Characterization of Pseudomonas aeruginosa fliO, a gene involved in flagellar biosynthesis and adherence

Mucin adhesion of serial cystic fibrosis airways Pseudomonas aeruginosa isolates

The chronic airway infections with Pseudomonas aeruginosa are the major co-morbidity in people with cystic fibrosis (CF). Within CF lungs, P. aeruginosa persists in the conducting airways together with human mucins as the most abundant structural component of its microenvironment. We investigated the adhesion of 41 serial CF airway P. aeruginosa isolates to airway mucin preparations from CF sputa. Mucins and bacteria were retrieved from five modulator-naïve patients with advanced CF lung disease. The P. aeruginosa isolates from CF airways and non-CF reference strains showed a strain-specific signature in their adhesion to ovine, porcine and bovine submaxillary mucins and CF airway mucins ranging from no or low to moderate and strong binding. Serial CF clonal isolates and colony morphotypes from the same sputum sample were as heterogeneous in their affinity to mucin as representatives of other clones thus making 'mucin binding' one of the most variable intraclonal phenotypic traits of P. aeruginosa known to date. Most P. aeruginosa CF airway isolates did not adhere more strongly to CF airway mucins than to plastic surfaces. The strong binders, however, exhibited a strain-specific affinity gradient to O-glycans, CF airway and mammalian submaxillary mucins.

Distinct Screening Approaches Uncover PA14_36820 and RecA as Negative Regulators of Biofilm Phenotypes in Pseudomonas aeruginosa PA14

Pseudomonas aeruginosa commonly infects hospitalized patients and the lungs of individuals with cystic fibrosis. This species is known for forming biofilms, which are communities of bacterial cells held together and encapsulated by a self-produced extracellular matrix. The matrix provides extra protection to the constituent cells, making P. aeruginosa infections challenging to treat. We previously identified a gene, PA14_16550, which encodes a DNA-binding TetR-type repressor and whose deletion reduced biofilm formation. Here, we assessed the transcriptional impact of the 16550 deletion and found six differentially regulated genes. Among them, our results implicated PA14_36820 as a negative regulator of biofilm matrix production, while the remaining 5 had modest effects on swarming motility. We also screened a transposon library in a biofilm-impaired ΔamrZ Δ16550 strain for restoration of matrix production. Surprisingly, we found that disruption or deletion of recA increased biofilm matrix production, both in biofilm-impaired and wild-type strains. Because RecA functions both in recombination and in the DNA damage response, we asked which function of RecA is important with respect to biofilm formation by using point mutations in recA and lexA to specifically disable each function. Our results implied that loss of either function of RecA impacts biofilm formation, suggesting that enhanced biofilm formation may be one physiological response of P. aeruginosa cells to loss of either RecA function. IMPORTANCE Pseudomonas aeruginosa is a notorious human pathogen well known for forming biofilms, communities of bacteria that protect themselves within a self-secreted matrix. Here, we sought to find genetic determinants that impacted biofilm matrix production in P. aeruginosa strains. We identified a largely uncharacterized protein (PA14_36820) and, surprisingly, RecA, a widely conserved bacterial DNA recombination and repair protein, as negatively regulating biofilm matrix production. Because RecA has two main functions, we used specific mutations to isolate each function and found that both functions influenced matrix production. Identifying negative regulators of biofilm production may suggest future strategies to reduce the formation of treatment-resistant biofilms.

Establishment of a Novel Rat Model of Gram-Negative Periprosthetic Joint Infection Using Cementless Hip Hemiarthroplasty

BACKGROUND

Gram-negative periprosthetic joint infections (GN-PJIs) present unique challenges. Our aim was to establish a clinically representative GN-PJI model that recapitulates biofilm formation in vivo. We also hypothesized that biofilm formation on the implant surface would affect its ability to osseointegrate.

METHODS

Three-dimensionally-printed medical-grade titanium hip implants were used to replace the femoral heads of male Sprague-Dawley rats. GN-PJI was induced using 2 bioluminescent Pseudomonas aeruginosa strains: a reference strain (PA14-lux) and a mutant biofilm-defective strain (ΔflgK-lux). Infection was monitored in real time using an in vivo imaging system (IVIS) and magnetic resonance imaging (MRI). Bacterial loads were quantified utilizing the viable colony count. Biofilm formation at the bone-implant interface was visualized using field-emission scanning electron microscopy (FE-SEM). Implant stability, as an outcome, was directly assessed by quantifying osseointegration using microcomputed tomography, and indirectly assessed by identifying gait-pattern changes.

RESULTS

Bioluminescence detected by the IVIS was focused on the hip region and demonstrated localized infection, with greater ability of PA14-lux to persist in the model compared with the ΔflgK-lux strain, which is defective in biofilm formation. This was corroborated by MRI, as PA14-lux induced relatively larger implant-related abscesses. Biofilm formation at the bone-implant interface induced by PA14-lux was visualized using FE-SEM versus defective-biofilm formation by ΔflgK-lux. Quantitatively, the average viable colony count of the sonicated implants, in colony-forming units/mL, was 3.77 × 108 for PA14-lux versus 3.65 × 103 for ΔflgK-lux, with a 95% confidence interval around the difference of 1.45 × 108 to 6.08 × 108 (p = 0.0025). This difference in the ability to persist in the model was reflected significantly on implant osseointegration, with a mean intersection surface of 4.1 × 106 ± 1.99 × 106 μm2 for PA14-lux versus 6.44 × 106 ± 2.53 × 106 μm2 for ΔflgK-lux and 7.08 × 106 ± 1.55 × 106 μm2 for the noninfected control (p = 0.048).

CONCLUSIONS

To our knowledge, this proposed, novel in vivo biofilm-based model is the most clinically representative for GN-PJI to date, since animals can bear weight on the implant, poor osseointegration was associated with biofilm formation, and localized PJI was assessed by various modalities.

CLINICAL RELEVANCE

This model will allow for more reliable testing of novel biofilm-targeting therapeutics.

Efficient zinc uptake is critical for the ability of Pseudomonas aeruginosa to express virulence traits and colonize the human lung

We have recently shown that Pseudomonas aeruginosa, an opportunistic pathogen that chronically infects the lungs of patients with cystic fibrosis (CF) and other forms of lung disease, is extremely efficient in recruiting zinc from the environment and that this capability is required for its ability to cause acute lung infections in mice. To verify that P. aeruginosa faces zinc shortage when colonizing the lungs of human patients, we analyzed the expression of three genes that are highly induced under conditions of zinc deficiency (zrmA, dksA2 and rpmE2), in bacteria in the sputum of patients with inflammatory lung disease. All three genes were expressed in all the analyzed sputum samples to a level much higher than that of bacteria grown in zinc-containing laboratory medium, supporting the hypothesis that P. aeruginosa is under zinc starvation during lung infections. We also found that the expression of several virulence traits that play a central role in the ability of P. aeruginosa to colonize the lung is affected by disruption of the most important zinc importing systems. Virulence features dependent on zinc intake include swarming and swimming motility and the ability to form biofilms. Furthermore, alterations in zinc assimilation interfere with the synthesis of the siderophore pyoverdine, suggesting that zinc recruitment could modulate iron uptake and affect siderophore-mediated cell signaling. Our results reveal that zinc uptake is likely to play a key role in the ability of P. aeruginosa to cause chronic lung infections and strongly modulates critical virulence traits of the pathogen. Taking into account the recent discovery that zinc uptake in P. aeruginosa is promoted by the release of a small molecular weight molecule showing high affinity for zinc, our data suggest novel and effective possibilities to control lung infections by these bacteria.

Characterization of New Virulence Factors Involved in the Intracellular Growth and Survival of Burkholderia pseudomallei

Burkholderia pseudomallei, the causative agent of melioidosis, has complex and poorly understood extracellular and intracellular lifestyles. We used transposon-directed insertion site sequencing (TraDIS) to retrospectively analyze a transposon library that had previously been screened through a BALB/c mouse model to identify genes important for growth and survival in vivo. This allowed us to identify the insertion sites and phenotypes of negatively selected mutants that were previously overlooked due to technical constraints. All 23 unique genes identified in the original screen were confirmed by TraDIS, and an additional 105 mutants with various degrees of attenuation in vivo were identified. Five of the newly identified genes were chosen for further characterization, and clean, unmarked bpsl2248, tex, rpiR, bpsl1728, and bpss1528 deletion mutants were constructed from the wild-type strain K96243. Each of these mutants was tested in vitro and in vivo to confirm their attenuated phenotypes and investigate the nature of the attenuation. Our results confirm that we have identified new genes important to in vivo virulence with roles in different stages of B. pseudomallei pathogenesis, including extracellular and intracellular survival. Of particular interest, deletion of the transcription accessory protein Tex was shown to be highly attenuating, and the tex mutant was capable of providing protective immunity against challenge with wild-type B. pseudomallei, suggesting that the genes identified in our TraDIS screen have the potential to be investigated as live vaccine candidates.

Pseudomonas aeruginosa cystic fibrosis isolates of similar RAPD genotype exhibit diversity in biofilm forming ability in vitro

BACKGROUND

Pseudomonas aeruginosa is considered to grow in a biofilm in cystic fibrosis (CF) chronic lung infections. Bacterial cell motility is one of the main factors that have been connected with P. aeruginosa adherence to both biotic and abiotic surfaces. In this investigation, we employed molecular and microscopic methods to determine the presence or absence of motility in P. aeruginosa CF isolates, and statistically correlated this with their biofilm forming ability in vitro.

RESULTS

Our investigations revealed a wide diversity in the production, architecture and control of biofilm formation. Of 96 isolates, 49% possessed swimming motility, 27% twitching and 52% swarming motility, while 47% were non-motile. Microtitre plate assays for biofilm formation showed a range of biofilm formation ability from biofilm deficient phenotypes to those that formed very thick biofilms. A comparison of the motility and adherence properties of individual strains demonstrated that the presence of swimming and twitching motility positively affected biofilm biomass. Crucially, however, motility was not an absolute requirement for biofilm formation, as 30 non-motile isolates actually formed thick biofilms, and three motile isolates that had both flagella and type IV pili attached only weakly. In addition, CLSM analysis showed that biofilm-forming strains of P. aeruginosa were in fact capable of entrapping non-biofilm forming strains, such that these 'non-biofilm forming' cells could be observed as part of the mature biofilm architecture.

CONCLUSIONS

Clinical isolates that do not produce biofilms in the laboratory must have the ability to survive in the patient lung. We propose that a synergy exists between isolates in vivo, which allows "non biofilm-forming" isolates to be incorporated into the biofilm. Therefore, there is the potential for strains that are apparently non-biofilm forming in vitro to participate in biofilm-mediated pathogenesis in the CF lung.

The alternative sigma factor AlgT represses Pseudomonas aeruginosa flagellum biosynthesis by inhibiting expression of fleQ

Pseudomonas aeruginosa poses a serious risk in individuals suffering from cystic fibrosis (CF). Strains colonizing the CF lung are generally motile but frequently convert to a nonmotile phenotype as the disease progresses. In many cases, this is coordinately regulated with the overproduction of the exopolysaccharide alginate. Both the expression of alginate (mucoidy) and the loss of flagellum synthesis may provide the bacterium with a selective advantage in the CF lung. Previously published data showed that the regulation of alginate production and flagellum biosynthesis in the CF isolate FRD1 is inversely controlled by the alternative sigma factor AlgT. In this study, we observed that in CF isolates, the mucoid and the nonmotile phenotypes occur predominantly together. Using microarrays, we compared the transcriptomes of isogenic AlgT(+) and AlgT(-) P. aeruginosa and discovered that AlgT significantly downregulated the majority of flagellar genes. A pronounced inhibitory effect was observed in several genes essential for proper flagellum expression, including fleQ, which encodes an essential flagellar regulator. The microarray data were confirmed by reverse transcriptase PCR analysis and promoter fusion assays in isogenic AlgT(+) and AlgT(-) strains. Transmission electron microscopy, motility assays, and Western blots showed that ectopic expression of FleQ in mucoid, nonmotile CF isolates restored flagellum biosynthesis and motility. Together, these data show that AlgT mediates the negative control of flagellum expression by inhibiting the expression of the flagellar regulator fleQ.

Microarray analysis and functional characterization of the nitrosative stress response in nonmucoid and mucoid Pseudomonas aeruginosa

The type strain of Pseudomonas aeruginosa, PAO1, showed great upregulation of many nitrosative defense genes upon treatment with S-nitrosoglutathione, while the mucoid strain PAO578II showed no further upregulation above its constitutive upregulation of nor and fhp. NO* consumption however, showed that both strains mount functional, protein synthesis-dependent NO*-consumptive responses.

Lack of adherence of clinical isolates of Pseudomonas aeruginosa to asialo-GM(1) on epithelial cells

Numerous studies have reported that asialo-GM(1), gangliotetraosylceramide, or moieties serve as epithelial cell receptors for Pseudomonas aeruginosa. Usually this interaction is confirmed with antibodies to asialo-GM(1). However, few, if any, of these reports have evaluated the binding of fresh clinical isolates of P. aeruginosa to asialo-GM(1) or the specificity of the antibodies for the asialo-GM(1) antigen. We confirmed that asialo-GM(1) dissolved in dimethyl sulfoxide could be added to the apical membrane of Madin-Darby canine kidney cells growing as a polarized epithelium on Transwell membranes (J. C. Comolli, L. L. Waite, K. E. Mostov, and J. N. Engel, Infect. Immun. 67:3207-3214, 1999) and that such treatment enhanced the binding of P. aeruginosa strain PA103. However, no other P. aeruginosa strain, including eight different clinical isolates, exhibited enhanced binding to asialo-GM(1)-treated cells. Studies with commercially available antibodies to asialo-GM(1) showed that these preparations had high titers of antibody to P. aeruginosa antigens, including whole cells, purified lipopolysaccharide (LPS), and pili. Inhibition studies showed that adsorption of an antiserum to asialo-GM(1) with P. aeruginosa cells could remove the reactivity of antibodies to asialo-GM(1), and adsorption of this serum with asialo-GM(1) removed antibody binding to P. aeruginosa LPS. Antibodies in sera raised to asialo-GM(1) were observed to bind to P. aeruginosa cells by immunoelectron microscopy. Antibodies to asialo-GM(1) inhibited formation of a biofilm by P. aeruginosa in the absence of mammalian cells, indicating a direct inhibition of bacterial cell-cell interactions. These findings demonstrate that asialo-GM(1) is not a major cellular receptor for clinical isolates of P. aeruginosa and that commercially available antibodies raised to this antigen contain high titers of antibody to multiple P. aeruginosa antigens, which do not interfere with the binding of P. aeruginosa to mammalian cells but possibly interfere with the binding of P. aeruginosa cells to each other.

Pseudomonas aeruginosa mediated apoptosis requires the ADP-ribosylating activity of exoS

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that primarily infects immunocompromised individuals and patients with cystic fibrosis. Using a tissue culture system, invasive strains of P. aeruginosa were discovered to induce apoptosis at high frequency in HeLa and other epithelial and fibroblast cell lines. This apoptotic phenotype in the infected cells was determined by several criteria including (i) visual changes in cell morphology, (ii) induction of chromatin condensation and nuclear marginalization, (iii) the presence of a high percentage of cells with subG1 DNA content, and (iv) activation of caspase-3 activity. Induction of the type III secretion machinery, but not invasion of P. aeruginosa is required for induction of apoptosis. The apoptosis phenotype is independent of the cytoskeletal rearrangements that occur in the host cell early after infection. Mutants in P. aeruginosa exoS fail to induce apoptosis and complementation with wild-type exoS restored the apoptosis-inducing capacity, demonstrating that ExoS is the effector molecule. Analysis of exoS activity mutants shows that the ADP-ribosylating capacity of ExoS is essential for inducing the apoptotic pathway.

RpmA is required for nonopsonic phagocytosis of Pseudomonas aeruginosa

Pseudomonas aeruginosa causes severe respiratory tract infections in patients with cystic fibrosis (CF). We have been examining nonopsonic phagocytosis of P. aeruginosa by macrophages. To study the P. aeruginosa-macrophage interaction at the molecular level, we have constructed a transposon Tn5G bank in a clinical isolate of P. aeruginosa (strain 4020) and identified mutants resistant to nonopsonic phagocytosis. Phagocytosis-resistant mutants were enriched by passaging the transposon bank over 18 macrophage monolayers. Of 900 individual mutants isolated from this enriched pool in a nonopsonic phagocytosis assay, we identified 85 putative mutants that were resistant to phagocytosis. In this study, we have characterized one of these transposon mutants, P. aeruginosa 4020 H27A, which was poorly ingested. H27A possessed a Tn5G insertion in a gene encoding a protein with homology to the MotA proteins of several species of bacteria. We have called this gene rpmA for required for phagocytosis by macrophages. RpmA is one of two MotA paralogs in P. aeruginosa. This rpmA::Tn5G mutant was motile both on agar plates and in visual examination of wet mounts. The phagocytosis defect was partially complemented by providing the rpmA gene in trans and fully complemented when both rpmA and rpmB were provided. A rpmA null mutant was ingested by macrophages similar to the H27A transposon mutant. These data suggest that the rpmA and rpmB gene products are required for the efficient ingestion of P. aeruginosa by macrophages.

Functional characterization of a serine/threonine protein kinase of Pseudomonas aeruginosa

Protein kinases play a key role in signal transduction pathways in both eukaryotic and prokaryotic cells. Using in vivo expression technology, we have identified several promoters in Pseudomonas aeruginosa which are preferentially activated during infection of neutropenic mice. One of these promoters directs the transcription of a gene encoding a putative protein kinase similar to the enzymes found in eukaryotic cells. The full characterization of this protein, termed PpkA, is presented in this communication. The ppkA gene encodes a 1,032-amino-acid polypeptide with an N-terminal catalytic domain showing all of the conserved residues of protein kinases with the substrate phosphorylation specificities for serine and threonine residues. The catalytic domain is linked to the rest of the protein by a short proline-rich segment. The enzymes showed anomalous migration behavior when analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, which could be attributed to autophosphorylation activity. The full-length enzyme was expressed as an oligohistidine fusion protein and was shown to phosphorylate several artificial protein substrates. Both autophosphorylation and phosphorylation of added substrates were strongly reduced by a single-amino-acid substitution in the catalytic domain of PpkA. Although PpkA appears to be differentially phosphorylated by autocatalysis, the levels of phosphorylation have minimal effect on its overall enzymatic activity. Our results, therefore, indicate the operation of a novel protein phosphorylation mechanism during transduction of signals in P. aeruginosa, and this pathway may be important in regulating the expression of virulence factors by this pathogen during certain phases of infection.

Flagellar and twitching motility are necessary for Pseudomonas aeruginosa biofilm development

The formation of complex bacterial communities known as biofilms begins with the interaction of planktonic cells with a surface in response to appropriate environmental signals. We report the isolation and characterization of mutants of Pseudomonas aeruginosa PA14 defective in the initiation of biofilm formation on an abiotic surface, polyvinylchloride (PVC) plastic. These mutants are designated surface attachment defective (sad ). Two classes of sad mutants were analysed: (i) mutants defective in flagellar-mediated motility and (ii) mutants defective in biogenesis of the polar-localized type IV pili. We followed the development of the biofilm formed by the wild type over 8 h using phase-contrast microscopy. The wild-type strain first formed a monolayer of cells on the abiotic surface, followed by the appearance of microcolonies that were dispersed throughout the monolayer of cells. Using time-lapse microscopy, we present evidence that microcolonies form by aggregation of cells present in the monolayer. As observed with the wild type, strains with mutations in genes required for the synthesis of type IV pili formed a monolayer of cells on the PVC plastic. However, in contrast to the wild-type strain, the type IV pili mutants did not develop microcolonies over the course of the experiments, suggesting that these structures play an important role in microcolony formation. Very few cells of a non-motile strain (carrying a mutation in flgK) attached to PVC even after 8 h of incubation, suggesting a role for flagella and/or motility in the initial cell-to-surface interactions. The phenotype of these mutants thus allows us to initiate the dissection of the developmental pathway leading to biofilm formation.

Initiation of biofilm formation in Pseudomonas fluorescens WCS365 proceeds via multiple, convergent signalling pathways: a genetic analysis

Populations of surface-attached microorganisms comprising either single or multiple species are commonly referred to as biofilms. Using a simple assay for the initiation of biofilm formation (e.g. attachment to an abiotic surface) by Pseudomonas fluorescens strain WCS365, we have shown that: (i) P. fluorescens can form biofilms on an abiotic surface when grown on a range of nutrients; (ii) protein synthesis is required for the early events of biofilm formation; (iii) one (or more) extracytoplasmic protein plays a role in interactions with an abiotic surface; (iv) the osmolarity of the medium affects the ability of the cell to form biofilms. We have isolated transposon mutants defective for the initiation of biofilm formation, which we term surface attachment defective (sad). Molecular analysis of the sad mutants revealed that the ClpP protein (a component of the cytoplasmic Clp protease) participates in biofilm formation in this organism. Our genetic analyses suggest that biofilm formation can proceed via multiple, convergent signalling pathways, which are regulated by various environmental signals. Finally, of the 24 sad mutants analysed in this study, only three had defects in genes of known function. This result suggests that our screen is uncovering novel aspects of bacterial physiology.

The Pseudomonas aeruginosa flagellar cap protein, FliD, is responsible for mucin adhesion

Mucin-specific adhesion of Pseudomonas aeruginosa plays an important role in the initial colonization of this organism in the airways of cystic fibrosis patients. We report here that the flagellar cap protein, FliD, participates in this adhesion process. A polar chromosomal insertional mutation in the P. aeruginosa fliD gene made this organism nonadhesive to mucin in an in vitro mucin adhesion assay. The adhesive phenotype was restored by providing the fliD gene alone on a multicopy plasmid, suggesting involvement of this gene in mucin adhesion of P. aeruginosa. Further supporting this observation, the in vitro competition experiments demonstrated that purified FliD protein inhibited the mucin adhesion of nonpiliated P. aeruginosa PAK-NP, while the same concentrations of PilA and FlaG proteins of P. aeruginosa were ineffective in this function. The regulation of the fliD gene was studied and was found to be unique in that the transcription of the fliD gene was independent of the flagellar sigma factor sigma28. Consistent with this finding, no sigma28 binding sequence could be identified in the fliD promoter region. The results of the beta-galactosidase assays suggest that the fliD gene in P. aeruginosa is regulated by the newly described transcriptional regulator FleQ and the alternate sigma factor sigma54 (RpoN).

Cystic fibrosis transmembrane conductance regulator is an epithelial cell receptor for clearance of Pseudomonas aeruginosa from the lung

The cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride ion channel, but its relationship to the primary clinical manifestation of CF, chronic Pseudomonas aeruginosa pulmonary infection, is unclear. We report that CFTR is a cellular receptor for binding, endocytosing, and clearing P. aeruginosa from the normal lung. Murine cells expressing recombinant human wild-type CFTR ingested 30-100 times as many P. aeruginosa as cells lacking CFTR or expressing mutant DeltaF508 CFTR protein. Purified CFTR inhibited ingestion of P. aeruginosa by human airway epithelial cells. The first extracellular domain of CFTR specifically bound to P. aeruginosa and a synthetic peptide of this region inhibited P. aeruginosa internalization in vivo, leading to increased bacterial lung burdens. CFTR clears P. aeruginosa from the lung, indicating a direct connection between mutations in CFTR and the clinical consequences of CF.

A transcriptional activator, FleQ, regulates mucin adhesion and flagellar gene expression in Pseudomonas aeruginosa in a cascade manner

Previous work has demonstrated that fleR, the gene for a transcriptional activator belonging to the NtrC subfamily of response regulators, is involved in the regulation of mucin adhesion and flagellar expression by Pseudomonas aeruginosa. This report describes the identification and characterization of fleQ, the gene for another transcriptional regulator which also regulates mucin adhesion and motility in this organism. The complete nucleotide sequence of the fleQ gene was determined on both DNA strands, and an open reading frame (ORF) consisting of 1,493 nucleotides was identified. This ORF coded for a gene product of predicted molecular weight, as confirmed by the overexpression of the fleQ gene as a fusion protein under an inducible promoter. The fleQ gene is flanked by a flagellar operon, fliDSorf126, at the 5' end and the fleSR operon on the 3' end. FleQ also had striking homology to a number of proteins belonging to the NtrC subfamily of response regulators, which work in concert with the alternate sigma factor RpoN (sigma54) to activate transcription. However, FleQ lacks the residues corresponding to Asp-54 and Lys-104 of the NtrC protein which are conserved in most of the members belonging to this subfamily of regulators. In addition, unlike some of the other transcriptional activators of this group, FleQ does not appear to have a cognate sensor kinase. A chromosomal insertional mutation in the fleQ gene abolished mucin adhesion and motility of P. aeruginosa PAK and PAK-NP. Both of these functions were regained by providing the complete fleQ gene on a multicopy plasmid. The location of fleQ immediately upstream of the fleSR operon, which is also necessary for the same process, suggested that these regulators may interact in some way. We therefore examined the regulation of the fleSR operon by fleQ and vice versa. Promoter fusion experiments showed that the fleSR operon was regulated by RpoN and FleQ. On the other hand, the fleQ promoter was independent of RpoN and FleR. FleQ, thus, adds another level of regulation to motility and adhesion in P. aeruginosa, above that of fleSR. We therefore propose the existence of a regulatory cascade which consists of at least two transcriptional regulators, FleQ and FleR, in the control of motility and adhesion in P. aeruginosa.

Recognition of mucin by the adhesin-flagellar system of Pseudomonas aeruginosa

Pseudomonas aeruginosa colonizes the mucus of patients with chronic lung diseases by a specific mechanism involving an adhesin-receptor system. Several adhesins have been implicated in the adhesion of P. aeruginosa to cells, but the identity of the principal adhesin(s) involved in adhesion to mucin is unknown. Mutagenesis studies have indicated that P. aeruginosa adhesion is under the control of the rpoN gene, which also regulates pilin synthesis, flagellum formation, and other functions. Mutagenesis of certain flagellar genes that are not controlled by RpoN, e.g., flif, also indicates a close relationship between adhesion and flagellar genes and not necessarily an independent effect of rpoN on adhesion. Mutants of certain early flagellar genes lead to the loss of both adhesion and motility, whereas mutants of certain late genes, e.g., fliC, the gene for flagellin, lose motility but retain adhesion. Recent studies indicate that both motility and adhesion are regulated by a two-component regulatory system called fleS-R, which in turn is controlled by another regulator in a cascade that involves rpoN. A fleR mutant possessing pili adheres poorly to mucins, definitively showing that pili do not play a major role in adhesion to mucin. It is unclear whether the adhesin is a flagellar protein or another protein that uses the flagellar export apparatus for localization or both. Finding the gene under control of rpoN may provide answers to these questions.

Differential gene expression by Pseudomonas aeruginosa during interaction with respiratory mucus

Pseudomonas aeruginosa is a common respiratory tract pathogen that causes serious infections in patients with cystic fibrosis. A number of putative virulence factors have been characterized in several laboratories, and some have been implicated in human infections, based on criteria such as the phenotype of isolates from infected patients, an immune response to a particular antigenic factor, and the effect of a virulence factor on infectivity in an animal model. We have developed a series of genetic tools to study the selective regulation of expression of P. aeruginosa genes during interactions of the pathogen with host tissues. These tools are based on direct enrichment of bacteria, when a particular promoter is induced or repressed. We have found that interaction of bacteria with mucus from patients with cystic fibrosis results in marked induction of expression of several genes, including one that encodes a lipopolysaccharide biosynthetic enzyme, a gene for a protein responsible for uptake of the ferric pyochelin siderophore, and a new gene homologous with a class of iron-responsive repressors. The tools described here are useful for identification of induced or repressed genes in various animal models of infection or in controlled laboratory conditions that mimic natural infections of humans. Such genes might not be detectable when bacteria are cultured in laboratory conditions, and these tools are therefore useful for general probing of a bacterial genome for genes regulated during different stages of infection.

Cloning and characterization of Pseudomonas aeruginosa fliF, necessary for flagellar assembly and bacterial adherence to mucin

Pseudomonas aeruginosa adheres to the mucosal surfaces of the lungs. This process appears to be mediated by nonpilus adhesins which bind to mucin. To find this nonpilus adhesin(s), mutagenesis of a nonpiliated mutant of P. aeruginosa with transposon Tn5G, followed by a screen for mucin adhesion, was used to isolate a series of mutants unable to adhere to mucin. All of these mutants were also found to be defective in motility. One such mutant, PAK-RR20, is characterized here. The site of the transposon insertion in PAK-RR20 was localized to a gene which is homologous to the fliF gene of other organisms and was flanked by other motility-related genes, fliE and fliG. Both adhesion and motility defects in PAK-RR20 were complemented by providing the fliF gene in trans. Since complementation could have been due to the presence of an internal promoter in the fliF gene or in the Tn5G transposon, which allowed the transcription of the downstream genes, another chromosomal mutant of the fliF gene was constructed by insertional inactivation with an antibiotic resistance cassette. This mutant was also nonmotile and nonadhesive. However, the two defects in this new mutant could not be complemented by the fliF gene in trans, consistent with the interpretation that there is no internal fliF promoter but possibly a functional promoter in the Tn5G transposon. The complete nucleotide sequences of the fliE and fliF genes and a partial nucleotide sequence of the fliG gene of P. aeruginosa were determined. Control of the promoter upstream of the fliE gene was analyzed by construction of a fliE-lacZ fusion and the introduction of this construct into strains of P. aeruginosa with mutations in several regulatory genes. Beta-Galactosidase expression measurements indicated that the fliE promoter does not utilize RpoF (sigma(28)) or RpoN (sigma(54)) sigma factors. The characterization of this gene as being responsible for the loss of adhesion indicates that basal body structures are probably important for localization of the adhesin.

Functions of bacterial flagella

Many bacterial species are motile by means of flagella. The structure and implantation of flagella seems related to the specific environments the cells live in. In some cases, the bacteria even adapt their flagellation pattern in response to the environmental conditions they encounter. Swarming cell differentiation is a remarkable example of this phenomenon. Flagella seem to have more functions than providing motility alone. For many pathogenic species, studies have been performed on the contribution of flagella to the virulence, but the result is not clear in all cases. Flagella are generally accepted as being important virulence factors, and expression and repression of flagellation and virulence have in several cases been shown to be linked. Providing motility is always an important feature of flagella of pathogenic bacteria, but adhesive and other properties also have been attributed to these flagella. In nonpathogenic bacterial colonization, flagella are important locomotive and adhesive organelles as well. In several cases where competition between several bacterial species exists, motility by means of flagella is shown to provide a specific advantage for a bacterium. This review gives an overview of studies that have been performed on the significance of flagellation in a wide variety of processes where flagellated bacteria are involved.

Cloning and phenotypic characterization of fleS and fleR, new response regulators of Pseudomonas aeruginosa which regulate motility and adhesion to mucin

This work has identified two genes (designated fleS and fleR) in Pseudomonas aeruginosa which are highly homologous to members of the subclass of two-component systems involved in transcriptional regulation of a diverse array of genes from sigma 54 promoters. The genes are located upstream from fliE, a flagellar gene of P. aeruginosa, and they are arranged in a putative fleSR operon. FleS has a predicted molecular mass of 43.87 kDa and shows strong homology to histidine kinases which in other two-component systems have been shown to be sensor proteins. FleR has a predicted molecular mass of 51.26 kDa and is homologous to other regulatory proteins that bind to specific upstream activating elements to enhance transcription of genes with sigma 54 promoters. The fleSR system is believed to control both flagellar synthesis and adhesion to mucin. Several lines of evidence are presented. (i) A nonpiliated mutant of P. aeruginosa PAK containing a gentamicin cassette in fleR is nonmotile and nonadhesive. (ii) The fleR mutant regained motility and adhesion when complemented with a wild-type copy of fleR. (iii) A Western blot (immunoblot) of the fleR mutant showed no synthesis of flagellin, and electron microscopy of the fleR mutant confirmed the lack of flagella. Previous work has shown that flagellar mutants with mutations in fliA (sigma 28) or fliC (the structural gene for flagellin) retain adhesion; therefore, these new observations suggest that FleSR regulates both the expression of flagella and the nonpilus adhesin(s) for mucin or that one of the flagellar proteins (other than flagellin) may be responsible for adhesion to mucins.