The antigen 43 structure reveals a molecular Velcro-like mechanism of autotransporter-mediated bacterial clumping

Mechanisms of Resistance to the Contact-Dependent Bacteriocin CdzC/D in Caulobacter crescentus

The Cdz bacteriocin system allows the aquatic oligotrophic bacterium Caulobacter crescentus to kill closely related species in a contact-dependent manner. The toxin, which aggregates on the surfaces of producer cells, is composed of two small hydrophobic proteins, CdzC and CdzD, each bearing an extended glycine-zipper motif, that together induce inner membrane depolarization and kill target cells. To further characterize the mechanism of Cdz delivery and toxicity, we screened for mutations that render a target strain resistant to Cdz-mediated killing. These mutations mapped to four loci, including a TonB-dependent receptor, a three-gene operon (named zerRAB for zipper envelope resistance), and perA (for pentapeptide envelope resistance). Mutations in the zerRAB locus led to its overproduction and to potential changes in cell envelope composition, which may diminish the susceptibility of cells to Cdz toxins. The perA gene is also required to maintain a normal cell envelope, but our screen identified mutations that confer resistance to Cdz toxins without substantially affecting the cell envelope functions of PerA. We demonstrate that PerA, which encodes a pentapeptide repeat protein predicted to form a quadrilateral β-helix, localizes primarily to the outer membrane of cells, where it may serve as a receptor for the Cdz toxins. Collectively, these results provide new insights into the function and mechanisms of an atypical, contact-dependent bacteriocin system.IMPORTANCE Bacteriocins are commonly used by bacteria to kill neighboring cells that compete for resources. Although most bacteriocins are secreted, the aquatic, oligotrophic bacterium Caulobacter crescentus produces a two-peptide bacteriocin, CdzC/D, that remains attached to the outer membranes of cells, enabling contact-dependent killing of cells lacking the immunity protein CdzI. The receptor for CdzC/D has not previously been reported. Here, we describe a genetic screen for mutations that confer resistance to CdzC/D. One locus identified, perA, encodes a pentapeptide repeat protein that resides in the outer membrane of target cells, where it may act as the direct receptor for CdzC/D. Collectively, our results provide new insight into bacteriocin function and diversity.

Molecular characterization of enterotoxigenic Escherichia coli isolated from neonatal calves diarrhea

Enterotoxigenic Escherichia coli (ETEC) produces different virulence factors allowing the bacterium to colonize and develop watery diarrhea. Proteomics studies have also introduced new protein belonging to the secretion pathways, antigen 43 (Ag43), which plays important role in E. coli pathogenesis. The objective of this study was to investigate O-types and virulence factors of E. coli isolates from neonatal calves diarrhea. Total of 120 isolates from diarrheic calves were genotyped for their O groups and the presence of virulence genes K99, F41 and STa as well as Ag43. The predominant O-type was O101 (51.00%) and the prevalence of K99, F41 and STa was 7 (5.80%). The Ag43 was detected in all samples with three different allelic patterns. Our results indicated that K99 positive isolates certainly have one of each 2200 bp or 1800 bp or both copies of Ag43 passenger domain, while negative K99 isolates lack the Ag43. The results reported here provide informative data regarding the prevalence of E. coli O-types and their virulence factors in enteric colibacillosis. The Ag43 that was more found in K99 positive isolates might be associated with diarrhea-causing E. coli strains in neonatal calves.

Biofilms 2018: A diversity of microbes and mechanisms

The 8th ASM Conference on Biofilms was held in Washington D.C. on October 7-11, 2018. This very highly subscribed meeting represented a wide breadth of current research in biofilms, and included over 500 attendees, 12 sessions with 64 oral presentations, and four poster sessions with about 400 posters.

Type V Secretion in Gram-Negative Bacteria

Type V, or "autotransporter," secretion is a term used to refer to several simple protein export pathways that are found in a wide range of Gram-negative bacteria. Autotransporters are generally single polypeptides that consist of an extracellular ("passenger") domain and a β barrel domain that anchors the protein to the outer membrane (OM). Although it was originally proposed that the passenger domain is secreted through a channel formed solely by the covalently linked β barrel domain, experiments performed primarily on the type Va, or "classical," autotransporter pathway have challenged this hypothesis. Several lines of evidence strongly suggest that both the secretion of the passenger domain and the membrane integration of the β barrel domain are catalyzed by the barrel assembly machinery (Bam) complex, a conserved hetero-oligomer that plays an essential role in the assembly of most integral OM proteins. The secretion reaction appears to be driven at least in part by the folding of the passenger domain in the extracellular space. Although many aspects of autotransporter biogenesis remain to be elucidated, it will be especially interesting to determine whether the different classes of proteins that fall under the type V rubric-most of which have not been examined in detail-are assembled by the same basic mechanism as classical autotransporters.

Velcro-mimicking surface based on polymer loop brushes

We herein report the fabrication of a Velcro-mimicking surface based on polymer brushes. Using poly(ε-caprolactone) (PCL) as the model polymer, polymer loop brushes (PLBs) and singly tethered polymer brushes (STPBs) with nearly identical tethering point density and brush heights were synthesized using a polymer single crystal (PSC)-assisted grafting-to method. Atomic force microscopy-based single molecular force spectroscopy (AFM-SMFS) and macroscale lap-shear experiments both demonstrated that the PLBs led to strong adhesion that is up to ∼10 times greater than the STPBs, which is attributed to the enriched chain entanglement between the probing polymer and the brushes. We envisage that our results will pave the way towards a new materials design for strong adhesives and nanocomposites.

CdrA Interactions within the Pseudomonas aeruginosa Biofilm Matrix Safeguard It from Proteolysis and Promote Cellular Packing

Biofilms are robust multicellular aggregates of bacteria that are encased in an extracellular matrix. Different bacterial species have been shown to use a range of biopolymers to build their matrices. Pseudomonas aeruginosa is a model organism for the laboratory study of biofilms, and past work has suggested that exopolysaccharides are a required matrix component. However, we found that expression of the matrix protein CdrA, in the absence of biofilm exopolysaccharides, allowed biofilm formation through the production of a CdrA-rich proteinaceous matrix. This represents a novel function for CdrA. Similar observations have been made for other species such as Escherichia coli and Staphylococcus aureus, which can utilize protein-dominant biofilm matrices. However, we found that these CdrA-containing matrices were susceptible to both exogenous and self-produced proteases. We previously reported that CdrA directly binds the biofilm matrix exopolysaccharide Psl. Now we have found that when CdrA bound to Psl, it was protected from proteolysis. Together, these results support the idea of the importance of multibiomolecular components in matrix stability and led us to propose a model in which CdrA-CdrA interactions can enhance cell-cell packing in an aggregate that is resistant to physical shear, while Psl-CdrA interactions enhance aggregate integrity in the presence of self-produced and exogenous proteases.IMPORTANCEPseudomonas aeruginosa forms multicellular aggregates or biofilms using both exopolysaccharides and the CdrA matrix adhesin. We showed for the first time that P. aeruginosa can use CdrA to build biofilms that do not require known matrix exopolysaccharides. It is appreciated that biofilm growth is protective against environmental assaults. However, little is known about how the interactions between individual matrix components aid in this protection. We found that interactions between CdrA and the exopolysaccharide Psl fortify the matrix by preventing CdrA proteolysis. When both components-CdrA and Psl-are part of the matrix, robust aggregates form that are tightly packed and protease resistant. These findings provide insight into how biofilms persist in protease-rich host environments.

Intercellular adhesion promotes clonal mixing in growing bacterial populations

Dense bacterial communities, known as biofilms, can have functional spatial organization driven by self-organizing chemical and physical interactions between cells, and their environment. In this work, we investigated intercellular adhesion, a pervasive property of bacteria in biofilms, to identify effects on the internal structure of bacterial colonies. We expressed the self-recognizing ag43 adhesin protein in Escherichia coli to generate adhesion between cells, which caused aggregation in liquid culture and altered microcolony morphology on solid media. We combined the adhesive phenotype with an artificial colony patterning system based on plasmid segregation, which marked clonal lineage domains in colonies grown from single cells. Engineered E. coli were grown to colonies containing domains with varying adhesive properties, and investigated with microscopy, image processing and computational modelling techniques. We found that intercellular adhesion elongated the fractal-like boundary between cell lineages only when both domains within the colony were adhesive, by increasing the rotational motion during colony growth. Our work demonstrates that adhesive intercellular interactions can have significant effects on the spatial organization of bacterial populations, which can be exploited for biofilm engineering. Furthermore, our approach provides a robust platform to study the influence of intercellular interactions on spatial structure in bacterial populations.

Discovery of New Genes Involved in Curli Production by a Uropathogenic Escherichia coli Strain from the Highly Virulent O45:K1:H7 Lineage

Curli are bacterial surface-associated amyloid fibers that bind to the dye Congo red (CR) and facilitate uropathogenic Escherichia coli (UPEC) biofilm formation and protection against host innate defenses. Here we sequenced the genome of the curli-producing UPEC pyelonephritis strain MS7163 and showed it belongs to the highly virulent O45:K1:H7 neonatal meningitis-associated clone. MS7163 produced curli at human physiological temperature, and this correlated with biofilm growth, resistance of sessile cells to the human cationic peptide cathelicidin, and enhanced colonization of the mouse bladder. We devised a forward genetic screen using CR staining as a proxy for curli production and identified 41 genes that were required for optimal CR binding, of which 19 genes were essential for curli synthesis. Ten of these genes were novel or poorly characterized with respect to curli synthesis and included genes involved in purine de novo biosynthesis, a regulator that controls the Rcs phosphorelay system, and a novel repressor of curli production (referred to as rcpA). The involvement of these genes in curli production was confirmed by the construction of defined mutants and their complementation. The mutants did not express the curli major subunit CsgA and failed to produce curli based on CR binding. Mutation of purF (the first gene in the purine biosynthesis pathway) and rcpA also led to attenuated colonization of the mouse bladder. Overall, this work has provided new insight into the regulation of curli and the role of these amyloid fibers in UPEC biofilm formation and pathogenesis.IMPORTANCE Uropathogenic Escherichia coli (UPEC) strains are the most common cause of urinary tract infection, a disease increasingly associated with escalating antibiotic resistance. UPEC strains possess multiple surface-associated factors that enable their colonization of the urinary tract, including fimbriae, curli, and autotransporters. Curli are extracellular amyloid fibers that enhance UPEC virulence and promote biofilm formation. Here we examined the function and regulation of curli in a UPEC pyelonephritis strain belonging to the highly virulent O45:K1:H7 neonatal meningitis-associated clone. Curli expression at human physiological temperature led to increased biofilm formation, resistance of sessile cells to the human cationic peptide LL-37, and enhanced bladder colonization. Using a comprehensive genetic screen, we identified multiple genes involved in curli production, including several that were novel or poorly characterized with respect to curli synthesis. In total, this study demonstrates an important role for curli as a UPEC virulence factor that promotes biofilm formation, resistance, and pathogenesis.

Identification and Characterization of a Phase-Variable Element That Regulates the Autotransporter UpaE in Uropathogenic Escherichia coli

Uropathogenic Escherichia coli (UPEC) is the most common etiologic agent of uncomplicated urinary tract infection (UTI). An important mechanism of gene regulation in UPEC is phase variation that involves inversion of a promoter-containing DNA element via enzymatic activity of tyrosine recombinases, resulting in biphasic, ON or OFF expression of target genes. The UPEC reference strain CFT073 has five tyrosine site-specific recombinases that function at two previously characterized promoter inversion systems, fimS and hyxS Three of the five recombinases are located proximally to their cognate target elements, which is typical of promoter inversion systems. The genes for the other two recombinases, IpuA and IpuB, are located distal from these sites. Here, we identified and characterized a third phase-variable invertible element in CFT073, ipuS, located proximal to ipuA and ipuB The inversion of ipuS is catalyzed by four of the five CFT073 recombinases. Orientation of the element drives transcription of a two-gene operon containing ipuR, a predicted LuxR-type regulator, and upaE, a predicted autotransporter. We show that the predicted autotransporter UpaE is surface located and facilitates biofilm formation as well as adhesion to extracellular matrix proteins in a K-12 recombinant background. Consistent with this phenotype, the ipuS ON condition in CFT073 results in defective swimming motility, increased adherence to human kidney epithelial cells, and a positive competitive kidney colonization advantage in experimental mouse UTIs. Overall, the identification of a third phase switch in UPEC that is regulated by a shared set of recombinases describes a complex phase-variable virulence network in UPEC.IMPORTANCE Uropathogenic Escherichia coli (UPEC) is the most common cause of urinary tract infection (UTI). ON versus OFF phase switching by inversion of small DNA elements at two chromosome sites in UPEC regulates the expression of important virulence factors, including the type 1 fimbria adhesion organelle. In this report, we describe a third invertible element, ipuS, in the UPEC reference strain CFT073. The inversion of ipuS controls the phase-variable expression of upaE, an autotransporter gene that encodes a surface protein involved in adherence to extracellular matrix proteins and colonization of the kidneys in a murine model of UTI.

Motility and chemotaxis of bacteria-driven microswimmers fabricated using antigen 43-mediated biotin display

Bacteria-driven biohybrid microswimmers (bacteriabots) combine synthetic cargo with motile living bacteria that enable propulsion and steering. Although fabrication and potential use of such bacteriabots have attracted much attention, existing methods of fabrication require an extensive sample preparation that can drastically decrease the viability and motility of bacteria. Moreover, chemotactic behavior of bacteriabots in a liquid medium with chemical gradients has remained largely unclear. To overcome these shortcomings, we designed Escherichia coli to autonomously display biotin on its cell surface via the engineered autotransporter antigen 43 and thus to bind streptavidin-coated cargo. We show that the cargo attachment to these bacteria is greatly enhanced by motility and occurs predominantly at the cell poles, which is greatly beneficial for the fabrication of motile bacteriabots. We further performed a systemic study to understand and optimize the ability of these bacteriabots to follow chemical gradients. We demonstrate that the chemotaxis of bacteriabots is primarily limited by the cargo-dependent reduction of swimming speed and show that the fabrication of bacteriabots using elongated E. coli cells can be used to overcome this limitation.

Ecology and evolution of metabolic cross-feeding interactions in bacteria

Literature covered: early 2000s to late 2017Bacteria frequently exchange metabolites with other micro- and macro-organisms. In these often obligate cross-feeding interactions, primary metabolites such as vitamins, amino acids, nucleotides, or growth factors are exchanged. The widespread distribution of this type of metabolic interactions, however, is at odds with evolutionary theory: why should an organism invest costly resources to benefit other individuals rather than using these metabolites to maximize its own fitness? Recent empirical work has shown that bacterial genotypes can significantly benefit from trading metabolites with other bacteria relative to cells not engaging in such interactions. Here, we will provide a comprehensive overview over the ecological factors and evolutionary mechanisms that have been identified to explain the evolution and maintenance of metabolic mutualisms among microorganisms. Furthermore, we will highlight general principles that underlie the adaptive evolution of interconnected microbial metabolic networks as well as the evolutionary consequences that result for cells living in such communities.

Molecular basis for the folding of β-helical autotransporter passenger domains

Bacterial autotransporters comprise a C-terminal β-barrel domain, which must be correctly folded and inserted into the outer membrane to facilitate translocation of the N-terminal passenger domain to the cell exterior. Once at the surface, the passenger domains of most autotransporters are folded into an elongated β-helix. In a cellular context, key molecules catalyze the assembly of the autotransporter β-barrel domain. However, how the passenger domain folds into its functional form is poorly understood. Here we use mutational analysis on the autotransporter Pet to show that the β-hairpin structure of the fifth extracellular loop of the β-barrel domain has a crucial role for passenger domain folding into a β-helix. Bioinformatics and structural analyses, and mutagenesis of a homologous autotransporter, suggest that this function is conserved among autotransporter proteins with β-helical passenger domains. We propose that the autotransporter β-barrel domain is a folding vector that nucleates folding of the passenger domain.

Autotransporter passenger domains are presented on or released from the bacterial surface upon translocation through an outer membrane β-barrel anchor. Here the authors study the two E. coli autotransporters Pet and EspP and propose that the β-barrel anchor acts as a vector to nucleate the folding of the passenger domain.

Intra-operative biopsy in chronic sinusitis detects pathogenic Escherichia coli that carry fimG/H, fyuA and agn43 genes coding biofilm formation

The aim of this study was to investigate whether or not surgical biopsy of sinus tissue in chronic sinusitis, not responsive to treatment, would detect E. coli. We intended to evaluate E. coli virulence genes, therefore dispute the causal role of such an unusual microorganism in chronic sinusitis, as well as consider effective pathogen-targeted therapy. Patients with E. coli isolated by intra-operative puncture biopsy were included in the study. Genetic analysis of E. coli isolates, including phylogenetic grouping and virulence factor characteristics, were done by multiplex PCR. We identified 26 patients with chronic sinusitis, in which 26 E. coli isolates were cultured. The E. coli isolates belonged mainly to pathogenic phylogenetic group B2, and carried multiple virulence genes. Three genes in particular were present in all (100%) of examined isolates, they were (1) marker agn43 gene for forming biofilm, (2) type 1 fimbriae (fimG/H gene) and (3) yersiniabactin receptor (fyuA). Furthermore, a pseudo-phylogenetic tree of virulence genes distribution revealed possible cooperation between agn43, fimG/H, and fyuA in the coding of biofilm formation. Intra-operative-biopsy and culture-based therapy, targeting the isolated E. coli, coincided with long-term resolution of symptoms. This is the first report demonstrating an association between a highly pathogenic E. coli, chronic sinus infection, and resolution of symptoms upon E. coli targeted therapy, a significant finding due to the fact that E. coli has not been considered to be a commensal organism of the oropharynx or sinuses. We postulate that the simultaneous presence of three genes, each coding biofilm formation, may in part account for the chronicity of E. coli sinusitis.

Bacterial autoaggregation

Many bacteria, both environmental and pathogenic, exhibit the property of autoaggregation. In autoaggregation (sometimes also called autoagglutination or flocculation), bacteria of the same type form multicellular clumps that eventually settle at the bottom of culture tubes. Autoaggregation is generally mediated by self-recognising surface structures, such as proteins and exopolysaccharides, which we term collectively as autoagglutinins. Although a widespread phenomenon, in most cases the function of autoaggregation is poorly understood, though there is evidence to show that aggregating bacteria are protected from environmental stresses or host responses. Autoaggregation is also often among the first steps in forming biofilms. Here, we review the current knowledge on autoaggregation, the role of autoaggregation in biofilm formation and pathogenesis, and molecular mechanisms leading to aggregation using specific examples.

Autoinducer 2-Dependent Escherichia coli Biofilm Formation Is Enhanced in a Dual-Species Coculture

Biofilms in nature typically consist of multiple species, and microbial interactions are likely to have crucial effects on biofilm development, structure, and functions. The best-understood form of communication within bacterial communities involves the production, release, and detection of signal molecules (autoinducers), known as quorum sensing. Although autoinducers mainly promote intraspecies communication, autoinducer 2 (AI-2) is produced and detected by a variety of bacteria, thus principally allowing interspecies communication. Here we show the importance of AI-2-mediated signaling in the formation of mixed biofilms by Enterococcus faecalis and Escherichia coli. Our results demonstrate that AI-2 produced by E. faecalis promotes collective behaviors of E. coli at lower cell densities, enhancing autoaggregation of E. coli but also leading to chemotaxis-dependent coaggregation between the two species. Finally, we show that formation of such mixed dual-species biofilms increases the stress resistance of both E. coli and E. faecalis.

IMPORTANCE The role of interspecies communication in the development of mixed microbial communities is becoming increasingly apparent, but specific examples of such communication remain limited. The universal signal molecule AI-2 is well known to regulate cell-density-dependent phenotypes of many bacterial species but, despite its potential for interspecies communication, the role of AI-2 in the establishment of multispecies communities is not well understood. In this study, we explore AI-2 signaling in a dual-species community containing two bacterial species that naturally cooccur in their mammalian hosts, i.e., Escherichia coli and Enterococcus faecalis. We show that active production of AI-2 by E. faecalis allows E. coli to perform collective behaviors at low cell densities. Additionally, AI-2- and chemotaxis-dependent coaggregation with E. faecalis creates nucleation zones for rapid growth of E. coli microcolonies in mixed biofilms and enhances the stress resistance of both species.

Identification of the Autochaperone Domain in the Type Va Secretion System (T5aSS): Prevalent Feature of Autotransporters with a β-Helical Passenger

Autotransporters (ATs) belong to a family of modular proteins secreted by the Type V, subtype a, secretion system (T5aSS) and considered as an important source of virulence factors in lipopolysaccharidic diderm bacteria (archetypical Gram-negative bacteria). While exported by the Sec pathway, the ATs are further secreted across the outer membrane via their own C-terminal translocator forming a β-barrel, through which the rest of the protein, namely the passenger, can pass. In several ATs, an autochaperone domain (AC) present at the C-terminal region of the passenger and upstream of the translocator was demonstrated as strictly required for proper secretion and folding. However, considering it was functionally characterised and identified only in a handful of ATs, wariness recently fells on the commonality and conservation of this structural element in the T5aSS. To circumvent the issue of sequence divergence and taking advantage of the resolved three-dimensional structure of some ACs, identification of this domain was performed following structural alignment among all AT passengers experimentally resolved by crystallography before searching in a dataset of 1523 ATs. While demonstrating that the AC is indeed a conserved structure found in numerous ATs, phylogenetic analysis further revealed a distribution into deeply rooted branches, from which emerge 20 main clusters. Sequence analysis revealed that an AC could be identified in the large majority of SAATs (self-associating ATs) but not in any LEATs (lipase/esterase ATs) nor in some PATs (protease autotransporters) and PHATs (phosphatase/hydrolase ATs). Structural analysis indicated that an AC was present in passengers exhibiting single-stranded right-handed parallel β-helix, whatever the type of β-solenoid, but not with α-helical globular fold. From this investigation, the AC of type 1 appears as a prevalent and conserved structural element exclusively associated to β-helical AT passenger and should promote further studies about the protein secretion and folding via the T5aSS, especially toward α-helical AT passengers.

Structural basis of host recognition and biofilm formation by Salmonella Saf pili

Pili are critical in host recognition, colonization and biofilm formation during bacterial infection. Here, we report the crystal structures of SafD-dsc and SafD-SafA-SafA (SafDAA-dsc) in Saf pili. Cell adherence assays show that SafD and SafA are both required for host recognition, suggesting a poly-adhesive mechanism for Saf pili. Moreover, the SafDAA-dsc structure, as well as SAXS characterization, reveals an unexpected inter-molecular oligomerization, prompting the investigation of Saf-driven self-association in biofilm formation. The bead/cell aggregation and biofilm formation assays are used to demonstrate the novel function of Saf pili. Structure-based mutants targeting the inter-molecular hydrogen bonds and complementary architecture/surfaces in SafDAA-dsc dimers significantly impaired the Saf self-association activity and biofilm formation. In summary, our results identify two novel functions of Saf pili: the poly-adhesive and self-associating activities. More importantly, Saf-Saf structures and functional characterizations help to define a pili-mediated inter-cellular oligomerizaiton mechanism for bacterial aggregation, colonization and ultimate biofilm formation.

Autotransporter Adhesins in Escherichia coli Pathogenesis

Most bacteria produce adhesion molecules to facilitate the interaction with host cells and establish successful infections. An important group of bacterial adhesins belong to the autotransporter (AT) superfamily, the largest group of secreted and outer membrane proteins in Gram-negative bacteria. AT adhesins possess diverse functions that facilitate bacterial colonisation, survival and persistence, and as such are often associated with increased bacterial fitness and pathogenic potential. In this review, we will describe AIDA-I type AT adhesins, which comprise the biggest and most diverse group in the AT family. We will focus on Escherichia coli proteins and define general aspects of their biogenesis, distribution, structural properties and key roles in infection.

The Shigella Virulence Factor IcsA Relieves N-WASP Autoinhibition by Displacing the Verprolin Homology/Cofilin/Acidic (VCA) Domain

Shigella flexneri is a bacterial pathogen that invades cells of the gastrointestinal tract, causing severe dysentery. Shigella mediates intracellular motility and spreading via actin comet tail formation. This process is dependent on the surface-exposed, membrane-embedded virulence factor IcsA, which recruits the host actin regulator N-WASP. Although it is clear that Shigella requires N-WASP for this process, the molecular details of this interaction and the mechanism of N-WASP activation remain poorly understood. Here, we show that co-expression of full-length IcsA and the Shigella membrane protease IcsP yields highly pure IcsA passenger domain (residues 53-758). We show that IcsA is monomeric and describe the solution structure of the passenger domain obtained by small-angle X-ray scattering (SAXS) analysis. The SAXS-derived models suggest that IcsA has an elongated shape but, unlike most other autotransporter proteins, possesses a central kink revealing a distinctly curved structure. Pull-down experiments show direct binding of the IcsA passenger domain to both the WASP homology 1 (WH1) domain and the GTPase binding domain (GBD) of N-WASP and no binding to the verprolin homology/cofilin/acidic (VCA) region. Using fluorescence polarization experiments, we demonstrate that IcsA binding to the GBD region displaces the VCA peptide and that this effect is synergistically enhanced upon IcsA binding to the WH1 region. Additionally, domain mapping of the IcsA interaction interface reveals that different regions of IcsA bind to the WH1 and GBD domains of N-WASP. Taken together, our data support a model where IcsA and N-WASP form a tight complex releasing the N-WASP VCA domain to recruit the host cell machinery for actin tail formation.

Type V Secretion Systems in Bacteria

Type V secretion denotes a variety of secretion systems that cross the outer membrane in Gram-negative bacteria but that depend on the Sec machinery for transport through the inner membrane. They are possibly the simplest bacterial secretion systems, because they consist only of a single polypeptide chain (or two chains in the case of two-partner secretion). Their seemingly autonomous transport through the outer membrane has led to the term "autotransporters" for various subclasses of type V secretion. In this chapter, we review the structure and function of these transporters and review recent findings on additional factors involved in the secretion process, which have put the term "autotransporter" to debate.

Coating polypropylene surfaces with protease weakens the adhesion and increases the dispersion of Candida albicans cells

OBJECTIVES

To investigate the ability of the proteases, subtilisin and α-chymotrypsin (aCT), to inhibit the adhesion of Candida albicans biofilm to a polypropylene surface.

RESULTS

The proteases were immobilized on plasma-treated polypropylene by covalently linking them with either glutaraldehyde (GA) or N'-diisopropylcarbodiimide (DIC) and N-hydroxysuccinimide (NHS). The immobilization did not negatively affect the enzyme activity and in the case of subtilisin, the activity was up to 640% higher than that of the free enzyme when using N-acetyl phenylalanine ethyl ester as the substrate. The efficacies against biofilm dispersal for the GA-linked SubC and aCT coatings were 41 and 55% higher than the control (polypropylene coated with only GA), respectively, whereas no effect was observed with enzymes immobilized with DIC and NHS. The higher dispersion efficacy observed for the proteases immobilized with GA could be both steric (proper orientation of the active site) and dynamic (higher protein mobility/flexibility).

CONCLUSIONS

Proteases immobilized on a polypropylene surface reduced the adhesion of C. albicans biofilms and therefore may be useful in developing anti-biofilm surfaces based on non-toxic molecules and sustainable strategies.

Adaptation of Escherichia coli traversing from the faecal environment to the urinary tract

The majority of extraintestinal pathogenic Escherichia coli (ExPEC) causing urinary tract infections (UTI) are found in the patient's own gut flora, but only limited knowledge is available on the potential adaptation that may occur in the bacteria in order to traverse the perineum and successfully infect the urinary tract. Here, matching pairs of faecal and UTI isolates from 42 patients were compared pairwise using in-depth whole-genome sequencing to investigate whether genetic changes were evident for successful colonization in these two different environments. The identified non-synonymous mutations (0-12 substitutions in each pair) were primarily associated to genes encoding virulence factors and nutrient metabolism; and indications of parallel evolution were observed in genes encoding the major phase-variable protein antigen 43, a toxin/antitoxin locus and haemolysin B. No differences in virulence potential were observed in a mouse UTI model for five matching faecal and UTI isolates with or without mutations in antigen 43 and haemolysin B. Variations in plasmid content were observed in only four of the 42 pairs. Although, we observed mutations in known UTI virulence genes for a few pairs, the majority showed no detectable differences with respect to mutations or mobilome when compared to their faecal counterpart. The results show that UPECs are successful in colonizing both the bladder and gut without adaptation.

Chemotaxis towards autoinducer 2 mediates autoaggregation in Escherichia coli

Bacteria communicate by producing and sensing extracellular signal molecules called autoinducers. Such intercellular signalling, known as quorum sensing, allows bacteria to coordinate and synchronize behavioural responses at high cell densities. Autoinducer 2 (AI-2) is the only known quorum-sensing molecule produced by Escherichia coli but its physiological role remains elusive, although it is known to regulate biofilm formation and virulence in other bacterial species. Here we show that chemotaxis towards self-produced AI-2 can mediate collective behaviour-autoaggregation-of E. coli. Autoaggregation requires motility and is strongly enhanced by chemotaxis to AI-2 at physiological cell densities. These effects are observed regardless whether cell-cell interactions under particular growth conditions are mediated by the major E. coli adhesin (antigen 43) or by curli fibres. Furthermore, AI-2-dependent autoaggregation enhances bacterial stress resistance and promotes biofilm formation.

Amyloid Structures as Biofilm Matrix Scaffolds

Recent insights into bacterial biofilm matrix structures have induced a paradigm shift toward the recognition of amyloid fibers as common building block structures that confer stability to the exopolysaccharide matrix. Here we describe the functional amyloid systems related to biofilm matrix formation in both Gram-negative and Gram-positive bacteria and recent knowledge regarding the interaction of amyloids with other biofilm matrix components such as extracellular DNA (eDNA) and the host immune system. In addition, we summarize the efforts to identify compounds that target amyloid fibers for therapeutic purposes and recent developments that take advantage of the amyloid structure to engineer amyloid fibers of bacterial biofilm matrices for biotechnological applications.

Characterization of Fusobacterium nucleatum ATCC 23726 adhesins involved in strain-specific attachment to Porphyromonas gingivalis

Bacterial adherence is an essential virulence factor in pathogenesis and infection. Fusobacterium nucleatum has a central role in oral biofilm architecture by acting as a bridge between early Gram-positive and late Gram-negative colonizers that do not otherwise adhere to each other. In this study, we survey a key adherence interaction of F. nucleatum with Porphyromonas gingivalis, and present evidence that multiple fusobacterial adhesins have a role in the attachment of F. nucleatum ATCC 23726 to P. gingivalis in a highly strain-dependent manner. Interaction between these species displayed varying sensitivities to arginine, galactose and lactose. Arginine was found to hamper coaggregation by at least 62% and up to 89% with several P. gingivalis strains and galactose inhibition ranged from no inhibition up to 58% with the same P. gingivalis strains. Lactose consistently inhibited F. nucleatum interaction with these P. gingivalis strains ranging from 40% to 56% decrease in coaggregation. Among the adhesins involved are the previously described Fap2 and surprisingly, RadD, which was described in an earlier study for its function in attachment of F. nucleatum to Gram-positive species. We also provide evidence for the presence of at least one additional adhesin that is sensitive to arginine but unlike Fap2 and RadD, is not a member of the autotransporter family type of fusobacterial large outer membrane proteins. The strain-specific binding profile of multiple fusobacterial adhesins to P. gingivalis highlights the heterogeneity and complexity of interspecies interactions in the oral cavity.

Kin Recognition in Bacteria

The ability of bacteria to recognize kin provides a means to form social groups. In turn these groups can lead to cooperative behaviors that surpass the ability of the individual. Kin recognition involves specific biochemical interactions between a receptor(s) and an identification molecule(s). Recognition specificity, ensuring that nonkin are excluded and kin are included, is critical and depends on the number of loci and polymorphisms involved. After recognition and biochemical perception, the common ensuing cooperative behaviors include biofilm formation, quorum responses, development, and swarming motility. Although kin recognition is a fundamental mechanism through which cells might interact, microbiologists are only beginning to explore the topic. This review considers both molecular and theoretical aspects of bacterial kin recognition. Consideration is also given to bacterial diversity, genetic relatedness, kin selection theory, and mechanisms of recognition.

Self-association motifs in the enteroaggregative Escherichia coli heat-resistant agglutinin 1

The heat-resistant agglutinin 1 (Hra1) is an integral outer membrane protein found in strains of Escherichia coli that are exceptional colonizers. Hra1 from enteroaggregative E. coli strain 042 is sufficient to confer adherence to human epithelial cells and to cause bacterial autoaggregation. Hra1 is closely related to the Tia invasin, which also confers adherence, but not autoaggregation. Here, we have demonstrated that Hra1 mediates autoaggregation by self-association and we hypothesize that at least some surface-exposed amino acid sequences that are present in Hra1, but absent in Tia, represent autoaggregation motifs. We inserted FLAG tags along the length of Hra1 and used immune-dot blots to verify that four in silico-predicted outer loops were indeed surface exposed. In Hra1 we swapped nine candidate motifs in three of these loops, ranging from one to ten amino acids in length, to the corresponding sequences in Tia. Three of the motifs were required for Hra1-mediated autoaggregation. The database was searched for other surface proteins containing these motifs; the GGXWRDDXK motif was also present in a surface-exposed region of Rck, a Salmonella enterica serotype Typhimurium complement resistance protein. Cloning and site-specific mutagenesis demonstrated that Rck can confer weak, GGXWRDDXK-dependent autoaggregation by self-association. Hra1 and Rck appear to form heterologous associations and GGXWRDDXK is required on both molecules for Hra1-Rck association. However, a GGYWRDDLKE peptide was not sufficient to interfere with Hra1-mediated autoaggregation. In the present study, three autoaggregation motifs in an integral outer membrane protein have been identified and it was demonstrated that at least one of them works in the context of a different cell surface.

Adhesins Involved in Attachment to Abiotic Surfaces by Gram-Negative Bacteria

During the first step of biofilm formation, initial attachment is dictated by physicochemical and electrostatic interactions between the surface and the bacterial envelope. Depending on the nature of these interactions, attachment can be transient or permanent. To achieve irreversible attachment, bacterial cells have developed a series of surface adhesins promoting specific or nonspecific adhesion under various environmental conditions. This article reviews the recent advances in our understanding of the secretion, assembly, and regulation of the bacterial adhesins during biofilm formation, with a particular emphasis on the fimbrial, nonfimbrial, and discrete polysaccharide adhesins in Gram-negative bacteria.

Molecular and Structural Characterization of a Novel Escherichia coli Interleukin Receptor Mimic Protein

Urinary tract infection (UTI) is a disease of extremely high incidence in both community and nosocomial settings. UTIs cause significant morbidity and mortality, with approximately 150 million cases globally per year. Uropathogenic Escherichia coli (UPEC) is the primary cause of UTI and is generally treated empirically. However, the rapidly increasing incidence of UTIs caused by multidrug-resistant UPEC strains has led to limited available treatment options and highlights the urgent need to develop alternative treatment and prevention strategies. In this study, we performed a comprehensive analysis to define the regulation, structure, function, and immunogenicity of recently identified UPEC vaccine candidate C1275 (here referred to as IrmA). We showed that the irmA gene is highly prevalent in UPEC, is cotranscribed with the biofilm-associated antigen 43 gene, and is regulated by the global oxidative stress response OxyR protein. Localization studies identified IrmA in the UPEC culture supernatant. We determined the structure of IrmA and showed that it adopts a unique domain-swapped dimer architecture. The dimeric structure of IrmA displays similarity to those of human cytokine receptors, including the interleukin-2 receptor (IL-2R), interleukin-4 receptor (IL-4R), and interleukin-10 receptor (IL-10R) binding domains, and we showed that purified IrmA can bind to their cognate cytokines. Finally, we showed that plasma from convalescent urosepsis patients contains high IrmA antibody titers, demonstrating the strong immunogenicity of IrmA. Taken together, our results indicate that IrmA may play an important role during UPEC infection.

Uropathogenic E. coli (UPEC) is the primary cause of urinary tract infection (UTI), a disease of major significance to human health. Globally, the incidence of UPEC-mediated UTI is strongly associated with increasing antibiotic resistance, making this extremely common infection a major public health concern. In this report, we describe the regulatory, structural, functional, and immunogenic properties of a candidate UPEC vaccine antigen, IrmA. We demonstrate that IrmA is a small UPEC protein that forms a unique domain-swapped dimer with structural mimicry to several human cytokine receptors. We also show that IrmA binds to IL-2, IL-4, and IL-10, is strongly immunogenic in urosepsis patients, and is coexpressed with factors associated with biofilm formation. Overall, this work suggests a potential novel contribution for IrmA in UPEC infection.

Development of a light-regulated cell-recovery system for non-photosynthetic bacteria

Background

Recent advances in the understanding of photosensing in biological systems have enabled the use of photoreceptors as novel genetic tools. Exploiting various photoreceptors that cyanobacteria possess, a green light-inducible gene expression system was previously developed for the regulation of gene expression in cyanobacteria. However, the applications of cyanobacterial photoreceptors are not limited to these bacteria but are also available for non-photosynthetic microorganisms by the coexpression of a cyanobacterial chromophore with a cyanobacteria-derived photosensing system. An Escherichia coli-derived self-aggregation system based on Antigen 43 (Ag43) has been shown to induce cell self-aggregation of various bacteria by exogenous introduction of the Ag43 gene.

Results

An E. coli transformant harboring a plasmid encoding the Ag43 structural gene under a green light-regulated gene expression system derived from the cyanobacterium Synechocystis sp. PCC6803 was constructed. Ag43 was inserted downstream of the cpcG₂ promoter P_cpcG2, and its expression was regulated by green light induction, which was achieved by the functional expression of cyanobacterial CcaS/CcaR by coexpressing its chromophore synthesis gene cassette in E. coli. E. coli transformants harboring this designed system self-aggregated under green light exposure and precipitated, whereas transformants lacking the green light induction system did not. The green light induction system effectively functioned before the cell culture entered the stationary growth phase, and approximately 80 % of the cell culture was recovered by simple decantation.

Conclusion

This study demonstrated the construction of a cell recovery system for non-photosynthetic microorganisms induced by exposure of cells to green light. The system was regulated by a two-component regulatory system from cyanobacteria, and cell precipitation was mediated by an autotransporter protein, Ag43. Although further strict control and an increase of cell recovery efficiency are necessary, the system represents a novel tool for future bioprocessing with reduced energy and labor required for cell recovery.

Deconvoluting the Effect of the Hydrophobic and Hydrophilic Domains of an Amphiphilic Integral Membrane Protein in Lipid Bicontinuous Cubic Mesophases

Lipidic bicontinuous cubic mesophases with encapsulated amphiphilic proteins are widely used in a range of biological and biomedical applications, including in meso crystallization, as drug delivery vehicles for therapeutic proteins, and as biosensors and biofuel cells. However, the effect of amphiphilic protein encapsulation on the cubic phase nanostructure is not well-understood. In this study, we illustrate the effect of incorporating the bacterial amphiphilic membrane protein Ag43, and its individual hydrophobic β(43) and hydrophilic α(43) domains, in bicontinuous cubic mesophases. For the monoolein, monoalmitolein, and phytantriol cubic phases with and without 8% w/w cholesterol, the effect of the full length amphiphilic protein Ag43 on the cubic phase nanostructure was more significant than the sum of the individual hydrophobic β(43) and hydrophilic α(43) domains. Several factors were found to potentially influence the impact of the hydrophobic β(43) domain on the cubic phase internal nanostructure. These include the size of the hydrophobic β(43) domain relative to the thickness of the lipid bilayer, as well as its charge and diameter. The size of the hydrophilic α(43) domain relative to the water channel radius of the cubic mesophase was also found to be important. The secondary structure of the Ag43 proteins was affected by the hydrophobic thickness and physicochemical properties of the lipid bilayer and the water channel diameter of the cubic phase. Such structural changes may be small but could potentially affect membrane protein function.

Detection of genes encoding multidrug resistance and biofilm virulence factor in uterine pathogenic bacteria in postpartum dairy cows

Reckless use of antibiotics and/or development of biofilm are the rationale for the development of multidrug resistance (MDR) of pathogenic bacteria. The objective of the present study was to detect MDR genes in Trueperella pyogenes and to detect biofilm virulence factor (VF) genes in Escherichia coli isolated from the uterus of postpartum dairy cows. Uterine secretions from different parity postpartum Holstein cows (n = 40) were collected using cytobrush technique after a sterile procedure from cows with varying degree of uterine inflammatory conditions. The cytobrush was stored in a specimen collector, placed in a cooler with ice, and transported to the laboratory within 2 hours. The pathogens were isolated and were identified initially by their colony morphology and biochemical characteristics. To further identify and classify the single species, and to determine the presence of MDR and VF genes, the genes fragments were amplified using the respective primers by either singleplex or multiplex polymerase chain reaction protocol, and amplicons were detected by electrophoresis method. T pyogenes was isolated in 17 of 40 (42.5%) cows in the study population as recognized by the 16S rRNA gene. Of the positive T pyogenes samples, 8 of 17 (42.1%) were positive for integron type 1 (intI I), and none were positive for integron type 2 (intI II). Of those 8 positive for intI I, six of eight (66.7%) were positive for amplicons aadA5 and aadA24-ORF1 at 1048 and 1608 bp, respectively, associated with specific drug resistance. Presence of addA5 indicated resistance to sulfadiazine, bacitracin, florfenicol, and ceftiofur. Presence of addA24-ORF1 indicated resistant to sulfadiazine, bacitracin, penicillin, clindamycin, and erythromycin. E coli was isolated in 18 of 40 (45.0%) cows in the study population. The genes for VF, Agn43a, and Agn43 b, associated with biofilm production, were found in 6 of 18 (33.3%) of the positive isolates. Both T pyogenes MDR gene and E coli biofilm VF existed in more severe form of uterine diseases than subclinical endometritis. In conclusion, 35% of T pyogenes isolates found were positive for a gene cassette associated with antibiotic resistance, and 33% of the E coli isolates contained genes for the VF associated with biofilm production.

The LapG protein plays a role in Pseudomonas aeruginosa biofilm formation by controlling the presence of the CdrA adhesin on the cell surface

Pseudomonas aeruginosa is a clinically relevant species involved in biofilm-based chronic infections. We provide evidence that the P. aeruginosa LapG protein functions as a periplasmic protease that can cleave the protein adhesin CdrA off the cell surface, and thereby plays a role in biofilm formation and biofilm dispersal. The P. aeruginosa LapG protein is shown to be a functional homolog of the Pseudomonas putida LapG protein which has previously been shown to function as a periplasmic protease that targets the surface adhesin LapA. Transposon mutagenesis and characterization of defined knockout mutants provided evidence that the CdrA adhesin is a target of LapG in P. aeruginosa. A wspF lapG double mutant was hyper-aggregating and hyper biofilm forming, whereas a wspF lapG cdrA triple mutant lost these phenotypes. In addition, western blot detection of CdrA in culture supernatants and whole-cell protein fractions showed that CdrA was retained in the whole-cell protein fraction when LapG was absent, whereas it was found in the culture supernatant when LapG was present. The finding that CdrA is a target of LapG in P. aeruginosa is surprising because CdrA has no homology to LapA.

Structure, Function, and Assembly of Adhesive Organelles by Uropathogenic Bacteria

Bacteria assemble a wide range of adhesive proteins, termed adhesins, to mediate binding to receptors and colonization of surfaces. For pathogenic bacteria, adhesins are critical for early stages of infection, allowing the bacteria to initiate contact with host cells, colonize different tissues, and establish a foothold within the host. The adhesins expressed by a pathogen are also critical for bacterial-bacterial interactions and the formation of bacterial communities, including biofilms. The ability to adhere to host tissues is particularly important for bacteria that colonize sites such as the urinary tract, where the flow of urine functions to maintain sterility by washing away non-adherent pathogens. Adhesins vary from monomeric proteins that are directly anchored to the bacterial surface to polymeric, hair-like fibers that extend out from the cell surface. These latter fibers are termed pili or fimbriae, and were among the first identified virulence factors of uropathogenic Escherichia coli. Studies since then have identified a range of both pilus and non-pilus adhesins that contribute to bacterial colonization of the urinary tract, and have revealed molecular details of the structures, assembly pathways, and functions of these adhesive organelles. In this review, we describe the different types of adhesins expressed by both Gram-negative and Gram-positive uropathogens, what is known about their structures, how they are assembled on the bacterial surface, and the functions of specific adhesins in the pathogenesis of urinary tract infections.

Engineering the Controlled Assembly of Filamentous Injectisomes in E. coli K-12 for Protein Translocation into Mammalian Cells

Bacterial pathogens containing type III protein secretion systems (T3SS) assemble large needle-like protein complexes in the bacterial envelope, called injectisomes, for translocation of protein effectors into host cells. The application of these “molecular syringes” for the injection of proteins into mammalian cells is hindered by their structural and genomic complexity, requiring multiple polypeptides encoded along with effectors in various transcriptional units (TUs) with intricate regulation. In this work, we have rationally designed the controlled expression of the filamentous injectisomes found in enteropathogenic Escherichia coli (EPEC) in the nonpathogenic strain E. coli K-12. All structural components of EPEC injectisomes, encoded in a genomic island called the locus of enterocyte effacement (LEE), were engineered in five TUs (eLEEs) excluding effectors, promoters and transcriptional regulators. These eLEEs were placed under the control of the IPTG-inducible promoter Ptac and integrated into specific chromosomal sites of E. coli K-12 using a marker-less strategy. The resulting strain, named synthetic injector E. coli (SIEC), assembles filamentous injectisomes similar to those in EPEC. SIEC injectisomes form pores in the host plasma membrane and are able to translocate T3-substrate proteins (e.g., translocated intimin receptor, Tir) into the cytoplasm of HeLa cells reproducing the phenotypes of intimate attachment and polymerization of actin-pedestals elicited by EPEC bacteria. Hence, SIEC strain allows the controlled expression of functional filamentous injectisomes for efficient translocation of proteins with T3S-signals into mammalian cells.

The yin-yang driving urinary tract infection and how proteomics can enhance research, diagnostics, and treatment

Bacterial urinary tract infections (UTIs) afflict millions of people worldwide both in the community and the hospital setting. The onset, duration, and severity of infection depend on the characteristics of the invading pathogen (yin), as well as the immune response elicited by the infected individual (yang). Uropathogenic Escherichia coli (UPEC) account for the majority of UTIs, and extensive investigations by many scientific groups have elucidated an elaborate pathogenic UPEC life cycle, involving the occupation of extracellular and intracellular niches and the expression of an arsenal of virulence factors that facilitate niche occupation. This review will summarize the current knowledge on UPEC pathogenesis; the host immune responses elicited to combat infection; and it will describe proteomics approaches used to understand UPEC pathogenesis, as well as drive diagnostics and treatment options. Finally, new strategies are highlighted that could be applied toward furthering our knowledge regarding host-bacterial interactions during UTI.

Intra- and inter-species interactions within biofilms of important foodborne bacterial pathogens

A community-based sessile life style is the normal mode of growth and survival for many bacterial species. Under such conditions, cell-to-cell interactions are inevitable and ultimately lead to the establishment of dense, complex and highly structured biofilm populations encapsulated in a self-produced extracellular matrix and capable of coordinated and collective behavior. Remarkably, in food processing environments, a variety of different bacteria may attach to surfaces, survive, grow, and form biofilms. Salmonella enterica, Listeria monocytogenes, Escherichia coli, and Staphylococcus aureus are important bacterial pathogens commonly implicated in outbreaks of foodborne diseases, while all are known to be able to create biofilms on both abiotic and biotic surfaces. Particularly challenging is the attempt to understand the complexity of inter-bacterial interactions that can be encountered in such unwanted consortia, such as competitive and cooperative ones, together with their impact on the final outcome of these communities (e.g., maturation, physiology, antimicrobial resistance, virulence, dispersal). In this review, up-to-date data on both the intra- and inter-species interactions encountered in biofilms of these pathogens are presented. A better understanding of these interactions, both at molecular and biophysical levels, could lead to novel intervention strategies for controlling pathogenic biofilm formation in food processing environments and thus improve food safety.

The Autotransporter BpaB Contributes to the Virulence of Burkholderia mallei in an Aerosol Model of Infection

Burkholderia mallei is a highly pathogenic bacterium that causes the zoonosis glanders. Previous studies indicated that the genome of the organism contains eight genes specifying autotransporter proteins, which are important virulence factors of Gram-negative bacteria. In the present study, we report the characterization of one of these autotransporters, BpaB. Database searches identified the bpaB gene in ten B. mallei isolates and the predicted proteins were 99-100% identical. Comparative sequence analyses indicate that the gene product is a trimeric autotransporter of 1,090 amino acids with a predicted molecular weight of 105-kDa. Consistent with this finding, we discovered that recombinant bacteria expressing bpaB produce a protein of ≥300-kDa on their surface that is reactive with a BpaB-specific monoclonal antibody. Analysis of sera from mice infected with B. mallei indicated that animals produce antibodies against BpaB during the course of disease, thus establishing production of the autotransporter in vivo. To gain insight on its role in virulence, we inactivated the bpaB gene of B. mallei strain ATCC 23344 and determined the median lethal dose of the mutant in a mouse model of aerosol infection. These experiments revealed that the bpaB mutation attenuates virulence 8-14 fold. Using a crystal violet-based assay, we also discovered that constitutive production of BpaB on the surface of B. mallei promotes biofilm formation. To our knowledge, this is the first report of a biofilm factor for this organism.

Looks can be deceiving: recent insights into the mechanism of protein secretion by the autotransporter pathway

Autotransporters are a large superfamily of cell surface proteins produced by Gram-negative bacteria that consist of an N-terminal extracellular domain ('passenger domain') and a C-terminal β-barrel domain that resides in the outer membrane (OM). Although it was originally proposed that the passenger domain is translocated across the OM through a channel formed exclusively by the covalently linked β-barrel domain, this idea has been strongly challenged by a variety of observations. Recent experimental results have suggested a new model in which both the translocation of the passenger domain and the membrane integration of the β-barrel domain are facilitated by the Bam complex, a highly conserved heteroligomer that plays a general role in OM protein assembly. Other factors, including periplasmic chaperones and inner membrane proteins, have also recently been implicated in the biogenesis of at least some members of the autotransporter superfamily. New results have raised intriguing questions about the energetics of the secretion reaction and the relationship between the assembly of autotransporters and the assembly of other classes of OM proteins. Concomitantly, new mechanistic and structural insights have expanded the utility of the autotransporter pathway for the surface display of heterologous peptides and proteins of interest.

The passenger-associated transport repeat promotes virulence factor secretion efficiency and delineates a distinct autotransporter subtype

Autotransporters are a superfamily of virulence factors secreted by Gram negative bacteria. They are comprised of an N-terminal passenger domain that is translocated across the outer membrane and a C-terminal domain that inserts into the outer membrane forming a β-barrel anchor. It is still poorly understood how the passenger is efficiently translocated in the absence of external energy inputs. Several mechanisms have been proposed in solution of this problem, yet due to the vast diversity of size, sequence and function of the passenger, it is not clear how widely these mechanisms are employed. In this study we functionally characterize a conserved repeat found in many passengers that we designate the Passenger-associated Transport Repeat (PATR). Using the autotransporter IcsA from the enteropathogen Shigella flexneri, we identified conserved PATR residues that are required for efficient export of the passenger during growth and infection. Furthermore, PATR-containing autotransporters are significantly larger than non-PATR autotransporters, with PATR copy number correlating with passenger size. We also show that PATR-containing autotransporters delineate a subgroup that associates with specific virulence traits and architectures. These results advance our understanding of autotransporter composition and indicate that an additional transport mechanism is important for thousands of these proteins.

Giving structure to the biofilm matrix: an overview of individual strategies and emerging common themes

Biofilms are communities of microbial cells that underpin diverse processes including sewage bioremediation, plant growth promotion, chronic infections and industrial biofouling. The cells resident in the biofilm are encased within a self-produced exopolymeric matrix that commonly comprises lipids, proteins that frequently exhibit amyloid-like properties, eDNA and exopolysaccharides. This matrix fulfils a variety of functions for the community, from providing structural rigidity and protection from the external environment to controlling gene regulation and nutrient adsorption. Critical to the development of novel strategies to control biofilm infections, or the capability to capitalize on the power of biofilm formation for industrial and biotechnological uses, is an in-depth knowledge of the biofilm matrix. This is with respect to the structure of the individual components, the nature of the interactions between the molecules and the three-dimensional spatial organization. We highlight recent advances in the understanding of the structural and functional role that carbohydrates and proteins play within the biofilm matrix to provide three-dimensional architectural integrity and functionality to the biofilm community. We highlight, where relevant, experimental techniques that are allowing the boundaries of our understanding of the biofilm matrix to be extended using Escherichia coli, Staphylococcus aureus, Vibrio cholerae, and Bacillus subtilis as exemplars.

Examining the structure and function of the biofilm extracellular matrix.

Fatal attraction: how bacterial adhesins affect host signaling and what we can learn from them

The ability of bacterial species to colonize and infect host organisms is critically dependent upon their capacity to adhere to cellular surfaces of the host. Adherence to cell surfaces is known to be essential for the activation and delivery of certain virulence factors, but can also directly affect host cell signaling to aid bacterial spread and survival. In this review we will discuss the recent advances in the field of bacterial adhesion, how we are beginning to unravel the effects adhesins have on host cell signaling, and how these changes aid the bacteria in terms of their survival and evasion of immune responses. Finally, we will highlight how the exploitation of bacterial adhesins may provide new therapeutic avenues for the treatment of a wide range of bacterial infections.

Of linkers and autochaperones: an unambiguous nomenclature to identify common and uncommon themes for autotransporter secretion

Autotransporter (AT) proteins provide a diverse array of important virulence functions to Gram-negative bacterial pathogens, and have also been adapted for protein surface display applications. The 'autotransporter' moniker refers to early models that depicted these proteins facilitating their own translocation across the bacterial outer membrane. Although translocation is less autonomous than originally proposed, AT protein segments upstream of the C-terminal transmembrane β-barrel have nevertheless consistently been found to contribute to efficient translocation and/or folding of the N-terminal virulence region (the 'passenger'). However, defining the precise secretion functions of these AT regions has been complicated by the use of multiple overlapping and ambiguous terms to define AT sequence, structural, and functional features, including 'autochaperone', 'linker' and 'junction'. Moreover, the precise definitions and boundaries of these features vary among ATs and even among research groups, leading to an overall murky picture of the contributions of specific features to translocation. Here we propose a unified, unambiguous nomenclature for AT structural, functional and conserved sequence features, based on explicit criteria. Applied to 16 well-studied AT proteins, this nomenclature reveals new commonalities for translocation but also highlights that the autochaperone function is less closely associated with a conserved sequence element than previously believed.

The dynamics and pH-dependence of Ag43 adhesins' self-association probed by atomic force spectroscopy

Self-associating auto-transporter (SAAT) adhesins are two-domain cell surface proteins involved in bacteria auto-aggregation and biofilm formation. Antigen 43 (Ag43) is a SAAT adhesin commonly found in Escherichia coli whose variant Ag43a has been shown to promote persistence of uropathogenic E. coli within the bladder. The recent resolution of the tri-dimensional structure of the 499 amino-acids' β-domain in Ag43a has shed light on the possible mechanism governing the self-recognition of SAAT adhesins, in particular the importance of trans-interactions between the L shaped β-helical scaffold of two α-domains of neighboring adhesins. In this study, we use single-molecule force spectroscopy (SMFS) and dynamic force spectroscopy (DFS) to unravel the dynamics of Ag43-self association under various pH and molecular elongation rate conditions that mimic the situations encountered by E. coli in its natural environment. Results evidenced an important stretchability of Ag43α with unfolding of sub-domains leading to molecular extension as long as 150 nm. Nanomechanical analysis of molecular stretching data suggested that self-association of Ag43 can lead to the formation of dimers and tetramers driven by rapid and weak cis- as well as slow but strong trans-interaction forces with a magnitude as large as 100-250 pN. The dynamics of cis- and trans-interactions were demonstrated to be strongly influenced by pH and applied shear force, thus suggesting that environmental conditions can modulate Ag43-mediated aggregation of bacteria at the molecular level.

The meningococcal autotransporter AutA is implicated in autoaggregation and biofilm formation

Autotransporters (ATs) are proteins secreted by Gram-negative bacteria that often play a role in virulence. Eight different ATs have been identified in Neisseria meningitidis, but only six of them have been characterized. AutA is one of the remaining ATs. Its expression remains controversial. Here, we show that the autA gene is present in many neisserial species, but its expression is often disrupted by various genetic features; however, it is expressed in certain strains of N. meningitidis. By sequencing the autA gene in large panels of disease isolates and Western blot analysis, we demonstrated that AutA expression is prone to phase variation at AAGC nucleotide repeats located within the DNA encoding the signal sequence. AutA is not secreted into the extracellular medium, but remains associated with the bacterial cell surface. We further demonstrate that AutA expression induces autoaggregation in a process that, dependent on the particular strain, may require extracellular DNA (eDNA). This property influences the organization of bacterial communities like lattices and biofilms. In vitro assays evidenced that AutA is a self-associating AT that binds DNA. We suggest that AutA-mediated autoaggregation might be particularly important for colonization and persistence of the pathogen in the nasopharynx of the host.

Characterization of an autotransporter adhesin protein shared by Burkholderia mallei and Burkholderia pseudomallei

Background

Autotransporters form a large family of outer membrane proteins specifying diverse biological traits of Gram-negative bacteria. In this study, we report the identification and characterization of a novel autotransporter gene product of Burkholderia mallei (locus tag BMA1027 in strain ATCC 23344).

Results

Database searches identified the gene in at least seven B. mallei isolates and the encoded proteins were found to be 84% identical. Inactivation of the gene encoding the autotransporter in the genome of strain ATCC 23344 substantially reduces adherence to monolayers of HEp-2 laryngeal cells and A549 type II pneumocytes, as well as to cultures of normal human bronchial epithelium (NHBE). Consistent with these findings, expression of the autotransporter on the surface of recombinant E. coli bacteria increases adherence to these cell types by 5–7 fold. The gene specifying the autotransporter was identified in the genome of 29 B. pseudomallei isolates and disruption of the gene in strain DD503 reduced adherence to NHBE cultures by 61%. Unlike B. mallei, the mutation did not impair binding of B. pseudomallei to A549 or HEp-2 cells. Analysis of sera from mice infected via the aerosol route with B. mallei and B. pseudomallei revealed that animals inoculated with as few as 10 organisms produce antibodies against the autotransporter, therefore indicating expression in vivo.

Conclusions

Our data demonstrate that we have identified an autotransporter protein common to the pathogenic species B. mallei and B. pseudomallei which mediates adherence to respiratory epithelial cells and is expressed in vivo during the course of aerosol infection.