Genetic diversity and virulence potential of environmental Vibrio cholerae population in a cholera-endemic area

To understand the evolutionary events and possible selection mechanisms involved in the emergence of pathogenic Vibrio cholerae, we analyzed diverse strains of V. cholerae isolated from environmental waters in Bangladesh by direct enrichment in the intestines of adult rabbits and by conventional laboratory culture. Strains isolated by conventional culture were mostly (99.2%) negative for the major virulence gene clusters encoding toxin-coregulated pilus (TCP) and cholera toxin (CT) and were nonpathogenic in animal models. In contrast, all strains selected in rabbits were competent for colonizing infant mice, and 56.8% of these strains carried genes encoding TCP alone or both TCP and CT. Ribotypes of toxigenic O1 and O139 strains from the environment were similar to pandemic strains, whereas ribotypes of non-O1 non-O139 strains and TCP(-) nontoxigenic O1 strains diverged widely from the seventh pandemic O1 and the O139 strains. Results of this study suggest that (i) the environmental V. cholerae population in a cholera-endemic area is highly heterogeneous, (ii) selection in the mammalian intestine can cause enrichment of environmental strains with virulence potential, (iii) pathogenicity of V. cholerae involves more virulence genes than currently appreciated, and (iv) most environmental V. cholerae strains are unlikely to attain a pandemic potential by acquisition of TCP and CT genes alone. Because most of the recorded cholera pandemics originated in the Ganges Delta region, this ecological setting presumably favors extensive genetic exchange among V. cholerae strains and thus promotes the rare, multiple-gene transfer events needed to assemble the critical combination of genes required for pandemic spread.

Reemergence of epidemic Vibrio cholerae O139, Bangladesh

During March and April 2002, a resurgence of Vibrio cholerae O139 occurred in Dhaka and adjoining areas of Bangladesh with an estimated 30,000 cases of cholera. Patients infected with O139 strains were much older than those infected with O1 strains (p<0.001). The reemerged O139 strains belong to a single ribotype corresponding to one of two ribotypes that caused the initial O139 outbreak in 1993. Unlike the strains of 1993, the recent strains are susceptible to trimethoprim, sulphamethoxazole, and streptomycin but resistant to nalidixic acid. The new O139 strains carry a copy of the Calcutta type CTX^Calc prophage in addition to the CTX^ET prophage carried by the previous strains. Thus, the O139 strains continue to evolve, and the adult population continues to be more susceptible to O139 cholera, which suggests a lack of adequate immunity against this serogroup. These findings emphasize the need for continuous monitoring of the new epidemic strains.

Pathogenicity islands and phages in Vibrio cholerae evolution

The identification of accessory genetic elements (plasmids, phages and chromosomal 'pathogenicity islands') encoding virulence-associated genes has facilitated our efforts to understand the origination of pathogenic microorganisms. Toxigenic Vibrio cholerae, the etiologic agent of cholera, represents a paradigm for this process in that this organism evolved from environmental nonpathogenic V. cholerae by acquisition of virulence genes. The major virulence genes in V. cholerae, which are clustered in several chromosomal regions, appear to have been recently acquired from phages or through undefined horizontal gene transfer events. Evidence is accumulating that the interactions of phages with each other can also influence the emergence of pathogenic clones of V. cholerae. Therefore, to track the evolution of pathogens from their nonpathogenic progenitors, it is also crucial to identify and characterize secondary genetic elements that mediate lateral transfer of virulence genes in trans. Understanding the evolutionary events that lead to the emergence of pathogenic clones might provide new approaches to the control of cholera and other infectious diseases.

Examination of diverse toxin-coregulated pilus-positive Vibrio cholerae strains fails to demonstrate evidence for Vibrio pathogenicity island phage

The major virulence factors of toxigenic Vibrio cholerae are cholera toxin, which is encoded by a lysogenic filamentous bacteriophage (CTXPhi), and toxin-coregulated pilus (TCP), an essential colonization factor that is also the receptor for CTXPhi. The genes involved in the biosynthesis of TCP reside in a pathogenicity island, which has been reported to correspond to the genome of another filamentous phage (designated VPIPhi) and to encode functions necessary for the production of infectious VPIPhi particles. We examined 46 V. cholerae strains having diverse origins and carrying different genetic variants of the TCP island for the production of the VPIPhi and CTXPhi in different culture conditions, including induction of prophages with mitomycin C and UV irradiation. Although 9 of 10 V. cholerae O139 strains and 12 of 15 toxigenic El Tor strains tested produced extracellular CTXPhi, none of the 46 TCP-positive strains produced detectable VPIPhi in repeated assays, which detected as few as 10 particles of a control CTX phage per ml. These results contradict the previous report regarding VPIPhi-mediated horizontal transfer of the TCP genes and suggest that the TCP island is unable to support the production of phage particles. Further studies are necessary to understand the mechanism of horizontal transfer of the TCP island.

Emergence and evolution of Vibrio cholerae O139

The emergence of Vibrio cholerae O139 Bengal during 1992-1993 was associated with large epidemics of cholera in India and Bangladesh and, initially, with a total displacement of the existing V. cholerae O1 strains. However, the O1 strains reemerged in 1994 and initiated a series of disappearance and reemergence of either of the two serogroups that was associated with temporal genetic and phenotypic changes sustained by the strains. Since the initial emergence of the O139 vibrios, new variants of the pathogen derived from multiple progenitors have been isolated and characterized. The clinical and epidemiological characteristics of these strains have been studied. Rapid genetic reassortment in O139 strains appears to be a response to the changing epidemiology of V. cholerae O1 and also a strategy for persistence in competition with strains of the O1 serogroup. The emergence of V. cholerae O139 has provided a unique opportunity to witness genetic changes in V. cholerae that may be associated with displacement of an existing serogroup by a newly emerging one and, thus, provide new insights into the epidemiology of cholera. The genetic changes and natural selection involving both environmental and host factors are likely to influence profoundly the genetics, epidemiology, and evolution of toxigenic V. cholerae, not only in the Ganges Delta region of India and Bangladesh, but also in other areas of endemic and epidemic cholera.

CTXphi-independent production of the RS1 satellite phage by Vibrio cholerae

The cholera toxin genes of Vibrio cholerae are encoded by the filamentous phage, CTXphi. Chromosomal CTXphi prophage DNA is often found flanked by copies of a related genetic element designated RS1, and RS1 DNA can be packaged into filamentous phage particles (designated RS1phi) by using the CTXphi morphogenesis genes. RS1phi is a satellite phage that further controls expression and dissemination of CTXphi. Here we describe a CTXphi-independent mechanism for production of RS1phi. A nontoxigenic environmental V. cholerae strain (55V71) was identified that supports production of RS1phi. However, newly infected CTX-negative strains did not produce RS1phi, indicating that additional 55V71 genes were involved in production of RS1phi. Analysis of nucleic acids from phage preparations of 55V71 revealed a 7.5-kb single-stranded DNA, whose corresponding replicative form was found in plasmid preparations. This DNA likely corresponds to the genome of a new filamentous phage, which we have designated KSF-1phi. The replicative form DNA of KSF-1phi was cloned into pUC18, and the resulting construct pKSF-1.1 supported the production of RS1phi particles by CTX-negative V. cholerae strains. RS1phi particles produced in this way infect recipient V. cholerae strains by a mechanism that is independent of the CTXphi receptor, the toxin-coregulated pilus. Thus, KSF-1phi is capable of facilitating the transfer of the RS1 element to strains that do not express toxin coregulated pilus. Given that RS1phi can enhance coproduction of CTXphi particles, KSF-1phi-mediated dissemination of RS1 may indirectly promote the spread of toxin genes among V. cholerae strains. This study also shows that filamentous phages can package diverse DNA elements and thus may play a role in horizontal transfer of more genes than previously appreciated.

Pathogenic potential of environmental Vibrio cholerae strains carrying genetic variants of the toxin-coregulated pilus pathogenicity island

The major virulence factors of toxigenic Vibrio cholerae are cholera toxin (CT), which is encoded by a lysogenic bacteriophage (CTXPhi), and toxin-coregulated pilus (TCP), an essential colonization factor which is also the receptor for CTXPhi. The genes for the biosynthesis of TCP are part of a larger genetic element known as the TCP pathogenicity island. To assess their pathogenic potential, we analyzed environmental strains of V. cholerae carrying genetic variants of the TCP pathogenicity island for colonization of infant mice, susceptibility to CTXPhi, and diarrheagenicity in adult rabbits. Analysis of 14 environmental strains, including 3 strains carrying a new allele of the tcpA gene, 9 strains carrying a new allele of the toxT gene, and 2 strains carrying conventional tcpA and toxT genes, showed that all strains colonized infant mice with various efficiencies in competition with a control El Tor biotype strain of V. cholerae O1. Five of the 14 strains were susceptible to CTXPhi, and these transductants produced CT and caused diarrhea in adult rabbits. These results suggested that the new alleles of the tcpA and toxT genes found in environmental strains of V. cholerae encode biologically active gene products. Detection of functional homologs of the TCP island genes in environmental strains may have implications for understanding the origin and evolution of virulence genes of V. cholerae.

A 4-year study of the epidemiology of Vibrio cholerae in four rural areas of Bangladesh

How Vibrio cholerae spreads around the world and what determines its seasonal peaks in endemic areas are not known. These features of cholera have been hypothesized to be primarily the result of environmental factors associated with aquatic habitats that can now be identified. Since 1997, fortnightly surveillance in 4 widely separated geographic locations in Bangladesh has been performed to identify patients with cholera and to collect environmental data. A total of 5670 patients (53% <5 years of age) have been studied; 14.3% had cholera (10.4% due to V. cholerae O1 El Tor, 3.8% due to O139). Both serogroups were found in all locations; outbreaks were seasonal and often occurred simultaneously. Water-use patterns showed that bathing and washing clothes in tube-well water was significantly protective in two of the sites. These data will be correlated with environmental factors, to develop a model for prediction of cholera outbreaks.

Cloning and characterization of genes encoding homologues of the B subunit of cholera toxin and the Escherichia coli heat-labile enterotoxin from clinical isolates of Citrobacter freundii and E. coli

We identified and characterized a gene encoding a homologue of the B subunits of cholera toxin (CTB) and heat-labile enterotoxin (LTB) of Escherichia coli from a clinical isolate of Citrobacter freundii that was found to produce a factor in the culture supernatant that cross-reacted with antibodies to CTB and LTB when assayed by enzyme-linked immunosorbent assay (ELISA). The gene encoding the ELISA-positive factor, cfxB, consisted of 375 nucleotides and was located downstream of an 852-nucleotide open reading frame, cfxA, with a 56-nucleotide intergenic space. The cfxB gene was predicted to encode a 125-amino-acid polypeptide, which had 73.8 and 72.8% identities with the amino acid sequences of LTB and CTB, respectively. However, the amino acid sequence of the deduced polypeptide CFXA had no homologies to those of the A subunits of CT or LT. DNA probes developed from the sequences of cfxA and cfxB were used to screen 67 C. freundii isolates and 152 E. coli isolates from diarrheal patients by colony blot hybridization. Two strains, C. freundii 48 and E. coli 176, reacted with both DNA probes under conditions of high stringency. We cloned homologues of the cfxA and cfxB genes from E. coli 176 and designated them ecxA and ecxB, respectively. The ecxA gene and the ecxB gene comprise 855 and 375 nucleotides, respectively, with a 50-nucleotide intergenic space, and encode a 285- and a 125-amino-acid residue polypeptides, respectively. The results of the present study may provide important clues to the origin and evolution of immunologically related factors sharing a common enterotoxin-like A and B subunit structures.

Molecular ecology of toxigenic Vibrio cholerae

Toxigenic Vibrio cholerae is the etiological agent of cholera, an acute dehydrating diarrhea that occurs in epidemic form in many developing countries. Although V. cholerae is a human pathogen, aquatic ecosystems are major habitats of Vibrio species, which includes both pathogenic and nonpathogenic strains that vary in their virulence gene content. V. cholerae belonging to the 01 and 0139 serogroups is commonly known to carry a set of virulence genes necessary for pathogenesis in humans. Recent studies have indicated that virulence genes or their homologues are also dispersed among environmental strains of V. cholerae belonging to diverse serogroups, which appear to constitute an environmental reservoir of virulence genes. Although the definitive roles of the virulence-associated factors in the environment, and the environmental selection pressures for V. cholerae-carrying virulence genes or their homologues is not clear, the potential for origination of new epidemic strains from environmental progenitors seems real. It is likely that the aquatic environment harbors different virulence-associated genes scattered among environmental vibrios, which possess a lower virulence potential than the epidemic strains. The ecosystem comprising the aquatic environment, V. cholerae, genetic elements mediating gene transfer, and the mammalian host appears to support the clustering of critical virulence genes in a proper combination leading to the origination of new V. cholerae strains with epidemic potential.

New variants of Vibrio cholerae O1 biotype El Tor with attributes of the classical biotype from hospitalized patients with acute diarrhea in Bangladesh

The sixth pandemic of cholera and, presumably, the earlier pandemics were caused by the classical biotype of Vibrio cholerae O1, which was progressively replaced by the El Tor biotype representing the seventh cholera pandemic. Although the classical biotype of V. cholerae O1 is extinct, even in southern Bangladesh, the last of the niches where this biotype prevailed, we have identified new varieties of V. cholerae O1, of the El Tor biotype with attributes of the classical biotype, from hospitalized patients with acute diarrhea in Bangladesh. Twenty-four strains of V. cholerae O1 isolated between 1991 and 1994 from hospitalized patients with acute diarrhea in Matlab, a rural area of Bangladesh, were examined for the phenotypic and genotypic traits that distinguish the two biotypes of V. cholerae O1. Standard reference strains of V. cholerae O1 belonging to the classical and El Tor biotypes were used as controls in all of the tests. The phenotypic traits commonly used to distinguish between the El Tor and classical biotypes, including polymyxin B sensitivity, chicken cell agglutination, type of tcpA and rstR genes, and restriction patterns of conserved rRNA genes (ribotypes), differentiated the 24 strains of toxigenic V. cholerae O1 into three types designated the Matlab types. Although all of the strains belonged to ribotypes that have been previously found among El Tor vibrios, type I strains had more traits of the classical biotype while type II and III strains appeared to be more like the El Tor biotype but had some classical biotype properties. These results suggest that, although the classical and El Tor biotypes have different lineages, there are possible naturally occurring genetic hybrids between the classical and El Tor biotypes that can cause cholera and thus provide new insight into the epidemiology of cholera in Bangladesh. Furthermore, the existence of such novel strains may have implications for the development of a cholera vaccine.

Isolation of Shigella dysenteriae type 1 and S. flexneri strains from surface waters in Bangladesh: comparative molecular analysis of environmental Shigella isolates versus clinical strains

Bacillary dysentery caused by Shigella species is a public health problem in developing countries including Bangladesh. Although, shigellae-contaminated food and drinks are often the source of the epidemic's spread, the possible presence of the pathogen and transmission of it through environmental waters have not been adequately examined. We analyzed surface waters collected in Dhaka, Bangladesh, for the presence of shigellae by a combination of PCR assays followed by concentration and culturing of PCR-positive samples. Analysis of 128 water samples by PCR assays for Shigella-specific virulence genes including ipaBCD, ipaH, and stx1 identified 14 (10.9%) samples which were positive for one or more of these virulence genes. Concentration of the PCR-positive samples by filtration followed by culturing identified live Shigella species in 11 of the 14 PCR-positive samples. Analysis of rRNA gene restriction patterns (ribotype) showed that the environmental isolates shared ribotypes with a collection of clinical isolates, but in contrast to the clinical isolates, 10 of the 11 environmental isolates were either negative or carried deletions in the plasmid-encoded invasion-associated genes ipaB, ipaC, and ipaD. However, all environmental Shigella isolates were positive for the chromosomal multicopy invasion-associated gene ipaH and all Shigella dysenteriae type 1 isolates were positive for the stx1 gene in addition to ipaH. This study demonstrated the presence of Shigella in the aquatic environment and dispersion of different virulence genes among these isolates which appear to constitute an environmental reservoir of Shigella-specific virulence genes. Since critical virulence genes in Shigella are carried by plasmids or mobile genetic elements, the environmental gene pool may contribute to an optimum combination of genes, causing the emergence of virulent Shigella strains which is facilitated in particular by close contact of the population with surface waters in Bangladesh.

Prevalence of the pandemic genotype of Vibrio parahaemolyticus in Dhaka, Bangladesh, and significance of its distribution across different serotypes

Sixty-six strains of Vibrio parahaemolyticus belonging to 14 serotypes were isolated from hospitalized patients in Dhaka, Bangladesh, from January 1998 to December 2000. Among these, 48 strains belonging to four serotypes had the pandemic genotype and possessed the tdh gene. A marker (open reading frame ORF8) for a filamentous phage previously thought to correspond to the pandemic genotype was found to have a poor correlation with the pandemic genotype.

RS1 element of Vibrio cholerae can propagate horizontally as a filamentous phage exploiting the morphogenesis genes of CTXphi

In toxigenic Vibrio cholerae, cholera toxin is encoded by the CTX prophage, which consists of a core region carrying ctxAB genes and genes required for CTXPhi morphogenesis, and an RS2 region encoding regulation, replication, and integration functions. Integrated CTXPhi is often flanked by another genetic element known as RS1 which carries all open reading frames (ORFs) found in RS2 and an additional ORF designated rstC. We identified a single-stranded circularized form of the RS1 element, in addition to the CTXPhi genome, in nucleic acids extracted from phage preparations of 32 out of 83 (38.5%) RS1-positive toxigenic V. cholerae strains analyzed. Subsequently, the corresponding double-stranded replicative form (RF) of the RS1 element was isolated from a representative strain and marked with a kanamycin resistance (Km(r)) marker in an intergenic site to construct pRS1-Km. Restriction and PCR analysis of pRS1-Km and sequencing of a 300-bp region confirmed that this RF DNA was the excised RS1 element which formed a novel junction between ig1 and rstC. Introduction of pRS1-Km into a V. cholerae O1 classical biotype strain, O395, led to the production of extracellular Km(r) transducing particles, which carried a single-stranded form of pRS1-Km, thus resembling the genome of a filamentous phage (RS1-KmPhi). Analysis of V. cholerae strains for susceptibility to RS1-KmPhi showed that classical biotype strains were more susceptible to the phage compared to El Tor and O139 strains. Nontoxigenic (CTX(-)) O1 and O139 strains which carried genes encoding the CTXPhi receptor toxin-coregulated pilus (TCP) were also more susceptible (>1,000-fold) to the phage compared to toxigenic El Tor or O139 strains. Like CTXPhi, the RS1Phi genome also integrated into the host chromosomes by using the attRS sequence. However, only transductants of RS1-KmPhi which also harbored the CTXPhi genome produced a detectable level of extracellular RS1-KmPhi. This suggested that the core genes of CTXPhi are also required for the morphogenesis of RS1Phi. The results of this study showed for the first time that RS1 element, which encodes a site-specific recombination system in V. cholerae, can propagate horizontally as a filamentous phage, exploiting the morphogenesis genes of CTXPhi.

Molecular analysis of antibiotic resistance gene clusters in vibrio cholerae O139 and O1 SXT constins

Many recent Asian clinical Vibrio cholerae E1 Tor O1 and O139 isolates are resistant to the antibiotics sulfamethoxazole (Su), trimethoprim (Tm), chloramphenicol (Cm), and streptomycin (Sm). The corresponding resistance genes are located on large conjugative elements (SXT constins) that are integrated into prfC on the V. cholerae chromosome. We determined the DNA sequences of the antibiotic resistance genes in the SXT constin in MO10, an O139 isolate. In SXT(MO10), these genes are clustered within a composite transposon-like structure found near the element's 5' end. The genes conferring resistance to Cm (floR), Su (sulII), and Sm (strA and strB) correspond to previously described genes, whereas the gene conferring resistance to Tm, designated dfr18, is novel. In some other O139 isolates the antibiotic resistance gene cluster was found to be deleted from the SXT-related constin. The El Tor O1 SXT constin, SXT(ET), does not contain the same resistance genes as SXT(MO10). In this constin, the Tm resistance determinant was located nearly 70 kbp away from the other resistance genes and found in a novel type of integron that constitutes a fourth class of resistance integrons. These studies indicate that there is considerable flux in the antibiotic resistance genes found in the SXT family of constins and point to a model for the evolution of these related mobile elements.

Diminished diarrheal response to Vibrio cholerae strains carrying the replicative form of the CTX(Phi) genome instead of CTX(Phi) lysogens in adult rabbits

Toxigenic Vibrio cholerae strains are lysogens of CTX(Phi), a filamentous bacteriophage which encodes cholera toxin (CT). Following infection of recipient V. cholerae cells by CTX(Phi), the phage genome either integrates into the host chromosome at a specific attachment site (attRS) or exists as a replicative-form (RF) plasmid. We infected naturally occurring attRS-negative nontoxigenic V. cholerae or attenuated (CTX(-) attRS negative) derivatives of wild-type toxigenic strains with CTX(Phi) and examined the diarrheagenic potential of the strains carrying the RF of the CTX(Phi) genome using the adult rabbit diarrhea model. Under laboratory conditions, strains carrying the RF of CTX(Phi) produced more CT than corresponding lysogens as assayed by a G(M1)-based enzyme-linked immunosorbent assay and by fluid accumulation in ligated ileal loops of rabbits. However, when tested for diarrhea in rabbits, the attRS-negative strains (which carried the CTX(Phi) genome as the RF) were either negative or produced mild diarrhea, whereas the attRS-positive strains with integrated CTX(Phi) produced severe fatal diarrhea. Analysis of the strains after intestinal passage showed that the attRS-negative strains lost the phage genome at approximately a fivefold higher frequency than under in vitro conditions, and 75 to 90% of cells recovered from challenged rabbits after 24 h were CT negative. These results suggested that strains carrying the RF of CTX(Phi) are unable to cause severe disease due to rapid loss of the phage in vivo, and the gastrointestinal environment thus provides selection of toxigenic strains with an integrated CTX(Phi) genome. These results may have implications for the development of live V. cholerae vaccine candidates impaired in chromosomal integration of CTX(Phi). These findings may also contribute to understanding of the etiology of diarrhea occasionally associated with nontoxigenic V. cholerae strains.

Local production of anti-vibrio cholerae mucosal antibody in reproductive tract tissues after cholera

To investigate whether intestinal presentation of an antigen by Vibrio cholerae, a noninvasive organism, could induce an anatomically distant mucosal immune response in reproductive tract tissues, the endocervical immune responses of women in Bangladesh were evaluated after cholera. Endocervical secretions were analyzed for secretory IgA (sIgA) antibody against the B subunit of cholera toxin (CtxB) in 9 women with cholera and 8 women with diarrhea caused by neither V. cholerae nor heat labile enterotoxin-producing Escherichia coli. Women infected with V. cholerae developed significant sIgA anti-CtxB responses in endocervical samples (P< or =.02). Antibody subtype analysis of endocervical IgA was consistent with local mucosal production (P< or =.001). Women with cholera did not develop sIgA anti-CtxB responses in serum. The ability to generate specific mucosal immune responses in reproductive tract tissues after intestinal presentation of antigen could facilitate development of vaccines effective against reproductive tract pathogens.

Molecular comparison of toxigenic clinical & non-toxigenic environmental strains of Vibrio cholerae O1 Ogawa isolated during an outbreak of cholera in south India

BACKGROUND & OBJECTIVES

While investigating a cholera outbreak in south India, toxigenic and nontoxigenic strains of Vibrio cholerae O1 were isolated from patients and from the environment, respectively. This study was performed to compare the genetic relatedness of the patient and environmental strains to determine clonal relationships among these strains and thereby determine the source of the cholera outbreak.

METHODS

The 16 strains of V. cholerae isolated from hospitalized patients and 8 environmental V. cholerae strains isolated from the environment were phenotypically and genotypically characterized using a variety of standard techniques.

RESULTS

Sixteen toxigenic clinical strains and 2 nontoxigenic environmental strains belonged to O1 serogroup, Ogawa serotype and El Tor biotype. The remaining 6 nontoxigenic environmental strains were classified as non-O1, non-O139 V. cholerae. The drug resistance pattern of the clinical and environmental strains of V. cholerae showed marked differences with the patient strains being resistant to more number of drugs as compared to the environmental strains. DNA fingerprinting of the strains showed considerable diversity between toxigenic clinical and nontoxigenic environmental O1 Ogawa isolates and between the O1 and non-O1, non-O139 isolates.

INTERPRETATION & CONCLUSION

In this outbreak of cholera, the O1 strains of V. cholerae from clinical and environmental sources belonged to two different clones and the environmental strains could perhaps be the future cholera outbreak causing clones.

The efficacy of bismuth subsalicylate in the treatment of acute diarrhoea and the prevention of persistent diarrhoea

A controlled, randomized, double-blind study in Bangladeshi children (ages 4-36 mo) with acute diarrhoea was undertaken to determine whether bismuth subsalicylate (BSS) would prevent the development of persistent diarrhoea (PD) in young children. The children were randomized to two groups: 226 were given liquid oral BSS, (as Pepto-Bismol), 100 mg/kg/d for 5 d; 225 were given placebo of identical appearance. On admission to the study, the two groups were comparable both clinically and microbiologically. Rotavirus was found in 56% of all the children, and enterotoxigenic E. coli in 31% of a subsample studied. Children treated with BSS had less severe and less prolonged illness than those treated with placebo (p = 0.057). There was, however, no difference in the development of PD between the two groups (8% and 11%). Unexpectedly, patients treated with BSS gained significantly more weight (2.3%) than those treated with placebo (0.5%; p < 0.001) during the course of the study. No toxicity of BSS was detected.

CONCLUSION

Treatment with BSS had a modest therapeutic effect on acute diarrhoea, as has been previously demonstrated, but with no suggestion of a therapeutic effect on the prevention of persistent diarrhoea in this group of patients.

Immune response of Bangladeshi children with acute diarrhea who subsequently have persistent diarrhea

BACKGROUND

Because altered immune responses may be a risk factor for persistent diarrhea, various aspects of the immune response were examined to elucidate the underlying immune mechanisms that may be involved in the development of persistent diarrhea.

METHODS

Children (7-12 months of age) with watery diarrhea for 6 to 8 days from the Dhaka Hospital of the International Centre for Diarrheal Disease Research, Bangladesh (ICDDR,B), were enrolled. Children were classified as having acute diarrhea (AD) or persistent diarrhea (PD) if diarrhea resolved within 14 days or persisted for more than 14 days, respectively. Uninfected control children (n = 13), from the Nutrition Follow-Up Unit of ICDDR,B were also enrolled. Of the 123 children with diarrhea who were enrolled, 85 had AD and 38 had PD. Comparisons were performed for clinical features, nutritional status (weight for age, plasma transferrin, and serum albumin levels), and immune responses: neutrophil function; peripheral blood mononuclear cell function, delayed-type hypersensitivity (DTH) responses, plasma levels of immunoglobulins, tumor necrosis factor-alpha, and interferon-gamma. Univariate analyses were conducted to assess differences among the three groups of children and between children with AD and PD. Logistic regression was performed to determine risk factors for PD.

RESULTS

There were no differences in clinical features and nutritional status among the groups of children studied. More children in whom PD developed had a negative DTH response to tuberculin than those with AD (P = 0.021). Also, a negative DTH response to tuberculin was a significant risk factor for PD (odds ratio [OR] = 3.8, 95% confidence interval [CI] = 1.5-9.9).

CONCLUSIONS

Children with acute diarrhea with a negative DTH response to tuberculin are more likely to have development of persistent diarrhea.

The O139 serogroup of Vibrio cholerae comprises diverse clones of epidemic and nonepidemic strains derived from multiple V. cholerae O1 or non-O1 progenitors

Sixty-four representative strains of Vibrio cholerae O139 were analyzed, to re-examine the origin of this serogroup. Ribotyping differentiated the strains into 3 HindIII and 7 BglI ribotypes. One HindIII and 5 BglI ribotypes were shared by all toxigenic O139 strains. Of 6 nontoxigenic O139 strains, 3 shared ribotypes with the toxigenic strains, carried genes encoding toxin coregulated pilus, and were susceptible to the cholera toxin-converting bacteriophage CTXPhi. The remaining 3 strains belonged to 2 different ribotypes distinct from toxigenic O139 strains and were resistant to CTXPhi and JA-1, an O139-specific lytic bacteriophage. Polymerase chain reaction amplicons corresponding to the gmhD gene carried by these 3 strains also differed from those of the toxigenic O139 strains but were identical to those of 15 environmental non-O1-non-O139 strains. Thus, the O139 antigen is present in different lineages, and this serogroup appears to comprise epidemic and nonepidemic strains derived separately from different progenitors.

Virulence genes in environmental strains of Vibrio cholerae

The virulence of a pathogen is dependent on a discrete set of genetic determinants and their well-regulated expression. The ctxAB and tcpA genes are known to play a cardinal role in maintaining virulence in Vibrio cholerae, and these genes are believed to be exclusively associated with clinical strains of O1 and O139 serogroups. In this study, we examined the presence of five virulence genes, including ctxAB and tcpA, as well as toxR and toxT, which are involved in the regulation of virulence, in environmental strains of V. cholerae cultured from three different freshwater lakes and ponds in the eastern part of Calcutta, India. PCR analysis revealed the presence of these virulence genes or their homologues among diverse serotypes and ribotypes of environmental V. cholerae strains. Sequencing of a part of the tcpA gene carried by an environmental strain showed 97.7% homology to the tcpA gene of the classical biotype of V. cholerae O1. Strains carrying the tcpA gene expressed the toxin-coregulated pilus (TCP), demonstrated by both autoagglutination analysis and electron microscopy of the TCP pili. Strains carrying ctxAB genes also produced cholera toxin, determined by monosialoganglioside enzyme-linked immunosorbent assay and by passage in the ileal loops of rabbits. Thus, this study demonstrates the presence and expression of critical virulence genes or their homologues in diverse environmental strains of V. cholerae, which appear to constitute an environmental reservoir of virulence genes, thereby providing new insights into the ecology of V. cholerae.

Sunlight-induced propagation of the lysogenic phage encoding cholera toxin

In toxigenic Vibrio cholerae, the cholera enterotoxin (CT) is encoded by CTXPhi, a lysogenic bacteriophage. The propagation of this filamentous phage can result in the origination of new toxigenic strains. To understand the nature of possible environmental factors associated with the propagation of CTXPhi, we examined the effects of temperature, pH, salinity, and exposure to direct sunlight on the induction of the CTX prophage and studied the transmission of the phage to potential recipient strains. Exposure of cultures of CTXPhi lysogens to direct sunlight resulted in approximately 10,000-fold increases in phage titers. Variation in temperature, pH, or salinity of the culture did not have a substantial effect on the induction of the prophage, but these factors influenced the stability of CTXPhi particles. Exposure of mixed cultures of CTXPhi lysogens and potential recipient strains to sunlight significantly increased both the in vitro and in vivo (in rabbit ileal loops) transduction of the recipient strains by CTXPhi. Included in these transduction experiments were two environmental nontoxigenic (CTXPhi(-)) strains of V. cholerae O139. These two O139 strains were transduced at high efficiency by CTXPhi, and the phage genome integrated into the O139 host chromosome. The resulting CTXPhi lysogens produced biologically active CT both in vitro and in rabbit ileal loops. This finding suggests a possible mechanism explaining the origination of toxigenic V. cholerae O139 strains from nontoxigenic progenitors. This study indicates that sunlight is a significant inducer of the CTX prophage and suggests that sunlight-induced transmission of CTXPhi may constitute part of a natural mechanism for the origination of new toxigenic strains of V. cholerae.

Treatment of enterotoxigenic and enteropathogenic Escherichia coli-induced diarrhoea in children with bovine immunoglobulin milk concentrate from hyperimmunized cows: a double-blind, placebo-controlled, clinical trial

BACKGROUND

Enterotoxigenic Escherichia coli (ETEC) and enteropathogenic Escherichia coli (EPEC) are important causes of diarrhoea in young children and are associated with significant mortality rates. Passive immunization with antibodies from immunized cows has previously been shown to be effective as prophylaxis against E. coli-induced diarrhoea and therapeutically against rotavirus and cryptosporidia-induced diarrhoea.

METHODS

We tested the therapeutic efficacy of an oral bovine immunoglobulin milk concentrate (BIC) from cows hyperimmunized with ETEC and EPEC strains, in a randomized, placebo-controlled study in children with E. coli-induced diarrhoea. Eighty-six children between 4-24 months of age attending the International Centre for Diarrhoeal Disease Research, Bangladesh (ICDDR, B) with E. coli-induced diarrhoea (63 EPEC/ETEC and 23 with other diarrhoeagenic E. coli) were randomly assigned to receive orally administered BIC (20 g) containing anti-ETEC/EPEC antibodies or a placebo preparation daily for 4 consecutive days. Daily stool output, intake of oral rehydration solution (ORS), stool frequency, and presence of diarrhoeagenic E. coli strains in the stool were monitored for 4 days.

RESULTS

Children in the treatment group tolerated the BIC with no side effects. There were no significant differences between the two groups with regard to ORS intake, stool output, frequency of diarrhoea, or clearance of pathogen. Nor was there any significant alteration in the duration of diarrhoea.

CONCLUSIONS

In contrast to the prophylactic efficacy of anti-E. coli BIC and the therapeutic efficacy of a similarly prepared anti-rotavirus BIC, antibodies from hyperimmunized cows appear to have no significant therapeutic benefit in the treatment of acute diarrhoea due to EPEC/ETEC.