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Volume 147,
Issue 4,
2001
Volume 147, Issue 4, 2001
- Review Article
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- Antigens And Immunity
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Antibody response against Escherichia coli heat-stable enterotoxin expressed as fusions to flagellin
More LessThe heat-stable toxin (ST) produced by enterotoxigenic Escherichia coli strains causes diarrhoea by altering the fluid secretion in intestinal epithelial cells. Here, the effectiveness of a flagellin fusion protein of Salmonella containing a 19-amino-acid sequence derived from the ST sequence (FLA–ST) in generating antibodies capable of neutralizing the toxic activity of ST was evaluated. This fusion protein, and an alternative construction where two cysteine residues in the ST sequence were substituted by alanines (STmt), were delivered to the immune system by three distinct strategies: (i) orally, using an attenuated Salmonella strain expressing FLA–ST; (ii) intraperitoneally, by injection of purified FLA–ST; (iii) orally, using attenuated Salmonella carrying a eukaryotic expression plasmid (pCDNA3) with the gene encoding FLA–ST. The results showed that the flagellin system can be used as a carrier to generateST-neutralizing antibodies. However, it should be mentioned that humoral immune response against ST was only obtained when the mutated ST sequence was employed. FLA–ST was found to be non-immunogenic when delivered via the oral route with attenuated Salmonella strains. However, a flagellin antibody response was obtained by immunizing mice with Salmonella carrying pCDNA3/FLA-STmt. Oral immunization with Salmonella carrying the eukaryotic expression plasmid (pCDNA3/FLA–STmt) seems to be a promising method to elicit an appropriate response against fusions to flagellin.
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- Biochemistry
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Clostridium botulinum type A haemagglutinin-positive progenitor toxin (HA+-PTX) binds to oligosaccharides containing Galβ1-4GlcNAc through one subcomponent of haemagglutinin (HA1)
Haemagglutinin (HA) activity of Clostridium botulinum type A 19S and 16S toxins (HA-positive progenitor toxin; HA+-PTX) was characterized. HA titres against human erythrocytes of HA+-PTX were inhibited by the addition of lactose, D-galactose, N-acetyl-D-galactosamine and D-fucose to the reaction mixtures. A direct glycolipid binding test demonstrated that type A HA+-PTX strongly bound to paragloboside and some neutral glycolipids, but did not bind to gangliosides. Type A HA+-PTX also bound to asialoglycoproteins (asialofetuin, neuraminidase-treated transferrin), but not to sialoglycoproteins (fetuin, transferrin). Although glycopeptidase F treatment of asialofetuin abolished the binding of HA+-PTX, endo-α-N-acetylgalactosaminidase treatment did not. Thus these results can be interpreted as indicating that type A HA+-PTX detects and binds to Galβ1-4GlcNAc in paragloboside and the N-linked oligosaccharides of glycoproteins. Regardless of neuraminidase treatment, type A HA+-PTX bound to glycophorin A which is a major sialoglycoprotein on the surface of erythrocytes. Both native glycophorin A and neuraminidase-treated glycophorin A inhibited the binding of erythrocytes to type A HA+-PTX. Since the N-linked oligosaccharide of glycophorin A is di-branched and more than 50% of this sugar chain is monosialylated, type A HA+-PTX probably bound to the unsialylated branch of the N-linked oligosaccharide of glycophorin A and agglutinated erythrocytes. One subcomponent of HA, designated HA1, did not agglutinate native erythrocytes, although it did bind to erythrocytes, paragloboside and asialoglycoproteins in a manner quite similar to that of HA+-PTX. These results indicate that type A HA+-PTX binds to oligosaccharides through HA1.
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- Bioenergetics And Transport
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Intracellular pH regulation by Mycobacterium smegmatis and Mycobacterium bovis BCG
More LessMycobacteria are likely to encounter acidic pH in the environments they inhabit; however intracellular pH homeostasis has not been investigated in these bacteria. In this study, Mycobacterium smegmatis and Mycobacterium bovis [Bacille Calmette–Guérin (BCG)] were used as examples of fast- and slow-growing mycobacteria, respectively, to study biochemical and physiological responses to acidic pH. M. smegmatis and M. bovis BCG were able to grow at pH values of 4·5 and 5·0, respectively, suggesting the ability to regulate internal pH. Both species of mycobacteria maintained their internal pH between pH 6·1 and 7·2 when exposed to decreasing external pH and the maximum ΔpH observed was approximately 2·1 to 2·3 units for both bacteria. The ΔpH of M. smegmatis at external pH 5·0 was dissipated by protonophores (e.g. carbonyl cyanide m-chlorophenylhydrazone), ionophores (e.g. monensin and nigericin) and N,N′-dicyclohexylcarbodiimide (DCCD), an inhibitor of the proton-translocating F1F0-ATPase. These results demonstrate that permeability of the cytoplasmic membrane to protons and proton extrusion by the F1F0-ATPase plays a key role in maintaining internal pH near neutral. Correlations between measured internal pH and cell viability indicated that the lethal internal pH for both strains of mycobacteria was less than pH 6·0. Compounds that decreased internal pH caused a rapid decrease in cell survival at acidic pH, but not at neutral pH. These data indicate that both strains of mycobacteria exhibit intracellular pH homeostasis and this was crucial for the survival of these bacteria at acidic pH values.
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- Development And Structure
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Frozen motion of gliding bacteria outlines inherent features of the motility apparatus
More LessHigh-resolution data of actively gliding wild-type bacteria of four different species and of four different gliding mutants of Myxococcus xanthus were obtained from scanning electron micrographs. By shock freezing and freeze drying, motility-associated surface patterns could be fixed and consequently distinct intermediate states of motion could be observed for the first time. It is shown that these topographic patterns are immediately lost when gliding motility is stopped by blocking the respiratory chain with potassium cyanide or sodium azide. From the surface topography, the mode of action of the gliding apparatus of all four bacterial species examined can be described as a twisted circularly closed ‘band’. During gliding, groups of nodes of the supertwisted apparatus show evidence of travelling like waves along the trichomes. However, the spacing between the nodes is not constant but varies within a certain range. This indicates that they are flexibly modulated as a consequence of the gliding state of the individual trichome.
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Defining the mycoplasma ‘cytoskeleton’: the protein composition of the Triton X-100 insoluble fraction of the bacterium Mycoplasma pneumoniae determined by 2-D gel electrophoresis and mass spectrometry
More LessAfter treating Mycoplasma pneumoniae cells with the nonionic detergent Triton X-100, an undefined, structured protein complex remains that is called the ‘Triton X-100 insoluble fraction’ or ‘Triton shell’. By analogy with eukaryotic cells and supported by ultrastructural analyses it is supposed that this fraction contains the components of a bacterial cytoskeleton-like structure. In this study, the composition of the Triton X-100 insoluble fraction was defined by electron microscopic screening for possible structural elements, and by two-dimensional (2-D) gel electrophoresis and MS to identify the proteins present. Silver staining of 2-D gels revealed about 100 protein spots. By staining with colloidal Coomassie blue, about 50 protein spots were visualized, of which 41 were identified by determining the mass and partial sequence of tryptic peptides of individual proteins. The identified proteins belonged to several functional categories, mainly energy metabolism, translation and heat-shock response. In addition, lipoproteins were found and most of the proteins involved in cytadherence that were previously shown to be components of the Triton X-100 insoluble fraction. There were also 11 functionally unassigned proteins. Based on sequence-derived predictions, some of these might be potential candidates for structural components. Quantitatively, the most prevalent proteins were the heat-shock protein DnaK, elongation factor Tu and subunits α and β of the pyruvate dehydrogenase complex (PdhA, PdhB), but definite conclusions regarding the composition of the observed structures can only be drawn after specific proteins are assigned to them, for example by immunocytochemistry.
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- Environmental Microbiology
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Transcriptional regulation of styrene degradation in Pseudomonas putida CA-3
More LessThe GenBank accession number for the sequence determined in this work is AF257095.
The styrene degradative pathway in Pseudmonas putida CA-3 has previously been shown to be divided into an upper pathway involving the conversion of styrene to phenylacetic acid and a lower pathway for the subsequent degradation of phenylacetic acid. It is reported here that expression of the regulatory genes styS and styR is essential for transcription of the upper pathway, but not for degradation of the lower pathway inducer, phenylacetic acid. The presence of phenylacetic acid in the growth medium completely repressed the upper pathway enzymes even in the presence of styrene, the upper pathway inducer. This repression is mediated at the transcription level by preventing expression of the styS and styR regulatory genes. Finally, an examination was made of the various stages of the diauxic growth curve obtained when P. putida CA-3 was grown on styrene together with an additional carbon source and it is reported that catabolite repression may involve a different mechanism to transcriptional repression by an additional carbon source.
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- Genetics And Molecular Biology
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Feedback control of polyketide metabolism during tylosin production
More LessTylosin is produced by Streptomyces fradiae via a combination of polyketide metabolism and synthesis of three deoxyhexose sugars, of which mycaminose is the first to be added to the polyketide aglycone, tylactone (protylonolide). Previously, disruption of the gene (tylMII) encoding attachment of mycaminose to the aglycone unexpectedly abolished accumulation of the latter, raising the possibility of a link between polyketide metabolism and deoxyhexose biosynthesis in S. fradiae. However, at that time, it was not possible to eliminate an alternative explanation, namely, that downstream effects on the expression of other genes, not involved in mycaminose metabolism, might have contributed to this phenomenon. Here, it is shown that disruption of any of the four genes (tylMI–III and tylB) specifically involved in mycaminose biosynthesis elicits a similar response, confirming that production of mycaminosyl-tylactone directly influences polyketide metabolism in S. fradiae. Under similar conditions, when mycaminose biosynthesis was specifically blocked by gene disruption, accumulation of tylactone could be restored by exogenous addition of glycosylated tylosin precursors. Moreover, certain other macrolides, not of the tylosin pathway, were also found to elicit qualitatively similar effects. Comparison of the structures of stimulatory macrolides will facilitate studies of the stimulatory mechanism.
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The signal transducer (BlaRI) and the repressor (BlaI) of the Staphylococcus aureus β-lactamase operon are inducible
More LessThe precise start points for transcription of the blaZ and of the blaRI/blaI genes of the Staphylococcus aureus β-lactamase operon have been determined by primer extension analysis. Consequently the overlapping promoter sequences were deduced. Northern blots showed that the synthesis of the 2100 nt mRNA from blaRI is inducible and that a blaI probe hybridized to the same mRNA as the blaRI probe. The gene cat, encoding chloramphenicol acetyltransferase, was fused separately to the blaZ and blaRI/blaI promoters, and used to compare their strengths. The promoter for blaZ is about six times stronger than that for blaRI/blaI and the synthesis of chloramphenicol acetyltransferase from both promoters is inducible, supporting the results from the Northern blots.
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Surface exposure and protease insensitivity of Borrelia burgdorferi Erp (OspEF-related) lipoproteins
Borrelia burgdorferi can encode numerous lipoproteins of the Erp family. Although initially described as outer surface proteins, the technique used in that earlier study has since been demonstrated to disrupt bacterial membranes and allow labelling of subsurface proteins. Data are now presented from additional analyses indicating that Erp proteins are indeed surface exposed in the outer membrane. Surface localization of these infection-associated proteins indicates the potential for interactions of Erp proteins with vertebrate tissues. Some Erp proteins were resistant to in situ digestion by certain proteases, suggesting that those proteins fold in manners which hide protease cleavage sites, or that they interact with other protective membrane components. Additionally, cultivation of B. burgdorferi in the presence of antibodies directed against Erp proteins inhibited bacterial growth.
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Type IV O antigen modification genes in the genome of Shigella flexneri NCTC 8296
More LessThe GenBank accession number for the sequence reported in this paper is AF288197.
The genes encoding type IV O antigen glucosylation were characterized from both Escherichia coli and Shigella flexneri. The putative O antigen modification genes from E. coli, o120 o306 o443, were PCR-amplified and introduced into S. flexneri serotype Y strain SFL124. Immunogold labelling and phage sensitivity indicated the presence of both serotype Y and serotype 4a O antigens on the cell surface of the resulting recombinant SFL124 strains, suggesting that only partial serotype conversion was conferred by the E. coli genes. The type IV O antigen modification genes were then isolated and characterized from S. flexneri serotype 4a strain NCTC 8296. A 3·8 kb chromosomal fragment conferred complete conversion to serotype 4a when introduced into SFL124. Sequence analysis of the fragment revealed the presence of three genes, gtrA IV gtrB IV gtrIV Sf. DNAs homologous to bacteriophage int and attP were located upstream of gtrA IV, suggesting that this region of the NCTC 8296 genome may have originated from a bacteriophage; however, a serotype-converting phage could not be induced from this strain nor from other strains used in this study. Comparison of the GtrIVSf and GtrIVEc (o443) proteins revealed that they are 41% identical and 63% similar, which is the highest degree of similarity reported among the S. flexneri O antigen glucosyltransferases.
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Detergent-independent in vitro activity of a truncated Bacillus signal peptidase
The Gram-positive eubacterium Bacillus subtilis contains five chromosomally encoded type I signal peptidases (SPases) for the processing of secretory pre-proteins. Even though four of these SPases, denoted SipS, SipT, SipU and SipV, are homologous to the unique SPase I of Escherichia coli, they are structurally different from that enzyme, being almost half the size and containing one membrane anchor instead of two. To investigate whether the unique membrane anchor of Bacillus SPases is required for in vitro activity, soluble forms of SipS of B. subtilis, SipS of Bacillus amyloliquefaciens and SipC of the thermophile Bacillus caldolyticus were constructed. Of these three proteins, only a hexa-histidine-tagged soluble form of SipS of B. amyloliquefaciens could be isolated in significant quantities. This protein displayed optimal activity at pH 10, which is remarkable considering the fact that the catalytic domain of SPases is located in an acidic environment at the outer surface of the membrane of living cells. Strikingly, in contrast to what has been previously reported for the soluble form of the E. coli SPase, soluble SipS was active in the absence of added detergents. This observation can be explained by the fact that a highly hydrophobic surface domain of the E. coli SPase, implicated in detergent-binding, is absent from SipS.
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The Bacillus subtilis yabQ gene is essential for formation of the spore cortex
More LessAn extensive screening for transcripts with probes specific for the genes in a 108 kb region from rrnO to spo0H of the Bacillus subtilis chromosome led to identification of an operon, yabP–yabQ–divIC–yabR, the expression of which was initiated at the second hour of sporulation and in a σE-dependent manner. Among three y genes in the operon, deletion of the yabQ gene, which is predicted to encode a protein product of 468 residues with five membrane-spanning domains, resulted in a large decrease in numbers of chloroform-, lysozyme- and heat-resistant spores, compared to findings with the wild-type strain. Electron microscopy revealed that development of the spore cortex was blocked in the yabQ mutant. In addition, although the spore coat was visible, the inner coat layer of the mutant seemed partially detached from the outer coat. In sporangia of the strains harbouring an in-frame fusion of the green fluorescent protein gene to yabQ, fluorescence was detected around the forespore. This localization did not depend on SpoIVA or on CotE functions, both of which determine proper localization of coat proteins and cortex formation. The yabQ deletion did not affect expression of genes involved in cortex synthesis. These results suggest that the YabQ protein localizes in the membrane of the forespore and plays an important role in cortex formation.
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New members of the ctrA regulon: the major chemotaxis operon in Caulobacter is CtrA dependent
More LessThe Caulobacter crescentus che promoter region consists of two divergent promoters, directing expression of the major chemotaxis operon and a novel gene cagA (chemotaxis associated gene A). Analyses of start sites by primer extension and alignment of the divergent promoters revealed significant similarities between them at the −35 promoter region. Both mcpA and cagA are differentially expressed in the cell cycle, with maximal activation of transcription in predivisional cells. The main difference between the mcpA and cagA promoters is that, in common with the fljK flagellin, cagA is expressed in swarmer cells. A cagA–lacZ promoter fusion that contains 36 bases of untranslated mRNA has sufficient information to segregate the lacZ transcript to swarmer cells. Expression of mcpA and cagA was dependent on DNA replication. Transcriptional epistasis experiments were performed to identify potential regulators in the flagellar hierarchy. The sigma factor RpoN, which is required for flagellar biogenesis, is not required for mcpA and cagA expression. Mutations in the genes for the MS-ring and the switch complex (flagellar class II mutants) do not affect expression of mcpA and cagA. However, CtrA, an essential response regulator of flagellar gene transcription, is required.
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A new simvastatin (mevinolin)-resistance marker from Haloarcula hispanica and a new Haloferax volcanii strain cured of plasmid pHV2
More LessThe GenBank accession number for the sequence reported in this paper is AF123438.
The mevinolin-resistance determinant, hmg, encodes the enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and is a commonly used selectable marker in halobacterial genetics. Plasmids bearing this marker suffer from instability in Haloferax volcanii because the resistance gene was derived from the genome of this species and is almost identical in sequence to the chromosomal copy. In order to reduce the level of homologous recombination between introduced plasmid vectors and the chromosome of Haloferax, a homologue of the hmg determinant was obtained from the distantly related organism, Haloarcula hispanica. The nucleotide sequences of the wild-type genes (hmgA) of these two species are only 78% identical, and the predicted protein sequences show 71% identity. In comparison to the wild-type hmgA gene, the resistance gene from a mutant resistant to simvastatin (an analogue of mevinolin) showed a single base substitution in the putative promoter. Plasmids constructed using the new resistance determinant were stably maintained under selection in Hfx. volcanii and possessed very low recombination rates with the chromosome of this species. In addition, an improved strain of Hfx. volcanii was developed to overcome the plasmid instability and growth reduction observed in the commonly used WFD11 strain.
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The product of the ybdE gene of the Escherichia coli chromosome is involved in detoxification of silver ions
More LessTranscription of the ybcZ–ylcA ylcBCD–ybdE region of the Escherichia coli K38 chromosome was analysed by Northern RNA–DNA hybridization, RT-PCR and primer extension. Transcription of a dicistronic ybcZ–ylcA mRNA and a tetracistronic ylcBCD–ybdE mRNA was induced by silver and was initiated from the sigma-70 promoters ylcAp and ylcBp. Expression of β-galactosidase activity from a Φ(ylcBp–lacZ) operon fusion was also induced by Ag+ and Cu2+, but not by Zn2+. In-frame deletion of ybdE from the chromosome yielded a silver-sensitive E. coli mutant strain which did not differ in its copper resistance from its wild-type strain. On the other hand, deletion of the copA gene for the copper-exporting P-type ATPase CopA resulted in copper sensitivity, but not in silver sensitivity. A ΔybdE ΔcopA double mutant strain behaved towards copper as the ΔcopA strain and towards silver as the ΔybdE strain. Thus, in E. coli, the YlcBCD–YbdE system may be involved in silver- but not in copper resistance, and CopA may be involved in copper- but not in silver resistance.
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Topological investigations of the FomA porin from Fusobacterium nucleatum and identification of the constriction loop L6
More LessThe GenBank accession number for the sequence reported in this paper is X72583.
Porin FomA in the outer membrane of Fusobacterium nucleatum is a trimeric protein, which exhibits permeability properties similar to that of the well-known enterobacterial diffusion porins. The proposed topology model of the FomA monomer depicts the β-barrel motif typical of diffusion porins, consisting of 16 antiparallel β-strands. To investigate the accuracy of the FomA model and assess the topological relationship with other porins, individual deletions of variable size in seven of the eight surface-exposed regions of the porin were genetically engineered. Deletions in the predicted loops L1 to L7 were tolerated by the FomA porins, as judged by a normal assembly in the outer membrane of Escherichia coli and a sustained pore-forming ability. Deletions in the largest proposed external region, loop L6, made the FomA porins considerably more permeable to antibiotics, indicating larger pore channels. The distinctly increased uptake rates and size exclusion limits displayed by the L6 deletion mutant porins, suggest that loop L6 folds back into the β-barrel thereby constricting the native FomA channel. Thus, the position of the channel constriction loop appears to be shifted towards the C terminus in the FomA porin, as compared to the crystal structures of five non-specific diffusion porins.
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The gene yghK linked to the glc operon of Escherichia coli encodes a permease for glycolate that is structurally and functionally similar to L-lactate permease
More LessIn Escherichia coli the glc operon involved in glycolate utilization is located at 67·3 min and formed by genes encoding the enzymes glycolate oxidase (glcDEF) and malate synthase G (glcB). Their expression from a single promoter upstream of glcD is induced by growth on glycolate and regulated by the activator encoded by the divergently transcribed gene glcC. Gene yghK, located 350 bp downstream of glcB, encodes a hydrophobic protein highly similar to the L-lactate permease encoded by lldP. Expression studies have shown that the yghK gene (proposed name glcA) is transcribed from the same promoter as the other glc structural genes and thus belongs to the glc operon. Characterization of a glcA::cat mutant showed that GlcA acts as glycolate permease and that glycolate can also enter the cell through another transport system. Evidence is presented of the involvement of L-lactate permease in glycolate uptake. Growth on this compound was abolished in a double mutant of the paralogous genes glcA and lldP, and restored with plasmids expressing either GlcA or LldP. Characterization of the putative substrates for these two related permeases showed, in both cases, specificity for the 2-hydroxymonocarboxylates glycolate, L-lactate and D-lactate. Although both GlcA and LldP recognize D-lactate, mutant analysis proved that L-lactate permease is mainly responsible for its uptake.
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- Pathogenicity And Medical Microbiology
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Analysis of the role of flagellar activity in virulence gene expression in Vibrio cholerae
More LessExpression of virulence factors and motility of Vibrio cholerae are intimately linked by an as yet uncharacterized mechanism. Several lines of evidence indicate that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation, as alterations of the motility phenotype, either by mutation or by inhibitory drugs, result in varied levels of virulence factor production. The Na+-driven polar flagella of some Vibrio species are proposed to act as mechanosensors, sensing media viscosity. It has been suggested that the V. cholerae flagellum might act as a ‘voltmeter’, responding to changes in membrane potential, or might sense some environmental conditions that lead to the repression of virulence factors in V. cholerae. To test these hypotheses, β-galactosidase levels of several types of non-motile mutant derivatives of a V. cholerae toxT::lacZ reporter strain were analysed following changes in media viscosity, membrane potential and other environmental conditions. Like the parental strain, the non-motile strain showed increased toxT::lacZ expression in high-viscosity media, suggesting that the sensing of media viscosity does not occur via the flagella. Other molecules that might be able to detect changes in media viscosity could include mechanosensitive (MS) ion channels found in the bacterial membrane. However, a V. cholerae derivative strain mutated in two putative MS channels still showed increased toxT::lacZ expression in high-viscosity media, indicating that these putative ion channels of V. cholerae are not involved in the viscosity effect and suggesting an as yet uncharacterized mechanism for sensing of media viscosity. The flagellum does not appear to act as a voltmeter, as β-galactosidase activities of the non-flagellate derivative strain were found to be similar to those of the parental strain after artificially changing the sodium membrane bioenergetics. Similarly, several environmental conditions known to reduce toxin expression were equally effective in reducing toxT::lacZ expression in the motile or non-motile strains. In conclusion, the flagellum of V. cholerae does not act as a mechanosensor, voltmeter or signal transducer for environmental conditions. Thus, alternative mechanisms for the detection of these conditions must exist that likely do not involve the ToxR molecule, as the sensing of all of the tested parameters occurred when the TcpP/H proteins alone activated the toxT::lacZ reporter gene.
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