Virulence genes of clinical Vibrio cholerae O1 isolates in Thailand and their ribotypes

OBJECTIVE

To determine virulence associated-genes and ribotypes of Vibrio cholerae epidemic strains isolated from cholera patients in Thailand.

METHOD

A total of 240 V. cholerae El Tor, O1 strains, isolated from patients with cholera in Thailand during two different periods, i.e. 1999-2000 (200 strains; 193 Ogawa and 7 Inaba) and 2001-2002 (40 strains; all Inaba), were analyzed for the presence of virulence genes, namely ctxA, ctxB, zot, ace, toxR, tcpA, hlyA, nanH and ninT by PCR. For ribotyping, genomic DNA segments of the 240 strains and 10 reference V. cholerae strains isolated before 1999 from Thailand and elsewhere were digested with BglI endonuclease, subjected to a 0.8% agarose gel electrophoresis, blotted onto a nylon membrane and probed with enzyme-labeled Escherichia coli rRNA. The DNA bands were visualized by autoradiography.

RESULTS

Genes encoding the A and B subunits of CT, Zot, Ace, ToxR, TcpA, HlyA, NanH and NinT could be amplified from all of the 10 V. cholerae O1 reference strains and from 239 of the 240 studied isolates. One Inaba isolate of 2001-2002 gave only amplicons of toxR and hlyA. For ribotyping, the 10 reference strains revealed six different patterns designated A to F. None of the 240 strains isolated in Thailand during the two periods had the A-C, E and F ribotypes. The isolates of 1999-2000 revealed ribotype D and three other ribotypes, designated G, H and I. The majority of the isolates of 2001-2002 showed ribotype G. The remaining showed other new ribotypes, J and K.

CONCLUSIONS

The clinical V. cholerae isolates of two epidemics from Thailand showed a sustained appearance of one epidemic V. cholerae clone, and a constant, but gradual and minor change in the genetic constituent of the other V. cholerae strains as indicated by the change of the ribotypes of the strains in the two study periods. Moreover, we found that a V. cholerae strain which cannot produce CT, Zot, Ace, TcpA, NanH and NinT can still cause symptomatic cholera.

An inducible lambdoid prophage encoding cytolethal distending toxin (Cdt-I) and a type III effector protein in enteropathogenic Escherichia coli

Cytolethal distending toxins (CDTs) are inhibitory cyclomodulins, which block eukaryotic cell proliferation and are produced by a diverse group of Gram-negative bacteria, including Escherichia coli strains associated with intestinal and extraintestinal infections. However, the mode of transmission of the toxin gene clusters among diverse bacterial pathogens is unclear. We found that Cdt-I produced by enteropathogenic E. coli strains associated with diarrhea is encoded by a lambdoid prophage, which is inducible and infectious. The genome of Cdt-I converting phage (CDT-1Phi) comprises 47,021 nucleotides with 60 predicted ORFs organized into six genomic regions encoding the head and tail, virulence, integrase, unknown functions, regulation, and lysis. The genomic organization of CDT-1Phi is similar to those of SfV, a serotype-converting phage of Shigella flexneri, and UTI89, a prophage identified in uropathogenic E. coli. Besides the cdtI gene cluster, the virulence region of CDT-1Phi genome contains sequences homologous to a truncated cycle inhibiting factor and a type 3 effector protein. Mutation analysis of susceptible E. coli strain C600 suggested that the outer membrane protein OmpC is a putative receptor for CDT-1Phi. CDT-1Phi genome was also found to integrate into the host bacterial chromosome forming lysogens, which produced biologically active Cdt-I. Furthermore, phage induction appeared to cause enhanced toxigenicity of the E. coli strains carrying lysogenic CDT-1Phi. Our results suggest that CDT-1Phi is the latest member of a growing family of lambdoid phages encoding bacterial cyclomodulins and that the phage may have a role in horizontal transfer of these virulence genes.

Seaweeds as a reservoir for diverse Vibrio parahaemolyticus populations in Japan

Gastroenteritis caused by Vibrio parahaemolyticus has recently been associated with foods prepared with seaweeds, but little is known about the bacterium's abundance and diversity among seaweeds in coastal environment. Therefore, we determined its phenotypic and genotypic diversity in relation to its seasonal abundance in seawater and seaweed samples from three areas of Kii Channel, Japan during June 2003 to May 2004. Isolates were obtained by selective enrichment of samples and detection of V. parahaemolyticus by colony hybridization with a species-specific probe. A total of 128 isolates comprising 16 from each source in each season were characterized by serotyping and ribotyping. V. parahaemolyticus was more abundant in seaweeds (3,762 isolates) than in water samples (2,238 isolates). Twenty and 17 serotypes were found among the selected seaweed and seawater isolates, respectively. Cluster analysis revealed 19, 11, 7 and 9 ribotypes during summer, autumn, winter and spring, respectively. Seaweeds supported a diverse V. parahaemolyticus population throughout the year and thus seaweeds are a reservoir for the organism. However, V. parahaemolyticus occurrence had positive correlation with water temperature and its abundance in seaweeds was at least 50 times higher during summer than in winter.

Multiply antibiotic-resistant Vibrio cholerae O1 biotype El Tor strains emerge during cholera outbreaks in Zambia

Antibiotic resistance data, made available from laboratory records during eight cholera outbreaks between 1990 and 2004 showed Vibrio cholerae serogroup O1 to have a low level of resistance (2-3%) to tetracycline during 1990-1991. Resistance increased for tetracycline (95%), chloramphenicol (78%), doxycycline (70%) and trimethoprim-sulphamethoxazole (97%) in subsequent outbreaks. A significant drop in resistance to tetracycline and chloramphenicol followed the adoption of a national policy to replace tetracycline with erythromycin for treating cholera. Sixty-nine strains from cholera outbreaks in Zambia between 1996 and 2004, were examined for antibiotic resistance and basic molecular traits. A 140 MDa conjugative, multidrug-resistant plasmid was found to encode tetracycline resistance in strains from 1996/1997 whereas strains from 2003/2004 were resistant to furazolidone, but susceptible to tetracycline, and lacked this plasmid. PCR revealed 25 of 27 strains from 1996/1997 harboured the intl1 class 1 integron but lacked SXT, a conjugative transposon element. Similar screening of 42 strains from 2003/2004 revealed all carried SXT but not the intl1 class 1 integron. All 69 strains, except two, one lacking ctxA and the other rstR and thus presumably truncated in the CTX prophage region, were positive for important epidemic markers namely rfbO1, ctxA, rstR2, and tcpA of El Tor biotype. Effective cholera management is dependent on updated reports on culture and sensitivity to inform the choice of antibiotic. Since the emergence of antibiotic resistance may significantly influence strategies for controlling cholera, continuous monitoring of epidemic strains is crucial.

Genetic characterization ofVibrio choleraestrains by inter simple sequence repeat-PCR

The utility of inter simple sequence repeat-PCR (ISSR-PCR) assay in the characterization and elucidation of the phylogenetic relationship between the pathogenic and nonpathogenic isolates of Vibrio cholerae is demonstrated. A total of 45 V. cholerae strains including 15 O1 El Tor, nine O139 and 21 non-O1/non-O139 strains were analyzed using eight ISSR primers. These primers, which are essentially simple sequence repeats (SSR) with additional nonrepeat bases at the 5' or 3' end, amplify genomic regions interspersed between closely spaced SSRs. Neighbor-joining analysis showed that the strains belonging to the same serogroup clustered together with the exception of one O1 and two O139 strains. The absence of pathogenicity islands in these strains, as confirmed by PCR, suggested their non-O1/non-O139 origin. Thus the ISSR-PCR-based phylogeny was consistent with the classification of V. cholerae based on serological methods. A finer resolution of the clustering of the toxinogenic O1 El Tor and toxinogenic O139 subtypes was obtained by ISSR-PCR analysis as compared with the Enterobacterial Repetitive Intergenic Consensus sequences-based PCR analysis for the same set of strains. Thus, it is proposed that ISSR-PCR is an efficient tool in phylogenetic classification of prokaryotic genomes in general and diagnostic genotyping of microbial pathogens in particular.

A novel serovar of Shigella dysenteriae from patients with diarrhoea in Bangladesh

Every year, around 3 % of isolates from patients with diarrhoea at Dhaka Hospital, ICDDR,B, are identified as Shigella-like organisms (SLOs) based on their activity in biochemical tests. These isolates do not react with any of the current Shigella antisera including all existing and provisional serotypes. Among these SLOs, a unique cluster of seven isolates with an identical plasmid profile was found and these isolates were further characterized by phenotypic and genotypic techniques. All were nonlactose fermenters, with an identical biochemical pattern typical of Shigella dysenteriae. They were classified as invasive since they harboured the 140 MDa invasive plasmid, were able to bind Congo red, produced keratoconjunctivitis in the guinea pig eye, and were positive by PCR for the ipaH gene and Shigella enterotoxin 2 [ShET-2] gene. All isolates were resistant to ampicillin, tetracycline and sulfamethoxazole-trimethoprim but were susceptible to mecillinam, nalidixic acid, ceftriaxone and ciprofloxacin. Six of the isolates were identical in DNA pattern by PFGE with the seventh exhibiting a closely related pattern; both patterns were distinguishable from all other Shigella and Escherichia coli patterns. An antiserum prepared against one of the isolates reacted with all isolates and did not cross-react with other Shigella and E. coli serotype reference strains. It is therefore proposed that these isolates represent a new provisional serovar of S. dysenteriae, type strain KIVI 162.

Increasing spectrum in antimicrobial resistance of Shigella isolates in Bangladesh: resistance to azithromycin and ceftriaxone and decreased susceptibility to ciprofloxacin

Antimicrobial resistance of Shigella isolates in Bangladesh, during 2001-2002, was studied and compared with that of 1991-1992 to identify the changes in resistance patterns and trends. A significant increase in resistance to trimethoprim-sulphamethoxazole (from 52% to 72%, p < 0.01) and nalidixic acid (from 19% to 51%, p < 0.01) was detected. High, but unchanged, resistance to tetracycline, ampicillin, and chloramphenicol, low resistance to mecillinam (resistance 3%, intermediate 3%), and to emergence of resistance to azithromycin (resistance 16%, intermediate 62%) and ceftriaxone/cefixime (2%) were detected in 2001-2002. Of 266 recent isolates, 63% were resistant to > or =3 anti-Shigella drugs (multidrug-resistant [MDR]) compared to 52% of 369 strains (p < 0.007) in 1991-1992. Of 154 isolates tested by E-test in 2001-2002, 71% were nalidixic acid-resistant (minimum inhibitory concentration [MIC] > or =32 microg/mL) and had 10-fold higher MIC90 (0.25 microg/mL) to ciprofloxacin than that of nalidixic acid-susceptible strains exhibiting decreased ciprofloxacin susceptibility, which were detected as ciprofloxacin-susceptible and nalidixic acid-resistant by the disc-diffusion method. These strains were frequently associated with MDR traits. High modal MICs were observed to azithromycin (MIC 6 microg/mL) and nalidixic acid (MIC 128 micdrog/mL) and low to ceftriaxone (MIC 0.023 microg/mL). Conjugative R-plasmids-encoded extended-spectrum beta-lactamase was responsible for resistance to ceftriaxone/cefixime. The growing antimicrobial resistance of Shigella is worrying and mandates monitoring of resistance. Pivmecillinam or ciprofloxacin might be considered for treating shigellosis with caution.

Vibrio cholerae O1 strains are facultative intracellular bacteria, able to survive and multiply symbiotically inside the aquatic free-living amoeba Acanthamoeba castellanii

Vibrio cholerae species are extracellular, waterborne, gram-negative bacteria that are overwhelmed by predators in aquatic environments. The unencapsulated serogroup V. cholerae O1 and encapsulated V. cholerae O139 cause epidemic and pandemic outbreaks of cholera. It has recently been shown that the aquatic and free-living amoeba Acanthamoeba castellanii is not a predator to V. cholerae O139; rather, V. cholerae O139 has shown an intracellular compatibility with this host. The aim of this study was to examine the ability of V. cholerae O1 classical and El Tor strains to grow and survive in A. castellanii. The interaction between A. castellanii and V. cholerae O1 strains was studied by means of amoeba cell counts and viable counts of the bacteria in the absence or presence of amoebae. The viable count of intracellularly growing bacteria was estimated by utilizing gentamicin assay. Confocal microscopy and electron microscopy were used to determine the intracellular localization of V. cholerae in A. castellanii. The results showed that V. cholerae O1 classical and El Tor strains grew and survived intracellularly in the cytoplasm of trophozoites, and that the bacteria were also found in the cysts of A. castellanii. The interaction showed a facultative intracellular behaviour of V. cholerae O1 classical and El Tor strains and a possible role of A. castellanii as an environmental host of V. cholerae species.

Global dissemination of Vibrio parahaemolyticus serotype O3:K6 and its serovariants

Vibrio parahaemolyticus is recognized as a cause of food-borne gastroenteritis, particularly in the Far East, where raw seafood consumption is high. An unusual increase in admissions of V. parahaemolyticus cases was observed at the Infectious Diseases Hospital in Calcutta, a city in the northeastern part of India, beginning February 1996. Analysis of the strains revealed that a unique serotype, O3:K6, not previously isolated during the surveillance in Calcutta accounted for 50 to 80% of the infections in the following months. After this report, O3:K6 isolates identical to those isolated in Calcutta were reported from food-borne outbreaks and from sporadic cases in Bangladesh, Chile, France, Japan, Korea, Laos, Mozambique, Peru, Russia, Spain, Taiwan, Thailand, and the United States. Other serotypes, such as O4:K68, O1:K25, and O1:KUT (untypeable), that had molecular characteristics identical to that of the O3:K6 serotype were subsequently documented. These serotypes appeared to have diverged from the O3:K6 serotype by alteration of the O:K antigens and were defined as "serovariants" of the O3:K6 isolate. O3:K6 and its serovariants have now spread into Asia, America, Africa, and Europe. This review traces the genesis, virulence features, molecular characteristics, serotype variants, environmental occurrence, and global spread of this unique clone of V. parahaemolyticus.

Characterization of enterotoxigenic Escherichia coli from diarrhoeal patients in Bangladesh using phenotyping and genetic profiling

A total of 99 isolates out of 370 colonization factor (CF)-positive, well-characterized enterotoxigenic Escherichia coli (ETEC) strains belonging to 13 different CF types isolated from diarrhoeal patients admitted to the hospital of the International Centre for Diarrhoeal Disease Research, Bangladesh, were tested. The isolates were selected at random based on expression of the major CFs prevailing in Dhaka, Bangladesh, from 1996 to 1998. These isolates were characterized by O-antigenic serotyping, randomly amplified polymorphic DNA (RAPD) analysis and biochemical fingerprinting using the PhenePlate (PhP) system. The 99 ETEC isolates belonged to 10 O serogroups, the predominant ones being O6 (n=28), O115 (n=20) and O128 (n=20). Most isolates of serogroup O6 (CS1+CS3, 11/14; CS2+CS3, 5/8) belonged to the same PhP/RAPD type (H/f), whereas other isolates of serogroup O6 (n=12) belonged to different PhP/RAPD types (Si/f and F/c). Eleven serogroup O128 (CFA/I) isolates belonged to the same PhP/RAPD type (E/b), whereas the other O128 isolates formed different PhP/RAPD types. Fifteen (75%) serogroup O115 isolates (together with fourteen isolates from serogroups O25, O114, O142 and O159) demonstrated two closely related common groups by PhP typing (A and A1) and belonged to the same PhP/RAPD type (A/a). Three major clonal groups were identified among the ETEC strains in this study, largely based on O-antigenic type, CF expression pattern and toxin profile.

Genomic analysis of the Mozambique strain of Vibrio cholerae O1 reveals the origin of El Tor strains carrying classical CTX prophage

Cholera outbreaks in subSaharan African countries are caused by strains of the El Tor biotype of toxigenic Vibrio cholerae O1. The El Tor biotype is the causative agent of the current seventh cholera pandemic, whereas the classical biotype, which was associated with the sixth pandemic, is now extinct. Besides other genetic differences the CTX prophages encoding cholera toxin in the two biotypes of V. cholerae O1 have distinct repressor (rstR) genes. However, recent incidences of cholera in Mozambique were caused by an El Tor biotype V. cholerae O1 strain that, unusually, carries a classical type (CTX(class)) prophage. We conducted genomic analysis of the Mozambique strain and its CTX prophage together with chromosomal phage integration sites to understand the origin of this atypical strain and its evolutionary relationship with the true seventh pandemic strain. These analyses showed that the Mozambique strain carries two copies of CTX(class) prophage located on the small chromosome in a tandem array that allows excision of the prophage, but the excised phage genome was deficient in replication and did not produce CTX(class) virion. Comparative genomic microarray analysis revealed that the strain shares most of its genes with the typical El Tor strain N16961 but did not carry the TLC gene cluster, and RS1 sequence, adjacent to the CTX prophage. Our data are consistent with the Mozambique strain's having evolved from a progenitor similar to the seventh pandemic strain, involving multiple recombination events and suggest a model for origination of El Tor strains carrying the classical CTX prophage.

Comparison of enterotoxigenicEscherichia coliisolated from surface water and diarrhoeal stool samples in Bangladesh

Enterotoxigenic Escherichia coli (ETEC) is a common cause of bacterial infection leading to acute watery diarrhea in infants and young children. Although the prevalence of ETEC is high in Bangladesh and infections can be spread through food and contaminated water, limited information is available about ETEC in the surface water. We carried out studies to isolate ETEC from surface water samples from ponds, rivers, and a lake from a site close to field areas known to have a high incidence of diarrhea in Dhaka, Bangladesh, and Matlab, Bangladesh. ETEC strains isolated from the water sources were compared with ETEC strains isolated from patients with diarrhea at two hospitals in these areas. ETEC were isolated from 30% (45 of 150) of the samples from the surface water sources and 19% (518 of 2700) of the clinical specimens. One hundred ETEC strains isolated from patients with similar phenotypes as the environmental strains were compared for phenotypic and genotypic properties. The most common O serogroups on ETEC were O6, O25, O78, O115, and O126 in both types of strains. Pulsed-field gel electrophoresis analyses of the ETEC strains showed that multiple clones of ETEC were present within each colonization factor type and that some clones detected in the environment were also isolated from the stools of patients. The strains showed multiple and similar antibiotic resistance patterns. This study shows that ETEC is prevalent in surface water sources in Bangladesh suggesting a possible reason for the endemicity of this pathogen in Bangladesh.Key words: enterotoxigenic Escherichia coli (ETEC), surface water samples, colonization factors, pulsed-field gel electrophoresis, toxin types.

Isolation of Vibrio cholerae O1 strains similar to pre-seventh pandemic El Tor strains during an outbreak of gastrointestinal disease in an island resort in Fiji

Five strains of Vibrio cholerae O1, one each from an Australian and a New Zealand tourist with gastrointestinal illness returning from an island resort in Fiji and the remaining three from water sources located in the same resort, were extensively characterized. Conventional phenotypic traits that are used for biotyping of O1 V. cholerae categorized all five strains as belonging to the El Tor biotype. Genetic screening of 11 regions that are associated with virulence in V. cholerae showed variable results. The absence of genes comprising Vibrio seventh pandemic island-I (VSP-I) and VSP-II in all the strains indicated that these strains were very similar to the pre-seventh pandemic V. cholerae O1 El Tor strains. The ctxAB genes were absent in all strains whereas orfU and zot were present in four strains, indicating that the strains were non-toxigenic. Four strains carried a truncated CTX prophage. Although epidemiological and molecular studies suggested that these strains did not cause cholera amongst tourists at the resort, their similarity to pre-seventh pandemic strains, their prior association with gastrointestinal illness and their presence in the island resort setting warrant more attention.

Genetic organization of pre-CTX and CTX prophages in the genome of an environmental Vibrio cholerae non-O1, non-O139 strain

The cholera toxin (CT) is a critical determinant of the virulence of epidemic Vibrio cholerae strains. The ctxAB operon encoding CT is part of the genome of a filamentous bacteriophage CTXΦ, which may integrate as a single copy or as multiple copies in the genome of V. cholerae. The CTXΦ genome is composed of RS2 (2.4 kb) and core (4.5 kb) regions. In the present study extensive genetic mapping analyses indicated that two copies of tandemly arrayed CTX prophages are integrated in the small chromosome of an environmental V. cholerae strain, VCE232, belonging to serogroup O4. Further mapping revealed that the integration of prophages has occurred in the same genetic locus of the small chromosome of VCE232 as that of V. cholerae O1 biotype El Tor strains. Interestingly, a new type of RS2-like element 3.5 kb in size was found in the CTX prophage genome in the small chromosome of VCE232. Cloning followed by sequencing of the new RS2-like element of VCE232 revealed the presence of three ORFs, which probably encode highly divergent types of phage regulatory proteins. Furthermore, the strain VCE232 also harbours two copies of a tandemly arranged CTX prophage devoid of the ctxAB genes, called pre-CTX prophage, in its large chromosome. The presence of multiple copies of diverse CTX prophages in both the chromosomes of VCE232 suggests that toxigenic environmental V. cholerae non-O1, non-O139 strains could play a role in the emergence of new epidemic clones.

Genetic characteristics of Matlab variants of Vibrio cholerae O1 that are hybrids between classical and El Tor biotypes

The Matlab variants of Vibrio cholerae O1, defined as hybrids between the classical and El Tor biotypes, were first isolated from hospitalized patients with acute secretory diarrhoea in Matlab, a rural area of Bangladesh. These variants could not be categorized as classical or El Tor biotypes by phenotypic and genotypic tests, and had representative traits of both the biotypes. A number of virulence-associated genes and/or gene clusters were screened by PCR and DNA sequencing. El Tor-specific gene clusters, Vibrio seventh-pandemic islands (VSP)-I and -II and repeat toxin (RTX) were present in the genome of these variants, indicating their El Tor lineage, whereas the nucleotide-sequence-derived CtxB amino acid sequence of these strains grouped them under the classical biotype. Matlab variants possessed all the necessary genes to initiate pandemics. The genetic relatedness of Matlab variants to the V. cholerae strains recently isolated in Mozambique is another important observation of this study, which underscores the epidemiological significance of Matlab variants.

Cholera due to altered El Tor strains of Vibrio cholerae O1 in Bangladesh

We determined the types of cholera toxin (CT) produced by a collection of 185 Vibrio cholerae O1 strains isolated in Bangladesh over the past 45 years. All of the El Tor strains of V. cholerae O1 isolated since 2001 produced CT of the classical biotype, while those isolated before 2001 produced CT of the El Tor biotype.

Antibiotic resistance and genetic diversity of Shigella sonnei isolated from patients with diarrhoea between 1999 and 2003 in Bangladesh

Shigella sonnei is a significant cause of diarrhoeal infection in both developing and industrialized countries. From 1999 to 2003, 445 strains of Shigella sonnei were isolated from patients admitted to the diarrhoea treatment centre of the International Center for Diarrhoeal Disease Research, Bangladesh. More than 60% of the isolates were resistant to nalidixic acid, 89% to sulfamethoxazole-trimethoprim and 9.5% to ampicillin. In addition, 4% of strains were resistant to multiple antibiotics (AmpR TetR SxtR StrR) and 4.2% of strains were sensitive to all antibiotics tested. None of the strains were positive for the set1 gene, whereas 46% were positive for the sen gene. Forty-six per cent of the strains (stored at -70 degrees C) harboured the 120 MDa invasive plasmid and representative strains produced keratoconjunctivitis in the guinea pig eye. In addition, three plasmids of approximately 5, 1.8 and 1.4 MDa were found to be present in more than 90% of the strains. A self-transmissible, middle-ranged plasmid (35-80 MDa) carrying the multiple antibiotic resistance gene was found in some strains. PFGE analysis of the strains identified five unique types with many subtypes, which were characterized into four unique types by ribotyping analysis. It can be concluded that endemic strains of Shigella sonnei isolated from patients in Bangladesh are diverse in their genetic pattern.

A multicentre study of Shigella diarrhoea in six Asian countries: disease burden, clinical manifestations, and microbiology

BACKGROUND

The burden of shigellosis is greatest in resource-poor countries. Although this diarrheal disease has been thought to cause considerable morbidity and mortality in excess of 1,000,000 deaths globally per year, little recent data are available to guide intervention strategies in Asia. We conducted a prospective, population-based study in six Asian countries to gain a better understanding of the current disease burden, clinical manifestations, and microbiology of shigellosis in Asia.

METHODS AND FINDINGS

Over 600,000 persons of all ages residing in Bangladesh, China, Pakistan, Indonesia, Vietnam, and Thailand were included in the surveillance. Shigella was isolated from 2,927 (5%) of 56,958 diarrhoea episodes detected between 2000 and 2004. The overall incidence of treated shigellosis was 2.1 episodes per 1,000 residents per year in all ages and 13.2/1,000/y in children under 60 months old. Shigellosis incidence increased after age 40 years. S. flexneri was the most frequently isolated Shigella species (1,976/2,927 [68%]) in all sites except in Thailand, where S. sonnei was most frequently detected (124/146 [85%]). S. flexneri serotypes were highly heterogeneous in their distribution from site to site, and even from year to year. PCR detected ipaH, the gene encoding invasion plasmid antigen H in 33% of a sample of culture-negative stool specimens. The majority of S. flexneri isolates in each site were resistant to amoxicillin and cotrimoxazole. Ciprofloxacin-resistant S. flexneri isolates were identified in China (18/305 [6%]), Pakistan (8/242 [3%]), and Vietnam (5/282 [2%]).

CONCLUSIONS

Shigella appears to be more ubiquitous in Asian impoverished populations than previously thought, and antibiotic-resistant strains of different species and serotypes have emerged. Focusing on prevention of shigellosis could exert an immediate benefit first by substantially reducing the overall diarrhoea burden in the region and second by preventing the spread of panresistant Shigella strains. The heterogeneous distribution of Shigella species and serotypes suggest that multivalent or cross-protective Shigella vaccines will be needed to prevent shigellosis in Asia.

Development and validation of a PulseNet standardized pulsed-field gel electrophoresis protocol for subtyping of Vibrio cholerae

PulseNet is a network that utilizes standardized pulsed-field gel electrophoresis (PFGE) protocols with the purpose of conducting laboratory-based surveillance of foodborne pathogens. PulseNet standardized PFGE protocols are subject to rigorous testing during the developmental phase and careful evaluation during a validation process assessing its robustness and reproducibility in different laboratories. Here we describe the development and validation of a rapid PFGE protocol for subtyping Vibrio cholerae for use in PulseNet International activities. While the protocol was derived from the existing PulseNet protocol for Escherichia coli O157, various aspects of this protocol were optimized for use with V. cholerae, most notably a change of the primary and secondary restriction enzyme to SfiI and NotI, respectively, and the use of a two-block electrophoresis program. External validation of this protocol was undertaken through a collaboration between three PulseNet Asia Pacific laboratories (Public Health Laboratory Centre, Hong Kong, National Institute of Infectious Diseases, Japan, and International Center for Diarrhoeal Diseases Research-Bangladesh) and PulseNet USA. Comparison of PFGE patterns generated by each of the participating laboratories demonstrated that the protocol is robust and reproducible.

Seasonal cholera caused by Vibrio cholerae serogroups O1 and O139 in the coastal aquatic environment of Bangladesh

Since Vibrio cholerae O139 first appeared in 1992, both O1 El Tor and O139 have been recognized as the epidemic serogroups, although their geographic distribution, endemicity, and reservoir are not fully understood. To address this lack of information, a study of the epidemiology and ecology of V. cholerae O1 and O139 was carried out in two coastal areas, Bakerganj and Mathbaria, Bangladesh, where cholera occurs seasonally. The results of a biweekly clinical study (January 2004 to May 2005), employing culture methods, and of an ecological study (monthly in Bakerganj and biweekly in Mathbaria from March 2004 to May 2005), employing direct and enrichment culture, colony blot hybridization, and direct fluorescent-antibody methods, showed that cholera is endemic in both Bakerganj and Mathbaria and that V. cholerae O1, O139, and non-O1/non-O139 are autochthonous to the aquatic environment. Although V. cholerae O1 and O139 were isolated from both areas, most noteworthy was the isolation of V. cholerae O139 in March, July, and September 2004 in Mathbaria, where seasonal cholera was clinically linked only to V. cholerae O1. In Mathbaria, V. cholerae O139 emerged as the sole cause of a significant outbreak of cholera in March 2005. V. cholerae O1 reemerged clinically in April 2005 and established dominance over V. cholerae O139, continuing to cause cholera in Mathbaria. In conclusion, the epidemic potential and coastal aquatic reservoir for V. cholerae O139 have been demonstrated. Based on the results of this study, the coastal ecosystem of the Bay of Bengal is concluded to be a significant reservoir for the epidemic serogroups of V. cholerae.

Distribution and characterization of integrons in various serogroups of Vibrio cholerae strains isolated from diarrhoeal patients between 1992 and 2000 in Kolkata, India

A total of 133 clinical strains of Vibrio cholerae comprising 44 strains of O1, 45 strains of O139 and 44 strains of non-O1, non-O139 serogroups isolated from hospitalized patients in Kolkata, India, from 1992 to 2000 was examined for the presence of class 1, 2 and 4 integrons. By PCR and DNA sequencing, seven strains of O1, one strain of O139 and six strains of non-O1, non-O139 serogroups were found to contain class 1 integron-harbouring genes aadA1, aadA2 (encoding resistance to streptomycin and spectinomycin), blaP1 (resistance to beta-lactams), aar-3 (resistance to rifampicin), aacA4 (resistance to kanamycin and gentamicin), and dfrA1 and dfrA15 (resistance to trimethoprim). Most strains produced one or two bands using primers specific for the amplification of the variable region where the antibiotic-resistance genes are located, and their sizes ranged from 700 to 1250 bp. However, one strain of V. cholerae O1 isolated in 1994 gave a 2483 bp fragment, the largest fragment so far found in a class 1 integron of V. cholerae O1. No strain was positive for the intI2 gene. All V. cholerae strains, regardless of serogroup, were positive for the intI4 gene by PCR and using a colony hybridization test. Amplification of the intI4 gene by PCR yielded a 2200 bp fragment (1260 bp larger than the expected size) from three V. cholerae O139 strains isolated in 1999. Sequence analysis of this amplicon revealed an insertion of IS1359 in the middle of the intI4 gene. These data indicate that a class 1 integron is present in some clinical strains of V. cholerae isolated in Kolkata, India, and that a class 4 integron is ubiquitously distributed among V. cholerae strains regardless of serogroup.

The emerging strains of Shigella dysenteriae type 2 in Bangladesh are clonal

A total of 113 strains of Shigella dysenteriae type 2 isolated from patients attending the Dhaka diarrhoea treatment centre of ICDDR,B: Centre for Health and Population Research during the period 1999-2004 were studied. Serotype of the isolates was confirmed using commercially available antisera. Except for arabinose fermentation, all the strains had similar biochemical reactions. More than 60% of the strains were sensitive to commonly used antibiotics; only 6% (n=7) of the strains were resistant to nalidixic acid, and none of the strains were resistant to mecillinam and ciprofloxacin. All strains were invasive as demonstrated by the presence of a 140 MDa plasmid, ial, sen and ipaH genes, Congo Red absorption ability and by the Sereny test performed on representative strains. Plasmid patterns were heterogeneous but more than 50% of strains were confined to a single pattern. All strains possessed a 1.6 MDa plasmid and 87% of the strains contained a 4 MDa plasmid. Middle-range plasmids (90 MDa to 30 MDa) present in 36% of the strains were not associated with antibiotic resistance. All the strains were clustered within a single type with four subtypes by pulsed-field gel electrophoresis while ribotyping patterns of all the strains were identical.