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UNLABELLED - Bacterial biofilms are ubiquitous in nature, and their resilience is derived in part from a complex extracellular matrix that can be tailored to meet environmental demands. Although common developmental stages leading to biofilm formation have been described, how the extracellular components are organized to allow three-dimensional biofilm development is not well understood. Here we show that uropathogenic Escherichia coli (UPEC) strains produce a biofilm with a highly ordered and complex extracellular matrix (ECM). We used electron microscopy (EM) techniques to image floating biofilms (pellicles) formed by UPEC. EM revealed intricately constructed substructures within the ECM that encase individual, spatially segregated bacteria with a distinctive morphology. Mutational and biochemical analyses of these biofilms confirmed curli as a major matrix component and revealed important roles for cellulose, flagella, and type 1 pili in pellicle integrity and ECM infrastructure. Collectively, the findings of this study elucidated that UPEC pellicles have a highly organized ultrastructure that varies spatially across the multicellular community.
IMPORTANCE - Bacteria can form biofilms in diverse niches, including abiotic surfaces, living cells, and at the air-liquid interface of liquid media. Encasing these cellular communities is a self-produced extracellular matrix (ECM) that can be composed of proteins, polysaccharides, and nucleic acids. The ECM protects biofilm bacteria from environmental insults and also makes the dissolution of biofilms very challenging. As a result, formation of biofilms within humans (during infection) or on industrial material (such as water pipes) has detrimental and costly effects. In order to combat bacterial biofilms, a better understanding of components required for biofilm formation and the ECM is required. This study defined the ECM composition and architecture of floating pellicle biofilms formed by Escherichia coli.
UNLABELLED - Helicobacter pylori contains four genes that are predicted to encode proteins secreted by the autotransporter (type V) pathway. One of these, the pore-forming toxin VacA, has been studied in great detail, but thus far there has been very little investigation of three VacA-like proteins. We show here that all three VacA-like proteins are >250 kDa in mass and localized on the surface of H. pylori. The expression of the three vacA-like genes is upregulated during H. pylori colonization of the mouse stomach compared to H. pylori growth in vitro, and a wild-type H. pylori strain outcompeted each of the three corresponding isogenic mutant strains in its ability to colonize the mouse stomach. One of the VacA-like proteins localizes to a sheath that overlies the flagellar filament and bulb, and therefore, we designate it FaaA (flagella-associated autotransporter A). In comparison to a wild-type H. pylori strain, an isogenic faaA mutant strain exhibits decreased motility, decreased flagellar stability, and an increased proportion of flagella in a nonpolar site. The flagellar localization of FaaA differs markedly from the localization of other known autotransporters, and the current results reveal an important role of FaaA in flagellar localization and motility.
IMPORTANCE - The pathogenesis of most bacterial infections is dependent on the actions of secreted proteins, and proteins secreted by the autotransporter pathway constitute the largest family of secreted proteins in pathogenic Gram-negative bacteria. In this study, we analyzed three autotransporter proteins (VacA-like proteins) produced by Helicobacter pylori, a Gram-negative bacterium that colonizes the human stomach and contributes to the pathogenesis of gastric cancer and peptic ulcer disease. We demonstrate that these three proteins each enhance the capacity of H. pylori to colonize the stomach. Unexpectedly, one of these proteins (FaaA) is localized to a sheath that overlies H. pylori flagella. The absence of FaaA results in decreased H. pylori motility as well as a reduction in flagellar stability and a change in flagellar localization. The atypical localization of FaaA reflects a specialized function of this autotransporter designed to optimize H. pylori colonization of the gastric niche.
BACKGROUND - LuxS may function as a metabolic enzyme or as the synthase of a quorum sensing signalling molecule, auto-inducer-2 (AI-2); hence, the mechanism underlying phenotypic changes upon luxS inactivation is not always clear. In Helicobacter pylori, we have recently shown that, rather than functioning in recycling methionine as in most bacteria, LuxS (along with newly-characterised MccA and MccB), synthesises cysteine via reverse transsulphuration. In this study, we investigated whether and how LuxS controls motility of H. pylori, specifically if it has its effects via luxS-required cysteine metabolism or via AI-2 synthesis only.
RESULTS - We report that disruption of luxS renders H. pylori non-motile in soft agar and by microscopy, whereas disruption of mccAHp or mccBHp (other genes in the cysteine provision pathway) does not, implying that the lost phenotype is not due to disrupted cysteine provision. The motility defect of the DeltaluxSHp mutant was complemented genetically by luxSHp and also by addition of in vitro synthesised AI-2 or 4, 5-dihydroxy-2, 3-pentanedione (DPD, the precursor of AI-2). In contrast, exogenously added cysteine could not restore motility to the DeltaluxSHp mutant, confirming that AI-2 synthesis, but not the metabolic effect of LuxS was important. Microscopy showed reduced number and length of flagella in the DeltaluxSHp mutant. Immunoblotting identified decreased levels of FlaA and FlgE but not FlaB in the DeltaluxSHp mutant, and RT-PCR showed that the expression of flaA, flgE, motA, motB, flhA and fliI but not flaB was reduced. Addition of DPD but not cysteine to the DeltaluxSHp mutant restored flagellar gene transcription, and the number and length of flagella.
CONCLUSIONS - Our data show that as well as being a metabolic enzyme, H. pylori LuxS has an alternative role in regulation of motility by modulating flagellar transcripts and flagellar biosynthesis through production of the signalling molecule AI-2.
Bacteria sense environmental cues and regulate gene expression accordingly so as to persist in diverse niches. QseC is a membrane sensor kinase shown in enterohemorrhagic Escherichia coli to respond to host and bacterial signals by phosphorylating the QseB response regulator at residue D51, resulting in QseB activation and presumably upregulation of virulence genes. We studied QseBC in uropathogenic E. coli (UPEC). UPEC establish infection by colonizing and invading bladder cells. After invasion, UPEC can escape into the cytoplasm where they can form intracellular bacterial communities. Deletion of qseC significantly attenuated intracellular bacterial community formation and virulence, whereas paradoxically qseB deletion did not impact pathogenesis. We found that QseB upregulates its own expression in the qseC mutant, arguing that it is activated even in the absence of QseC. However, expression of QseB, but not a QseB_D51A mutant, in the absence of QseC resulted in downregulation of type 1 pili, curli and flagella. We observed similar phenotypes with enterohemorrhagic E. coli, showing that this is not a UPEC-specific phenomenon. Target gene expression is restored when QseC is present. We discovered that QseC has phosphatase activity required for QseB dephosphorylation. Thus, the QseC phosphatase capacity is critical for modulating QseB activity and subsequent gene expression.
For successful generation of different cell types by asymmetric cell division, cell differentiation should be initiated only after completion of division. Here, we describe a control mechanism by which Caulobacter couples the initiation of a developmental program to the completion of cytokinesis. Genetic evidence indicates that localization of the signaling protein DivK at the flagellated pole prevents premature initiation of development. Photobleaching and FRET experiments show that polar localization of DivK is dynamic with rapid pole-to-pole shuttling of diffusible DivK generated by the localized activities of PleC phosphatase and DivJ kinase at opposite poles. This shuttling is interrupted upon completion of cytokinesis by the segregation of PleC and DivJ to different daughter cells, resulting in disruption of DivK localization at the flagellated pole and subsequent initiation of development in the flagellated progeny. Thus, dynamic polar localization of a diffusible protein provides a control mechanism that monitors cytokinesis to regulate development.
Intraflagellar transport (IFT) is a process required for flagella and cilia assembly that describes the dynein and kinesin mediated movement of particles along axonemes that consists of an A and a B complex, defects in which disrupt retrograde and anterograde transport, respectively. Herein, we describe a novel Caenorhabditis elegans gene, xbx-1, that is required for retrograde IFT and shares homology with a mammalian dynein light intermediate chain (D2LIC). xbx-1 expression in ciliated sensory neurons is regulated by the transcription factor DAF-19, as demonstrated previously for genes encoding IFT complex B proteins. XBX-1 localizes to the base of the cilia and undergoes anterograde and retrograde movement along the axoneme. Disruption of xbx-1 results in cilia defects and causes behavioral abnormalities observed in other cilia mutants. Analysis of cilia in xbx-1 mutants reveals that they are shortened and have a bulb like structure in which IFT proteins accumulate. The role of XBX-1 in IFT was further confirmed by analyzing the effect that other IFT mutations have on XBX-1 localization and movement. In contrast to other IFT proteins, retrograde XBX-1 movement was detected in complex A mutants. Our results suggest that the DLIC protein XBX-1 functions together with the CHE-3 dynein in retrograde IFT, downstream of the complex A proteins.
Cilia are present on cells of many eukaryotic organisms and recent data in the mouse suggest that ciliary defects can cause severe developmental abnormalities and disease. Studies across eukaryotic systems indicate that cilia are constructed and maintained through a highly conserved process termed intraflagellar transport (IFT), for which many of the proteins involved have yet to be identified. IFT describes the movement of large protein particles consisting of an A and a B complex along the cilia axoneme in anterograde and retrograde directions. Herein we describe a novel C. elegans gene, F59C6.7/9, that is required for cilia assembly and whose function is disrupted in che-13 ciliogenic mutants. As previously shown for all IFT complex B genes identified to date, expression of che-13 (F59C6.7/9) is regulated by the RFX-type transcription factor DAF-19, suggesting a conserved transcriptional pathway in ciliogenesis. Fluorescent-tagged CHE-13 protein concentrates at the base of cilia and moves along the axoneme as expected for an IFT protein. Furthermore, loss of che-13 differentially affects the localization of two known IFT complex B proteins, OSM-5 and OSM-6, implying that CHE-13 functions as part of this complex. Overall, our data confirm that CHE-13 is an IFT protein and further that the IFT particle assembles in an ordered process through specific protein-protein interactions.
Eukaryotic organisms utilize microtubule-dependent motors of the kinesin and dynein superfamilies to generate intracellular movement. To identify new genes involved in the regulation of axonal transport in Drosophila melanogaster, we undertook a screen based upon the sluggish larval phenotype of known motor mutants. One of the mutants identified in this screen, roadblock (robl), exhibits diverse defects in intracellular transport including axonal transport and mitosis. These defects include intra-axonal accumulations of cargoes, severe axonal degeneration, and aberrant chromosome segregation. The gene identified by robl encodes a 97-amino acid polypeptide that is 57% identical (70% similar) to the 105-amino acid Chlamydomonas outer arm dynein-associated protein LC7, also reported here. Both robl and LC7 have homology to several other genes from fruit fly, nematode, and mammals, but not Saccharomyces cerevisiae. Furthermore, we demonstrate that members of this family of proteins are associated with both flagellar outer arm dynein and Drosophila and rat brain cytoplasmic dynein. We propose that roadblock/LC7 family members may modulate specific dynein functions.
Vibrio cholerae must colonize the human small intestine to cause diarrhoeal disease. V.cholerae strains N16961 (EI Tor, Inaba) and 395 (classical, Ogawa) adhered to the epithelial cell surface and the mucus layer of isolated human small intestinal epithelial cells. They adhered specifically to the mucosa and apical membrane in thin sections of small intestine. No binding to the basolateral membrane of dissected epithelial tissue or to intracellular components of the epithelial cells was observed by either light or indirect immunofluorescence microscopy. Based on these results, a modified ELISA was developed to quantitatively study adherence of V. cholerae to human small intestinal epithelial cells. The assay used homogenized human small intestinal mucosal tissue as the substrate for binding. Treatment of the epithelial cell homogenate with 2-mercaptoethanol to disrupt protein and glycoprotein secondary structure inhibited the binding of V. cholerae strains, suggesting that binding was to specific receptors. Several V. cholerae strains and mutants from both biotypes were tested for adherence in the modified ELISA. Wild-type strains of both biotypes and non-enterotoxigenic strains, which were known to colonize humans, adhered. V. cholerae mutants defective in motility, flagellar structure of chemotaxis, which were known to exhibit reduced colonization in animal models, exhibited decreased adherence. The specificity of the assay and its ability to quantify binding should facilitate identification and the study of adherence factors involved in the colonization of human small intestinal epithelial cells by V. cholerae.