Promoter activities in Vibrio cholerae ctx phi prophage

Cholera toxin expression by El Tor Vibrio cholerae in shallow culture growth conditions

Vibrio cholerae O1 classical, El Tor and O139 are the primary biotypes that cause epidemic cholera, and they also express cholera toxin (CT). Although classical V. cholerae produces CT in various settings, the El Tor and O139 strains require specific growth conditions for CT induction, such as the so-called AKI conditions, which consist of growth in static conditions followed by growth under aerobic shaking conditions. However, our group has demonstrated that CT production may also take place in shallow static cultures. How these type of cultures induce CT production has been unclear, but we now report that in shallow culture growth conditions, there is virtual depletion of dissolved oxygen after 2.5 h of growth. Concurrently, during the first three to 4 h, endogenous CO2 accumulates in the media and the pH decreases. These findings may explain CT expression at the molecular level because CT production relies on a regulatory cascade, in which the key regulator AphB may be activated by anaerobiosis and by low pH. AphB activation stimulates TcpP synthesis, which induces ToxT production, and ToxT directly stimulates ctxAB expression, which encodes CT. Importantly, ToxT activity is enhanced by bicarbonate. Therefore, we suggest that in shallow cultures, AphB is activated by initial decreases in oxygen and pH, and subsequently, ToxT is activated by intracellular bicarbonate that has been generated from endogenous CO2. This working model would explain CT production in shallow cultures and, possibly, also in other growth conditions.

A Vibrio cholerae relaxed (relA) mutant expresses major virulence factors, exhibits biofilm formation and motility, and colonizes the suckling mouse intestine

We have constructed a relaxed mutant of El Tor biotype Vibrio cholerae strain C7258 by disruption of the RelA catalytic domain. The ability of the V. cholerae relaxed mutant to biosynthesize guanosine tetraphosphate and pentaphosphate was severely affected; the mutant showed a reduced growth rate in minimal medium that could be reversed by the addition of Casamino Acids, and it was thermosensitive. Contrary to published findings, the new relA mutant still produced significant cholera toxin and toxin-coregulated pilus. The V. cholerae relA mutant was motile, produced normal biofilms, and colonized the suckling mouse intestine. Our data suggest that levels of basal guanosine nucleotides pppGpp and ppGpp, rather than the availability of a stringent response, could influence expression of virulence factors, depending on strain and culture conditions. Production of hemagglutinin (HA)/protease, which requires HapR, RpoS, and the cyclic AMP receptor protein, was not strongly affected. Nevertheless, overexpression of RelA protein from an isopropyl-beta-d-thiogalactopyranoside-inducible promoter posttranscriptionally diminished production of HA/protease.

The Vibrio cholerae vieSAB locus encodes a pathway contributing to cholera toxin production

The genes encoding cholera toxin (CT), ctxAB, are coregulated with those for other Vibrio cholerae virulence factors by a cascade of transcriptional activators, including ToxR, TcpP, and ToxT. Additional regulators that modulate expression of ctxAB during infection were recently identified in a genetic selection. A transposon insertion in vieS, the sensor kinase of the VieSAB three-component signal transduction system, resulted in failure to induce expression of a ctxA-recombinase fusion during murine infection. To determine which components of the VieSAB system are essential for CT regulation, ctxAB transcript levels were assessed by RNase protection assay in various vieSAB in-frame deletion mutants after growth in vitro under virulence gene inducing conditions. A threefold reduction in ctxAB transcript levels was observed for the (Delta)vieSAB strain; consistent with this, the (Delta)vieSAB strain produced twofold less CT protein than the wild type, and this defect was complementable in trans. These results suggest that the VieSAB three-component system is required for full activation of the ctxAB operon during in vitro growth as well as during infection. The VieSAB system may regulate ctxAB expression indirectly by affecting production of ToxT, because decreased toxT transcript levels were observed in the (Delta)vieSAB strain.

Expression of Vibrio cholerae zonula occludens toxin and analysis of its subcellular localization

Vibrio cholerae elaborates zonula occludens toxin (Zot), a protein that increases the permeability of small intestinal mucosa by opening intercellular tight junctions. The zot gene is located, together with the genes encoding CT and Ace enterotoxins, within the genome of V. cholerae filamentous phage CTXsmall ef, Cyrillic. Interestingly, Zot appears to be structurally and functionally related to the gene I product of other filamentous phages and it has been shown to be required for CTXsmall ef, Cyrillic morphogenesis. In this study we described the cloning of zot in several expression plasmid systems and we examined the subcellular localization of Zot by using affinity purified anti-Zot antibodies. We found that Zot localizes in the V. cholerae cell envelope with M(r);45 kDa which is consistent with the predicted primary translation product from the first methionine of zot (44.8 kDa). A second molecule, corresponding to the 33 kDa N-terminal region of Zot, was also detected. Both molecules are exposed at the bacterial cell surface. The production of the 33 kDa Zot, that might represent a processing product, was abolished in mutant ZotG59. N-terminal tagged 6xHis-Zot fusion protein retained the capability to reach the outer membrane and the 6xHis tag was not cleaved off during the translocation to the periplasm, whereas the presence of the tag partially blocked the formation of the 33 kDa molecule. Zot secretion and anchorage to the bacterial outer membrane was also observed in E. coli strains expressing Zot, suggesting that the toxin may be directed to the outer membrane via the same pathway in E. coli and V. cholerae. Zot cleavage might be due to a V. cholerae specific protease activity, since the 33 kDa protein was not efficiently produced in E. coli. On the basis of these data and Zot amino acid sequence analysis, we suggest that while the N-terminal part of the molecule is involved in the morphogenesis of CTXsmall ef, Cyrillic, the C-terminal region might carry the domain(s) responsible for Zot enterotoxic activity.

Zonula occludens toxin is a powerful mucosal adjuvant for intranasally delivered antigens

Zonula occludens toxin (Zot) is produced by toxigenic strains of Vibrio cholerae and has the ability to reversibly alter intestinal epithelial tight junctions, allowing the passage of macromolecules through the mucosal barrier. In the present study, we investigated whether Zot could be exploited to deliver soluble antigens through the nasal mucosa for the induction of antigen-specific systemic and mucosal immune responses. Intranasal immunization of mice with ovalbumin (Ova) and recombinant Zot, either fused to the maltose-binding protein (MBP-Zot) or with a hexahistidine tag (His-Zot), induced anti-Ova serum immunoglobulin G (IgG) titers that were approximately 40-fold higher than those induced by immunization with antigen alone. Interestingly, Zot also stimulated high anti-Ova IgA titers in serum, as well as in vaginal and intestinal secretions. A comparison with Escherichia coli heat-labile enterotoxin (LT) revealed that the adjuvant activity of Zot was only sevenfold lower than that of LT. Moreover, Zot and LT induced similar patterns of Ova-specific IgG subclasses. The subtypes IgG1, IgG2a, and IgG2b were all stimulated, with a predominance of IgG1 and IgG2b. In conclusion, our results highlight Zot as a novel potent mucosal adjuvant of microbial origin.

Preliminary assessment of the safety and immunogenicity of a new CTXPhi-negative, hemagglutinin/protease-defective El Tor strain as a cholera vaccine candidate

Vibrio cholerae 638 (El Tor, Ogawa), a new CTXPhi-negative hemagglutinin/protease-defective strain that is a cholera vaccine candidate, was examined for safety and immunogenicity in healthy adult volunteers. In a double-blind placebo-controlled study, no significant adverse reactions were observed in volunteers ingesting strain 638. Four volunteers of 42 who ingested strain 638 and 1 of 14 who received placebo experienced loose stools. The strain strongly colonized the human small bowel, as evidenced by its isolation from the stools of 37 of 42 volunteers. V. cholerae 638, at doses ranging from 4 x 10(7) to 2 x 10(9) vibrios, elicited significant serum vibriocidal antibody and anti-Ogawa immunoglobulin A antibody secreting cell responses.

Cholera toxin differentially regulates nitric oxide synthesis, tumor necrosis factor-alpha production and respiratory burst in murine macrophages

The aim of this study was to determine if cholera toxin can modulate the expression of several macrophage effector functions. The effect of cholera toxin on the induction of NO synthesis, production of tumor necrosis factor-alpha and induction of respiratory burst was examined in the J774.A2 macrophage cell line. Pre-incubation of cell cultures with cholera toxin significantly down-regulated lipopolysaccharide-induced NO synthesis and phorbol-12-myristate-13-acetate-induced respiratory burst. Concomitant addition of cholera toxin and lipopolysaccharide to cell cultures enhanced the production of tumor necrosis factor-alpha. These effects were abrogated when cholera toxin was inactivated by heat or treated with a specific monoclonal antibody.

ToxR-independent expression of cholera toxin from the replicative form of CTXphi

The ctxAB operon, which encodes cholera toxin, resides in the genome of CTXphi, a filamentous bacteriophage. Within Vibrio cholerae cells, the CTXphi genome can exist either as a replicating plasmid or as a prophage integrated into the chromosome. Previous work established that ToxR is required for chromosomal ctxAB expression. We have learned that strains harboring the CTXphi replicative form produce cholera toxin under all conditions tested, independently of ToxR. During passage of CTXphi lysogens through the infant mouse intestine, transduction of CTXphi to a recipient strain can be detected, indicating that phage excision and replication occur in vivo. These results suggest that phage induction might provide a novel mechanism for the regulation of cholera toxin production.