Mechanism of association of adenylate cyclase toxin with the surface of Bordetella pertussis: a role for toxin-filamentous haemagglutinin interaction

Bordetella filamentous hemagglutinin and adenylate cyclase toxin interactions on the bacterial surface are consistent with FhaB-mediated delivery of ACT to phagocytic cells

Bordetella species that cause respiratory infections in mammals include B. pertussis, which causes human whooping cough, and B. bronchiseptica, which infects nearly all mammals. Both bacterial species produce filamentous hemagglutinin (FhaB) and adenylate cyclase toxin (ACT), prominent surface-associated and secreted virulence factors that contribute to persistence in the lower respiratory tract by inhibiting clearance by phagocytic cells. FhaB and ACT proteins interact with themselves, each other, and host cells. Using immunoblot analyses, we showed that ACT binds to FhaB on the bacterial surface before it can be detected in culture supernatants. We determined that SphB1, a surface protease identified based on its requirement for FhaB cleavage, is also required for ACT cleavage, and we determined that the presence of ACT blocks SphB1-dependent and -independent cleavage of FhaB, but the presence of FhaB does not affect SphB1-dependent cleavage of ACT. The primary SphB1-dependent cleavage site on ACT is proximal to ACT's active site, in a region that is critical for ACT activity. We also determined that FhaB-bound ACT on the bacterial surface can intoxicate host cells producing CR3, the receptor for ACT. In addition to increasing our understanding of FhaB, ACT, and FhaB-ACT interactions on the Bordetella surface, our data are consistent with a model in which FhaB functions as a novel toxin delivery system by binding to ACT and allowing its release upon binding of ACT to its receptor, CR3, on phagocytic cells.IMPORTANCEBacteria need to control the variety, abundance, and conformation of proteins on their surface to survive. Members of the Gram-negative bacterial genus Bordetella include B. pertussis, which causes whooping cough in humans, and B. bronchiseptica, which causes respiratory infections in a broad range of mammals. These species produce two prominent virulence factors, the two-partner secretion (TPS) effector FhaB and adenylate cyclase toxin (ACT), that interact with themselves, each other, and host cells. Here, we determined that ACT binds FhaB on the bacterial surface before being detected in culture supernatants and that ACT bound to FhaB can be delivered to eukaryotic cells. Our data are consistent with a model in which FhaB delivers ACT specifically to phagocytic cells. This is the first report of a TPS system facilitating the delivery of a separate polypeptide toxin to target cells and expands our understanding of how TPS systems contribute to bacterial pathogenesis.

Comparison of Two Different Methods for the Extraction of Outer Membrane Vesicles from the Bordetella pertussis as a Vaccine Candidate

Despite the availability of a vaccine, pertussis is still a worldwide health problem. Outer membrane vesicles (OMVs) in gram-negative bacteria can stimulate the immune system due to several outer membrane proteins and are very good candidates in vaccine development. OMVs obtained from Bordetella pertussis contain several antigens, which are considered immunogenic, and could make them a potential candidate for vaccine production. The current study aimed to compare the current OMV extraction method (with ultracentrifuge) and a modified extraction method (without ultracentrifuge) and to evaluate the physicochemical properties as well as the expression of their main virulence factors. Vaccinal strain BP134 grown on Bordet Gengo agar were inoculated in Modified Stainer-Scholte medium for mass cultivation. OMVs were prepared using two different methods. They were then stained and examined with a transmission electron microscope. Protein contents were measured by the Bradford method, and then the protein profile was evaluated by SDS-PAGE. The presence of immunogenic antigens was detected by Western blotting. The size and shape of the OMVs obtained from the modified method without the use of ultracentrifuge were similar to the current method and had a size between 40 and 200 nm. The total protein yields of the OMV isolated using the current and modified methods were 800 and 600 µg/ml, respectively. Evaluating the protein profile of extracted OMVs showed the presence of different proteins. Finally, the presence of PTX, PRN, and FHA was observed in OMVs extracted from both methods. Comparison of the two OMV extraction methods showed that the obtained vesicles have a suitable and similar shape and size as well as the expression of three important pathogenic factors as immunogens. Despite the relatively low reduction in protein yield as the modified method does not require ultracentrifuge, this extraction method can be used as a suitable alternative for extracting the outer membrane vesicles from B. pertussis, especially in developing countries. It should be noted that further experiments including immunogenicity determination of OMVs obtained as vaccine candidates in animal models are required.

DegP Initiates Regulated Processing of Filamentous Hemagglutinin in Bordetella bronchiseptica

Filamentous hemagglutinin (FhaB) is a critical virulence factor for both Bordetella pertussis, the causal agent of whooping cough, and the closely related species Bordetella bronchiseptica. FhaB is an adhesin, suppresses inflammatory cytokine production, and protects against phagocytic cell clearance during infection. Regulated degradation of the FhaB C-terminal prodomain is required to establish a persistent infection in mice. Two proteases, CtpA in the periplasm and SphB1 on the bacterial surface, are known to mediate FhaB processing, and we recently determined that CtpA functions before, and controls the FhaB cleavage site of, SphB1. However, the data indicate that another periplasmic protease must initiate degradation of the prodomain by removing a portion of the FhaB C terminus that inhibits CtpA-mediated degradation. Using a candidate approach, we identified DegP as the initiating protease. Deletion of degP or substitution of its predicted catalytic residue resulted in reduced creation of FHA′ (the main product of FhaB processing) and an accumulation of full-length FhaB in whole-cell lysates. Also, FHA′ was no longer released into culture supernatants in degP mutants. Alterations of the FhaB C terminus that relieve inhibition of CtpA abrogate the need for DegP, consistent with DegP functioning prior to CtpA in the processing pathway. DegP is not required for secretion of FhaB through FhaC or for adherence of the bacteria to host cells, indicating that DegP acts primarily as a protease and not a chaperone for FhaB in B. bronchiseptica. Our results highlight a role for HtrA family proteases in activation of virulence factors in pathogenic bacteria.

Membrane Activity and Channel Formation of the Adenylate Cyclase Toxin (CyaA) of Bordetella pertussis in Lipid Bilayer Membranes

The Gram-negative bacterium Bordetella pertussis is the cause of whooping cough. One of its pathogenicity factors is the adenylate cyclase toxin (CyaA) secreted by a Type I export system. The 1706 amino acid long CyaA (177 kDa) belongs to the continuously increasing family of repeat in toxin (RTX) toxins because it contains in its C-terminal half a high number of nine-residue tandem repeats. The protein exhibits cytotoxic and hemolytic activities that target primarily myeloid phagocytic cells expressing the αMβ2 integrin receptor (CD11b/CD18). CyaA represents an exception among RTX cytolysins because the first 400 amino acids from its N-terminal end possess a calmodulin-activated adenylate cyclase (AC) activity. The entry of the AC into target cells is not dependent on the receptor-mediated endocytosis pathway and penetrates directly across the cytoplasmic membrane of a variety of epithelial and immune effector cells. The hemolytic activity of CyaA is rather low, which may have to do with its rather low induced permeability change of target cells and its low conductance in lipid bilayer membranes. CyaA forms highly cation-selective channels in lipid bilayers that show a strong dependence on aqueous pH. The pore-forming activity of CyaA but not its single channel conductance is highly dependent on Ca²⁺ concentration with a half saturation constant of about 2 to 4 mM.

Exofacial phospholipids at the plasma membrane: ill-defined targets for early infection processes

The eukaryotic plasma membrane (PM) consists largely of phospholipids and proteins, and separates the intracellular compartments from the extracellular space. It also serves as a signaling platform for cell-to-cell communication and an interaction platform for the molecular crosstalk between pathogens and their target cells. Much research has been done to elucidate the interactions between pathogens and host membrane proteins. However, little is known about the interactions between pathogens and membrane phospholipids, although reports have described a contribution of phospholipids to cell recognition and/or invasion during early infection by diverse pathogens. Thus, during adhesion to the host cell, the obligate intracellular bacterial pathogens Chlamydia spp., the facultative intracellular pathogen Helicobacter pylori and the facultative aerobic pathogen Vibrio parahaemolyticus, interact with exofacial phospholipids. This review focuses on several prominent instances of pathogen interaction with host-cell phospholipids.

Regulated, sequential processing by multiple proteases is required for proper maturation and release of Bordetella filamentous hemagglutinin

Filamentous hemagglutinin (FHA) is a critically important virulence factor produced by Bordetella species that cause respiratory infections in humans and other animals. It is also a prototypical member of the widespread two partner secretion (TPS) pathway family of proteins. First synthesized as a ~370 kDa protein called FhaB, its C-terminal ~1,200 amino acid 'prodomain' is removed during translocation to the cell surface via the outer membrane channel FhaC. Here, we identify CtpA as a periplasmic protease that is responsible for the regulated degradation of the prodomain and for creation of an intermediate polypeptide that is cleaved by the autotransporter protease SphB1 to generate FHA. We show that the central prodomain region is required to initiate degradation of the prodomain and that CtpA degrades the prodomain after a third, unidentified protease (P3) first removes the extreme C-terminus of the prodomain. Stepwise proteolysis by P3, CtpA and SphB1 is required for maturation of FhaB, release of FHA into the extracellular milieu, and full function in vivo. These data support a substantially updated model for the mechanism of secretion, maturation and function of this model TPS protein.

Role of Major Toxin Virulence Factors in Pertussis Infection and Disease Pathogenesis

Bordetella pertussis produces several toxins that affect host-pathogen interactions. Of these, the major toxins that contribute to pertussis infection and disease are pertussis toxin, adenylate cyclase toxin-hemolysin and tracheal cytotoxin. Pertussis toxin is a multi-subunit protein toxin that inhibits host G protein-coupled receptor signaling, causing a wide array of effects on the host. Adenylate cyclase toxin-hemolysin is a single polypeptide, containing an adenylate cyclase enzymatic domain coupled to a hemolysin domain, that primarily targets phagocytic cells to inhibit their antibacterial activities. Tracheal cytotoxin is a fragment of peptidoglycan released by B. pertussis that elicits damaging inflammatory responses in host cells. This chapter describes these three virulence factors of B. pertussis, summarizing background information and focusing on the role of each toxin in infection and disease pathogenesis, as well as their role in pertussis vaccination.

Bordetella pertussis and Bordetella bronchiseptica filamentous hemagglutinins are processed at different sites

Filamentous hemagglutinin (FHA) mediates adherence and plays an important role in lower respiratory tract infections by pathogenic Bordetellae. The mature FHA proteins of B. pertussis (Bp‐FHA) and the B. bronchiseptica (Bb‐FHA) are generated by processing of the respective FhaB precursors by the autotransporter subtilisin‐type protease SphB1. We have used bottom‐up proteomics with differential ¹⁶O/¹⁸O labeling and show that despite high‐sequence conservation of the corresponding FhaB segments, the mature Bp‐FHA (~ 230 kDa) and Bb‐FHA (~ 243 kDa) proteins are processed at different sites of FhaB, after the Ala‐2348 and Lys‐2479 residues, respectively. Moreover, protease surface accessibility probing by on‐column (on‐line) digestion of the Bp‐FHA and Bb‐FHA proteins yielded different peptide patterns, revealing structural differences in the N‐terminal and C‐terminal domains of the Bp‐FHA and Bb‐FHA proteins. These data indicate specific structural variations between the highly homologous FHA proteins.

Characterization of Post-Translational Modifications and Cytotoxic Properties of the Adenylate-Cyclase Hemolysin Produced by Various Bordetella pertussis and Bordetella parapertussis Isolates

Bordetella pertussis and Bordetella parapertussis are the causal agents of whooping cough in humans. They produce diverse virulence factors, including adenylate cyclase-hemolysin (AC-Hly), a secreted toxin of the repeat in toxins (RTX) family with cyclase, pore-forming, and hemolytic activities. Post-translational modifications (PTMs) are essential for the biological activities of the toxin produced by B. pertussis. In this study, we compared AC-Hly toxins from various clinical isolates of B. pertussis and B. parapertussis, focusing on (i) the genomic sequences of cyaA genes, (ii) the PTMs of partially purified AC-Hly, and (iii) the cytotoxic activity of the various AC-Hly toxins. The genes encoding the AC-Hly toxins of B. pertussis and B. parapertussis displayed very limited polymorphism in each species. Most of the sequence differences between the two species were found in the C-terminal part of the protein. Both toxins harbored PTMs, mostly corresponding to palmitoylations of the lysine 860 residue and palmoylations and myristoylations of lysine 983 for B. pertussis and AC-Hly and palmitoylations of lysine 894 and myristoylations of lysine 1017 for B. parapertussis AC-Hly. Purified AC-Hly from B. pertussis was cytotoxic to macrophages, whereas that from B. parapertussis was not.

Characterization of Post-Translational Modifications and Cytotoxic Properties of the Adenylate-Cyclase Hemolysin Produced by Various Bordetella pertussis and Bordetella parapertussis Isolates

Bordetella pertussis and Bordetella parapertussis are the causal agents of whooping cough in humans. They produce diverse virulence factors, including adenylate cyclase-hemolysin (AC-Hly), a secreted toxin of the repeat in toxins (RTX) family with cyclase, pore-forming, and hemolytic activities. Post-translational modifications (PTMs) are essential for the biological activities of the toxin produced by B. pertussis. In this study, we compared AC-Hly toxins from various clinical isolates of B. pertussis and B. parapertussis, focusing on (i) the genomic sequences of cyaA genes, (ii) the PTMs of partially purified AC-Hly, and (iii) the cytotoxic activity of the various AC-Hly toxins. The genes encoding the AC-Hly toxins of B. pertussis and B. parapertussis displayed very limited polymorphism in each species. Most of the sequence differences between the two species were found in the C-terminal part of the protein. Both toxins harbored PTMs, mostly corresponding to palmitoylations of the lysine 860 residue and palmoylations and myristoylations of lysine 983 for B. pertussis and AC-Hly and palmitoylations of lysine 894 and myristoylations of lysine 1017 for B. parapertussis AC-Hly. Purified AC-Hly from B. pertussis was cytotoxic to macrophages, whereas that from B. parapertussis was not.

Bordetella Adenylate Cyclase-Hemolysin Toxins

Adenylate cyclase-hemolysin toxin is secreted and produced by three classical species of the genus Bordetella: Bordetella pertussis, B. parapertussis and B. bronchiseptica. This toxin has several properties such as: (i) adenylate cyclase activity, enhanced after interaction with the eukaryotic protein, calmodulin; (ii) a pore-forming activity; (iii) an invasive activity. It plays an important role in the pathogenesis of these Bordetella species responsible for whooping cough in humans or persistent respiratory infections in mammals, by modulating host immune responses. In contrast with other Bordetella toxins or adhesins, lack of (or very low polymorphism) is observed in the structural gene encoding this toxin, supporting its importance as well as a potential role as a vaccine antigen against whooping cough. In this article, an overview of the investigations undertaken on this toxin is presented.

Albumin, in the Presence of Calcium, Elicits a Massive Increase in Extracellular Bordetella Adenylate Cyclase Toxin

Pertussis (whooping cough), caused by Bordetella pertussis, is resurging in the United States and worldwide. Adenylate cyclase toxin (ACT) is a critical factor in establishing infection with B. pertussis and acts by specifically inhibiting the response of myeloid leukocytes to the pathogen. We report here that serum components, as discovered during growth in fetal bovine serum (FBS), elicit a robust increase in the amount of ACT, and ≥90% of this ACT is localized to the supernatant, unlike growth without FBS, in which ≥90% is associated with the bacterium. We have found that albumin, in the presence of physiological concentrations of calcium, acts specifically to enhance the amount of ACT and its localization to the supernatant. Respiratory secretions, which contain albumin, promote an increase in amount and localization of active ACT that is comparable to that elicited by serum and albumin. The response to albumin is not mediated through regulation of ACT at the transcriptional level or activation of the Bvg two-component system. As further illustration of the specificity of this phenomenon, serum collected from mice that lack albumin does not stimulate an increase in ACT. These data, demonstrating that albumin and calcium act synergistically in the host environment to increase production and release of ACT, strongly suggest that this phenomenon reflects a novel host-pathogen interaction that is central to infection with B. pertussis and other Bordetella species.

Bordetella adenylate cyclase toxin interacts with filamentous haemagglutinin to inhibit biofilm formation in vitro

Bordetella pertussis, the causative agent of whooping cough, secretes and releases adenylate cyclase toxin (ACT), which is a protein bacterial toxin that targets host cells and disarms immune defenses. ACT binds filamentous haemagglutinin (FHA), a surface-displayed adhesin, and until now, the consequences of this interaction were unknown. A B. bronchiseptica mutant lacking ACT produced more biofilm than the parental strain; leading Irie et al. to propose the ACT-FHA interaction could be responsible for biofilm inhibition. Here we characterize the physical interaction of ACT with FHA and provide evidence linking that interaction to inhibition of biofilm in vitro. Exogenous ACT inhibits biofilm formation in a concentration-dependent manner and the N-terminal catalytic domain of ACT (AC domain) is necessary and sufficient for this inhibitory effect. AC Domain interacts with the C-terminal segment of FHA with ∼650 nM affinity. ACT does not inhibit biofilm formation by Bordetella lacking the mature C-terminal domain (MCD), suggesting the direct interaction between AC domain and the MCD is required for the inhibitory effect. Additionally, AC domain disrupts preformed biofilm on abiotic surfaces. The demonstrated inhibition of biofilm formation by a host-directed protein bacterial toxin represents a novel regulatory mechanism and identifies an unprecedented role for ACT.

Adenylate Cyclase Toxin promotes bacterial internalisation into non phagocytic cells

Bordetella pertussis causes whooping cough, a respiratory infectious disease that is the fifth largest cause of vaccine-preventable death in infants. Though historically considered an extracellular pathogen, this bacterium has been detected both in vitro and in vivo inside phagocytic and non-phagocytic cells. However the precise mechanism used by B. pertussis for cell entry, or the putative bacterial factors involved, are not fully elucidated. Here we find that adenylate cyclase toxin (ACT), one of the important toxins of B. pertussis, is sufficient to promote bacterial internalisation into non-phagocytic cells. After characterization of the entry route we show that uptake of "toxin-coated bacteria" proceeds via a clathrin-independent, caveolae-dependent entry pathway, allowing the internalised bacteria to survive within the cells. Intracellular bacteria were found inside non-acidic endosomes with high sphingomyelin and cholesterol content, or "free" in the cytosol of the invaded cells, suggesting that the ACT-induced bacterial uptake may not proceed through formation of late endolysosomes. Activation of Tyr kinases and toxin-induced Ca(2+)-influx are essential for the entry process. We hypothesize that B. pertussis might use ACT to activate the endocytic machinery of non-phagocytic cells and gain entry into these cells, in this way evading the host immune system.

New Insight into Filamentous Hemagglutinin Secretion Reveals a Role for Full-Length FhaB in Bordetella Virulence

Bordetella filamentous hemagglutinin (FHA), a primary component of acellular pertussis vaccines, contributes to virulence, but how it functions mechanistically is unclear. FHA is first synthesized as an ~370-kDa preproprotein called FhaB. Removal of an N-terminal signal peptide and a large C-terminal prodomain (PD) during secretion results in "mature" ~250-kDa FHA, which has been assumed to be the biologically active form of the protein. Deletion of two C-terminal subdomains of FhaB did not affect production of functional FHA, and the mutant strains were indistinguishable from wild-type bacteria for their ability to adhere to the lower respiratory tract and to suppress inflammation in the lungs of mice. However, the mutant strains, which produced altered FhaB molecules, were eliminated from the lower respiratory tract much faster than wild-type B. bronchiseptica, suggesting a defect in resistance to early immune-mediated clearance. Our results revealed, unexpectedly, that full-length FhaB plays a critical role in B. bronchiseptica persistence in the lower respiratory tract.

IMPORTANCE

The Bordetella filamentous hemagglutinin (FHA) is a primary component of the acellular pertussis vaccine and an important virulence factor. FHA is initially produced as a large protein that is processed during secretion to the bacterial surface. As with most processed proteins, the mature form of FHA has been assumed to be the functional form of the protein. However, our results indicate that the full-length form plays an essential role in virulence in vivo. Furthermore, we have found that FHA contains intramolecular regulators of processing and that this control of processing is integral to its virulence activities. This report highlights the advantage of studying protein maturation and function simultaneously, as a role for the full-length form of FHA was evident only from in vivo infection studies and not from in vitro studies on the production or maturation of FHA or even from in vitro virulence-associated activity assays.

Filamentous hemagglutinin of Bordetella pertussis: a key adhesin with immunomodulatory properties?

The filamentous hemagglutinin of pathogenic Bordetellae is a prototype of a large two-partner-system-secreted and β-structure-rich bacterial adhesin. It exhibits several binding activities that may facilitate bacterial adherence to airway mucosa and host phagocytes in the initial phases of infection. Despite three decades of research on filamentous hemagglutinin, there remain many questions on its structure-function relationships, integrin interactions and possible immunomodulatory signaling capacity. Here we review the state of knowledge on this important virulence factor and acellular pertussis vaccine component. Specific emphasis is placed on outstanding questions that are yet to be answered.

Antibody-mediated inhibition of Bordetella pertussis adenylate cyclase-haemolysin-induced macrophage cytotoxicity is influenced by variations in the bacterial population

Whooping cough is a vaccine-preventable disease presenting with epidemic cycles linked to natural and/or vaccine-driven evolution of the aetiological agent of the disease, Bordetella pertussis. Adenylate cyclase-haemolysin (AC-Hly) is a major toxin produced by this pathogen, which mediates macrophage apoptosis in vitro and in vivo. While current acellular pertussis vaccine (APV) formulations do not include AC-Hly, they all contain pertussis toxin and can comprise filamentous haemagglutinin (FHA), which interacts with AC-Hly, and pertactin (PRN), which has been hypothesized also to interact with AC-Hly. We aimed to study the capacity of specific antibodies to inhibit the in vitro B. pertussis AC-Hly-mediated cytotoxicity of J774A.1 murine macrophages in a background of a changing bacterial population. We demonstrate that: (i) clinical isolates of different types or PRN phenotype are all cytotoxic and lethal in the mouse model of respiratory infection; (ii) lack of PRN production does not impact AC-Hly-related phenotypes; (iii) anti-AC-Hly antibodies inhibit cell lysis whatever the phenotype of the isolate, while anti-PRN antibodies significantly inhibit cell lysis provided the isolate produces this antigen, which might be relevant in vivo for APV-induced immunity; and (iv) anti-FHA antibodies only inhibit lysis induced by isolates collected in 2012, maybe indicating specific characteristics of epidemic lineages of B. pertussis.

Evidence for phenotypic bistability resulting from transcriptional interference of bvgAS in Bordetella bronchiseptica

Bordetella species cause respiratory infections in mammals. Their master regulatory system BvgAS controls expression of at least three distinct phenotypic phases in response to environmental cues. The Bvg⁺ phase is necessary and sufficient for respiratory infection while the Bvg⁻ phase is required for survival ex vivo. We obtained large colony variants (LCVs) from the lungs of mice infected with B. bronchiseptica strain RBX9, which contains an in-frame deletion mutation in fhaB, encoding filamentous haemagglutinin. RBX9 also yielded LCVs when switched from Bvg⁻ phase conditions to Bvg⁺ phase conditions in vitro. We determined that LCVs are composed of both Bvg⁺ and Bvg⁻ phase bacteria and that they result from defective bvgAS positive autoregulation. The LCV phenotype was linked to the presence of a divergent promoter 5' to bvgAS, suggesting a previously undescribed mechanism of transcriptional interference that, in this case, leads to feedback-based bistability (FBM). Our results also indicate that a small proportion of RBX9 bacteria modulates to the Bvg⁻ phase in vivo. In addition to providing insight into transcriptional interference and FBM, our data provide an example of an in-frame deletion mutation exerting a 'polar' effect on nearby genes.

Epidemiology of whooping cough & typing of Bordetella pertussis

Bordetella pertussis is a Gram-negative human-restricted bacterium that evolved from the broad-range mammalian pathogen, Bordetella bronchiseptica. It causes whooping cough or pertussis in humans, which is the most prevalent vaccine-preventable disease worldwide. The introduction of the pertussis whole-cell vaccination for young children, followed by the introduction of the pertussis acellular vaccination (along with booster vaccination) for older age groups, has affected the bacterial population and epidemiology of the disease. B. pertussis is relatively monomorphic worldwide, but nevertheless, different countries are facing different epidemiological evolutions of the disease. Although it is tempting to link vaccine-driven phenotypic and genotypic evolution of the bacterium to epidemiology, many other factors should be considered and surveillance needs to continue, in addition to studies investigating the impact of current clinical isolates on vaccine efficacy.

Protective role of adenylate cyclase in the context of a live pertussis vaccine candidate

Despite high vaccination coverage, pertussis remains an important respiratory infectious disease and the least-controlled vaccine-preventable infectious disease in children. Natural infection with Bordetella pertussis is known to induce strong and long-lasting immunity that wanes later than vaccine-mediated immunity. Therefore, a live attenuated B. pertussis vaccine, named BPZE1, has been developed and has recently completed a phase I clinical trial in adult human volunteers. In this study, we investigated the contribution of adenylate cyclase (CyaA) in BPZE1-mediated protection against pertussis. A CyaA-deficient BPZE1 mutant was thus constructed. Absence of CyaA did not compromise the adherence properties of the bacteria onto mammalian cells. However, the CyaA-deficient mutant displayed a slight impairment in the ability to survive within macrophages compared to the parental BPZE1 strain. In vivo, whereas the protective efficacy of the CyaA-deficient mutant was comparable to the parental strain at a vaccine dose of 5 × 10(5) colony forming units (CFU), it was significantly impaired at a vaccine dose of 5 × 10(3) CFU. This impairment correlated with impaired lung colonization ability, and impaired IFN-γ production in the animal immunized with the CyaA-deficient BPZE1 mutant while the pertussis-specific antibody profile and Th17 response were comparable to those observed in BPZE1-immunized mice. Our findings thus support a role of CyaA in BPZE1-mediated protection through induction of cellular mediated immunity.

Quantification of the adenylate cyclase toxin of Bordetella pertussis in vitro and during respiratory infection

Whooping cough results from infection of the respiratory tract with Bordetella pertussis, and the secreted adenylate cyclase toxin (ACT) is essential for the bacterium to establish infection. Despite extensive study of the mechanism of ACT cytotoxicity and its effects over a range of concentrations in vitro, ACT has not been observed or quantified in vivo, and thus the concentration of ACT at the site of infection is unknown. The recently developed baboon model of infection mimics the prolonged cough and transmissibility of pertussis, and we hypothesized that measurement of ACT in nasopharyngeal washes (NPW) from baboons, combined with human and in vitro data, would provide an estimate of the ACT concentration in the airway during infection. NPW contained up to ≈ 10(8) CFU/ml B. pertussis and 1 to 5 ng/ml ACT at the peak of infection. Nasal aspirate specimens from two human infants with pertussis contained bacterial concentrations similar to those in the baboons, with 12 to 20 ng/ml ACT. When ≈ 10(8) CFU/ml of a laboratory strain of B. pertussis was cultured in vitro, ACT production was detected in 60 min and reached a plateau of ≈ 60 ng/ml in 6 h. Furthermore, when bacteria were brought into close proximity to target cells by centrifugation, intoxication was increased 4-fold. Collectively, these data suggest that at the bacterium-target cell interface during infection of the respiratory tract, the concentration of ACT can exceed 100 ng/ml, providing a reference point for future studies of ACT and pertussis pathogenesis.

An improved recombination-based in vivo expression technology-like reporter system reveals differential cyaA gene activation in Bordetella species

Bordetella pertussis and Bordetella bronchiseptica rely on the global two-component regulatory system BvgAS to control expression of distinct phenotypic phases. In the Bvg(-) phase, expression of vrg genes, including those required for motility in B. bronchiseptica, is activated and genes encoding virulence factors are not expressed. Conversely, in the Bvg(+) phase, genes encoding virulence factors are highly expressed while genes necessary for motility are repressed. Although several genetic analyses have demonstrated the importance of the Bvg(+) phase during respiratory infection, Bvg-regulated gene activation in B. bronchiseptica has not been investigated in vivo. To address this, we developed a plasmid, pGFLIP, that encodes a sensitive Flp recombinase-based fluorescent reporter system able to document gene activation both in vitro and in vivo. Using pGFLIP, we demonstrated that cyaA, considered to be a "late" Bvg(+) phase gene, is activated substantially earlier in B. bronchiseptica than B. pertussis following a switch from Bvg(-) to Bvg(+) phase conditions. We show that the altered activation of cyaA is not due to differences in the cyaA promoter or in the bvgAS alleles of B. bronchiseptica compared to B. pertussis, but appears to be species specific. Finally, we used pGFLIP to show that flaA remains repressed during infection, confirming that B. bronchiseptica does not modulate to the Bvg(-) phase in vivo.

Contribution of Bordetella filamentous hemagglutinin and adenylate cyclase toxin to suppression and evasion of interleukin-17-mediated inflammation

Bordetella pertussis and Bordetella bronchiseptica establish respiratory infections with notorious efficiency. Our previous studies showed that the fhaB genes of B. pertussis and B. bronchiseptica, which encode filamentous hemagglutinin (FHA), are functionally interchangeable and provided evidence that FHA-deficient B. bronchiseptica induces more inflammation in the lungs of mice than wild-type B. bronchiseptica. We show here that the robust inflammatory response to FHA-deficient B. bronchiseptica is characterized by the early and sustained influx of interleukin-17 (IL-17)-positive neutrophils and macrophages and, at 72 h postinoculation, IL-17-positive CD4(+) T cells, suggesting that FHA allows the bacteria to suppress the development of an IL-17-mediated inflammatory response. We also show that the cyaA genes of B. pertussis and B. bronchiseptica, which encode adenylate cyclase toxin (ACT), are functionally interchangeable and that ACT, specifically its catalytic activity, is required for B. bronchiseptica to resist phagocytic clearance but is neither required for nor inhibitory of the induction of inflammation if bacteria are present in numbers sufficient to persist during the first 3 days postinoculation. Incubation of bone marrow-derived macrophages with a ΔcyaA strain caused decreased production of IL-1β and increased production of tumor necrosis factor alpha (TNF-α) and IL-12, while incubation with a ΔcyaA ΔfhaB strain caused increased production of IL-23. These data suggest that FHA and ACT both contribute to suppress the recruitment of neutrophils and the development of an IL-17-mediated immune response. To our knowledge, this is the first demonstration of a microbial pathogen suppressing IL-17-mediated inflammation in vivo as a strategy to evade innate immunity.

RTX proteins: a highly diverse family secreted by a common mechanism

Repeats-in-toxin (RTX) exoproteins of Gram-negative bacteria form a steadily growing family of proteins with diverse biological functions. Their common feature is the unique mode of export across the bacterial envelope via the type I secretion system and the characteristic, typically nonapeptide, glycine- and aspartate-rich repeats binding Ca²⁺ ions. In this review, we summarize the current state of knowledge on the organization of rtx loci and on the biological and biochemical activities of therein encoded proteins. Applying several types of bioinformatic screens on the steadily growing set of sequenced bacterial genomes, over 1000 RTX family members were detected, with the biological functions of most of them remaining to be characterized. Activities of the so far characterized RTX family members are then discussed and classified according to functional categories, ranging from the historically first characterized pore-forming RTX leukotoxins, through the large multifunctional enzymatic toxins, bacteriocins, nodulation proteins, surface layer proteins, up to secreted hydrolytic enzymes exhibiting metalloprotease or lipase activities of industrial interest.

Role of HrpA in biofilm formation of Neisseria meningitidis and regulation of the hrpBAS transcripts

Two-partner secretion systems of gram-negative organisms are utilized in adherence, invasion, and biofilm formation. The HrpAB proteins of Neisseria meningitidis are members of a two-partner secretion system, and HrpA is established as being important to adherence and intracellular escape. This study set out to determine the expression pattern of members of the hrpBAS putative operon and to find a functional role for the HrpA protein. The upregulation of these genes was found in situations of anaerobiosis and cell contact. These observations prompted the study of the function of HrpA in biofilms on human bronchial epithelial cells. HrpA mutants in encapsulated and unencapsulated NMB strains demonstrated biofilm growth equivalent to that of the wild-type strain at 6 h but a decreased ability to form biofilms at 48 h. Biofilms formed by hrpA mutants for 48 h on collagen-coated coverslips demonstrated significant reductions compared to those of wild-type strains. Taken together, these observations imply a role for HrpA in the biofilm structure. Further analysis demonstrated the presence of HrpA on the surface of the bacterium.

Adenylate cyclase influences filamentous haemagglutinin-mediated attachment of Bordetella pertussis to epithelial alveolar cells

Attachment to epithelial cells in the respiratory tract is a key event in Bordetella pertussis colonization. Filamentous haemagglutinin (FHA) is an important virulence factor mediating adhesion to host cells. In this study, the relevance of the interaction between FHA and adenylate cyclase toxin (ACT) during bacterial attachment was investigated. Mutants lacking either FHA or ACT showed significantly decreased adherence to epithelial respiratory cells. The use of several ACT-specific monoclonal antibodies and antiserum showed that the decrease in attachment of strains lacking ACT expression could not be explained by the adhesin-like activity of ACT, or a change of any of the biological activities of ACT. Immunoblot analysis showed that the lack of ACT expression did not interfere with FHA localization. An heparin-inhibitable carbohydrate-binding site is crucial in the process of FHA-mediated bacterial binding to epithelial cells. In the presence of heparin attachment of wild-type B. pertussis, but not of the isogenic ACT defective mutant, to epithelial cells was significantly decreased. These results suggest that ACT enhances the adhesive functions of FHA, and modifies the performance of the FHA heparin-inhibitable carbohydrate binding site. We propose that the presence of ACT in the outer membrane of B. pertussis to play a role in the functionality of FHA.

Characterization of leukotoxin from a clinical strain of Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a Gram negative pathogen that is the etiologic agent of localized aggressive periodontitis (LAP), a rapidly progressing and severe disease of the oral cavity that affects predominantly adolescents. A. actinomycetemcomitans is also found in extraoral infections including infective endocarditis. As one of its many virulence determinants, A. actinomycetemcomitans produces the RTX (repeats in toxin) exotoxin, leukotoxin (LtxA). LtxA specifically kills leukocytes of humans and Old World Monkeys. All of our current knowledge of A. actinomycetemcomitans LtxA is based on the protein from strain JP2, a nonadherent laboratory isolate. Because laboratory isolates can lose virulence properties, we wished to examine LtxA from a clinical isolate, NJ4500. We show that localization patterns of LtxA do not differ between the strains. Subcellular localization studies with NJ4500 revealed that LtxA localizes to the outer membrane and that the interaction between LtxA and the surface of cells is specific. Surface localized LtxA was not removed with NaCl treatment and protease protection experiments revealed that approximately 10 kDa of LtxA is exposed. We purified secreted LtxA from NJ4500 and found that the specific activity of this toxin was greater than that of secreted LtxA from JP2. For other RTX toxins, fatty acid modification affects toxin activity, and A. actinomycetemcomitans LtxA is predicted to be modified. We show by two-dimensional gel electrophoresis that NJ4500 LtxA is more highly modified than JP2 LtxA, suggesting that the difference in activities could be due to differential modification. Studies of A. actinomycetemcomitans pathogenesis should therefore consider LtxA from clinical isolates.

Bordetella adenylate cyclase toxin: a swift saboteur of host defense

Bordetella that infect mammals produce a multifunctional repeat in toxin (RTX) adenylate cyclase toxin known as CyaA, an excellent example of bacterial sophistication in subverting host defense. Recent reports show that interaction of CyaA with tracheal epithelial cells aids adhesion of Bordetella to ciliated mucosa and induces production of the pro-inflammatory cytokine interleukin, IL-6. Myeloid phagocytes, attracted to the site of infection are the target of freshly secreted CyaA that binds to the alpha(M)beta2 integrin (CD11b/CD18), penetrates cells and promptly suppresses their bactericidal functions by converting cellular ATP to cAMP. Such uncontrolled cAMP signaling can also drive CD11b-expressing immature dendritic cells into a semi-mature state, possibly hijacking them to shape the local adaptive immune response towards tolerance of the pathogen.

Influence of CR3 (CD11b/CD18) expression on phagocytosis of Bordetella pertussis by human neutrophils

CR3 (CD11b/CD18) is expressed on neutrophils, and the engagement of CR3 can promote phagocytosis. CR3 serves as the receptor for the Bordetella pertussis adhesin filamentous hemagglutinin (FHA) and for the adenylate cyclase toxin (ACT), which blocks neutrophil function. The influence of CR3, FHA, and ACT on the phagocytosis of B. pertussis by human neutrophils was examined. The surface expression and function of CR3 are regulated. Tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) increased CR3 surface expression, but only TNF-alpha increased the ability of neutrophils to phagocytose B. pertussis, suggesting that elevated CR3 expression alone is not sufficient to promote phagocytosis. Purified FHA and pertussis toxin also increased the surface expression of CR3 on neutrophils, while ACT and the B subunit of pertussis toxin did not affect CR3 expression. FHA-mediated attachment to CR3 can lead to phagocytosis, especially in the absence of ACT. FHA mutants failed to attach and were not phagocytosed by neutrophils. Similarly, an antibody to CR3 blocked both attachment and phagocytosis. The addition of exogenous FHA enhanced the attachment and phagocytosis of wild-type B. pertussis and FHA mutants. Mutants lacking the SphB1 protease, which cleaves FHA and allows the release of FHA from the bacterial surface, were phagocytosed more efficiently than wild-type bacteria. ACT mutants were efficiently phagocytosed, but wild-type B. pertussis or ACT mutants plus exogenous ACT resisted phagocytosis. These studies suggest that the activation and surface expression of CR3, FHA expression, and the efficiency of ACT internalization all influence whether B. pertussis will be phagocytosed and ultimately killed by neutrophils.

Adenylate cyclase toxin (ACT) from Bordetella hinzii: characterization and differences from ACT of Bordetella pertussis

Bordetella hinzii is a commensal respiratory microorganism in poultry but is increasingly being recognized as an opportunistic pathogen in immunocompromised humans. Although associated with a variety of disease states, practically nothing is known about the mechanisms employed by this bacterium. In this study, we show by DNA sequencing and reverse transcription-PCR that both commensal and clinical strains of B. hinzii possess and transcriptionally express cyaA, the gene encoding adenylate cyclase toxin (ACT) in other pathogenic Bordetella species. By Western blotting, we also found that B. hinzii produces full-length ACT protein in quantities that are comparable to those made by B. pertussis. In contrast to B. pertussis ACT, however, ACT from B. hinzii is less extractable from whole bacteria, nonhemolytic, has a 50-fold reduction in adenylate cyclase activity, and is unable to elevate cyclic AMP levels in host macrophages (nontoxic). The decrease in enzymatic activity is attributable, at least in part, to a decreased binding affinity of B. hinzii ACT for calmodulin, the eukaryotic activator of B. pertussis ACT. In addition, we demonstrate that the lack of intoxication by B. hinzii ACT may be due to the absence of expression of cyaC, the gene encoding the accessory protein required for the acylation of B. pertussis ACT. These results demonstrate the expression of ACT by B. hinzii and represent the first characterization of a potential virulence factor of this organism.

Bordetella bronchiseptica adherence to cilia is mediated by multiple adhesin factors and blocked by surfactant protein A

In the virulent state (Bvg+), Bordetella bronchiseptica expresses adhesins and toxins that mediate adherence to the upper airway epithelium, an essential early step in pathogenesis. In this study, we used a rabbit tracheal epithelial cell binding assay to test how specific host or pathogen factors contribute to ciliary binding. The host antimicrobial agent surfactant protein A (SP-A) effectively reduced ciliary binding by Bvg+ B. bronchiseptica. To evaluate the relative contributions of bacterial adhesins and toxins to ciliary binding, we used mutant strains of B. bronchiseptica in the binding assay. When compared to Bvg+ or Bvg- phase-locked B. bronchiseptica strains, single-knockout strains lacking one of the known adhesins (filamentous hemagglutinin, pertactin, or fimbriae) displayed an intermediate ciliary binding capacity throughout the coincubation. A B. bronchiseptica strain deficient in adenylate cyclase-hemolysin toxin also displayed an intermediate level of adherence between Bvg+ and Bvg- strains and had the lowest ciliary affinity of any of the Bvg+ phase strains tested. A B. bronchiseptica strain that was missing dermonecrotic toxin also displayed intermediate binding; however, this strain displayed ciliary binding significantly higher than most of the adhesin knockouts tested. Taken together, these findings suggest that virulent-state B. bronchiseptica expresses multiple adhesins with overlapping contributions to ciliary adhesion and that host production of SP-A can provide innate immunity by blocking bacterial adherence to the ciliated epithelium.

Pertussis toxin and adenylate cyclase toxin provide a one-two punch for establishment of Bordetella pertussis infection of the respiratory tract

Previously we found that pertussis toxin (PT), an exotoxin virulence factor produced by Bordetella pertussis, plays an important early role in colonization of the respiratory tract by this pathogen, using a mouse intranasal infection model. In this study, we examined the early role played by another exotoxin produced by this pathogen, adenylate cyclase toxin (ACT). By comparing a wild-type strain to a mutant strain (DeltaCYA) with an in-frame deletion of the cyaA gene encoding ACT, we found that the lack of ACT confers a significant peak (day 7) colonization defect (1 to 2 log(10)). In mixed-infection experiments, the DeltaCYA strain was significantly outcompeted by the wild-type strain, and intranasal administration of purified ACT did not increase colonization by DeltaCYA. These data suggest that ACT benefits the bacterial cells that produce it and, unlike PT, does not act as a soluble factor benefiting the entire infecting bacterial population. Comparison of lower respiratory tract infections over the first 4 days after inoculation revealed that the colonization defect of the PT deletion strain was apparent earlier than that of DeltaCYA, suggesting that PT plays an earlier role than ACT in the establishment of B. pertussis infection. Examination of cells in the bronchoalveolar lavage fluid of infected mice revealed that, unlike PT, ACT does not appear to inhibit neutrophil influx to the respiratory tract early after infection but may combat neutrophil activity once influx has occurred.

Molecular pathogenesis, epidemiology, and clinical manifestations of respiratory infections due to Bordetella pertussis and other Bordetella subspecies

Bordetella respiratory infections are common in people (B. pertussis) and in animals (B. bronchiseptica). During the last two decades, much has been learned about the virulence determinants, pathogenesis, and immunity of Bordetella. Clinically, the full spectrum of disease due to B. pertussis infection is now understood, and infections in adolescents and adults are recognized as the reservoir for cyclic outbreaks of disease. DTaP vaccines, which are less reactogenic than DTP vaccines, are now in general use in many developed countries, and it is expected that the expansion of their use to adolescents and adults will have a significant impact on reducing pertussis and perhaps decrease the circulation of B. pertussis. Future studies should seek to determine the cause of the unique cough which is associated with Bordetella respiratory infections. It is also hoped that data gathered from molecular Bordetella research will lead to a new generation of DTaP vaccines which provide greater efficacy than is provided by today's vaccines.

A three-component regulatory system regulates biofilm maturation and type III secretion in Pseudomonas aeruginosa

Biofilms are structured communities found associated with a wide range of surfaces. Here we report the identification of a three-component regulatory system required for biofilm maturation by Pseudomonas aeruginosa strain PA14. A transposon mutation that altered biofilm formation in a 96-well dish assay originally defined this locus, which is comprised of genes for a putative sensor histidine kinase and two response regulators and has been designated sadARS. Nonpolar mutations in any of the sadARS genes result in biofilms with an altered mature structure but do not confer defects in growth or early biofilm formation, swimming, or twitching motility. After 2 days of growth under flowing conditions, biofilms formed by the mutants are indistinguishable from those formed by the wild-type (WT) strain. However, by 5 days, mutant biofilms appear to be more homogeneous than the WT in that they fail to form large and distinct macrocolonies and show a drastic reduction in water channels. We propose that the sadARS three-component system is required for later events in biofilm formation on an abiotic surface. Semiquantitative reverse transcription-PCR analysis showed that there is no detectable change in expression of the sadARS genes when cells are grown in a planktonic culture versus a biofilm, indicating that this locus is not itself induced during or in response to biofilm formation. DNA microarray studies were used to identify downstream targets of the SadARS system. Among the genes regulated by the SadARS system are those required for type III secretion. Mutations in type III secretion genes result in strains with enhanced biofilm formation. We propose a possible mechanism for the role that the SadARS system plays in biofilm formation.

Newly secreted adenylate cyclase toxin is responsible for intoxication of target cells by Bordetella pertussis

Adenylate cyclase (AC) toxin is present on the surface of Bordetella pertussis organisms and their addition to eukaryotic cells results in increases in intracellular cAMP. To test the hypothesis that surface-bound toxin is the source for intoxication of cells when incubated with B. pertussis, we characterized the requirements of intoxication from intact bacteria and found that this process is calcium-dependent and blocked by monoclonal antibody to AC toxin or antibody against CD11b, a surface glycoprotein receptor for the toxin. Increases in intracellular cAMP correlate with the number of adherent bacteria, not the total number present in the medium, suggesting that interaction of bacteria with target cells is important for efficient delivery of AC toxin. A filamentous haemagglutinin-deficient mutant (BP353) and a clinical isolate (GMT1), both of which have a marked reduction in AC toxin on their surface, and wild-type B. pertussis (BP338) from which surface AC toxin has been removed by trypsin, were fully competent for intoxicating target cells, demonstrating that surface-bound AC toxin is not responsible for intoxication. B. pertussis killed by gentamicin or gamma irradiation were unable to intoxicate, illustrating that toxin delivery requires viable bacteria. Furthermore, CCCP, a protonophore that disrupts the proton gradient necessary for the secretion of related RTX toxins, blocked intoxication by whole bacteria. These data establish that delivery of this toxin by intact B. pertussis is not dependent on the surface-associated AC toxin, but requires close association of live bacteria with target cells and the active secretion of AC toxin.

The Bvg virulence control system regulates biofilm formation in Bordetella bronchiseptica

Bordetella species utilize the BvgAS (Bordetella virulence gene) two-component signal transduction system to sense the environment and regulate gene expression among at least three phases: a virulent Bvg+ phase, a nonvirulent Bvg- phase, and an intermediate Bvgi phase. Genes expressed in the Bvg+ phase encode known virulence factors, including adhesins such as filamentous hemagglutinin (FHA) and fimbriae, as well as toxins such as the bifunctional adenylate cyclase/hemolysin (ACY). Previous studies showed that in the Bvgi phase, FHA and fimbriae continue to be expressed, but ACY expression is significantly downregulated. In this report, we determine that Bordetella bronchiseptica can form biofilms in vitro and that the generation of biofilm is maximal in the Bvgi phase. We show that FHA is required for maximal biofilm formation and that fimbriae may also contribute to this phenotype. However, expression of ACY inhibits biofilm formation, most likely via interactions with FHA. Therefore, the coordinated regulation of adhesins and ACY expression leads to maximal biofilm formation in the Bvgi phase in B. bronchiseptica.

Characterization of adenylate cyclase-hemolysin gene duplication in a Bordetella pertussis isolate

We describe a clinical isolate of Bordetella pertussis, the agent responsible for whooping cough, composed of at least two clones harboring one or two copies of the cya locus encoding one of the major toxins, adenylate cyclase-hemolysin. No difference was observed between the two clones in murine and cellular models, probably due to the high instability of the cya locus duplication.

Membrane restructuring by Bordetella pertussis adenylate cyclase toxin, a member of the RTX toxin family

Adenylate cyclase toxin (ACT) is secreted by Bordetella pertussis, the bacterium causing whooping cough. ACT is a member of the RTX (repeats in toxin) family of toxins, and like other members in the family, it may bind cell membranes and cause disruption of the permeability barrier, leading to efflux of cell contents. The present paper summarizes studies performed on cell and model membranes with the aim of understanding the mechanism of toxin insertion and membrane restructuring leading to release of contents. ACT does not necessarily require a protein receptor to bind the membrane bilayer, and this may explain its broad range of host cell types. In fact, red blood cells and liposomes (large unilamellar vesicles) display similar sensitivities to ACT. A varying liposomal bilayer composition leads to significant changes in ACT-induced membrane lysis, measured as efflux of fluorescent vesicle contents. Phosphatidylethanolamine (PE), a lipid that favors formation of nonlamellar (inverted hexagonal) phases, stimulated ACT-promoted efflux. Conversely, lysophosphatidylcholine, a micelle-forming lipid that opposes the formation of inverted nonlamellar phases, inhibited ACT-induced efflux in a dose-dependent manner and neutralized the stimulatory effect of PE. These results strongly suggest that ACT-induced efflux is mediated by transient inverted nonlamellar lipid structures. Cholesterol, a lipid that favors inverted nonlamellar phase formation and also increases the static order of phospholipid hydrocarbon chains, among other effects, also enhanced ACT-induced liposomal efflux. Moreover, the use of a recently developed fluorescence assay technique allowed the detection of trans-bilayer (flip-flop) lipid motion simultaneous with efflux. Lipid flip-flop further confirms the formation of transient nonlamellar lipid structures as a result of ACT insertion in bilayers.

Lipopolysaccharide 3-deoxy-D-manno-octulosonic acid (Kdo) core determines bacterial association of secreted toxins

In contrast to cholera toxin (CT), which is secreted solubly by Vibrio cholerae across the outer membrane, heat-labile enterotoxin (LT) is retained on the surface of enterotoxigenic Escherichia coli (ETEC) via an interaction with lipopolysaccharide (LPS). We examined the nature of the association between LT and LPS. Soluble LT binds to the surface of LPS deep-rough biosynthesis mutants but not to lipid A, indicating that only the Kdo (3-deoxy-d-manno-octulosonic acid) core is required for binding. Although capable of binding truncated LPS and Kdo, LT has a higher affinity for longer, more complete LPS species. A putative LPS binding pocket is proposed based on the crystal structure of the toxin. The ability to bind LPS and remain associated with the bacterial surface is not unique to LT, as CT also binds to E. coli LPS. However, neither LT nor CT is capable of binding to the surface of Vibrio. The core structures of Vibrio and E. coli LPS differ in that Vibrio contains a phosphorylated single Kdo-lipid A, and E. coli LPS contains unphosphorylated Kdo2-lipid A. We determined that the phosphate group on the Kdo core of Vibrio LPS prevents CT from binding, resulting in the secretion of soluble toxin. Because LT binds E. coli LPS, it remains associated with the extracellular bacterial surface and is released in association with outer membrane vesicles. We propose that difference in the extracellular fates of LT and CT contribute to the differences in disease caused by ETEC and Vibrio cholerae.