Characterization of an Enterotoxin Produced by Vibrio cholerae O139

A cholera-like enterotoxin was purified from Vibrio cholerae O139 strain AI-1841 isolated from a diarrheal patient in Bangladesh. Its characteristics were compared with that of cholera toxins (CTs) of classical strain 569B and El Tor strain KT25. Al-1841 produced as much toxin as O1 strains. The toxins were indistinguishable in terms of their migration profiles in conventional polyacrylamide gel disc electrophoresis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and isoelectrofocusing as well as their affinity for hydroxyapatite. The skin permeability factor activity and the fluid accumulation induced in rabbit ileal loops of the toxin of AI-1841 were identical to those of the CTs. Three toxins equally reacted against anti-569B CT antiserum in Western blotting, and their B subunits formed a precipitin line against any anti-B subunit antiserum by double gel immunodiffusion. Anti-569B CTB antibody neutralized the three toxins in their PF activities and enterotoxicities. The amino acid sequence of 1841 toxin B subunit was identical with that of KT25 CTB, corresponding to the DNA sequence of ctxB from El Tor strains of the seventh pandemic. We concluded 1841 toxin was identical to CT of the seventh pandemic El Tor vibrios.     

Immunization with Cholera Toxin B Subunit Induces High-Level Protection in the Suckling Mouse Model of Cholera

Cholera toxin (CT) is the primary virulence factor responsible for severe cholera. Vibrio cholerae strains unable to produce CT show severe attenuation of virulence in animals and humans. The pentameric B subunit of CT (CTB) contains the immunodominant epitopes recognized by antibodies that neutralize CT. Although CTB is a potent immunogen and a promising protective vaccine antigen in animal models, immunization of humans with detoxified CT failed to protect against cholera. We recently demonstrated however that pups reared from mice immunized intraperitoneally (IP) with 3 doses of recombinant CTB were well protected against a highly lethal challenge dose of V. cholerae N16961. The present study investigated how the route and number of immunizations with CTB could influence protective efficacy in the suckling mouse model of cholera. To this end female mice were immunized with CTB intranasally (IN), IP, and subcutaneously (SC). Serum and fecal extracts were analyzed for anti-CTB antibodies by quantitative ELISA, and pups born to immunized mothers were challenged orogastrically with a lethal dose of V. cholerae. Pups from all immunized groups were highly protected from death by 48 hours (64–100% survival). Cox regression showed that percent body weight loss at 24 hours predicted death by 48 hours, but we were unable to validate a specific amount of weight loss as a surrogate marker for protection. Although CTB was highly protective in all regimens, three parenteral immunizations showed trends toward higher survival and less weight loss at 24 hours post infection. These results demonstrate that immunization with CTB by any of several routes and dosing regimens can provide protection against live V. cholerae challenge in the suckling mouse model of cholera. Our data extend the results of previous studies and provide additional support for the inclusion of CTB in the development of a subunit vaccine against V. cholerae.     

Immunizing Adult Female Mice with a TcpA-A2-CTB Chimera Provides a High Level of Protection for Their Pups in the Infant Mouse Model of Cholera

Vibrio cholerae expresses two primary virulence factors, cholera toxin (CT) and the toxin-coregulated pilus (TCP). CT causes profuse watery diarrhea, and TCP (composed of repeating copies of the major pilin TcpA) is required for intestinal colonization by V. cholerae. Antibodies to CT or TcpA can protect against cholera in animal models. We developed a TcpA holotoxin-like chimera (TcpA-A2-CTB) to elicit both anti-TcpA and anti-CTB antibodies and evaluated its immunogenicity and protective efficacy in the infant mouse model of cholera. Adult female CD-1 mice were immunized intraperitoneally three times with the TcpA-A2-CTB chimera and compared with similar groups immunized with a TcpA+CTB mixture, TcpA alone, TcpA with Salmonella typhimurium flagellin subunit FliC as adjuvant, or CTB alone. Blood and fecal samples were analyzed for antigen-specific IgG or IgA, respectively, using quantitative ELISA. Immunized females were mated; their reared offspring were challenged orogastrically with 10 or 20 LD₅₀ of V. cholerae El Tor N16961; and vaccine efficacy was assessed by survival of the challenged pups at 48 hrs. All pups from dams immunized with the TcpA-A2-CTB chimera or the TcpA+CTB mixture survived at both challenge doses. In contrast, no pups from dams immunized with TcpA+FliC or CTB alone survived at the 20 LD₅₀ challenge dose, although the anti-TcpA or anti-CTB antibody level elicited by these immunizations was comparable to the corresponding antibody level achieved by immunization with TcpA-A2-CTB or TcpA+CTB. Taken together, these findings comprise strong preliminary evidence for synergistic action between anti-TcpA and anti-CTB antibodies in protecting mice against cholera. Weight loss analysis showed that only immunization of dams with TcpA-A2-CTB chimera or TcpA+CTB mixture protected their pups against excess weight loss from severe diarrhea. These data support the concept of including both TcpA and CTB as immunogens in development of an effective multivalent subunit vaccine against V. cholerae.     

Synthesis of cholera toxin B subunit gene: cloning and expression of a functional 6XHis-tagged protein in Escherichia coli

Cholera toxin B subunit (CTB) has been extensively studied as immunogen, adjuvant, and oral tolerance inductor depending on the antigen conjugated or coadministered. It has been already expressed in several bacterial and yeast systems. In this study, we synthesized a versatile gene coding a 6XHis-tagged CTB (359 bp). The sequence was designed according to codon usage of Escherichia coli, Lactobacillus casei, and Salmonella typhimurium. The gene assembly was based on a polymerase chain reaction, in which the polymerase extends DNA fragments from a pool of overlapping oligonucleotides. The synthetic gene was amplified, cloned, and expressed in E. coli in an insoluble form, reaching levels about 13 mg of purified active pentameric rCTB per liter of induced culture. Western blot and ELISA analyses showed that recombinant CTB is strongly and specifically recognized by polyclonal antibodies against the cholera toxin. The ability to form the functional pentamers was observed in cell culture by the inhibition of cholera toxin activity on Y1 adrenal cells in the presence of recombinant CTB. The 6XHis-tagged CTB provides a simple way to obtain functional CTB through Ni²⁺-charged resin after refolding and also free of possible CTA contaminants as in the case of CTB obtained from Vibrio cholerae cultures.     

霍乱毒素B亚单位工程菌MM2在含乳酸培养基中的高表达

确定了工程菌MM2中霍乱毒素B亚单位(CTB)在含乳酸培养基中的高表达方案。采用两阶段控制培养温度(30℃→37℃)可提高CTB产量4倍,提高后期pH值(72→84)可提高CTB比表达水平214倍,中间补加乙酸钠可提高CTB产量65%。在5L发酵罐培养菌密度OD₆₀₀达30,CTB产量达1867mg/L,产物在培养液中以多聚体形式存在,具有抗原性。     

Studies on the immunogenic potential of plant-expressed cholera toxin B subunit

Nicotiana tabacum var. Samsun was transformed via Agrobacterium-mediated transformation with a gene encoding the cholera toxin B subunit (CTB) of Vibrio cholerae, modified to contain a sequence coding for an endoplasmic reticulum retention signal (SEKDEL), under the control of the cauliflower mosaic virus 35S promoter. Total protein from the transgenic leaf tissue was isolated and an aliquot containing 5 g recombinant CTB was injected intradermally into Balb/c (H2K^d) mice. CTB-specific serum IgG was detected in animals that had been administered plant-expressed or native purified CTB. A T-cell proliferation study using splenocytes and cytokine estimations in supernatants generated by in vitro stimulation of macrophages isolated from the immuno-primed animals was carried out. Inhibition of proliferation of T lymphocytes was observed in splenic T lymphocytes isolated from animals injected with either native or plant-expressed CTB. Macrophages isolated from mice immunised with native or plant-expressed CTB showed enhanced secretion of interleukin-10 but secretion of lipopolysaccharide-induced interleukin-12 and tumor necrosis factor alpha was inhibited. These studies suggest that plant-expressed protein behaved like native CTB with regards to effects on T-cell proliferation and cytokine levels, indicating the suitability of plant expression systems for the production of bacterial antigens, which could be used as edible vaccine. The transgene was found to be inherited in the progeny and was expressed to yield a pentameric form of CTB as evident by its interaction with G_M1 ganglioside.Abbreviations BAP 6-Benzylaminopurine - Con A Concanavilin A - CTB Cholera toxin B subunit - ctxB Gene encoding cholera toxin B subunit - ELISA Enzyme-linked immunosorbent assay - HRP Horseradish peroxidase - IL-10 Interleukin-10 - IL-12 Interleukin-12 - LPS Lipopolysaccharide - NAA Naphthaleneacetic acid - PBS Phosphate-buffered saline - TNF Tumour necrosis factor alphaCommunicated by H. Uchimiya     

Overexpression of a Mutant B Subunit in Toxigenic Vibrio cholerae Diminishes Production of Active Cholera Toxin In Vivo

A mutant cholera toxin B subunit containing a G33E substitution was constructed and expressed in V. cholerae. The G33E amino acid substitution did not affect the amount of recombinant CTB secreted to the culture medium. The overexpression of the mutant B subunits in wild-type toxigenic cholera vibrios led to an 80% decrease in production of active cholera toxin in vitro and in vivo. Overexpression of B^G33E subunits could be instrumental in the increase of the biosafety of live attenuated cholera candidate vaccine strains. Received: 29 January 1998 / Accepted: 30 April 1998     

Binding efficiencies of carbohydrate ligands with different genotypes of cholera toxin B: molecular modeling, dynamics and docking simulation studies

Vibrio cholerae produces cholera toxin (CT) that consists of two subunits, A and B, and is encoded by a filamentous phage CTXΦ. The A subunit carries enzymatic activity that ribosylates ADP, whereas the B subunit binds to monosialoganglioside (GM1) receptor in epithelial cells. Molecular analysis of toxigenic V. cholerae strains indicated the presence of multiple ctxB genotypes. In this study, we employed a comparative modeling approach to define the structural features of all known variants of ctxB found in O139 serogroup V. cholerae. Modeling, molecular dynamics and docking simulations studies suggested subtle variations in the binding ability of ctxB variants to carbohydrate ligands of GM1 (galactose, sialic acid and N-acetyl galactosamine). These findings throw light on the molecular efficiencies of pathogenic isolates of V. cholerae harboring natural variants of ctxB in causing the disease, thus suggesting the need to consider ctxB variations when designing vaccines against cholera.     

Fluorescently labeled liposomes for monitoring cholera toxin binding to epithelial cells

Vibrio cholerae, the causative agent for cholera, expresses a toxin required for virulence consisting of two subunits: the pentameric cholera toxin B (CTB) and cholera toxin A (CTA). CTB is frequently used as an indicator of the presence of pathogenic V. cholerae and binds to the G_M1 ganglioside on the surface of epithelial cells. To study V. cholerae virulence (CTB expression) in the presence of human epithelia, we devised an inexpensive, simple, and rapid method for quantifying CTB bound on epithelial surfaces in microtiter plates. G_M1 ganglioside was incorporated into the lipid bilayer of liposomes both encapsulating the fluorescent dye sulforhodamine B (SRB) and with SRB tagged to lipids in the bilayer (BEGs). In addition, G_M1-embedded liposomes encapsulating SRB only (EGs) and with SRB in their bilayers only (BGs) were synthesized. The three types of liposomes were compared with respect to their efficacy for both visualizing and quantifying CTB attached to the surface of Caco-2 cells. The BEGs were the most effective overall, providing both visualization under a fluorescence microscope and quantification after lysis in a microtiter plate reader. A limit of detection corresponding to 0.28 μg/ml applied CTB was attained for the on-cell assay using the microtiter plate reader approach, whereas as low as 2 μg/ml applied CTB could be observed under the fluorescence microscope.     

Comparison of a Reversed Passive Latex Agglutination and a Polymerase Chain Reaction for Identification of Cholera Toxin Producing Vibrio cholerae O1

Production of cholera toxin (CT) in AKI medium and conservation of CT gene (ctx) of 49 strains of Vibrio cholerae O1 were compared by reversed passive latex agglutination (RPLA) and polymerase chain reaction (PCR). The production of CT agreed with conservation of the ctx in 48 out of the 49 strains. Ten strains were positive, and 38 strains were negative by both methods. Only one strain was negative in RPLA and positive in PCR. This suggested that the combination of AKI-SW and RPLA is comparable to PCR to identify CT-producing V. cholerae O1.     

Transcutaneous immunization with cholera toxin B subunit adjuvant suppresses IgE antibody responses via selective induction of Th1 immune responses

Topical application of cholera toxin (CT) onto mouse skin can induce a humoral immune response to CT as well as to coadministered Ags. In this study, we examined the nontoxic cell-binding B subunit of CT (CTB) as a potential adjuvant for cutaneous immune responses when coadministered with the prototype protein Ag, OVA. CTB applied onto skin induced serum Ab responses to itself with magnitudes comparable to those evoked by CT but was poorly efficient at promoting systemic Ab responses to coadministered OVA. However, transcutaneous immunization (TCI) with either CT or CTB and OVA led to vigorous OVA-specific T cell proliferative responses. Furthermore, CTB potentiated Th1-driven responses (IFN-gamma production) whereas CT induced both Th1 and Th2 cytokine production. Coadministration of the toxic subunit CTA, together with CTB and OVA Ag, led to enhanced Th1 and Th2 responses. Moreover, whereas TCI with CT enhanced serum IgE responses to coadministered OVA, CTB suppressed these responses. TCI with either CT or CTB led to an increased accumulation of dendritic cells in the exposed epidermis and the underlying dermis. Thus, in contrast to CT, CTB appears to behave very differently when given by the transcutaneous as opposed to a mucosal route and the results suggest that the adjuvanticity of CT on Th1- and Th2-dependent responses induced by TCI involves two distinct moieties, the B and the A subunits, respectively.     

Characterization of a multifunctional feather-degrading <Emphasis Type="Italic">Bacillus subtilis</Emphasis> isolated from forest soil

In this study, we isolated and characterized a novel feather-degrading bacterium that shows keratinolytic, antifungal and plant growth-promoting activities. A bacterium S8 was isolated from forest soil and confirmed to belong to Bacillus subtilis by BIOLOG system and 16S rRNA gene analysis. The improved culture conditions for the production of keratinolytic protease were 0.1% (w/v) sorbitol, 0.3% (w/v) KNO₃, 0.1% (w/v) K₂HPO₄, 0.06% (w/v) KH₂PO₄ and 0.04% (w/v) MgCl₂·6H₂O (pH 8.0 and 30°C), respectively. In the improved medium containing 0.1% (w/v) feather, keratinolytic protease production was around 53.3 ± 0.3 U/ml at 4 day; this value was 10-fold higher than the yield in the basal feather medium (5.3 ± 0.1 U/ml). After cultivation for 5 days in the improved medium, intact feather was completely degraded. Feather degradation resulted in free –SH group, soluble protein and amino acids production. The concentration of free –SH group in the culture medium was 15.5 ± 0.2 μM at 4 days. Nineteen amino acids including all essential amino acids were produced in the culture medium; the concentration of total amino acid produced was 3360.4 μM. Proline (2809.9 μM), histidine (371.3 μM) and phenylalanine (172.0 μM) were the major amino acids released in the culture medium. B. subtilis S8 showed the properties related to plant growth promotion: hydrolytic enzymes, ammonification, indoleacetic acid (IAA), phosphate solubilization, and broad-spectrum antimicrobial activity. Interestingly, the strain S8 grown in the improved medium produced IAA and antifungal activity, indicating simultaneous production of keratinolytic and antifungal activities and IAA by B. subtilis S8. These results suggest that B. subtilis S8 could be not only used to improve the nutritional value of feather wastes but also is useful in situ biodegradation of feather wastes. Furthermore, it could also be a potential biofertilizer or biocontrol agent applicable to crop plant soil.     

Continuous acetone-butanol fermentation: a global approach for the improvement in the solvent productivity in synthetic medium

Summary The present chemostat culture studies were performed in a partial gas recycle system using Clostridium acetobutylicum ATCC 824. Initiation of chemostat cultures at two different levels of vitamins has demonstrated a more than 3 fold improvement in the solvent productivity. The production of extracellular autobacteriocin was increased markedly, when strict anaerobic conditions of the feed vessel were not maintained. When pH was regulated at 4.4, very little variation was seen in the concentration of acids. The duration of solvent production was reduced significantly when NH₄OH was replaced by NaOH for pH regulation, whereas a marked increase in the production of extracellular autobacteriocin was observed. In the optimised medium conditions, a solvent productivity of 2.09 g l^-1 h^-1 (highest ever) could be obtained in the synthetic medium.     

Expression of a synthetic cholera toxin B subunit in tobacco using ubiquitin promoter and bar gene as a selectable marker

A protocol has been developed to produce a cholera toxin B subunit (CTB) in tobacco tolerant to the herbicide phosphinothricin (PPT) by means of in vitro selection. The synthetic CTB subunit gene was altered to modify the codon usage to that of tobacco plant genes. The gene was then cloned into a plant expression vector and was under the control of the ubiquitin promoter and transformed into tobacco plants by Agrobacterium-mediated transformation. Transgenic plantlets were selected in a medium supplemented with 5 mg/L PPT. Polymerase chain reaction analysis confirmed stable integration of the synthetic CTB gene into a chromosomal DNA. A high level of CTB (1.8% of total soluble protein) was expressed in transgenic plants, which was 18-fold higher than that under the control of the expressed CaMV 35S promoter with native gene. The transgenic plants when transferred to a greenhouse proved to be resistant to 2% PPT.     

Medium pH, carbon and nitrogen concentrations modulate the phosphate solubilization efficiency of Penicillium purpurogenum through organic acid production

Aims: To study phosphate solubilization in Penicillium purpurogenum as function of medium pH, and carbon and nitrogen concentrations. Methods and Results: Tricalcium phosphate (CP) solubilization efficiency of P. purpurogenum was evaluated at acid or alkaline pH using different C and N sources. Glucose‐ and (NH₄)₂SO₄‐based media showed the highest P solubilization values followed by fructose. P. purpurogenum solubilizing ability was higher in cultures grown at pH 6·5 than cultures at pH 8·5. Organic acids were detected in both alkaline and neutral media, but the relative percentages of each organic acid differed. Highest P release coincided with the highest organic acids production peak, especially gluconic acid. When P. purpurogenum grew in alkaline media, the nature and concentration of organic acids changed at different N and C concentrations. A factorial categorical experimental design showed that the highest P‐solubilizing activity, coinciding with the highest organic acid production, corresponded to the highest C concentration and lowest N concentration. Conclusions: The results described in the present study show that medium pH and carbon and nitrogen concentrations modulate the P solubilization efficiency of P. purpurogenum through the production of organic acids and particularly that of gluconic acid. In the P solubilization optimization studies, glucose and (NH₄)₂SO₄ as C and N sources allowed a higher solubilization efficiency at high pH. Significance and Impact of the Study: This organism is a potentially proficient soil inoculant, especially in P‐poor alkaline soils where other P solubilizers fail to release soluble P. Further work is necessary to elucidate whether these results can be extrapolated to natural soil ecosystems, where different pH values are present. Penicillium purpurogenum could be used to develop a bioprocess for the manufacture of phosphatic fertilizer with phosphate calcium minerals.     

Characterization of Vibrio cholerae O139 Synonym Bengal Isolated from Patients with Cholera-Like Disease in Bangladesh

Vibrio cholerae O139 (synonym Bengal), a novel serovar of V. cholerae, is the causative agent of large outbreaks of cholera-like illness currently sweeping India and Bangladesh. Eight randomly selected V. cholerae O139 isolates were studied for their biological properties, which were compared with those of V. cholerae O1 and other V. cholerae non-O1. The V. cholerae O139 isolates were characterized by the production of large amount of cholera toxin, hemagglutination, weak hemolytic properties, resistance to polymyxin B, lysogeny with, and production of, kappa type phage (4/8 isolates only), and resistance to both classical and El Tor-specific phages. Thus, V. cholerae O139 isolates had an overall similarity with V. cholerae O1 El Tor.     

Very efficient extracellular production of cholera toxin B subunit using Bacillus brevis

Abstract We have constructed a very efficient synthesis and secretion system for cholera toxin B subunit (CTB) of Vibrio cholerae 569B using Bacillus brevis . The constructed expression-secretion vector has the multiple promoters and the signal peptide coding region of the mwp gene, a structural gene for one of the major cell wall proteins of B. brevis strain 47, directly followed by the gene encoding the mature CTB. A large amount of mature CTB (1.4 g per liter of culture) was secreted into the medium. It had the same amino terminal amino acid sequence as that of authentic CTB and was fully active in GM1 ganglioside binding assay.     

Production of l-Lysine by Leucine Auxotrophs Derived from AEC Resistant Mutant of Brevibacterium lactofermentum

The effect of amino acids was examined on the production of l-lysine by AEC resistant mutant of B. lactofermentum. Among amino acids tested, only leucine showed strong specific inhibition. In order to release the production of l-lysine from this negative effect of leucine, leucine auxotrophs were derived from AEC resistant strain of B. lactofermentum. Most of these leucine auxotrophs produced larger amount of l-lysine (maximally 41 mg/ml) than the parental strain which produced about 18 mg/ml of l-lysine. It was confirmed that leucine auxotrophs derived from AEC resistant mutant of other glutamate producing bacteria, B. saccharolyticum and Corynebacterium glutamicum. These results suggested that leucine might directly or indirectly affect the biosynthesis of lysine.

However, this increase in lysine productivity of leucine auxotrophs could not be explained by the alteration of aspartokinase (EC 2.7.2.4) and homoserine dehydrogenase (EC 1.1.1.3). These enzymes are key enzymes in lysine and threonine biosynthesis, respectively.