Genetic mapping of Vibrio cholerae enterotoxin structural genes

The structural genes which constitute the cholera toxin operon, ctxAB, were genetically mapped in the Vibrio cholerae El Tor strain RV79. This strain of V. cholerae contains two copies of the ctx operon located on a 7-kilobase-pair tandemly duplicated region. We began by isolating a vibriophage VcA1 insertion mutation in one of the two ctxA genes located in this region. The mutant carrying this ctxA::VcA1 insertion, DC24, was converted to a VcA1-facilitated donor by introduction of the conjugal plasmid pSJ15, which carries an inserted copy of a defective VcA1-like prophage. The donor characteristics of DC24(pSJ15) indicated that the ctxA::VcA1 insertion mutation was near the trp region of the V. cholerae chromosome. Subsequent RV79 three-factor crosses were performed between VcA1-facilitated donors and recipient strains carrying one of two structural gene mutations in ctx, either delta ctxA23P Kmr or delta ctx-7922. The former was constructed by an in vivo marker exchange procedure and could be scored either by its kanamycin resistance phenotype or by its lack of DNA sequences homologous to the ctxA region. The delta ctx-7922 mutation is a total deletion of both ctx copies of strain RV79. The three-factor cross data strongly suggest that the two ctx loci of RV79 map between the nal and his genes of V. cholerae in the trp nal his linkage group. Physical analysis and heterologous crosses between an RV79 El Tor donor and a 569B classical recipient indicates that one of the two 569B ctx operon copies maps in the same region as the RV79 ctx loci (i.e., linked to nal). Together with previously published observations, these data show that the ctx structural genes are not closely linked to other genes known to affect toxin production in V. cholerae.     

A gene for the enterotoxin zonula occludens toxin is present in Vibrio mimicus and Vibrio cholerae O139

Abstract The presence of the zonula occludens toxin (ZOT) gene, which encodes an enterotoxin produced by serotype O1 strains of the pathogenic bacterium, Vibrio cholerae , in addition to cholera toxin, was investigated in selected strains of V. mimicus and the new pandemic V. cholerae non-O1 serotype O139. The zot gene was detected by polymerase chain reaction (PCR) amplification, using sets of primers based on the sequence of the V. cholerae O1 zot sequence. PCR amplification of genomic DNAs of both cholera toxin gene ( ctx ) positive and ctx⁻ strains of V. mimicus detected the presence of zot gene. An Acc -I- Eco RV V. cholerae zot gene fragment designed to overlap PCR products was used as a probe. Southern hybridization studies confirmed that the PCR fragments from V. mimicus and V. cholerae O139 were strongly homologous to the V. cholerae O1 zot gene. The zot gene was found with 3 to 5 strains of V. mimicus of which only one strain harbored the ctx gene. The presence of a zot gene in ctx⁻ toxigenic V. mimicus indicates a possible role of ZOT in the toxigenicity of this species. We conclude that, in addition to ctx, V. mimicus and V. cholerae O139 have the potential to produce ZOT.     

Distribution of the zot (zonula occludens toxin) gene among strains of Vibrio cholerae 01 and non-01

Abstract The distribution of the zot gene that encodes the zonula occludens toxin, a newly described toxin of Vibrio cholerae , among clinical, environmental and food isolates of V. cholerae 01 and non-01 was investigated. Both the zot gene and the ctx gene that encode cholera toxin were found in 247 of 257 clinical strains and 62 of 415 environmental or food isolates of V. cholerae 01. The zot gene, but not the ctx gene was found in 37 strains (one clinical strain and 36 environmental or food isolates). In addition, two of 31 clinical strains and six of 98 environmental or food isolates of V. cholerae non-01 possessed both the zot gene and the ctx gene. These results demonstrated the predominantly concurrent occurrence of the zot gene and ctx genes among strains of V. cholerae 01 which suggests a possible synergistic role of ZOT in the causation of acute dehydrating diarrhea produced by V. cholerae 01.     

Duplication and amplification of toxin genes in Vibrio cholerae

Vibrio cholerae strains of the classical biotype all contain two widely separated copies of the cholera toxin operon ctxAB. In contrast, EI Tor strains containing multiple copies of ctx have their copies arranged on large tandem repeats which are either 7 or 9.7 kb in length. The variation in size among these large tandem duplications was due to a difference in the copy number of a smaller, 2.7 kb, tandemly repeated sequence (RS1) that is located at the novel joint of these duplications, as well as upstream and downstream of ctx. Southern blot hybridization analysis indicated that amplification of a DNA region carrying ctx and flanked by direct repeats of RS1 may be responsible for the hypertoxinogenic phenotype of EI Tor variants selected by intraintestinal growth in rabbits.     

Testing the biological activity of Vibrio cholerae on cell subcultures

Testing the supernatants of ctx(+) strains of V. cholerae eltor and V. cholerae O139 on cell subcultures confirmed the possibility of the synthesis of hemolysin by V. cholerae under the condition of growing them in tripton medium lacking FeCl3. At the same time ctx(+) strains of V. cholerae of both serogroups retained, simultaneously with hemolysin production, their capacity for the synthesis of cholera toxin.     

Distribution of Vibrio cholerae virulence genes among different Vibrio species isolated in Sardinia, Italy

The members of the genus Vibrio include harmless aquatic strains as well as strains capable of causing epidemics of cholera. Diarrhoea caused by Vibrio cholerae is attributed to cholerae enterotoxin (CT) codified by the ctx operon and regulated by a number of virulence genes such as toxT, toxR and toxS. Fifty-two Vibrio strains were isolated from different aquatic environments in and around Sardinia and searched by PCR for the presence of ctxA, zot, ace, toxR, toxS, toxT, tcpA and vpi virulence genes in the genomes of the isolates. The toxR operon was found in 27 Vibrio alginolyticus strains out of 42 analysed, in three out of four V. cholerae non-O1 strains and in three Vibrio parahaemolyticus isolates. A positive amplification for the virulence pathogenic island (vpi) was produced by five V. alginolyticus strains. Finally, the ace expected amplification fragment was found in two V. alginolyticus isolates whereas the amplification with zot primers produced the expected fragment in one V. alginolyticus isolate. Differentiation of these strains with a PCR fingerprinting technique revealed no association between the presence of virulence genes and a particular fingerprinting pattern. Although most Vibrio species are considered non-pathogenic or only potentially harmful to humans, the finding of V. cholerae virulence genes in other members of the genus Vibrio, and the recent reports of the creation and evolution of pandemic strains of V. cholerae, may give a new perspective to the significance of these results.     

Incidence and toxigenicity of Vibrio cholerae in a freshwater lake during the epidemic of cholera caused by serogroup O139 Bengal in Calcutta, India

Abstract The extent of contamination of a freshwater lake with Vibrio cholerae 0139 Bengal and the toxigenicity of all the V. cholerae isolates recovered during the period of the study were examined during and after an explosive outbreak of 0139 cholera in Calcutta. Strains biochemically characterized as V. cholerae could be isolated throughout the period of study examined from the freshwater lake samples. Most probable number of V. cholerae belonging to the 0139 serogroup in surface waters was 3 to 4 per 100 ml during major part of the study but isolation of this serogroup from sediment and plankton samples was infrequent. Of the total of 150 strains recovered, 23 (15.3%) agglutinated with the 0139 antiserum while the remaining belonged to the non-O1 non-O139 serogroups. None of the strains agglutinated with the O1 antiserum. All the 23 strains of V. cholerae O139 produced cholera toxin while 7.9% of the 127 non-O1 non-O139 strains also produced cholera toxin. Resistance to ampilicillin, furazolidone and streptomycin was encountered among strains belonging to both V. cholerae O139 and V. cholerae non-O1 non-O139 strains, but the percentage of resistant strains in the former was much higher than in the latter. During this cholera epidemic, possibly due to the introduction of large numbers of toxigenic V. cholerae such as the O139 serogroup, there was an increase in the number of toxigenic vibrios among the innocuous aquatic residents. This presumably occured through genetic exchange and, if substantiated, could play an important role in the re-emergence of epidemics.     

Single multiplex polymerase chain reaction for environmental surveillance of toxigenic—Pathogenic O1 and Non-O1vibrio cholerae

A multiplex PCR assay was developed for the detection of toxigenic and pathogenic V. cholerae from direct water sources using specific primers targeting diverse genes, viz. outer membrane protein (ompW), cholera toxin (ctxB), ORF specific for O1 (rfbG), zonula occludens (zot) and toxin co-regulated pilus (tcpB); among these genes, ompW acts as internal control for V. cholerae, the ctx gene as a marker for toxigenicity and tcp for pathogenicity. The sensitivity of multiplex PCR was 5 x 10(4) V. cholerae cells per reaction. The procedure was simplified as direct bacterial cells were used as template and there was no need for DNA extraction. The assay was specific as no amplification occurred with the other bacteria used. Toxigenic V. cholerae were artificially spiked in different water samples, filtered through a 0.45 microm membrane, and the filters containing bacteria were enriched in APW for 6 h. PCR following filtration and enrichment could detect as little as 8 V. cholerae cells per mL in different spiked water samples. Various environmental potable water samples were screened for the presence of V. cholerae using this assay procedure. The proposed method is rapid, sensitive and specific for environmental surveillance for the presence of toxigenic-pathogenic and nonpathogenic V. cholerae.     

Inheritance and expression of cholera toxin genes introduced into Vibrio cholerae el tor cells in a hybrid plasmid

Two replicons, pOX38 (in F-factor derivative lacking all IS elements) and pCT105 (containing cholera toxin operon cloned in pBR322) have been fused to produce recombinant plasmid, pCO109, which was then introduced into Vibrio cholerae eltor by conjugation. Restriction analysis showed pCO109 to dissociate in V. cholerae cells at a higher frequency than in Escherichia coli strains, its pOX38 component being lost, while the pCT105 component demonstrated relative stability. V. cholerae eltor RV79 (pCT105) produced 4-5 micrograms/ml of cholera toxin. Occasional insertion of cloned vctA, B operon into RV79 chromosome was also observed.     

Comparative study of the diagnostic value of new cholera eltor bacteriophages ctx+, ctx-, and KhDF-3, 4, and 5

The comparative evaluation of the diagnostic value of new cholera eltor bacteriophages ctx+ and ctx-, as well as monophages X[symbol: see text]-3, 4, 5, demonstrated their high activity and specificity. Using of these bacteriophages epidemic potential of 95% Vibrio cholerae eltor strains ctx+ and 84.5% of V. cholerae eltor stains ctx- was determined. Commercial monophages X[symbol: see text]-3, 4, 5 were inferior to bacteriophages ctx+ and ctx- in their diagnostic value: only 55% of strains having gene ctxAB were found to be epidemically dangerous, i.e. 45% of strains capable of causing the disease were not detected. On the basis of the results obtained in this investigation cholera eltor bacteriophages ctx+ and ctx- were recommended for introduction into practical use, while further production of cholera diagnostic monophages X[symbol: see text]-3, 4, 5 was recommended to be stopped.     

Detection of heat-stable enterotoxin genes among Australian Vibrio cholerae O1 strains

Abstract DNA probes derived from the heat-stable enterotoxin gene of Vibrio cholerae non-O1 ( stn ), and the cholera toxin gene (etc), were used to screen 199 strains of V. cholerae O1, which were isolated within Australia from 1977–1986. 13 environmental strains isolated from the riverine environment in Southeast Queensland in 1980 and 1981, hybridized with the stn and ctx DNA probes. The concentrated supernatant of 6 of these strains elicited fluid accumulation in the infant mouse assay both before and after heating at 100 °C for 5 min. Genetic relationships among the 13 stn ⁺ strains were studied by a comparison of the rRNA-RFLPs (ribotyping) and by Southern blot analysis with a stn gene probe. The results indicate that there is a clonal relationship among the Australian stn ⁺ strains and that there is an environmental reservoir of stn genes among Australian V. cholerae O1 isolates.     

Genome sequencing reveals unique mutations in characteristic metabolic pathways and the transfer of virulence genes between V. mimicus and V. cholerae

Vibrio mimicus, the species most similar to V. cholerae, is a microbe present in the natural environmental and sometimes causes diarrhea and internal infections in humans. It shows similar phenotypes to V. cholerae but differs in some biochemical characteristics. The molecular mechanisms underlying the differences in biochemical metabolism between V. mimicus and V. cholerae are currently unclear. Several V. mimicus isolates have been found that carry cholera toxin genes (ctxAB) and cause cholera-like diarrhea in humans. Here, the genome of the V. mimicus isolate SX-4, which carries an intact CTX element, was sequenced and annotated. Analysis of its genome, together with those of other Vibrio species, revealed extensive differences within the Vibrionaceae. Common mutations in gene clusters involved in three biochemical metabolism pathways that are used for discrimination between V. mimicus and V. cholerae were found in V. mimicus strains. We also constructed detailed genomic structures and evolution maps for the general types of genomic drift associated with pathogenic characters in polysaccharides, CTX elements and toxin co-regulated pilus (TCP) gene clusters. Overall, the whole-genome sequencing of the V. mimicus strain carrying the cholera toxin gene provides detailed information for understanding genomic differences among Vibrio spp. V. mimicus has a large number of diverse gene and nucleotide differences from its nearest neighbor, V. cholerae. The observed mutations in the characteristic metabolism pathways may indicate different adaptations to different niches for these species and may be caused by ancient events in evolution before the divergence of V. cholerae and V. mimicus. Horizontal transfers of virulence-related genes from an uncommon clone of V. cholerae, rather than the seventh pandemic strains, have generated the pathogenic V. mimicus strain carrying cholera toxin genes.     

Characterization of VPI pathogenicity island and CTXphi prophage in environmental strains of Vibrio cholerae

Environmental isolates of Vibrio cholerae of eight randomly amplified polymorphic DNA (RAPD) fingerprint types from Calcutta, India, that were unusual in containing toxin-coregulated pilus or cholera toxin genes but not O1 or O139 antigens of epidemic strains were studied by PCR and sequencing to gain insights into V. cholerae evolution. We found that each isolate contained a variant form of the VPI pathogenicity island. Distinguishing features included (i) four new alleles of tcpF (which encodes secreted virulence protein; its exact function is unknown), 20 to 70% divergent (at the protein level) from each other and canonical tcpF; (ii) a new allele of toxT (virulence regulatory gene), 36% divergent (at the protein level) in its 5' half and nearly identical in its 3' half to canonical toxT; (iii) a new tcpA (pilin) gene; and (iv) four variant forms of a regulatory sequence upstream of toxT. Also found were transpositions of an IS903-related element and function-unknown genes to sites in VPI. Cholera toxin (ctx) genes were found in isolates of two RAPD types, in each case embedded in CTXphi-like prophages. Fragments that are inferred to contain only putative repressor, replication, and integration genes were present in two other RAPD types. New possible prophage repressor and replication genes were also identified. Our results show marked genetic diversity in the virulence-associated gene clusters found in some nonepidemic V. cholerae strains, suggest that some of these genes contribute to fitness in nature, and emphasize the potential importance of interstrain gene exchange in the evolution of this species.     

Rapid polymerase chain reaction method for detection of Vibrio cholerae in foods.

The polymerase chain reaction was used to selectively amplify sequences within the cholera toxin operon from Vibrio cholerae O1. Oysters, crabmeat, shrimp, and lettuce were seeded with V. cholerae and then homogenized or washed with alkaline peptone water, followed by short-term (6- to 8-h) enrichment. A detection limit of as few as 1 V. cholerae CFU per 10 g of food was obtained with amplification reactions from crude bacterial lysates. The method is extremely rapid and obviates the need for DNA isolation from a variety of complex food matrices.     

Detection of toxigenic Vibrio cholerae from environmental water samples by an enrichment broth cultivation-pit-stop semi-nested PCR procedure

A pit-stop semi-nested PCR assay for the detection of toxigenic Vibrio cholerae in environmental water samples was developed and its performance evaluated. The PCR technique amplifies sequences within the cholera toxin operon specific for toxigenic V. cholerae. The PCR procedure coupled with an enrichment culture detected as few as four V. cholerae organisms in pure culture. Treated sewage, surface, ground and drinking water samples were seeded with V. cholerae and following enrichment, a detection limit of as few as 1 V. cholerae cfu ml(-1) was obtained with amplification reactions from crude bacterial lysates. The proposed method, which includes a combination of enrichment, rapid sample preparation and a pit-stop semi-nested PCR, could be applicable in the rapid detection of toxigenic V. cholerae in environmental water samples.     

A new type of conjugative transposon encodes resistance to sulfamethoxazole, trimethoprim, and streptomycin in Vibrio cholerae O139.

Vibrio cholerae O139 is the first non-O1 serogroup of V. cholerae to give rise to epidemic cholera. Apparently, this new serogroup arose from an El Tor O1 strain of V cholerae, but V. cholerae O139 is distinguishable from V. cholerae El Tor O1 by virtue of its novel antigenic structure and also its characteristic pattern of resistances to the antibiotics sulfamethoxazole, trimethoprim, streptomycin, and furazolidone. We found that the first three of these antibiotic resistances are carried on an approximately 62-kb self-transmissible, chromosomally integrating genetic element which we have termed the SXT element. This novel conjugative transposon-like element could be conjugally transferred from V. cholerae O139 to V cholerae O1 and Escherichia coli strains, where it integrated into the recipient chromosomes in a site-specific manner independent of recA. To study the potential virulence properties of the SXT element as well as to improve upon the live attenuated O139 vaccine strain Bengal-2, a large internal deletion in the SXT element was crossed on to the Bengal-2 chromosome. The resulting strain, Bengal-2.SXT(s), is sensitive to sulfamethoxazole and trimethoprim and colonizes the intestines of suckling mice as well as wild-type strains do, suggesting that the SXT element does not encode a colonization factor. Derivatives of Bengal-2.SXT(s) are predicted to be safe, antibiotic-sensitive, live attenuated vaccines for cholera due to the O139 serogroup.     

Characterization of a clinical Vibrio cholerae O139 isolate from Mexico

Pathogenic strains of Vibrio cholerae O139 possess the cholera toxin A subunit (ctxA) gene as well as the gene for toxin co-regulated pili (tcpA). We report the isolation of a ctxA-negative, tcpA-negative V. cholerae O139 strain (INDREI) from a patient in Mexico diagnosed with gastrointestinal illness. Certain phenotypic characteristics of this strain were identical to those of V. cholerae O1 biotype El Tor. Unlike ctxA-positive V. cholerae O139 strains, this strain was sensitive to a wide panel of antibiotics, including ampicillin, chloramphenicol, ciprofloxacin, gentamicin, furazolidone, nalidixic acid, nitrofurantoin, tetracycline, trimethoprim-sulfamethoxazole, and streptomycin, but was resistant to polymyxin B. Ribotype and pulsed-field gel electrophoresis profiles of INDRE1 differed from those of ctxA-positive V. cholerae O139 and other V. cholerae strains. Phenotypic characteristics of the Mexico strain were similar to those reported for V. cholerae O139 isolates from Argentina and Sri Lanka.     

AB5肠毒素A、B亚基比例表达调控机理的研究

霍乱毒素（ＣＴ）及大肠杆菌势不稳定甩的毒素（ＬＴ）的Ｂ亚基基因的表达水平是Ａ亚基的５￣７倍,研究发现Ａ基因内部存在着１个８０ｂｐ长的翻译控区,含有３个翻译起始序列（ＴＩＲ）,其第２个翻译起始序列的终止密码与第３个翻译起始序列的起始序列的起始密码重叠,因而形成翻译偶联,继续翻译至Ａ基因的终止密码。将ＴＩＲ３的超始密码ＡＴＧ突变为ＡＴＣ后ＴＩＲ２及ＴＩＲ３失去功能,这时下游报告基因（类似于ｃｔｘＡ、Ｂ