vivo Mixed populations of mucoid isolates and nonmucoid revertants are recovered from CF lungs, suggesting a selective benefit for coexistence of these variants. In this study, cocultures of mucoid and nonmucoid variants exhibited enhanced resistance to two host antimicrobials: LL-37, a cationic antimicrobial peptide, and hydrogen peroxide (H₂O₂). Alginate production by mucoid isolates protected nonmucoid variants in consortia from LL-37, as addition of alginate exogenously to nonmucoid variants abrogated LL-37 killing. Conversely, nonmucoid revertants shielded mucoid variants from H₂O₂ stress via catalase (KatA) production, which was transcriptionally repressed by AlgT and AlgR, central regulators of alginate biosynthesis. Furthermore, extracellular release of KatA by nonmucoid revertants was dependent on lys, encoding an endolysin implicated in autolysis and extracellular DNA (eDNA) release. Overall, these data provide a rationale to study interactions of P. aeruginosa mucoid and nonmucoid variants as contributors to evasion of innate immunity and persistence within the CF lung.IMPORTANCEP. aeruginosa mucoid conversion within lungs of cystic fibrosis (CF) patients is a hallmark of chronic infection and predictive of poor prognosis. The selective benefit of mixed populations of mucoid and nonmucoid variants, often isolated from chronically infected CF patients, has not been explored. Here, we show that mixed-variant communities of P. aeruginosa demonstrate advantages in evasion of innate antimicrobials via production of shared goods: alginate and catalase. These data argue for therapeutically targeting multiple constituents (both mucoid and nonmucoid variants) within diversified P. aeruginosa communities in vivo, as these variants can differentially shield one another from components of the host response.

Nitric Oxide, an Old Molecule With Noble Functions in Pseudomonas aeruginosa Biology

Pseudomonas aeruginosa, a Gram-negative bacterium, is characterized by its versatility that enables persistent survival under adverse conditions. It can grow on diverse energy sources and readily acquire resistance to antimicrobial agents. As an opportunistic human pathogen, it also causes chronic infections inside the anaerobic mucus airways of cystic fibrosis patients. As a strict respirer, P. aeruginosa can grow by anaerobic nitrate ( [Formula: see text] ) respiration. Nitric oxide (NO) produced as an intermediate during anaerobic respiration exerts many important effects on the biological characteristics of P. aeruginosa. This review provides information regarding (i) how P. aeruginosa grows by anaerobic respiration, (ii) mechanisms by which NO is produced under such growth, and (iii) bacterial adaptation to NO. We also review the clinical relevance of NO in the fitness of P. aeruginosa and the use of NO as a potential therapeutic for treating P. aeruginosa infection.

Surface Sensing for Biofilm Formation in Pseudomonas aeruginosa

Aggregating and forming biofilms on biotic or abiotic surfaces are ubiquitous bacterial behaviors under various conditions. In clinical settings, persistent presence of biofilms increases the risks of healthcare-associated infections and imposes huge healthcare and economic burdens. Bacteria within biofilms are protected from external damage and attacks from the host immune system and can exchange genomic information including antibiotic-resistance genes. Dispersed bacterial cells from attached biofilms on medical devices or host tissues may also serve as the origin of further infections. Understanding how bacteria develop biofilms is pertinent to tackle biofilm-associated infections and transmission. Biofilms have been suggested as a continuum of growth modes for adapting to different environments, initiating from bacterial cells sensing their attachment to a surface and then switching cellular physiological status for mature biofilm development. It is crucial to understand bacterial gene regulatory networks and decision-making processes for biofilm formation upon initial surface attachment. Pseudomonas aeruginosa is one of the model microorganisms for studying bacterial population behaviors. Several hypotheses and studies have suggested that extracellular macromolecules and appendages play important roles in bacterial responses to the surface attachment. Here, I review recent studies on potential molecular mechanisms and signal transduction pathways for P. aeruginosa surface sensing.

The Pseudomonas aeruginosa Two-Component Regulator AlgR Directly Activates rsmA Expression in a Phosphorylation-Independent Manner

Pseudomonas aeruginosa is an important pathogen of the immunocompromised, causing both acute and chronic infections. In cystic fibrosis (CF) patients, P. aeruginosa causes chronic disease. The impressive sensory network of P. aeruginosa allows the bacterium to sense and respond to a variety of stimuli found in diverse environments. Transcriptional regulators, including alternative sigma factors and response regulators, integrate signals changing gene expression, allowing P. aeruginosa to cause infection. The two-component transcriptional regulator AlgR is important in P. aeruginosa pathogenesis in both acute and chronic infections. In chronic infections, AlgR and the alternative sigma factor AlgU activate the genes responsible for alginate production. Previous work demonstrated that AlgU controls rsmA expression. RsmA is a posttranscriptional regulator that is antagonized by two small RNAs, RsmY and RsmZ. In this work, we demonstrate that AlgR directly activates rsmA expression from the same promoter as AlgU. In addition, phosphorylation was not necessary for AlgR activation of rsmA using algR and algZ mutant strains. AlgU and AlgR appear to affect the antagonizing small RNAs rsmY and rsmZ indirectly. RsmA was active in a mucA22 mutant strain using leader fusions of two RsmA targets, tssA1 and hcnA AlgU and AlgR were necessary for posttranscriptional regulation of tssA1 and hcnA Altogether, our work demonstrates that the alginate regulators AlgU and AlgR are important in the control of the RsmA posttranscriptional regulatory system. These findings suggest that RsmA plays an unknown role in mucoid strains due to AlgU and AlgR activities.IMPORTANCE P. aeruginosa infections are difficult to treat and frequently cause significant mortality in CF patients. Understanding the mechanisms of persistence is important. Our work has demonstrated that the alginate regulatory system also significantly impacts the posttranscriptional regulator system RsmA/Y/Z. We demonstrate that AlgR directly activates rsmA expression, and this impacts the RsmA regulon. This leads to the possibility that the RsmA/Y/Z system plays a role in helping P. aeruginosa persist during chronic infection. In addition, this furthers our understanding of the reach of the alginate regulators AlgU and AlgR.

The multi-output incoherent feedforward loop constituted by the transcriptional regulators LasR and RsaL confers robustness to a subset of quorum sensing genes in Pseudomonas aeruginosa

Thelasmulti-output IFFL-1 splits the QS regulon into two distinct sub-regulons with different robustness with respect to LasR fluctuations.

Pseudomonas aeruginosa AmrZ Binds to Four Sites in the algD Promoter, Inducing DNA-AmrZ Complex Formation and Transcriptional Activation

During late stages of cystic fibrosis pulmonary infections, Pseudomonas aeruginosa often overproduces the exopolysaccharide alginate, protecting the bacterial community from host immunity and antimicrobials. The transcription of the alginate biosynthesis operon is under tight control by a number of factors, including AmrZ, the focus of this study. Interestingly, multiple transcription factors interact with the far-upstream region of this promoter (PalgD), in which one AmrZ binding site has been identified previously. The mechanisms of AmrZ binding and subsequent activation remain unclear and require more-detailed investigation. In this study, in-depth examinations elucidated four AmrZ binding sites, and their disruption eliminated AmrZ binding and promoter activation. Furthermore, our in vitro fluorescence resonance energy transfer experiments suggest that AmrZ holds together multiple binding sites in PalgD and thereafter induces the formation of higher-order DNA-AmrZ complexes. To determine the importance of interactions between those AmrZ oligomers in the cell, a DNA phasing experiment was performed. PalgD transcription was significantly impaired when the relative phase between AmrZ binding sites was reversed (5 bp), while a full-DNA-turn insertion (10 bp) restored promoter activity. Taken together, the investigations presented here provide a deeper mechanistic understanding of AmrZ-mediated binding to PalgD IMPORTANCE: Overproduction of the exopolysaccharide alginate provides protection to Pseudomonas aeruginosa against antimicrobial treatments and is associated with chronic P. aeruginosa infections in the lungs of cystic fibrosis patients. In this study, we combined a variety of microbiological, genetic, biochemical, and biophysical approaches to investigate the activation of the alginate biosynthesis operon promoter by a key transcription factor named AmrZ. This study has provided important new information on the mechanism of activation of this extremely complex promoter.

Vfr Directly Activates exsA Transcription To Regulate Expression of the Pseudomonas aeruginosa Type III Secretion System

The Pseudomonas aeruginosa cyclic AMP (cAMP)-Vfr system (CVS) is a global regulator of virulence gene expression. Regulatory targets include type IV pili, secreted proteases, and the type III secretion system (T3SS). The mechanism by which CVS regulates T3SS gene expression remains undefined. Single-cell expression studies previously found that only a portion of the cells within a population express the T3SS under inducing conditions, a property known as bistability. We now report that bistability is altered in avfr mutant, wherein a substantially smaller fraction of the cells express the T3SS relative to the parental strain. Since bistability usually involves positive-feedback loops, we tested the hypothesis that virulence factor regulator (Vfr) regulates the expression of exsA ExsA is the central regulator of T3SS gene expression and autoregulates its own expression. Although exsA is the last gene of the exsCEBA polycistronic mRNA, we demonstrate that Vfr directly activates exsA transcription from a second promoter (PexsA) located immediately upstream of exsA PexsA promoter activity is entirely Vfr dependent. Direct binding of Vfr to a PexsA promoter probe was demonstrated by electrophoretic mobility shift assays, and DNase I footprinting revealed an area of protection that coincides with a putative Vfr consensus-binding site. Mutagenesis of that site disrupted Vfr binding and PexsA promoter activity. We conclude that Vfr contributes to T3SS gene expression through activation of the PexsA promoter, which is internal to the previously characterized exsCEBA operon.

IMPORTANCE

Vfr is a cAMP-dependent DNA-binding protein that functions as a global regulator of virulence gene expression in Pseudomonas aeruginosa Regulation by Vfr allows for the coordinate production of related virulence functions, such as type IV pili and type III secretion, required for adherence to and intoxication of host cells, respectively. Although the molecular mechanism of Vfr regulation has been defined for many target genes, a direct link between Vfr and T3SS gene expression had not been established. In the present study, we report that Vfr directly controls exsA transcription, the master regulator of T3SS gene expression, from a newly identified promoter located immediately upstream of exsA.

Modulation of Type III Secretion System in Pseudomonas aeruginosa: Involvement of the PA4857 Gene Product

Pseudomonas aeruginosa is an opportunistic pathogen that causes serious acute or chronic infections in humans. Acute infections typically involve the type III secretion systems (T3SSs) and bacterial motility, whereas chronic infections are often associated with biofilm formation and the type VI secretion system. To identify new genes required for pathogenesis, a transposon mutagenesis library was constructed and the gene PA4857, named tspR, was found to modulate T3SS gene expression. Deletion of P. aeruginosa tspR reduced the virulence in a mouse acute lung infection model and diminished cytotoxicity. Suppression of T3SS gene expression in the tspR mutant resulted from compromised translation of the T3SS master regulator ExsA. TspR negatively regulated two small RNAs, RsmY and RsmZ, which control RsmA. Our data demonstrated that defects in T3SS expression and biofilm formation in retS mutant could be partially restored by overexpression of tspR. Taken together, our results demonstrated that the newly identified retS-tspR pathway is coordinated with the retS-gacS system, which regulates the genes associated with acute and chronic infections and controls the lifestyle choice of P. aeruginosa.

Biogenesis of Pseudomonas aeruginosa type IV pili and regulation of their function

Type IV pili (T4P) are bacterial virulence factors involved in a wide variety of functions including deoxyribonucleic acid uptake, surface attachment, biofilm formation and twitching motility. While T4P are common surface appendages, the systems that assemble them and the regulation of their function differ between species. Pseudomonas aeruginosa, Neisseria spp. and Myxococcus xanthus are common model systems used to study T4P biology. This review focuses on recent advances in P. aeruginosa T4P structural biology, and the regulatory pathways controlling T4P biogenesis and function.

The RNA Helicase DeaD Stimulates ExsA Translation To Promote Expression of the Pseudomonas aeruginosa Type III Secretion System

The Pseudomonas aeruginosa type III secretion system (T3SS) is a primary virulence factor important for phagocytic avoidance, disruption of host cell signaling, and host cell cytotoxicity. ExsA is the master regulator of T3SS transcription. The expression, synthesis, and activity of ExsA is tightly regulated by both intrinsic and extrinsic factors. Intrinsic regulation consists of the well-characterized ExsECDA partner-switching cascade, while extrinsic factors include global regulators that alter exsA transcription and/or translation. To identify novel extrinsic regulators of ExsA, we conducted a transposon mutagenesis screen in the absence of intrinsic control. Transposon disruptions within gene PA2840, which encodes a homolog of the Escherichia coli RNA-helicase DeaD, significantly reduced T3SS gene expression. Recent studies indicate that E. coli DeaD can promote translation by relieving inhibitory secondary structures within target mRNAs. We report here that PA2840, renamed DeaD, stimulates ExsA synthesis at the posttranscriptional level. Genetic experiments demonstrate that the activity of an exsA translational fusion is reduced in a deaD mutant. In addition, exsA expression in trans fails to restore T3SS gene expression in a deaD mutant. We hypothesized that DeaD relaxes mRNA secondary structure to promote exsA translation and found that altering the mRNA sequence of exsA or the native exsA Shine-Dalgarno sequence relieved the requirement for DeaD in vivo. Finally, we show that purified DeaD promotes ExsA synthesis using in vitro translation assays. Together, these data reveal a novel regulatory mechanism for P. aeruginosa DeaD and add to the complexity of global regulation of T3SS.

IMPORTANCE

Although members of the DEAD box family of RNA helicases are appreciated for their roles in mRNA degradation and ribosome biogenesis, an additional role in gene regulation is now emerging in bacteria. By relaxing secondary structures in mRNAs, DEAD box helicases are now thought to promote translation by enhancing ribosomal recruitment. We identify here an RNA helicase that plays a critical role in promoting ExsA synthesis, the central regulator of the Pseudomonas aeruginosa type III secretion system, and provide additional evidence that DEAD box helicases directly stimulate translation of target genes. The finding that DeaD stimulates exsA translation adds to a growing list of transcriptional and posttranscriptional regulatory mechanisms that control type III gene expression.

Fluxome study of Pseudomonas fluorescens reveals major reorganisation of carbon flux through central metabolic pathways in response to inactivation of the anti-sigma factor MucA

Background

The bacterium Pseudomonas fluorescens switches to an alginate-producing phenotype when the pleiotropic anti-sigma factor MucA is inactivated. The inactivation is accompanied by an increased biomass yield on carbon sources when grown under nitrogen-limited chemostat conditions. A previous metabolome study showed significant changes in the intracellular metabolite concentrations, especially of the nucleotides, in mucA deletion mutants compared to the wild-type. In this study, the P. fluorescens SBW25 wild-type and an alginate non-producing mucA- ΔalgC double-knockout mutant are investigated through model-based ¹³C-metabolic flux analysis (¹³C-MFA) to explore the physiological consequences of MucA inactivation at the metabolic flux level. Intracellular metabolite extracts from three carbon labelling experiments using fructose as the sole carbon source are analysed for ¹³C-label incorporation in primary metabolites by gas and liquid chromatography tandem mass spectrometry.

Results

From mass isotopomer distribution datasets, absolute intracellular metabolic reaction rates for the wild type and the mutant are determined, revealing extensive reorganisation of carbon flux through central metabolic pathways in response to MucA inactivation. The carbon flux through the Entner-Doudoroff pathway was reduced in the mucA- ΔalgC mutant, while flux through the pentose phosphate pathway was increased. Our findings also indicated flexibility of the anaplerotic reactions through down-regulation of the pyruvate shunt in the mucA- ΔalgC mutant and up-regulation of the glyoxylate shunt.

Conclusions

Absolute metabolic fluxes and metabolite levels give detailed, integrated insight into the physiology of this industrially, medically and agriculturally important bacterial species and suggest that the most efficient way of using a mucA- mutant as a cell factory for alginate production would be to use non-growing conditions and nitrogen deprivation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12918-015-0148-0) contains supplementary material, which is available to authorized users.

Expression analysis of the Pseudomonas aeruginosa AlgZR two-component regulatory system

Pseudomonas aeruginosa virulence components are subject to complex regulatory control primarily through two-component regulatory systems that allow for sensing and responding to environmental stimuli. In this study, the expression and regulation of the P. aeruginosa AlgZR two-component regulatory system were examined. Primer extension and S1 nuclease protection assays were used to identify two transcriptional initiation sites for algR within the algZ coding region, and two additional start sites were identified upstream of the algZ coding region. The two algR transcriptional start sites, RT1 and RT2, are directly regulated by AlgU, consistent with previous reports of increased algR expression in mucoid backgrounds, and RpoS additionally plays a role in algR transcription. The expression of the first algZ promoter, ZT1, is entirely dependent upon Vfr for expression, whereas Vfr, RpoS, or AlgU does not regulate the second algZ promoter, ZT2. Western blot, real-time quantitative PCR (RT-qPCR), and transcriptional fusion analyses show that algZR expression is Vfr dependent. The algZ and algR genes also are cotranscribed in both nonmucoid and mucoid backgrounds. Furthermore, algZR was found to be cotranscribed with hemCD by RT-PCR. RT-qPCR confirmed that hemC transcription in the PAO1 ΔalgZ mutant was 40% of the level of the wild-type strain. Taken together, these results indicate that algZR transcription involves multiple factors at multiple start sites that control individual gene expression as well as coexpression of this two-component system with heme biosynthetic genes.

The Pseudomonas aeruginosa AlgZR two-component system coordinates multiple phenotypes

Pseudomonas aeruginosa is an opportunistic pathogen that causes a multitude of infections. These infections can occur at almost any site in the body and are usually associated with a breach of the innate immune system. One of the prominent sites where P. aeruginosa causes chronic infections is within the lungs of cystic fibrosis patients. P. aeruginosa uses two-component systems that sense environmental changes to differentially express virulence factors that cause both acute and chronic infections. The P. aeruginosa AlgZR two component system is one of its global regulatory systems that affects the organism's fitness in a broad manner. This two-component system is absolutely required for two P. aeruginosa phenotypes: twitching motility and alginate production, indicating its importance in both chronic and acute infections. Additionally, global transcriptome analyses indicate that it regulates the expression of many different genes, including those associated with quorum sensing, type IV pili, type III secretion system, anaerobic metabolism, cyanide and rhamnolipid production. This review examines the complex AlgZR regulatory network, what is known about the structure and function of each protein, and how it relates to the organism's ability to cause infections.

The mucoid switch in Pseudomonas aeruginosa represses quorum sensing systems and leads to complex changes to stationary phase virulence factor regulation

The opportunistic pathogen Pseudomonas aeruginosa chronically infects the airways of Cystic Fibrosis (CF) patients during which it adapts and undergoes clonal expansion within the lung. It commonly acquires inactivating mutations of the anti-sigma factor MucA leading to a mucoid phenotype, caused by excessive production of the extracellular polysaccharide alginate that is associated with a decline in lung function. Alginate production is believed to be the key benefit of mucA mutations to the bacterium in the CF lung. A phenotypic and gene expression characterisation of the stationary phase physiology of mucA22 mutants demonstrated complex and subtle changes in virulence factor production, including cyanide and pyocyanin, that results in their down-regulation upon entry into stationary phase but, (and in contrast to wildtype strains) continued production in prolonged stationary phase. These findings may have consequences for chronic infection if mucoid P. aeruginosa were to continue to make virulence factors under non-growing conditions during infection. These changes resulted in part from a severe down-regulation of both AHL-and AQ (PQS)-dependent quorum sensing systems. In trans expression of the cAMP-dependent transcription factor Vfr restored both quorum sensing defects and virulence factor production in early stationary phase. Our findings have implications for understanding the evolution of P. aeruginosa during CF lung infection and it demonstrates that mucA22 mutation provides a second mechanism, in addition to the commonly occurring lasR mutations, of down-regulating quorum sensing during chronic infection this may provide a selection pressure for the mucoid switch in the CF lung.

A gacS deletion in Pseudomonas aeruginosa cystic fibrosis isolate CHA shapes its virulence

Pseudomonas aeruginosa, a human opportunistic pathogen, is capable of provoking acute and chronic infections that are associated with defined sets of virulence factors. During chronic infections, the bacterium accumulates mutations that silence some and activate other genes. Here we show that the cystic fibrosis isolate CHA exhibits a unique virulence phenotype featuring a mucoid morphology, an active Type III Secretion System (T3SS, hallmark of acute infections), and no Type VI Secretion System (H1-T6SS). This virulence profile is due to a 426 bp deletion in the 3′ end of the gacS gene encoding an essential regulatory protein. The absence of GacS disturbs the Gac/Rsm pathway leading to depletion of the small regulatory RNAs RsmY/RsmZ and, in consequence, to expression of T3SS, while switching off the expression of H1-T6SS and Pel polysaccharides. The CHA isolate also exhibits full ability to swim and twitch, due to active flagellum and Type IVa pili. Thus, unlike the classical scheme of balance between virulence factors, clinical strains may adapt to a local niche by expressing both alginate exopolysaccharide, a hallmark of membrane stress that protects from antibiotic action, host defences and phagocytosis, and efficient T3S machinery that is considered as an aggressive virulence factor.

Pseudomonas aeruginosa Genome Evolution in Patients and under the Hospital Environment

Pseudomonas aeruginosa is a Gram-negative environmental species and an opportunistic microorganism, establishing itself in vulnerable patients, such as those with cystic fibrosis (CF) or those hospitalized in intensive care units (ICU). It has become a major cause of nosocomial infections worldwide and a serious threat to Public Health because of overuse and misuse of antibiotics that have selected highly resistant strains against which very few therapeutic options exist. Herein is illustrated the intraclonal evolution of the genome of sequential isolates collected in a single CF patient from the early phase of pulmonary colonization to the fatal outcome. We also examined at the whole genome scale a pair of genotypically-related strains made of a drug susceptible, environmental isolate recovered from an ICU sink and of its multidrug resistant counterpart found to infect an ICU patient. Multiple genetic changes accumulated in the CF isolates over the disease time course including SNPs, deletion events and reduction of whole genome size. The strain isolated from the ICU patient displayed an increase in the genome size of 4.8% with major genetic rearrangements as compared to the initial environmental strain. The annotated genomes are given in free access in an interactive web application WallGene  designed to facilitate large-scale comparative analysis and thus allowing investigators to explore homologies and syntenies between P. aeruginosa strains, here PAO1 and the five clinical strains described.

AmrZ is a global transcriptional regulator implicated in iron uptake and environmental adaption in P. fluorescens F113

Background

AmrZ, a RHH transcriptional regulator, regulates motility and alginate production in pseudomonads. Expression of amrZ depends on the environmental stress sigma factor AlgU. amrZ and algU mutants have been shown to be impaired in environmental fitness in different pseudomonads with different lifestyles. Considering the importance of AmrZ for the ecological fitness of pseudomonads and taking advantage of the full sequencing and annotation of the Pseudomonas fluorescens F113 genome, we have carried out a ChIP-seq analysis from a pool of eight independent ChIP assays in order to determine the AmrZ binding sites and its implication in the regulation of genes involved in environmental adaption.

Results

154 enriched regions (AmrZ binding sites) were detected in this analysis, being 76% of them located in putative promoter regions. 18 of these peaks were validated in an independent ChIP assay by qPCR. The 154 peaks were assigned to genes involved in several functional classes such as motility and chemotaxis, iron homeostasis, and signal transduction and transcriptional regulators, including genes encoding proteins implicated in the turn-over of c-diGMP. A putative AmrZ binding site was also observed by aligning the 154 regions with the MEME software. This motif was present in 75% of the peaks and was similar to that described in the amrZ and algD promoters in P. aeruginosa. We have analyzed the role of AmrZ in the regulation of iron uptake genes, to find that AmrZ represses their expression under iron limiting conditions.

Conclusions

The results presented here show that AmrZ is an important global transcriptional regulator involved in environmental sensing and adaption. It is also a new partner in the complex iron homeostasis regulation.

BrlR from Pseudomonas aeruginosa is a c-di-GMP-responsive transcription factor

The transcriptional regulator BrlR is a member of the MerR family of multidrug transport activators that contributes to the high-level drug tolerance of Pseudomonas aeruginosa biofilms. While MerR regulators are known to activate both the expression of multidrug efflux pump genes and their own transcription upon inducer binding, little is known about BrlR activation. We demonstrate using promoter reporter strains, in vivo and in vitro DNA-binding assays combined with 5'RACE, that BrlR binds to its own promoter, likely via a MerR-like palindromic sequence. Unlike known MerR multidrug transport activators, BrlR and brlR expression are not activated by multidrug transporter substrates. Instead, BrlR-DNA binding was enhanced by the secondary messenger c-di-GMP. In addition to enhanced BrlR-DNA binding, c-di-GMP levels contributed to PbrlR promoter activity in initial attached cells with elevated c-di-GMP levels correlating with increased expression of brlR. While not harbouring amino acid motifs resembling previously defined c-di-GMP-binding domains, BrlR was found to bind c-di-GMP in vitro at a ratio of one c-di-GMP per two BrlR. Cross-linking assays confirmed dimer formation to be enhanced in the presence of elevated c-di-GMP levels. Our findings demonstrate BrlR to be an unusual MerR-family member in that BrlR function and expression require the secondary messenger c-di-GMP.

Analysis of the small RNA spf in the plant pathogen Pseudomonas syringae pv. tomato strain DC3000

Bacteria contain small non-coding RNAs (ncRNAs) that are typically responsible for altering transcription, translation or mRNA stability. ncRNAs are important because they often regulate virulence factors and susceptibility to various stresses. Here, the regulation of a recently described ncRNA of Pseudomonas syringae DC3000, spot 42 (now referred to as spf), was investigated. A putative RpoE binding site was identified upstream of spf in strain DC3000. RpoE is shown to regulate the expression of spf. Also, deletion of spf results in increased sensitivity to hydrogen peroxide compared with the wild-type strain, suggesting that spf plays a role in susceptibility to oxidative stress. Furthermore, expression of alg8 is shown to be influenced by spf, suggesting that this ncRNA plays a role in alginate biosynthesis. Structural and comparative genomic analyses show this ncRNA is well conserved among the pseudomonads. The findings provide new information on the regulation and role of this ncRNA in P. syringae.

The AlgZR two-component system recalibrates the RsmAYZ posttranscriptional regulatory system to inhibit expression of the Pseudomonas aeruginosa type III secretion system

Pseudomonas aeruginosa causes chronic airway infections in cystic fibrosis (CF) patients. A classic feature of CF airway isolates is the mucoid phenotype. Mucoidy arises through mutation of the mucA anti-sigma factor and subsequent activation of the AlgU regulon. Inactivation of mucA also results in reduced expression of the Vfr transcription factor. Vfr regulates several important virulence factors, including a type III secretion system (T3SS). In the present study, we report that ExsA expression, the master regulator of T3SS gene expression, is further reduced in mucA mutants through a Vfr-independent mechanism involving the RsmAYZ regulatory system. RsmA is an RNA binding protein required for T3SS gene expression. Genetic experiments suggest that the AlgZR two-component system, part of the AlgU regulon, inhibits ExsA expression by increasing the expression of RsmY and RsmZ, two small noncoding RNAs that sequester RsmA from target mRNAs. Epistasis analyses revealed that increasing the concentration of free RsmA, through either rsmYZ deletion or increased RsmA expression, partially restored T3SS gene expression in the mucA mutant. Furthermore, increasing RsmA availability in combination with Vfr complementation fully restored T3SS expression. Recalibration of the RsmAYZ system by AlgZR, however, did not alter the expression of other selected RsmA-dependent targets. We account for this observation by showing that ExsA expression is more sensitive to changes in free RsmA than other members of the RsmA regulon. Together, these data indicate that recalibration of the RsmAYZ system partially accounts for reduced T3SS gene expression in mucA mutants.

Pseudomonas aeruginosa AlgR phosphorylation modulates rhamnolipid production and motility

AlgR is a key Pseudomonas aeruginosa transcriptional response regulator required for virulence. AlgR activates alginate production and twitching motility but represses the Rhl quorum-sensing (QS) system, including rhamnolipid production. The role of AlgR phosphorylation is enigmatic, since phosphorylated AlgR (AlgR-P) is required for twitching motility through the fimU promoter but is not required for the activation of alginate production. In order to examine the role of AlgR phosphorylation in vivo, a PAO1 algRD54E strain (with algR encoding a D-to-E change at position 54), which constitutively activates fimU transcription and exhibits twitching motility, was created. A corresponding PAO1 algRD54N strain (with algR encoding a D-to-N change at position 54) that does not activate fimU or twitching motility was compared to PAO1, PAO1 algRD54E, PAO1 ΔalgZ (deletion of the algZ [fimS] gene, encoding a putative histidine kinase), and PAO1 ΔalgR for swarming motility, rhamnolipid production, and rhlA transcription. PAO1 and PAO1 algRD54E produced approximately 2-fold-higher levels of rhamnolipids than PAO1 algRD54N and PAO1 ΔalgZ, thereby indicating that phosphorylated AlgR is required for normal rhamnolipid production. Examination of purified AlgR, AlgR-P, AlgR D54N, and AlgR D54E showed that AlgR-P and AlgR D54E bound preferentially to the fimU and rhlA promoters. Additionally, AlgR-P bound specifically to two sites within the rhlA promoter that were not bound by unphosphorylated AlgR. Taken together, these results indicate that phosphorylated AlgR-P has increased affinity for the rhlA promoter and is required for the coordinate activation of twitching motility, rhamnolipid production, and swarming motility in P. aeruginosa.

Sugar administration is an effective adjunctive therapy in the treatment of Pseudomonas aeruginosa pneumonia

Treatment of acute and chronic pulmonary infections caused by opportunistic pathogen Pseudomonas aeruginosa is limited by the increasing frequency of multidrug bacterial resistance. Here, we describe a novel adjunctive therapy in which administration of a mix of simple sugars-mannose, fucose, and galactose-inhibits bacterial attachment, limits lung damage, and potentiates conventional antibiotic therapy. The sugar mixture inhibits adhesion of nonmucoid and mucoid P. aeruginosa strains to bronchial epithelial cells in vitro. In a murine model of acute pneumonia, treatment with the sugar mixture alone diminishes lung damage, bacterial dissemination to the subpleural alveoli, and neutrophil- and IL-8-driven inflammatory responses. Remarkably, the sugars act synergistically with anti-Pseudomonas antibiotics, including β-lactams and quinolones, to further reduce bacterial lung colonization and damage. To probe the mechanism, we examined the effects of sugars in the presence or absence of antibiotics during growth in liquid culture and in an ex vivo infection model utilizing freshly dissected mouse tracheas and lungs. We demonstrate that the sugar mixture induces rapid but reversible formation of bacterial clusters that exhibited enhanced susceptibility to antibiotics compared with individual bacteria. Our findings reveal that sugar inhalation, an inexpensive and safe therapeutic, could be used in combination with conventional antibiotic therapy to more effectively treat P. aeruginosa lung infections.

The MerR-like regulator BrlR confers biofilm tolerance by activating multidrug efflux pumps in Pseudomonas aeruginosa biofilms

A defining characteristic of biofilms is antibiotic tolerance that can be up to 1,000-fold greater than that of planktonic cells. In Pseudomonas aeruginosa, biofilm tolerance to antimicrobial agents requires the biofilm-specific MerR-type transcriptional regulator BrlR. However, the mechanism by which BrlR mediates biofilm tolerance has not been elucidated. Genome-wide transcriptional profiling indicated that brlR was required for maximal expression of genes associated with antibiotic resistance, in particular those encoding the multidrug efflux pumps MexAB-OprM and MexEF-OprN. Chromatin immunoprecipitation (ChIP) analysis revealed a direct regulation of these genes by BrlR, with DNA binding assays confirming BrlR binding to the promoter regions of the mexAB-oprM and mexEF-oprN operons. Quantitative reverse transcriptase PCR (qRT-PCR) analysis further indicated BrlR to be an activator of mexAB-oprM and mexEF-oprN gene expression. Moreover, immunoblot analysis confirmed increased MexA abundance in cells overexpressing brlR. Inactivation of both efflux pumps rendered biofilms significantly more susceptible to five different classes of antibiotics by affecting MIC but not the recalcitrance of biofilms to killing by bactericidal agents. Overexpression of either efflux pump in a ΔbrlR strain partly restored tolerance of ΔbrlR biofilms to antibiotics. Expression of brlR in mutant biofilms lacking both efflux pumps partly restored antimicrobial tolerance of biofilms to wild-type levels. Our results indicate that BrlR acts as an activator of multidrug efflux pumps to confer tolerance to P. aeruginosa biofilms and to resist the action of antimicrobial agents.

Phenotypic and molecular characterization of antimicrobial resistance and virulence factors in Pseudomonas aeruginosa clinical isolates from Recife, State of Pernambuco, Brazil

INTRODUCTION: The emergence of carbapenem resistance mechanisms in Pseudomonas aeruginosa has been outstanding due to the wide spectrum of antimicrobial degradation of these bacteria, reducing of therapeutic options. METHODS: Sixty-one clinical strains of P. aeruginosa isolated from five public hospitals in Recife, Pernambuco, Brazil, were examined between 2006 and 2010, aiming of evaluating the profiles of virulence, resistance to antimicrobials, presence of metallo-β-lactamase (MBL) genes, and clonal relationship among isolates. RESULTS: A high percentage of virulence factors (34.4% mucoid colonies; 70.5% pyocyanin; 93.4% gelatinase positives; and 72.1% hemolysin positive) and a high percentage of antimicrobial resistance rates (4.9% pan-resistant and 54.1% multi-drug resistant isolates) were observed. Among the 29 isolates resistant to imipenem and/or ceftazidime, 44.8% (13/29) were MBL producers by phenotypic evaluation, and of these, 46.2% (6/13) were positive for the blaSPM-1 gene. The blaIMP and blaVIM genes were not detected. The molecular typing revealed 21 molecular profiles of which seven were detected in distinct hospitals and periods. Among the six positive blaSPM-1 isolates, three presented the same clonal profile and were from the same hospital, whereas the other three presented different clonal profiles. CONCLUSIONS: These results revealed that P. aeruginosa is able to accumulate different resistance and virulence factors, making the treatment of infections difficult. The identification of blaSPM-1 genes and the dissemination of clones in different hospitals, indicate the need for stricter application of infection control measures in hospitals in Recife, Brazil, aiming at reducing costs and damages caused by P. aeruginosa infections.

A dynamic and intricate regulatory network determines Pseudomonas aeruginosa virulence

Pseudomonas aeruginosa is a metabolically versatile bacterium that is found in a wide range of biotic and abiotic habitats. It is a major human opportunistic pathogen causing numerous acute and chronic infections. The critical traits contributing to the pathogenic potential of P. aeruginosa are the production of a myriad of virulence factors, formation of biofilms and antibiotic resistance. Expression of these traits is under stringent regulation, and it responds to largely unidentified environmental signals. This review is focused on providing a global picture of virulence gene regulation in P. aeruginosa. In addition to key regulatory pathways that control the transition from acute to chronic infection phenotypes, some regulators have been identified that modulate multiple virulence mechanisms. Despite of a propensity for chaotic behaviour, no chaotic motifs were readily observed in the P. aeruginosa virulence regulatory network. Having a ‘birds-eye’ view of the regulatory cascades provides the forum opportunities to pose questions, formulate hypotheses and evaluate theories in elucidating P. aeruginosa pathogenesis. Understanding the mechanisms involved in making P. aeruginosa a successful pathogen is essential in helping devise control strategies.

Calcium chelation by alginate activates the type III secretion system in mucoid Pseudomonas aeruginosa biofilms

The extracellular biofilm matrix includes primarily DNA and exopolysaccharides (EPS), which function to maintain aggregate structures and to protect biofilms from antibiotics and the immune response. Both polymers are anionic and have cation binding activity, however the impact of this activity on biofilms is not fully understood. Host cell contact is considered the primary signal for activation of most type III secretion systems (T3SS), although calcium limitation is frequently used as a trigger of contact-independent T3SS expression. We hypothesized that alginate, which is a known calcium binding exopolysaccharide produced in mucoid Pseudomonas aeruginosa isolates, can activate the T3SS in biofilms. The addition of exogenous purified alginate to planktonic, non-mucoid PAO1 cultures induced expression of exoS, exoT and exoY-lux reporters of the T3SS in a concentration-dependent manner. Induction by alginate was comparable to induction by the calcium chelator NTA. We extended our analysis of the T3SS in flow chamber-cultivated biofilms, and showed that hyperproduction of alginate in mucA22 mucoid isolates resulted in induction of the exoS-gfp transcriptional reporter compared to non-mucoid paired isolates. We confirmed the transcriptional effects of alginate on the T3SS expression using a FlAsH fluorescence method and showed high levels of the ExoT-Cys(4) protein in mucoid biofilms. Induction of the T3SS could be prevented in planktonic cultures and mucoid biofilms treated with excess calcium, indicating that Ca(2+) chelation by the EPS matrix caused contact-independent induction. However, mucoid isolates generally had reduced exoS-lux expression in comparison to paired, non-mucoid isolates when grown as planktonic cultures and agar colonies. In summary, we have shown a mucoid biofilm-specific induction of the type III secretion system and highlight a difference between planktonic and biofilm cultures in the production of virulence factors.

The two-component sensor KinB acts as a phosphatase to regulate Pseudomonas aeruginosa Virulence

Pseudomonas aeruginosa is an opportunistic pathogen that is capable of causing both acute and chronic infections. P. aeruginosa virulence is subject to sophisticated regulatory control by two-component systems that enable it to sense and respond to environmental stimuli. We recently reported that the two-component sensor KinB regulates virulence in acute P. aeruginosa infection. Furthermore, it regulates acute-virulence-associated phenotypes such as pyocyanin production, elastase production, and motility in a manner independent of its kinase activity. Here we show that KinB regulates virulence through the global sigma factor AlgU, which plays a key role in repressing P. aeruginosa acute-virulence factors, and through its cognate response regulator AlgB. However, we show that rather than phosphorylating AlgB, KinB's primary role in the regulation of virulence is to act as a phosphatase to dephosphorylate AlgB and alleviate phosphorylated AlgB's repression of acute virulence.

In vitro evaluation of tobramycin and aztreonam versus Pseudomonas aeruginosa biofilms on cystic fibrosis-derived human airway epithelial cells

OBJECTIVES

Aztreonam for inhalation solution (AZLI) was recently approved by the FDA for treating cystic fibrosis (CF) patients infected with Pseudomonas aeruginosa. Here we investigated the effect of aztreonam alone or in combination with tobramycin on P. aeruginosa biofilms grown on CF airway epithelial cells.

METHODS

P. aeruginosa biofilms, produced by laboratory strains or clinical isolates, were formed on confluent CF airway cells before treatment overnight with aztreonam or tobramycin alone or in combination. Alternatively, antibiotics were added 1 h after bacterial inoculation to assess their ability to impair biofilm formation at 5 h. Bacterial cfu remaining after treatment were then determined by plate counting.

RESULTS

In the absence of antibiotics, all strains developed biofilms that disrupted CF airway epithelial monolayers overnight. Tobramycin reduced the cfu of all strains grown as biofilms. Aztreonam reduced the cfu of some strains by ∼1 log unit without preserving the integrity of cystic fibrosis airway cell monolayers, while decreasing the biofilms of other clinical isolates by ∼4 log units and protecting the monolayers from being compromised. The combination of aztreonam and tobramycin reduced the cfu of two strains by an additional 0.5 and 2 log units, respectively. Of all the mechanisms explored, Psl exopolysaccharide production might explain the variations in biofilm tolerance to aztreonam in some of the strains.

CONCLUSIONS

Effects of aztreonam on P. aeruginosa biofilms in the in vitro co-culture model are strain-dependent. The simultaneous application of aztreonam and tobramycin may be beneficial for a subset of CF patients by eliminating susceptible P. aeruginosa strains.