Heterogeneity of the group B streptococcal type VII secretion system and influence on colonization of the female genital tract.

Type VIIb secretion systems (T7SSb) in Gram-positive bacteria facilitate physiology, interbacterial competition, and/or virulence via EssC ATPase-driven secretion of small ɑ-helical proteins and toxins. Recently, we characterized T7SSb in group B Streptococcus (GBS), a leading cause of infection in newborns and immunocompromised adults. GBS T7SS comprises four subtypes based on variation in the C-terminus of EssC and the repertoire of downstream effectors; however, the intraspecies diversity of GBS T7SS and impact on GBS-host interactions remains unknown. Bioinformatic analysis indicates that GBS T7SS loci encode subtype-specific putative effectors, which have low interspecies and inter-subtype homology but contain similar domains/motifs and therefore may serve similar functions. We further identify orphaned GBS WXG100 proteins. Functionally, we show that GBS T7SS subtype I and III strains secrete EsxA in vitro and that in subtype I strain CJB111, esxA1 appears to be differentially transcribed from the T7SS operon. Furthermore, we observe subtype-specific effects of GBS T7SS on host colonization, as CJB111 subtype I but not CNCTC 10/84 subtype III T7SS promotes GBS vaginal colonization. Finally, we observe that T7SS subtypes I and II are the predominant subtypes in clinical GBS isolates. This study highlights the potential impact of T7SS heterogeneity on host-GBS interactions.

Modification of cell wall polysaccharide guides cell division in Streptococcus mutans.

In ovoid-shaped, Gram-positive bacteria, MapZ guides FtsZ-ring positioning at cell equators. The cell wall of the ovococcus Streptococcus mutans contains peptidoglycan decorated with serotype c carbohydrates (SCCs). In the present study, we identify the major cell separation autolysin AtlA as an SCC-binding protein. AtlA binding to SCC is attenuated by the glycerol phosphate (GroP) modification. Using fluorescently labeled AtlA constructs, we mapped SCC distribution on the streptococcal surface, revealing enrichment of GroP-deficient immature SCCs at the cell poles and equators. The immature SCCs co-localize with MapZ at the equatorial rings throughout the cell cycle. In GroP-deficient mutants, AtlA is mislocalized, resulting in dysregulated cellular autolysis. These mutants display morphological abnormalities associated with MapZ mislocalization, leading to FtsZ-ring misplacement. Altogether, our data support a model in which maturation of a cell wall polysaccharide provides the molecular cues for the recruitment of cell division machinery, ensuring proper daughter cell separation and FtsZ-ring positioning.

Molecular basis for recognition of the Group A Carbohydrate backbone by the PlyC streptococcal bacteriophage endolysin.

Endolysins are peptidoglycan (PG) hydrolases that function as part of the bacteriophage (phage) lytic system to release progeny phage at the end of a replication cycle. Notably, endolysins alone can produce lysis without phage infection, which offers an attractive alternative to traditional antibiotics. Endolysins from phage that infect Gram-positive bacterial hosts contain at least one enzymatically active domain (EAD) responsible for hydrolysis of PG bonds and a cell wall binding domain (CBD) that binds a cell wall epitope, such as a surface carbohydrate, providing some degree of specificity for the endolysin. Whilst the EADs typically cluster into conserved mechanistic classes with well-defined active sites, relatively little is known about the nature of the CBDs and only a few binding epitopes for CBDs have been elucidated. The major cell wall components of many streptococci are the polysaccharides that contain the polyrhamnose (pRha) backbone modified with species-specific and serotype-specific glycosyl side chains. In this report, using molecular genetics, microscopy, flow cytometry and lytic activity assays, we demonstrate the interaction of PlyCB, the CBD subunit of the streptococcal PlyC endolysin, with the pRha backbone of the cell wall polysaccharides, Group A Carbohydrate (GAC) and serotype c-specific carbohydrate (SCC) expressed by the Group A Streptococcus and Streptococcus mutans, respectively.

PE5-PPE4-EspG3 heterotrimer structure from mycobacterial ESX-3 secretion system gives insight into cognate substrate recognition by ESX systems.

Mycobacterium tuberculosis has evolved numerous type VII secretion (ESX) systems to secrete multiple factors important for both growth and virulence across their cell envelope. ESX-1, ESX-3, and ESX-5 systems have been shown to each secrete a distinct set of substrates, including PE and PPE families of proteins, named for conserved Pro-Glu and Pro-Pro-Glu motifs in their N termini. Proper secretion of the PE-PPE proteins requires the presence of EspG, with each system encoding its own unique copy. There is no cross-talk between any of the ESX systems, and how each EspG recognizes its subset of PE-PPE proteins is currently unknown. The only current structural characterization of PE-PPE-EspG heterotrimers is from the ESX-5 system. Here we present the crystal structure of the PE5mt-PPE4mt-EspG3mm heterotrimer from the ESX-3 system. Our heterotrimer reveals that EspG3mm interacts exclusively with PPE4mt in a similar manner to EspG5, shielding the hydrophobic tip of PPE4mt from solvent. The C-terminal helical domain of EspG3mm is dynamic, alternating between "open" and "closed" forms, and this movement is likely functionally relevant in the unloading of PE-PPE heterodimers at the secretion machinery. In contrast to the previously solved ESX-5 heterotrimers, the PE-PPE heterodimer of our ESX-3 heterotrimer is interacting with its chaperone at a drastically different angle and presents different faces of the PPE protein to the chaperone. We conclude that the PPE-EspG interface from each ESX system has a unique shape complementarity that allows each EspG to discriminate among noncognate PE-PPE pairs.

Discovery of glycerol phosphate modification on streptococcal rhamnose polysaccharides.

Cell wall glycopolymers on the surface of Gram-positive bacteria are fundamental to bacterial physiology and infection biology. Here we identify gacH, a gene in the Streptococcus pyogenes group A carbohydrate (GAC) biosynthetic cluster, in two independent transposon library screens for its ability to confer resistance to zinc and susceptibility to the bactericidal enzyme human group IIA-secreted phospholipase A2. Subsequent structural and phylogenetic analysis of the GacH extracellular domain revealed that GacH represents an alternative class of glycerol phosphate transferase. We detected the presence of glycerol phosphate in the GAC, as well as the serotype c carbohydrate from Streptococcus mutans, which depended on the presence of the respective gacH homologs. Finally, nuclear magnetic resonance analysis of GAC confirmed that glycerol phosphate is attached to approximately 25% of the GAC N-acetylglucosamine side-chains at the C6 hydroxyl group. This previously unrecognized structural modification impacts host-pathogen interaction and has implications for vaccine design.

The molecular mechanism of N-acetylglucosamine side-chain attachment to the Lancefield group A carbohydrate in Streptococcus pyogenes.

In many Lactobacillales species (i.e. lactic acid bacteria), peptidoglycan is decorated by polyrhamnose polysaccharides that are critical for cell envelope integrity and cell shape and also represent key antigenic determinants. Despite the biological importance of these polysaccharides, their biosynthetic pathways have received limited attention. The important human pathogen, Streptococcus pyogenes, synthesizes a key antigenic surface polymer, the Lancefield group A carbohydrate (GAC). GAC is covalently attached to peptidoglycan and consists of a polyrhamnose polymer, with N-acetylglucosamine (GlcNAc) side chains, which is an essential virulence determinant. The molecular details of the mechanism of polyrhamnose modification with GlcNAc are currently unknown. In this report, using molecular genetics, analytical chemistry, and mass spectrometry analysis, we demonstrated that GAC biosynthesis requires two distinct undecaprenol-linked GlcNAc-lipid intermediates: GlcNAc-pyrophosphoryl-undecaprenol (GlcNAc-P-P-Und) produced by the GlcNAc-phosphate transferase GacO and GlcNAc-phosphate-undecaprenol (GlcNAc-P-Und) produced by the glycosyltransferase GacI. Further investigations revealed that the GAC polyrhamnose backbone is assembled on GlcNAc-P-P-Und. Our results also suggested that a GT-C glycosyltransferase, GacL, transfers GlcNAc from GlcNAc-P-Und to polyrhamnose. Moreover, GacJ, a small membrane-associated protein, formed a complex with GacI and significantly stimulated its catalytic activity. Of note, we observed that GacI homologs perform a similar function in Streptococcus agalactiae and Enterococcus faecalis In conclusion, the elucidation of GAC biosynthesis in S. pyogenes reported here enhances our understanding of how other Gram-positive bacteria produce essential components of their cell wall.

Structure of EspB, a secreted substrate of the ESX-1 secretion system of Mycobacterium tuberculosis.

Mycobacterium tuberculosis secretes multiple virulence factors during infection via the general Sec and Tat pathways, and via specialized ESX secretion systems, also referred to as type VII secretion systems. The ESX-1 secretion system is an important virulence determinant because deletion of ESX-1 leads to attenuation of M. tuberculosis. ESX-1 secreted protein B (EspB) contains putative PE (Pro-Glu) and PPE (Pro-Pro-Glu) domains, and a C-terminal domain, which is processed by MycP1 protease during secretion. We determined the crystal structure of PE-PPE domains of EspB, which represents an all-helical, elongated molecule closely resembling the structure of the PE25-PPE41 heterodimer despite limited sequence similarity. Also, we determined the structure of full-length EspB, which does not have interpretable electron density for the C-terminal domain confirming that it is largely disordered. Comparative analysis of EspB in cell lysate and culture filtrates of M. tuberculosis revealed that mature secreted EspB forms oligomers. Electron microscopy analysis showed that the N-terminal fragment of EspB forms donut-shaped particles. These data provide a rationale for the future investigation of EspB's role in M. tuberculosis pathogenesis.

Structure of the Mycobacterium tuberculosis type VII secretion system chaperone EspG5 in complex with PE25-PPE41 dimer.

The growth or virulence of Mycobacterium tuberculosis bacilli depends on homologous type VII secretion systems, ESX-1, ESX-3 and ESX-5, which export a number of protein effectors across membranes to the bacterial surface and environment. PE and PPE proteins represent two large families of highly polymorphic proteins that are secreted by these ESX systems. Recently, it was shown that these proteins require system-specific cytoplasmic chaperones for secretion. Here, we report the crystal structure of M. tuberculosis ESX-5-secreted PE25-PPE41 heterodimer in complex with the cytoplasmic chaperone EspG(5). EspG(5) represents a novel fold that is unrelated to previously characterized secretion chaperones. Functional analysis of the EspG(5) -binding region uncovered a hydrophobic patch on PPE41 that promotes dimer aggregation, and the chaperone effectively abolishes this process. We show that PPE41 contains a characteristic chaperone-binding sequence, the hh motif, which is highly conserved among ESX-1-, ESX-3- and ESX-5-specific PPE proteins. Disrupting the interaction between EspG(5) and three different PPE target proteins by introducing different point mutations generally affected protein secretion. We further demonstrate that the EspG(5) chaperone plays an important role in the ESX secretion mechanism by keeping aggregation-prone PE-PPE proteins in their soluble state.

Transcription of the Streptococcus pyogenes hyaluronic acid capsule biosynthesis operon is regulated by previously unknown upstream elements.

The important human pathogen Streptococcus pyogenes (group A Streptococcus [GAS]) produces a hyaluronic acid (HA) capsule that plays critical roles in immune evasion. Previous studies showed that the hasABC operon encoding the capsule biosynthesis enzymes is under the control of a single promoter, P1, which is negatively regulated by the two-component regulatory system CovR/S. In this work, we characterize the sequence upstream of P1 and identify a novel regulatory region controlling transcription of the capsule biosynthesis operon in the M1 serotype strain MGAS2221. This region consists of a promoter, P2, which initiates transcription of a novel small RNA, HasS, an intrinsic transcriptional terminator that inefficiently terminates HasS, permitting read-through transcription of hasABC, and a putative promoter which lies upstream of P2. Electrophoretic mobility shift assays, quantitative reverse transcription-PCR, and transcriptional reporter data identified CovR as a negative regulator of P2. We found that the P1 and P2 promoters are completely repressed by CovR, and capsule expression is regulated by the putative promoter upstream of P2. Deletion of hasS or of the terminator eliminates CovR-binding sequences, relieving repression and increasing read-through, hasA transcription, and capsule production. Sequence analysis of 44 GAS genomes revealed a high level of polymorphism in the HasS sequence region. Most of the HasS variations were located in the terminator sequences, suggesting that this region is under strong selective pressure. We discovered that the terminator deletion mutant is highly resistant to neutrophil-mediated killing and is significantly more virulent in a mouse model of GAS invasive disease than the wild-type strain. Together, these results are consistent with the naturally occurring mutations in this region modulating GAS virulence.

Structure and mechanism of biosynthesis of Streptococcus mutans cell wall polysaccharide.

Streptococcus mutans, the causative agent of human dental caries, expresses a cell wall attached Serotype c- specific Carbohydrate (SCC) that is critical for cell viability. SCC consists of a repeating →3)α-Rha(1→2)α-Rha(1→ polyrhamnose backbone, with glucose (Glc) side-chains and glycerol phosphate (GroP) decorations. This study reveals that SCC has one major and two minor Glc modifications. The major Glc modification, α-Glc, attached to position 2 of 3-rhamnose, is installed by SccN and SccM glycosyltransferases and is the site of the GroP addition. The minor Glc modifications are ß-Glc linked to position 4 of 3-rhamnose installed by SccP and SccQ glycosyltransferases, and α-Glc attached to position 4 of 2-rhamnose installed by SccN working in tandem with an unknown enzyme. Both the major and the minor ß-Glc modifications control bacterial morphology, but only the GroP and major Glc modifications are critical for biofilm formation.

Gestational diabetes augments group B Streptococcus infection by disrupting maternal immunity and the vaginal microbiota.

Group B Streptococcus (GBS) is a pervasive perinatal pathogen, yet factors driving GBS dissemination in utero are poorly defined. Gestational diabetes mellitus (GDM), a complication marked by dysregulated immunity and maternal microbial dysbiosis, increases risk for GBS perinatal disease. Using a murine GDM model of GBS colonization and perinatal transmission, we find that GDM mice display greater GBS in utero dissemination and subsequently worse neonatal outcomes. Dual-RNA sequencing reveals differential GBS adaptation to the GDM reproductive tract, including a putative glycosyltransferase (yfhO), and altered host responses. GDM immune disruptions include reduced uterine natural killer cell activation, impaired recruitment to placentae, and altered maternofetal cytokines. Lastly, we observe distinct vaginal microbial taxa associated with GDM status and GBS invasive disease status. Here, we show a model of GBS dissemination in GDM hosts that recapitulates several clinical aspects and identifies multiple host and bacterial drivers of GBS perinatal disease.

Disorder regulates homeostasis of extracytoplasmic proteins in streptococci.

Proteins harboring intrinsically disordered regions (IDRs) that lack regular secondary or tertiary structure are abundant across three domains of life. Here, using a deep neural network (DNN)-based method we predict IDRs in the extracytoplasmic proteome of Streptococcus mutans , Streptococcus pyogenes and Streptococcus pneumoniae . We identify a subset of the serine/threonine-rich IDRs and demonstrate that they are O -glycosylated with glucose by a GtrB-like glucosyltransferase in S. pyogenes and S. pneumoniae , and N-acetylgalactosamine by a Pgf-dependent mechanism in S. mutans . Loss of glycosylation leads to a defect in biofilm formation under ethanol-stressed conditions in S. mutans . We link this phenotype to a C-terminal IDR of peptidyl-prolyl isomerase PrsA which is protected from proteolytic degradation by O -glycosylation. The IDR length attenuates the efficiency of glycosylation and expression of PrsA. Taken together, our data support a model in which extracytoplasmic IDRs function as dynamic switches of protein homeostasis in streptococci.

SpyA is a membrane-bound ADP-ribosyltransferase of Streptococcus pyogenes which modifies a streptococcal peptide, SpyB.

All sequenced genomes of Streptococcus pyogenes (Group A Streptococcus, GAS) encode a protein, SpyA, with homology to C3-like ADP-ribosyltransferase toxins. SpyA is a novel virulence factor which plays a role in pathogenesis in a mouse model of soft-tissue infection. In this study we demonstrate that SpyA is a surface-exposed membrane protein which is anchored to the streptococcal membrane by an N-terminal transmembrane sequence. We identified a small gene upstream of spyA, designated spyB, which encodes a peptide of 35 amino acids, and is co-transcribed with spyA. Expression of spyBA is strongly influenced by translational coupling: mutational inactivation of spyB translation completely abolishes translation of spyA. spyB expression increases with increasing cell density and reaches its maximum at late exponential growth phase. The SpyB N-terminus is predicted to fold into an amphipathic α-helix, a structural motif that targets a protein to the cytoplasmic membrane. Consistent with the prediction, we found that a SpyB fusion with peptide affinity tags is located in the streptococcal membrane. An ADP-ribosylation assay with recombinant SpyA demonstrated that SpyA modifies SpyB. Thus, our study suggests that ADP-ribosylation of SpyB may be an important function of SpyA.

Research opportunities for the primordial prevention of rheumatic fever and rheumatic heart disease-streptococcal vaccine development: a national heart, lung and blood institute workshop report.

Streptococcus pyogenes, also known as group A streptococcus (StrepA), is a bacterium that causes a range of human diseases, including pharyngitis, impetigo, invasive infections, and post-infection immune sequelae such as rheumatic fever and rheumatic heart disease. StrepA infections cause some of the highest burden of disease and death in mostly young populations in low-resource settings. Despite decades of effort, there is still no licensed StrepA vaccine, which if developed, could be a cost-effective way to reduce the incidence of disease. Several challenges, including technical and regulatory hurdles, safety concerns and a lack of investment have hindered StrepA vaccine development. Barriers to developing a StrepA vaccine must be overcome in the future by prioritising key areas of research including greater understanding of StrepA immunobiology and autoimmunity risk, better animal models that mimic human disease, expanding the StrepA vaccine pipeline and supporting vaccine clinical trials. The development of a StrepA vaccine is a complex and challenging process that requires significant resources and investment. Given the global burden of StrepA infections and the potential for a vaccine to save lives and livelihoods, StrepA vaccine development is an area of research that deserves considerable support. This report summarises the findings of the Primordial Prevention Working Group-VAX, which was convened in November 2021 by the National Heart, Lung, and Blood Institute. The focus of this report is to identify research gaps within the current StrepA vaccine landscape and find opportunities and develop priorities to promote the rapid and successful advancement of StrepA vaccines.

Heterogeneity of the group B streptococcal type VII secretion system and influence on colonization of the female genital tract.

Type VIIb secretion systems (T7SSb) in Gram-positive bacteria facilitate physiology, interbacterial competition, and/or virulence via EssC ATPase-driven secretion of small ɑ-helical proteins and toxins. Recently, we characterized T7SSb in group B Streptococcus (GBS), a leading cause of infection in newborns and immunocompromised adults. GBS T7SS comprises four subtypes based on variation in the C-terminus of EssC and the repertoire of downstream effectors; however, the intra-species diversity of GBS T7SS and impact on GBS-host interactions remains unknown. Bioinformatic analysis indicates that GBS T7SS loci encode subtype-specific putative effectors, which have low inter-species and inter-subtype homology but contain similar domains/motifs and therefore may serve similar functions. We further identify orphaned GBS WXG100 proteins. Functionally, we show that GBS T7SS subtype I and III strains secrete EsxA in vitro and that in subtype I strain CJB111, esxA1 appears to be differentially transcribed from the T7SS operon. Further, we observe subtype-specific effects of GBS T7SS on host colonization, as subtype I but not subtype III T7SS promotes GBS vaginal persistence. Finally, we observe that T7SS subtypes I and II are the predominant subtypes in clinical GBS isolates. This study highlights the potential impact of T7SS heterogeneity on host-GBS interactions.

Apical expression of human full-length hCEACAM1-4L protein renders the Madin Darby Canine Kidney cells responsive to lipopolysaccharide leading to TLR4-dependent Erk1/2 and p38 MAPK signalling.

CEACAM1 expressed by granulocytes and epithelial cells is recognized as a membrane-associated receptor by some Gram-negative pathogens. Here we report a previously unsuspected role of human CEACAM1-4L (hCEACAM1-4L) in polarized epithelial cells. We find that in contrast with non-transfected cells, Madin Darby Canine Kidney strain II (MDCK) engineered for the apical expression of the long cytoplasmic chain protein hCEACAM1-4L showed a serum-independent increase in the phosphorylation of the extracellular signal-regulated kinase 1/2 (Erk1/2) and p38 mitogen-activated protein kinases (MAPKs) after treatment with lipopolysaccharide (LPS) of wild-type, diffusely adhering Afa/Dr Escherichia coli (Afa/Dr DAEC) strain IH11128. Aggregates of FITC-LPS bind the apical domain of MDCK-hCEACAM1-4L cells colocalizing with the apically expressed hCEACAM1-4L protein and do not bind MDCK-pCEP cells, and surface plasmon resonance analysis shows that LPS binds to the extracellular domain of the CEACAM1-4L protein. We showed that cell polarization and lipid rafts positively control the LPS-IH11128-induced phosphorylation of Erk1/2 in MDCK-hCEACAM1-4L cells. Structure-function analysis using mutated hCEACAM1-4L protein shows that the cytoplasmic domain of the protein is needed for LPS-induced MAPK signalling, and that phosphorylation of Tyr-residues is not increased in association with MAPK signalling. The hCEACAM1-4L-dependent Erk1/2 phosphorylation develops in the presence of lipid A and does not develop in the presence of penta-acylated LPS. Finally, small interfering RNA (siRNA) silencing of canine TLR4 abolishes the hCEACAM1-4L-dependent, LPS-induced phosphorylation of Erk1/2. Collectively, our results support the notion that the apically expressed, full-length hCEACAM1-4L protein functions as a novel LPS-conveying molecule at the mucosal surface of polarized epithelial cells for subsequent MD-2/TLR4 receptor-dependent MAPK Erk1/2 and p38 signalling.

Streptococcus pyogenes can support or inhibit growth of Haemophilus influenzae by supplying or restricting extracellular NAD.

Nicotinamide adenine dinucleotide (NAD+) is an essential co-factor for cellular metabolism and serves as a substrate in enzymatic processes. NAD+ is produced by de novo synthesis or salvage pathways in nearly all bacterial species. Haemophilus influenzae lacks the capacity for de novo synthesis, so it is dependent on import of NAD+ from the external environment or salvage biosynthetic pathways for recycling of NAD+ precursors and breakdown products. However, the actual sources of NAD+ utilized by H. influenzae in the respiratory tract are not well defined. In this study, we found that a variety of bacteria, including species found in the upper airway of humans, released NAD+ that was readily detectable in extracellular culture fluid, and which supported growth of H. influenzae in vitro. By contrast, certain strains of Streptococcus pyogenes (group A streptococcus or GAS) inhibited growth of H. influenzae in vitro by secreting NAD+-glycohydrolase (NADase), which degraded extracellular NAD+. Conversely, GAS strains that lacked enzymatically active NADase released extracellular NAD+, which could support H. influenzae growth. Our results suggest that many bacterial species, including normal flora of the upper airway, release NAD+ into the environment. GAS is distinctive in its ability to both release and degrade NAD+. Thus, colonization of the airway with H. influenzae may be promoted or restricted by co-colonization with GAS in a strain-specific manner that depends, respectively, on release of NAD+ or secretion of active NADase. We suggest that, in addition to its role as a cytotoxin for host cells, NADase may serve a separate function by restricting growth of H. influenzae in the human respiratory tract.

PplD is a de-N-acetylase of the cell wall linkage unit of streptococcal rhamnopolysaccharides.

The cell wall of the human bacterial pathogen Group A Streptococcus (GAS) consists of peptidoglycan decorated with the Lancefield group A carbohydrate (GAC). GAC is a promising target for the development of GAS vaccines. In this study, employing chemical, compositional, and NMR methods, we show that GAC is attached to peptidoglycan via glucosamine 1-phosphate. This structural feature makes the GAC-peptidoglycan linkage highly sensitive to cleavage by nitrous acid and resistant to mild acid conditions. Using this characteristic of the GAS cell wall, we identify PplD as a protein required for deacetylation of linkage N-acetylglucosamine (GlcNAc). X-ray structural analysis indicates that PplD performs catalysis via a modified acid/base mechanism. Genetic surveys in silico together with functional analysis indicate that PplD homologs deacetylate the polysaccharide linkage in many streptococcal species. We further demonstrate that introduction of positive charges to the cell wall by GlcNAc deacetylation protects GAS against host cationic antimicrobial proteins.

Type 1 fimbrial adhesin FimH elicits an immune response that enhances cell adhesion of Escherichia coli.

Escherichia coli causes about 90% of urinary tract infections (UTI), and more than 95% of all UTI-causing E. coli express type 1 fimbriae. The fimbrial tip-positioned adhesive protein FimH utilizes a shear force-enhanced, so-called catch-bond mechanism of interaction with its receptor, mannose, where the lectin domain of FimH shifts from a low- to a high-affinity conformation upon separation from the anchoring pilin domain. Here, we show that immunization with the lectin domain induces antibodies that exclusively or predominantly recognize only the high-affinity conformation. In the lectin domain, we identified four high-affinity-specific epitopes, all positioned away from the mannose-binding pocket, which are recognized by 20 separate clones of monoclonal antibody. None of the monoclonal or polyclonal antibodies against the lectin domain inhibited the adhesive function. On the contrary, the antibodies enhanced FimH-mediated binding to mannosylated ligands and increased by severalfold bacterial adhesion to urothelial cells. Furthermore, by natural conversion from the high- to the low-affinity state, FimH adhesin was able to shed the antibodies bound to it. When whole fimbriae were used, the antifimbrial immune serum that contained a significant amount of antibodies against the lectin domain of FimH was also able to enhance FimH-mediated binding. Thus, bacterial adhesins (or other surface antigens) with the ability to switch between alternative conformations have the potential to induce a conformation-specific immune response that has a function-enhancing rather than -inhibiting impact on the protein. These observations have implications for the development of adhesin-specific vaccines and may serve as a paradigm for antibody-mediated enhancement of pathogen binding.

Escherichia coli DraE adhesin-associated bacterial internalization by epithelial cells is promoted independently by decay-accelerating factor and carcinoembryonic antigen-related cell adhesion molecule binding and does not require the DraD invasin.

The Dr family of Escherichia coli adhesins are virulence factors associated with diarrhea and urinary tract infections. Dr fimbriae are comprised of two subunits. DraE/AfaE represents the major structural, antigenic, and adhesive subunit, which recognizes decay-accelerating factor (DAF) and carcinoembryonic antigen (CEA)-related cell adhesion molecules (CEACAMs) CEA, CEACAM1, CEACAM3, and CEACAM6 as binding receptors. The DraD/AfaD subunit caps fimbriae and has been implicated in the entry of Dr-fimbriated E. coli into host cells. In this study, we demonstrate that DAF or CEACAM receptors independently promote DraE-mediated internalization of E. coli by CHO cell transfectants expressing these receptors. We also found that DraE-positive recombinant bacteria adhere to and are internalized by primary human bladder epithelial cells which express DAF and CEACAMs. DraE-mediated bacterial internalization by bladder cells was inhibited by agents which disrupt lipid rafts, microtubules, and phosphatidylinositol 3-kinase (PI3K) activity. Immunofluorescence confocal microscopic examination of epithelial cells detected considerable recruitment of caveolin, beta(1) integrin, phosphorylated ezrin, phosphorylated PI3K, and tubulin, but not F-actin, by cell-associated bacteria. Finally, we demonstrate that the DraD subunit, previously implicated as an "invasin," is not required for beta(1) integrin recruitment or bacterial internalization.