Identification and occurrence of Vibrio cholerae flagellar core proteins in isolated outer membrane

Growth in a biofilm induces a hyperinfectious phenotype in Vibrio cholerae

Biofilm formation plays a multifaceted role in the life cycles of a wide variety of microorganisms. In the case of pathogenic Vibrio cholerae, biofilm formation in its native aquatic habitats is thought to aid in persistence during interepidemic seasons and to enhance infectivity upon oral ingestion. The structure of V. cholerae biofilms has been hypothesized to protect the bacteria during passage through the stomach. Here, we directly test the role of biofilm architecture in the infectivity of V. cholerae by comparing the abilities of intact biofilms, dispersed biofilms, and planktonic cells to colonize the mouse small intestine. Not only were V. cholerae biofilms better able to colonize than planktonic cells, but the structure of the biofilm was also found to be dispensable: intact and dispersed biofilms colonized equally, and both vastly out-colonized planktonic cells. The infectious dose for biofilm-derived V. cholerae was orders of magnitude lower than that of planktonic cells. This biofilm-induced hyperinfectivity may be due in part to a higher growth rate of biofilm-derived cells during infection. These results suggest that the infectious dose of naturally occurring biofilms of V. cholerae may be much lower than previously estimated using cells grown planktonically in vitro. Furthermore, this work implies the existence of factors specifically induced during growth in a biofilm that augment infection by V. cholerae.

Levels of the secreted Vibrio cholerae attachment factor GbpA are modulated by quorum-sensing-induced proteolysis

Vibrio cholerae is the etiologic agent of cholera in humans. Intestinal colonization occurs in a stepwise fashion, initiating with attachment to the small intestinal epithelium. This attachment is followed by expression of the toxin-coregulated pilus, microcolony formation, and cholera toxin (CT) production. We have recently characterized a secreted attachment factor, GlcNAc binding protein A (GbpA), which functions in attachment to environmental chitin sources as well as to intestinal substrates. Studies have been initiated to define the regulatory network involved in GbpA induction. At low cell density, GbpA was detected in the culture supernatant of all wild-type (WT) strains examined. In contrast, at high cell density, GbpA was undetectable in strains that produce HapR, the central regulator of the cell density-dependent quorum-sensing system of V. cholerae. HapR represses the expression of genes encoding regulators involved in V. cholerae virulence and activates the expression of genes encoding the secreted proteases HapA and PrtV. We show here that GbpA is degraded by HapA and PrtV in a time-dependent fashion. Consistent with this, Delta hapA Delta prtV strains attach to chitin beads more efficiently than either the WT or a Delta hapA Delta prtV Delta gbpA strain. These results suggest a model in which GbpA levels fluctuate in concert with the bacterial production of proteases in response to quorum-sensing signals. This could provide a mechanism for GbpA-mediated attachment to, and detachment from, surfaces in response to environmental cues.

Vibrio cholerae flagellins induce Toll-like receptor 5-mediated interleukin-8 production through mitogen-activated protein kinase and NF-kappaB activation

Vaccine reactogenicity has complicated the development of safe and effective live, oral cholera vaccines. Delta ctx Vibrio cholerae mutants have been shown to induce inflammatory diarrhea in volunteers and interleukin-8 (IL-8) production in cultured intestinal epithelial cells. Bacterial flagellins are known to induce IL-8 production through Toll-like receptor 5 (TLR5). Since the V. cholerae genome encodes five distinct flagellin proteins, FlaA to FlaE, with homology to conserved TLR5 recognition regions of Salmonella FliC, we hypothesized that V. cholerae flagellins may contribute to IL-8 induction through TLR5 and mitogen-activated protein kinase (MAPK) signaling. Each purified recombinant V. cholerae flagellin induced IL-8 production in T84 intestinal epithelial cells and also induced nuclear factor kappa B (NF-kappaB) activation in HEK293T/TLR5 transfectants, which was blocked by cotransfection with a TLR5 dominant-negative construct, demonstrating TLR5 specificity. Supernatants derived from Delta flaAC and Delta flaEDB mutants induced IL-8 production in HT-29 intestinal epithelial cells and in HEK293T cells overexpressing TLR5, whereas Delta flaABCDE supernatants induced significantly less IL-8 production, demonstrating the contribution of multiple flagellins in IL-8 induction. NF-kappaB activation by Delta flaABCDE supernatants was partially restored by flaA or flaAC complementation. Western analysis confirmed the presence of V. cholerae flagellins in culture supernatants. Purified recombinant V. cholerae FlaA activated the MAPKs p38, c-jun N-terminal kinase (JNK), and extracellular regulated kinase (ERK) in T84 cells. FlaA-induced IL-8 production in T84 cells was inhibited by the p38 inhibitor in combination with either the JNK or ERK inhibitors. Collectively, these data suggest that V. cholerae flagellins are present in culture supernatants and can induce TLR5- and MAPK-dependent IL-8 secretion in host cells.

Identification of proinflammatory flagellin proteins in supernatants of Vibrio cholerae O1 by proteomics analysis

The genome of Vibrio cholerae contains five flagellin genes that encode proteins (FlaA-E) of 39-41 kDa with 61-82% identity among them. Although the existing live oral attenuated vaccine strains against cholera are protective in humans, there is an intrinsic residual cytotoxic and inflammatory component associated with these candidate vaccine strains. Bacterial flagellins are known to be potent inducers of proinflammatory molecules via activation of Toll-like receptor 5. Here we found that purified flagella from wild type V. cholerae 395 induced significant release of interleukin (IL)-8 from cultured HT-29 human colonic epithelial cells. Furthermore we found that filtered supernatants of KKV90, a DeltaflaA isogenic strain unable to produce flagella, were still able to activate production of IL-8 albeit to significantly lower levels than the wild type, suggesting that other activators of proinflammatory molecules were still present in these supernatants. A comparative proteomics analysis of secreted proteins of V. cholerae 395 and KKV90 identified additional proteins with potential to induce IL-8 release in HT-29 cells. Secreted proteins in the range of 30-45 kDa identified by two-dimensional electrophoresis and mass spectrometry revealed the presence of two additional flagellins, FlaC and FlaD, that appeared to be secreted 3- and 6-fold more, respectively, in the mutant compared with the wild type. Double isogenic mutants flaAC and flaAD were unable to trigger IL-8 release from HT-29 cells. In sum, we have shown that purified flagella and secreted flagellin proteins (FlaC and FlaD) are inducers of IL-8 release from epithelial cells via Toll-like receptor 5. This observation may explain, in part, the observed reactogenicity of cholera vaccine strains in humans.

Cholera

Despite more than a century of study, cholera still presents challenges and surprises to us. Throughout most of the 20th century, cholera was caused by Vibrio cholerae of the O1 serogroup and the disease was largely confined to Asia and Africa. However, the last decade of the 20th century has witnessed two major developments in the history of this disease. In 1991, a massive outbreak of cholera started in South America, the one continent previously untouched by cholera in this century. In 1992, an apparently new pandemic caused by a previously unknown serogroup of V. cholerae (O139) began in India and Bangladesh. The O139 epidemic has been occurring in populations assumed to be largely immune to V. cholerae O1 and has rapidly spread to many countries including the United States. In this review, we discuss all aspects of cholera, including the clinical microbiology, epidemiology, pathogenesis, and clinical features of the disease. Special attention will be paid to the extraordinary advances that have been made in recent years in unravelling the molecular pathogenesis of this infection and in the development of new generations of vaccines to prevent it.

Isolation and characterization of Pseudomonas pseudomallei flagellin proteins

Flagellin proteins from several different strains of Pseudomonas pseudomallei have been isolated and purified to homogeneity by mechanical shearing and differential centrifugation techniques. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis yielded flagellin monomer protein bands with an estimated M(r) of 43,400. No lipopolysaccharide contamination of the purified protein preparations was detectable by silver staining of flagellin displayed on polyacrylamide gels and by Western immunoblotting with P. pseudomallei antilipopolysaccharide monoclonal antibody. NH2-terminal amino acid sequence analysis of the flagellin protein of P. pseudomallei 319a revealed significant homology with flagellins from Proteus mirabilis, Bordetella bronchiseptica, and Pseudomonas aeruginosa PAK. Rabbit polyclonal antiserum raised against the 319a flagellin protein reacted with 64 of 65 P. pseudomallei strains tested. The polyclonal antiserum proved effective in inhibiting the motility of these organisms in motility agar plates. Passive immunization studies demonstrated that 319a flagellin-specific antiserum was capable of protecting diabetic rats from challenge with a heterologous P. pseudomallei strain.

Identification of a mannose-binding pilus on Vibrio cholerae El Tor

The mannose-sensitive hemagglutinin (MSHA) that is associated with Vibrio cholerae strains of El Tor biotype is identified as a pilus composed of subunits with a molecular mass of approximately 17 kDa. In immunoelectron microscopy, a monoclonal antibody against MSHA that inhibited El Tor vibrio-mediated mannose-sensitive agglutination of chicken erythrocytes or El Tor bacterial binding to mannose-coated agarose beads, bound specifically to repetitive subunits along typical fimbriae extending from the surface of El Tor vibrios. No such pili were seen on the surface of MSHA negative classical vibrios, although non-surface exposed fimbrial subunits could be demonstrated in these bacteria by immunoblotting techniques.

Human immune response to Vibrio cholerae O1 whole cells and isolated outer membrane antigens

The serum immunoglobulin G (IgG) and mucosal secretory IgA (SIgA) response of human volunteers challenged with Vibrio cholerae O1 was analyzed for reactivity to V. cholerae O1 antigens by the immunoblot technique. Components of both in vitro- and in vivo (rabbit ligated ileal loop)-grown V. cholerae O1 were separated by sodium dodecyl sulfate-urea-polyacrylamide gel electrophoresis. Postchallenge serum IgG reacted uniquely with 15 antigens and with greater intensity than did prechallenge serum with at least 16 antigens. Serum IgG and SIgA reacted with antigens present in preparations from the homologous challenge strain of V. cholerae as well as antigens from strains of heterologous biotype or serotype. These heterologous antigens may represent antigens responsible for protection to rechallenge with a heterologous strain of V. cholerae. All the antigens detected by postchallenge jejunal fluid SIgA had an apparent molecular size of less than 25 kilodaltons. Serum IgG and jejunal fluid SIgA also reacted with antigens unique to in vivo-grown cells and several antigens in outer membrane preparations, suggesting that studies of protective immunity and V. cholerae O1 pathogenesis should include examination of both in vitro- and in vivo-grown V. cholerae O1 cellular antigens.

Coagglutination of Vibrio cholerae, Vibrio mimicus, and Vibrio vulnificus with anti-flagellar monoclonal antibody

Monoclonal antibodies (MAbs) with serological activity for purified flagellar (H) core protein prepared from Vibrio cholerae were identified by enzyme-linked immunosorbent assay. Four of these MAbs reacted with the flagella of V. cholerae and V. mimicus exclusively, while eight MAbs reacted with at least 1 of 30 heterologous Vibrio species tested by enzyme-linked immunosorbent assay or coagglutination. It appears that V. cholerae and V. mimicus express similar, if not identical, H determinants unique to these two Vibrio species. Staphylococcus aureus cells or latex beads armed with the four species-specific MAbs coagglutinated each of 47 isolates identified bacteriologically as V. cholerae or V. mimicus from among 103 Vibrio isolates tested. One coagglutination reagent armed with anti-V. vulnificus H MAb exhibited species specificity in that only V. vulnificus cells were coagglutinated from among the 31 Vibrio species examined. This reagent coagglutinated 20 isolates identified bacteriologically as V. vulnificus in a serological survey. MAb coagglutination reagents offer a rapid, specific, and economical alternative to the classical bacteriological approach to identify the human pathogens V. cholerae, V. mimicus, and V. vulnificus.

Rapid serological identification of Vibrio vulnificus by anti-H coagglutination

Staphylococcus aureus Cowan 1 cells were armed with anti-flagellar (anti-H) antibody produced in rabbits immunized with flagellar core protein prepared from Vibrio vulnificus. This reagent was assessed by coagglutination for its capacity to agglutinate and identify V. vulnificus. A species-specific H antigen is expressed in the core proteins of the polar flagella of V. vulnificus. Of 435 V. vulnificus isolates identified bacteriologically, 432 (99.3%) were agglutinated in the slide test within 2 min after the addition of the anti-V. vulnificus H coagglutination reagent. Other than Vibrio pelagius, the reagent did not agglutinate 19 heterologous Vibrio spp. tested, including 290 V. cholerae, 22 V. mimicus, 395 V. parahaemolyticus, and 16 V. fluvialis isolates recovered from seafood and the marine environment. The serological resolution of the coagglutination reaction was enhanced if the organism under test was suspended in 0.1 M Tris buffer-0.1 mM EDTA-1.0% Triton X-100 (TET) for 24 h before serological examination. The TET buffer also increased the sensitivity of the coagglutination reaction 100-fold over that for isolates suspended in 0.3% formalinized phosphate-buffered saline before testing. The anti-H coagglutination test is a rapid, serologically specific, and inexpensive procedure for identifying V. vulnificus one step beyond primary isolation.