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

Molecular Basis for the Activation of Human Innate Immune Response by the Flagellin Derived from Plant-Pathogenic Bacterium, Acidovorax avenae

Acidovorax avenae is a flagellated, pathogenic bacterium to various plant crops that has also been found in human patients with haematological malignancy, fever, and sepsis; however, the exact mechanism for infection in humans is not known. We hypothesized that the human innate immune system could be responsive to the purified flagellin isolated from A. avenae, named FLA-AA. We observed the secretion of inflammatory cytokines such as tumor necrosis factor-alpha (TNF-α), interleukin (IL)-6, and IL-8 by treating FLA-AA to human dermal fibroblasts, as well as macrophages. This response was exclusively through TLR5, which was confirmed by using TLR5-overexpression cell line, 293/hTLR5, as well as TLR5-specific inhibitor, TH1020. We also observed the secretion of inflammatory cytokine, IL-1β, by the activation of NLRC4 with FLA-AA. Overall, our results provide a molecular basis for the inflammatory response caused by FLA-AA in cell-based assays.

Mucins trigger dispersal of Pseudomonas aeruginosa biofilms

Mucus is a biological gel that lines all wet epithelia in the body, including the mouth, lungs, and digestive tract, and has evolved to protect the body from pathogenic infection. However, microbial pathogenesis is often studied in mucus-free environments that lack the geometric constraints and microbial interactions in physiological three-dimensional mucus gels. We developed fluid-flow and static test systems based on purified mucin polymers, the major gel-forming constituents of the mucus barrier, to understand how the mucus barrier influences bacterial virulence, particularly the integrity of Pseudomonas aeruginosa biofilms, which can become resistant to immune clearance and antimicrobial agents. We found that mucins separate the cells in P. aeruginosa biofilms and disperse them into suspension. Other viscous polymer solutions did not match the biofilm disruption caused by mucins, suggesting that mucin-specific properties mediate the phenomenon. Cellular dispersion depended on functional flagella, indicating a role for swimming motility. Taken together, our observations support a model in which host mucins are key players in the regulation of microbial virulence. These mucins should be considered in studies of mucosal pathogenesis and during the development of novel strategies to treat biofilms.

Biofilms are an important survival strategy for pathogenic bacteria including Pseudomonas aeruginosa and whilst mucins play a role the regulation of microbial virulence, microbial pathogenesis on mucosal tissues is often studied in mucin-free contexts. Here, Katharina Ribbeck and colleagues at the Massachusetts Institute of Technology used native purified mucin polymers and examined their effects on the integrity of Pseudomonas aeruginosa biofilms. The mucins dissolved the biofilms by separating the bacteria, which was not observed in other viscous alternative substances examined, but this did rely on functional bacterial motility. Here the authors provide evidence that mucins are involved in suppressing bacterial virulence and should be included in systems used to assess bacterial pathogenesis on mucosal tissues.

Immune Recognition of the Epidemic Cystic Fibrosis Pathogen Burkholderia dolosa

Burkholderia dolosa caused an outbreak in the cystic fibrosis (CF) clinic at Boston Children's Hospital from 1998 to 2005 and led to the infection of over 40 patients, many of whom died due to complications from infection by this organism. To assess whether B. dolosa significantly contributes to disease or is recognized by the host immune response, mice were infected with a sequenced outbreak B. dolosa strain, AU0158, and responses were compared to those to the well-studied CF pathogen Pseudomonas aeruginosa In parallel, mice were also infected with a polar flagellin mutant of B. dolosa to examine the role of flagella in B. dolosa lung colonization. The results showed a higher persistence in the host by B. dolosa strains, and yet, neutrophil recruitment and cytokine production were lower than those with P. aeruginosa The ability of host immune cells to recognize B. dolosa was then assessed, B. dolosa induced a robust cytokine response in cultured cells, and this effect was dependent on the flagella only when bacteria were dead. Together, these results suggest that B. dolosa can be recognized by host cells in vitro but may avoid or suppress the host immune response in vivo through unknown mechanisms. B. dolosa was then compared to other Burkholderia species and found to induce similar levels of cytokine production despite being internalized by macrophages more than Burkholderia cenocepacia strains. These data suggest that B. dolosa AU0158 may act differently with host cells and is recognized differently by immune systems than are other Burkholderia strains or species.

Cyclic diguanylate signaling in Gram-positive bacteria

The nucleotide second messenger 3'-5' cyclic diguanylate monophosphate (c-di-GMP) is a central regulator of the transition between motile and non-motile lifestyles in bacteria, favoring sessility. Most research investigating the functions of c-di-GMP has focused on Gram-negative species, especially pathogens. Recent work in Gram-positive species has revealed that c-di-GMP plays similar roles in Gram-positives, though the precise targets and mechanisms of regulation may differ. The majority of bacterial life exists in a surface-associated state, with motility allowing bacteria to disseminate and colonize new environments. c-di-GMP signaling regulates flagellum biosynthesis and production of adherence factors and appears to be a primary mechanism by which bacteria sense and respond to surfaces. Ultimately, c-di-GMP influences the ability of a bacterium to alter its transcriptional program, physiology and behavior upon surface contact. This review discusses how bacteria are able to sense a surface via flagella and type IV pili, and the role of c-di-GMP in regulating the response to surfaces, with emphasis on studies of Gram-positive bacteria.

c-di-GMP heterogeneity is generated by the chemotaxis machinery to regulate flagellar motility

Individual cell heterogeneity is commonly observed within populations, although its molecular basis is largely unknown. Previously, using FRET-based microscopy, we observed heterogeneity in cellular c-di-GMP levels. In this study, we show that c-di-GMP heterogeneity in Pseudomonas aeruginosa is promoted by a specific phosphodiesterase partitioned after cell division. We found that subcellular localization and reduction of c-di-GMP levels by this phosphodiesterase is dependent on the histidine kinase component of the chemotaxis machinery, CheA, and its phosphorylation state. Therefore, individual cell heterogeneity in c-di-GMP concentrations is regulated by the activity and the asymmetrical inheritance of the chemotaxis organelle after cell division. c-di-GMP heterogeneity results in a diversity of motility behaviors. The generation of diverse intracellular concentrations of c-di-GMP by asymmetric partitioning is likely important to the success and survival of bacterial populations within the environment by allowing a variety of motility behaviors.

DOI:http://dx.doi.org/10.7554/eLife.01402.001

Bacterial populations have traditionally been assumed to be made up of identical cells. However, while the bacteria within a population may be genetically identical, individual cells have different growth rates, metabolisms and motilities, among other things. This ‘phenotypic heterogeneity’ has been observed in many different species of bacteria, and in some cases it can be attributed to changes in the concentration of molecules called second messengers that help to relay signals from the external environment to targets within the cell.

It can be challenging to monitor changes in the concentration of specific molecules inside cells, but researchers recently developed a form of microscopy based on FRET (short for Forster resonance energy transfer) that can measure the levels of a second messenger molecule called cyclic di-guanylate (c-di-GMP) inside individual cells. This technique was used to study P. aeruginosa, a bacterium that has a single corkscrew-shaped propeller that enables it to swim through liquid. P. aeruginosa divides to form two daughter cells—one with a propeller and one without.

Although the daughter cell that does not have a propeller quickly grows one, FRET-based microscopy revealed that the daughter cell with a propeller had less c-di-GMP than the daughter without a propeller, but the reasons underlying this difference and its effects on bacterial behavior were not clear.

Now Kulasekara et al. show that the cell that inherits the propeller contains an enzyme that degrades c-di-GMP, and that the low levels of this second messenger molecule—caused by the enzyme being concentrated near the base of the propeller, and the presence of a protein (CheA) that enables the bacteria to swim towards sources of nutrients—result in faster swimming speeds and increased responsiveness to nutrients. In other words, although the two daughter cells are genetically identical, they behave quite differently because of the different levels of this second messenger molecule.

The existence of heterogeneity within a bacterial population likely leads to increased success and survival within changing diverse environments, and this work sets the stage for similar investigations into what establishes heterogeneity in other bacterial populations.

DOI:http://dx.doi.org/10.7554/eLife.01402.002

The role of the bacterial flagellum in adhesion and virulence

The bacterial flagellum is a complex apparatus assembled of more than 20 different proteins. The flagellar basal body traverses the cell wall, whereas the curved hook connects the basal body to the whip-like flagellar filament that protrudes several µm from the bacterial cell. The flagellum has traditionally been regarded only as a motility organelle, but more recently it has become evident that flagella have a number of other biological functions. The major subunit, flagellin or FliC, of the flagellum plays a well-documented role in innate immunity and as a dominant antigen of the adaptive immune response. Importantly, flagella have also been reported to function as adhesins. Whole flagella have been indicated as significant in bacterial adhesion to and invasion into host cells. In various pathogens, e.g., Escherichia coli, Pseudomonas aeruginosa and Clostridium difficile, flagellin and/or the distally located flagellar cap protein have been reported to function as adhesins. Recently, FliC of Shiga-toxigenic E. coli was shown to be involved in cellular invasion via lipid rafts. Here, we examine the latest or most important findings regarding flagellar adhesive and invasive properties, especially focusing on the flagellum as a potential virulence factor.

Inactivation of the fliY gene encoding a flagellar motor switch protein attenuates mobility and virulence of Leptospira interrogans strain Lai

Background

Pathogenic Leptospira species cause leptospirosis, a zoonotic disease of global importance. The spirochete displays active rotative mobility which may contribute to invasion and diffusion of the pathogen in hosts. FliY is a flagellar motor switch protein that controls flagellar motor direction in other microbes, but its role in Leptospira, and paricularly in pathogenicity remains unknown.

Results

A suicide plasmid for the fliY gene of Leptospira interrogans serogroup Icterohaemorrhagiae serovar Lai strain Lai that was disrupted by inserting the ampicillin resistance gene (bla) was constructed, and the inactivation of fliY gene in a mutant (fliY^-) was confirmed by PCR and Western Blot analysis. The inactivation resulted in the mRNA absence of fliP and fliQ genes which are located downstream of the fliY gene in the same operon. The mutant displayed visibly weakened rotative motion in liquid medium and its migration on semisolid medium was also markedly attenuated compared to the wild-type strain. Compared to the wild-type strain, the mutant showed much lower levels of adhesion to murine macrophages and apoptosis-inducing ability, and its lethality to guinea pigs was also significantly decreased.

Conclusion

Inactivation of fliY, by the method used in this paper, clearly had polar effects on downstream genes. The phentotypes observed, including lower pathogenicity, could be a consequence of fliY inactivation, but also a consequence of the polar effects.

The Lon protease of Pseudomonas aeruginosa is induced by aminoglycosides and is involved in biofilm formation and motility

Pseudomonas aeruginosa is an important nosocomial opportunistic human pathogen and a major cause of chronic lung infections in individuals with cystic fibrosis. Serious infections by this organism are often treated with a combination of aminoglycosides and semi-synthetic penicillins. Subinhibitory concentrations of antibiotics are now being recognized for their role in microbial persistence and the development of antimicrobial resistance, two very important clinical phenomena. An extensive screen of a P. aeruginosa PAO1 luciferase gene fusion library was performed to identify genes that were differentially regulated during exposure to subinhibitory gentamicin. It was demonstrated that subinhibitory concentrations of gentamicin and tobramycin induced a set of genes that are likely to affect the interaction of P. aeruginosa with host cells, including the gene encoding Lon protease, which is known to play a major role in protein quality control. Studies with a lon mutant compared to its parent and a complemented strain indicated that this protein was essential for biofilm formation and motility in P. aeruginosa.

Mucin-Pseudomonas aeruginosa interactions promote biofilm formation and antibiotic resistance

Pseudomonas aeruginosa is an opportunistic pathogen that causes chronic lung infections in people suffering from cystic fibrosis (CF). In CF airways, P. aeruginosa forms surface-associated communities called biofilms. Compared with free-swimming cultures, biofilms resist clearance by the host immune system and display increased resistance to antimicrobial agents. In this study we developed a technique to coat surfaces with molecules that are abundant in CF airways in order to investigate their impact on P. aeruginosa biofilm development. We found that P. aeruginosa biofilm development proceeds differently on surfaces coated with the glycoprotein mucin compared with biofilm development on glass and surfaces coated with actin or DNA. Biofilms formed on mucin-coated surfaces developed large cellular aggregates and had increased tolerance to the antibiotic tobramycin compared with biofilms grown on glass. Analysis of selected mutant backgrounds in conjunction with time-lapse microscopy revealed that surface-associated motility was blocked on the mucin surface. Furthermore, our data suggest that a specific adhesin-mucin interaction immobilizes the bacterium on the surface. Together, these experiments suggest that mucin, which may serve as an attachment surface in CF airways, impacts P. aeruginosa biofilm development and function.

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.

A four‐tiered transcriptional regulatory circuit controls flagellar biogenesis in Pseudomonas aeruginosa

The single polar flagellum of Pseudomonas aeruginosa is an important virulence and colonization factor of this opportunistic pathogen. In this study, the annotation of the genes belonging to the fla regulon was updated and their organization was analysed in strains PAK and PAO1, representative type-a and type-b strains of P. aeruginosa respectively. The flagellar genes are clustered in three non-contiguous regions of the chromosome. A polymorphic locus flanked by flgJ and fleQ in Region I contains a glycosylation island in PAK. The expression and ordered assembly of the complex multicomponent flagellum is intricately regulated. Dedicated flagellar genes fleQ, fleS, fleR, fliA, flgM and fleN encode proteins that participate in the regulation of the flagellar transcriptional circuit. In addition, expression of the flagellum is coordinately regulated with other P. aeruginosa virulence factors by the alternative sigma factor sigma54, encoded by rpoN. In order to gain insight into the hierarchical regulation of flagellar genes, deletion mutations were constructed in fleQ, fleR, fliA and rpoN. The transcriptional impact of these mutations was examined by transcriptional profiling using a P. aeruginosa whole genome microarray. Analysis of the transcriptomes generated for each of these mutants indicates a four-tiered (Classes I-IV) hierarchy of transcriptional regulation. Class I genes are constitutively expressed and include the transcriptional regulator fleQ and the alternative sigma factor fliA (sigma28). Class II genes including fleSR, encoding a two-component regulatory system require FleQ and RpoN (sigma54) for their transcriptional activation. Class III genes are positively regulated by the activated response regulator FleR in concert with RpoN. The transcription of Class IV genes is dependent on the availability of free FliA following the export of the FliA specific antisigma factor FlgM through the basal body rod-hook structure (assembled from Class II and III gene products). Two previously uncharacterized genes, which are coordinately regulated with known flagellar genes have been identified by genome-wide analysis and their role in flagellar biogenesis was analysed.

FleQ, the major flagellar gene regulator in Pseudomonas aeruginosa, binds to enhancer sites located either upstream or atypically downstream of the RpoN binding site

In Pseudomonas aeruginosa, flagellar genes are regulated in a cascade headed by FleQ, an NtrC/NifA-type activator. FleQ and RpoN positively regulate expression of flhA, fliE, fliL, and fleSR genes, among others. Direct interaction of FleQ with flhA, fliE, fliL, and fleSR promoters was demonstrated by gel shift assay, along with experiments to conclusively determine the specificity of its binding. DNase I footprinting was performed to determine the FleQ binding sites on flhA, fliE, fliL, and fleSR promoters. No sequence conservation among these binding sites was observed. Primer extension analysis revealed the transcription start sites (TSSs) to be localized above the FleQ binding sites in flhA, fliE, and fliL promoters. Analysis of the above data revealed FleQ binding to be in the leader sequence of these promoters, whereas FleQ binding was 67 bp upstream of the TSS in the fleSR promoter. Mutagenesis of the FleQ binding site in the flhA promoter confirmed its functionality in vivo. Deletion of the flhA promoter upstream of the RNA polymerase binding site did not result in a significant loss of promoter activity. These results point to two modes of regulation by an NtrC-type regulator in the flagellar hierarchy in P. aeruginosa, the first being the typical model of activation from a distance via looping in the fleSR promoter and the second involving flhA, fliE, and fliL promoters, where FleQ binds in the downstream vicinity of the promoter and activates transcription without looping.

A genomic island in Pseudomonas aeruginosa carries the determinants of flagellin glycosylation

Protein glycosylation has been long recognized as an important posttranslational modification process in eukaryotic cells. Glycoproteins, predominantly secreted or surface localized, have also been identified in bacteria. We have identified a cluster of 14 genes, encoding the determinants of the flagellin glycosylation machinery in Pseudomonas aeruginosa PAK, which we called the flagellin glycosylation island. Flagellin glycosylation can be detected only in bacteria expressing the a-type flagellin sequence variants, and the survey of 30 P. aeruginosa isolates revealed coinheritance of the a-type flagellin genes with at least one of the flagellin glycosylation island genes. Expression of the b-type flagellin in PAK, an a-type strain carrying the glycosylation island, did not lead to glycosylation of the b-type flagellin of PAO1, suggesting that flagellins expressed by b-type bacteria not only lack the glycosylation island, they cannot serve as substrates for glycosylation. Providing the entire glycosylation island of PAK, including its a-type flagellin in a flagellin mutant of a b-type strain, results in glycosylation of the heterologous flagellin. These results suggest that some or all of the 14 genes on the glycosylation island are the genes that are missing from strain PAO1 to allow glycosylation of an appropriate flagellin. Inactivation of either one of the two flanking genes present on this island abolished flagellin glycosylation. Based on the limited homologies of these gene products with enzymes involved in glycosylation, we propose that the island encodes similar proteins involved in synthesis, activation, or polymerization of sugars that are necessary for flagellin glycosylation.

Motility and the polar flagellum are required for Aeromonas caviae adherence to HEp-2 cells

Aeromonas caviae is increasingly being recognized as a cause of gastroenteritis, especially among the young. The adherence of aeromonads to human epithelial cells in vitro has been correlated with enteropathogenicity, but the mechanism is far from well understood. Initial investigations demonstrated that adherence of A. caviae to HEp-2 cells was significantly reduced by either pretreating bacterial cells with an antipolar flagellin antibody or by pretreating HEp-2 cells with partially purified flagella. To precisely define the role of the polar flagellum in aeromonad adherence, we isolated the A. caviae polar flagellin locus and identified five polar flagellar genes, in the order flaA, flaB, flaG, flaH, and flaJ. Each gene was inactivated using a kanamycin resistance cartridge that ensures the transcription of downstream genes, and the resulting mutants were tested for motility, flagellin expression, and adherence to HEp-2 cells. N-terminal amino acid sequencing, mutant analysis, and Western blotting demonstrated that A. caviae has a complex flagellum filament composed of two flagellin subunits encoded by flaA and flaB. The predicted molecular mass of both flagellins was approximately 31,700 Da; however, their molecular mass estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis was approximately 35,500 Da. This aberrant migration was thought to be due to their glycosylation, since the proteins were reactive in glycosyl group detection assays. Single mutations in either flaA or flaB did not result in loss of flagella but did result in decreased motility and adherence by approximately 50%. Mutation of flaH, flaJ, or both flagellin genes resulted in the complete loss of motility, flagellin expression, and adherence. However, mutation of flaG did not affect motility but did significantly reduce the level of adherence. Centrifugation of the flagellate mutants (flaA, flaB, and flaG) onto the cell monolayers did not increase adherence, whereas centrifugation of the aflagellate mutants (flaH, flaJ, and flaA flaB) increased adherence slightly. We conclude that maximum adherence of A. caviae to human epithelial cells in vitro requires motility and optimal flagellar function.

Typing of Pseudomonas aeruginosa strains in Norwegian cystic fibrosis patients

OBJECTIVES

Typing of Pseudomonas aeruginosa isolates from Norwegian cystic fibrosis (CF) patients with chronic Pseudomonas lung infection in order to see whether cross-infection might have occurred.

METHODS

Isolates from 60 patients were collected during the years 1994-98, and typed by pulsed field gel electrophoresis.

RESULTS

Seventy-one strains were identified. One large cluster of identical strains included 27 patients, and 13 smaller clusters of 2-4 patients were found (26 patients). Seven patients had a strain not shared by other patients (private strains). Harboring the main cluster strain was significantly associated with participation in summer camps and training courses (P = 0.004, chi-squared test). There were no associations with regular admissions to hospital (intravenous antibiotic courses) or smaller social gatherings of short duration. Small clusters and private strains were not associated with any of the risk factors. All strains were sensitive to colistin. The minimal inhibitory concentrations were generally lower in Norwegian P. aeruginosa strains compared with isolates from Danish patients.

CONCLUSIONS

Our results indicate that cross-infection with P. aeruginosa between cystic fibrosis patients has occurred.

Molecular characterization of fliD gene encoding flagellar cap and its expression among Clostridium difficile isolates from different serogroups

The fliD gene encoding the flagellar cap protein (FliD) of Clostridium difficile was studied in 46 isolates belonging to serogroups A, B, C, D, F, G, H, I, K, X, and S3, including 30 flagellated strains and 16 nonflagellated strains. In all but three isolates, amplification by PCR and reverse transcription-PCR demonstrated that the fliD gene is present and transcribed in both flagellated and nonflagellated strains. PCR-restriction fragment length polymorphism (RFLP) analysis of amplified fliD gene products revealed interstrain homogeneity, with one of two major patterns (a and b) found in all but one of the strains, which had pattern c. A polyclonal monospecific antiserum raised to the recombinant FliD protein reacted in immunoblots with crude flagellar preparations from 28 of 30 flagellated strains but did not recognize FliD from nonflagellated strains. The fliD genes from five strains representative of the three different RFLP groups were sequenced, and sequencing revealed 100% identity between the strains with the same pattern and 88% identity among strains with different patterns. Our results show that even though FliD is a structure exposed to the outer environment, the flagellar cap protein is very well conserved, and this high degree of conservation suggests that it has a very specific function in attachment to cell or mucus receptors.

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.

Phenotypic and genotypic diversity of the flagellin gene (fliC) among Clostridium difficile isolates from different serogroups

Phenotypic and genotypic diversity of the flagellin gene (fliC) of Clostridium difficile was studied in 47 isolates from various origins belonging to the serogroups A, B, C, D, F, G, H, I, K, X, and S3. Electron microscopy revealed 17 nonflagellated strains and 30 flagellated strains. PCR and reverse transcription-PCR demonstrated that the flagellin gene was present in all strains and that the fliC gene was expressed in both flagellated and nonflagellated strains. Southern blotting showed the presence of only one copy of the gene and three different hybridization patterns. DNA sequence analysis of fliC from the strains belonging to serogroups C, D, and X, representative of each profile, disclosed great variability in the central domain, whereas the N- and C-terminal domains were conserved. The variability of the flagellin gene fliC was further studied in the isolates by PCR-restriction fragment length polymorphism (RFLP) analysis. Nine different RFLP groups were identified (I to IX), among which three (I, VII, and VIII) corresponded to numerous serogroups whereas the six others (II, III, IV, V, VI, and IX) belonged to a single serogroup. Flagellin gene RFLP analysis could constitute an additional typing method employable in conjunction with other typing methods currently available.

Characterization of binding of Candida albicans to small intestinal mucin and its role in adherence to mucosal epithelial cells

In order to approximate and adhere to mucosal epithelial cells, Candida must traverse the overlying mucus layer. Interactions of Candida species with mucin and human buccal epithelial cells (BECs) were thus investigated in vitro. Binding of the Candida species to purified small intestinal mucin showed a close correlation with their hierarchy of virulence. Significant differences (P < 0.05) were found among three categories of Candida species adhering highly (C. dubliniensis, C. tropicalis, and C. albicans), moderately (C. parapsilosis and C. lusitaniae) or weakly (C. krusei and C. glabrata) to mucin. Adherence of C. albicans to BECs was quantitatively inhibited by graded concentrations of mucin. However, inhibition of adherence was reversed by pretreatment of mucin with pronase or C. albicans secretory aspartyl proteinase Sap2p but not with sodium periodate. Saturable concentration- and time-dependent binding of mucin to C. albicans was abrogated by pronase or Sap2p treatment of mucin but was unaffected by beta-mercaptoethanol, sodium periodate, neuraminidase, lectins, or potentially inhibitory sugars. Probing of membrane blots of the mucin with C. albicans revealed binding of the yeast to the 66-kDa cleavage product of the 118-kDa C-terminal glycopeptide of mucin. Although no evidence was found for the participation of C. albicans cell surface mannoproteins in specific receptor-ligand binding to mucin, inhibition of binding by p-nitrophenol (1 mM) and tetramethylurea (0.36 M) revealed that hydrophobic interactions are involved in adherence of C. albicans to mucin. These results suggest that C. albicans may both adhere to and enzymatically degrade mucins by the action of Saps, and that both properties may act to modulate Candida populations in the oral cavity and gastrointestinal tract.

A Clostridium difficile gene encoding flagellin

Six strains of Clostridium difficile examined by electron microscopy were found to carry flagella. The flagella of these strains were extracted and the N-terminal sequences of the flagellin proteins were determined. Four of the strains carried the N-terminal sequence MRVNTNVSAL exhibiting up to 90% identity to numerous flagellins. Using degenerate primers based on the N-terminal sequence and the conserved C-terminal sequence of several flagellins, the gene encoding the flagellum subunit (fliC) was isolated and sequenced from two virulent strains. The two gene sequences exhibited 91% inter-strain identity. The gene consists of 870 nt encoding a protein of 290 amino acids with an estimated molecular mass of 31 kDa, while the extracted flagellin has an apparent molecular mass of 39 kDa on SDS-PAGE. The FliC protein displays a high degree of identity in the N- and C-terminal amino acids whereas the central region is variable. A second ORF is present downstream of fliC displaying homology to glycosyltransferases. The fliC gene was expressed in fusion with glutathione S-transferase, purified and a polyclonal monospecific antiserum was obtained. Flagella of C. difficile do not play a role in adherence, since the antiserum raised against the purified protein did not inhibit adherence to cultured cells. PCR-RFLP analysis of amplified flagellin gene products and Southern analysis revealed inter-strain heterogeneity; this could be useful for epidemiological and phylogenetic studies of this organism.

Identification of two distinct types of flagellar cap proteins, FliD, in Pseudomonas aeruginosa

Binding of Pseudomonas aeruginosa strain PAK to mucin has been shown to be mediated by the flagellar cap protein, product of the fliD gene. Since the flagellar cap is very likely an exposed structure, the FliD polypeptide should be recognized by the host immune system, analogous to the recognition of dominant epitopes located in the exposed parts of the flagellin polypeptide within the assembled flagellum. In P. aeruginosa, a number of distinct flagellin variants are made, and these variable sequences presumably allow the newly infected P. aeruginosa to escape recognition by the antibody induced during a previous infection. Since similar mechanisms may direct the selection of FliD variants, we examined the extent of sequence heterogeneity among various FliD sequences among a selected group of P. aeruginosa. The results of PCR and nucleotide sequencing of the fliD region of eight different P. aeruginosa strains (laboratory strains PAK, PAO1, and PA103; clinical strains 1244, CS2, and CS32; cystic fibrosis strains CS29 and MDR) suggested that there were two distinct types of FliD in P. aeruginosa, which we named A type and B type. The results of Western blotting using the polyclonal antibodies raised against the purified FliD of A type (PAK) or B type (PAO1) further confirmed the existence of two distinct antigenic types of FliD proteins, with no cross-reactivity between the two serotypes. Further Western immunoblot analysis of the same strains using polyclonal FliC antibody showed that the strains with A-type FliD possessed a-type FliC and those with B-type FliD had b-type FliC. Similar Western blot analyses of 50 more P. aeruginosa strains obtained from varied sources revealed that all strains contained either A-type or B-type FliD, suggesting the existence of only two types of FliD in P. aeruginosa and indicating that fliC and fliD were coinherited. This limited diversity of FliC and FliD serotypes seems to be a unique feature of flagellar proteins. A chromosomal mutant having an insertion in the fliD gene of P. aeruginosa PAO1 was constructed. The motility defect of this mutant and a previously constructed PAK fliD mutant was better complemented with the fliD gene of the homologous types.

fleN, a gene that regulates flagellar number in Pseudomonas aeruginosa

The single polar flagellum of Pseudomonas aeruginosa plays an important role in the pathogenesis of infection by this organism. However, regulation of the assembly of this organelle has not been delineated. In analyzing the sequence available at the Pseudomonas genome database, an open reading frame (ORF), flanked by flagellar genes flhF and fliA, that coded for a protein (280 amino acids) with an ATP-binding motif at its N terminus was found. The ORF was inactivated by inserting a gentamicin cassette in P. aeruginosa PAK and PAO1. The resulting mutants were nonmotile on motility agar plates, but under a light microscope they exhibited random movement and tumbling behavior. Electron microscopic studies of the wild-type and mutant strains revealed that the mutants were multiflagellate, with three to six polar flagella per bacterium as rather than one as in the wild type, indicating that this ORF was involved in regulating the number of flagella and chemotactic motility in P. aeruginosa. The ORF was named fleN. An intact copy of fleN on a plasmid complemented the mutant by restoring motility and monoflagellate status. The beta-galactosidase activities of eight flagellar operon or gene promoters in the wild-type and fleN mutant strains revealed a direct correlation between six promoters that were upregulated in the fleN mutant (fliLMNOPQ, flgBCDE, fliEFG, fliDS orf126, fleSR, and fliC) and positive regulation by FleQ, an NtrC-like transcriptional regulator for flagellar genes. Based on these results, we propose a model where FleN influences FleQ activity (directly or indirectly) in regulating flagellar number in P. aeruginosa.

Effect of nutrient limitation on adhesion characteristics of Pseudomonas aeruginosa

Pseudomonas aeruginosa causes a variety of diseases in humans including lung and ocular infections. Infections of the cornea are usually associated with wearing contact lenses and can result in loss of vision. This study aimed to determine the effect of carbon or nitrogen limitation on the adhesion to contact lenses of a strain of Ps. aeruginosa isolated from contact lens-related corneal inflammation. Cells were grown in a continuous culture apparatus in varying levels of glucose or ammonia to effect nutrient limitation. Adhesion to contact lenses was measured as total counts and viable counts. The cell surface hydrophobicity and charge were measured using adhesion to surface-modified Sepharose. Changes in lipopolysaccharide were determined using 1D SDS-PAGE and changes in cell-surface proteins were measured using 2D gel electrophoresis. The more the cultures were nitrogen limited, the greater the increase in adhesion to unworn hydrogel contact lenses 0.3 x 10(3) - 2.2 x 10(3) cells/mm2 on Etafilon A lenses. Cells that were carbon limited showed a greater increase in adhesion to contact lenses when the lenses had been coated in artificial tears. It appeared that lipopolysaccharide may have been involved in the constitutive adhesion to unworn lenses that occurred during C-limitation, whereas changes in the outer membrane proteins contributed to the increased adhesion under nitrogen limitation, or the change in adhesion that occurred to carbon-limited cells using contact lenses coated in artificial tears. Nine cell-surface proteins appeared during nitrogen limitation with kDa/pI of 75/4.8, 4.9, 5.0; 62/5.6; 89/6.5; 38/6.4; 28/1.5; 18/6.4; 12/4.5. Any or all of these may have been involved in the increased adhesion and further experiments are underway to examine this possibility.

Molecular genetics of bacterial attachment and biofouling

Microbial adhesion to animate or inert surfaces is potentially mediated by nonspecific physical or specific ligand-receptor interactions. Growth and survival of the microbial community or biofilm then depends on adaptation to a series of changing environmental milieux. Within the realm of cell-cell interaction, recent advances suggest that flagella, fimbriae and other protein receptors are essential for bacterial attachment to surfaces. There has also been profound progress in the elucidation of genes and molecules necessary for bacterial attachments to surfaces and subsequent biofilm formation.

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).

Role of flagella in pathogenesis of Pseudomonas aeruginosa pulmonary infection

Pseudomonas aeruginosa strains are opportunistic pathogens associated with infections in immunocompromised hosts and patients with cystic fibrosis. Like many other mucosal pathogens, P. aeruginosa cells express flagella which provide motility and chemotaxis toward preferred substrates but also provide a ligand for clearance by phagocytic cells. We tested the role of flagella in the initial stages of respiratory tract infection by comparing the virulence of fliC mutants in a neonatal mouse model of pneumonia. In the absence of fliC, there was no mortality, compared with 30% mortality attributed to the parental strain PAK or 15% mortality associated with infection due to a pilA mutant PAK/NP (P < 0.0001). The fliC mutants caused pneumonia in only 25% of the mice inoculated, regardless of whether there was expression of the pilus, whereas the parental strain was associated with an 80% rate of pneumonia. Histopathological studies demonstrated that the fliC mutants caused very focal inflammation and that the organisms did not spread through the lungs as seen in infection due to either PAK or PAK/NP. Purified flagellin elicited an intense inflammatory response in the mouse lung. 125I-labeled flagellin bound to the glycolipids GM1 and GD1a and to asialoGM1 in an in vitro binding assay. However, flagellin-mediated binding to epithelial gangliosides was a relatively unusual event, as quantified by binding assays of wild-type or fliC mutant organisms to CHO Lec-2 cells with membrane-incorporated GM1. Fla+ organisms but not fliC mutants were efficiently taken up by murine macrophages. P. aeruginosa flagella are important in the establishment of respiratory tract infection and may act as a tether in initial interactions with epithelial membranes. This function is offset by the contribution of flagella to host clearance mechanisms facilitating phagocytic clearance and the role of flagellar genes in mucin binding and clearance.

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.

Inhibition of bacterial adherence to host tissue does not markedly affect disease in the murine model of Pseudomonas aeruginosa corneal infection

The prevention of bacterial infections by the inhibition of binding to host tissues is an oft-touted approach, but few studies with appropriate models of infection have tested its feasibility. Pseudomonas aeruginosa causes severe corneal infections in mice after inoculations with low doses, and infection is thought to depend upon an initial adherence of the bacteria to corneal cells. In vitro, adherence to corneal cells is mediated to a large degree by the complete-outer-core oligosaccharide of the bacterial lipopolysaccharide (LPS). However, bacteria adhering to tissues in vivo are difficult to differentiate from nonadherent bacteria. Since a direct correlate of P. aeruginosa adherence to corneal epithelial cells is the degree to which these cells internalize P. aeruginosa, the level of adherence in vivo can be approximated by measuring P. aeruginosa ingestion by cells by using gentamicin exclusion assays. To determine the degree to which inhibition of the corneal cell adherence affects the course of infection and disease in the murine model, we evaluated the ability of LPS-outer-core oligosaccharide to inhibit bacterial association and entry into corneal cells and to modulate the development of disease. Mice were anesthetized, and their corneas were scratched and inoculated with virulent P. aeruginosa 6294 or PAO1, along with either 50 microg of oligosaccharide derived from LPS from P. aeruginosa PAC557 (complete outer core but no O side chains) or oligosaccharide derived from LPS of P. aeruginosa PAC1RalgC::tet (incomplete-core oligosaccharide). After 4 h, there were no differences between groups in the counts of infecting and internalized bacteria. At 24 h, the complete-core oligosaccharide decreased the levels of bacteria per eye by 70 to 99.7% compared with the levels achieved by including the incomplete-core oligosaccharide in the infectious inoculum. Epithelial cell ingestion of bacteria was comparably affected. However, the effect on disease was modest and only evident at lower challenge doses that elicited mild disease in controls and when the bacterial association and ingestion were inhibited by >99%. Overall, it appears that in the murine model of P. aeruginosa corneal infection at challenge doses of bacteria 10-fold or greater than the minimal amount needed to cause disease, the absolute level of inhibition of bacterial adherence is insufficient to reduce the bacterial counts below that which elicits disease.

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.