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Biomedical subjects

David A Sack

Publications and source records attributed to David A Sack.

At least 19 recordsLinked to original sources
PubMed2005

Typing of human rotaviruses: nucleotide mismatches between the VP7 gene and primer are associated with genotyping failure.

BACKGROUND: Rotavirus genotyping is performed by using reverse transcription PCR with type-specific-primers. Because the high rotavirus mutation rate generates an extensive genomic variation, different G-type-specific primer sets are applied in different geographical locations. In Bangladesh, a significant proportion (36.9%) of the rotavirus strains isolated in 2002 could not be G-typed using the routinely used primer set. To investigate the reason why the strains were untypeable, nucleotide sequencing of the VP7 genes was performed. RESULTS: Four nucleotide substitutions at the G1 primer-binding site of the VP7 gene of Bangladeshi G1 rotaviruses rendered a major proportion of circulating strains untypeable using the routine primer set. Using an alternative primer set, we could identify G1 rotaviruses as the most prevalent genotype (44.8%), followed by G9 (21.7%), G2 (15.0%) and G4 (13.8%). CONCLUSION: Because of the natural variation in the rotaviral gene sequences, close monitoring of rotavirus genotyping methods is important.

Antigens, Viral↗
PubMed2005

Seasonal epidemics of cholera inversely correlate with the prevalence of environmental cholera phages.

The relationship among (i) the local incidence of cholera, (ii) the prevalence in the aquatic environment of Vibrio cholerae, and (iii) bacterial viruses that attack potentially virulent O1 and O139 serogroup strains of this organism (cholera phages) was studied in Dhaka, Bangladesh. Over nearly a 3-year period, we found that significantly more environmental water samples contained either a phage or a phage-susceptible V. cholerae strain than both (P < 0.00001). The number of cholera patients varied seasonally during this period and frequently coincided with the presence of pathogenic V. cholerae strains in water samples that otherwise lacked detectable cholera phages. Interepidemic periods were characterized by water samples containing cholera phages but no viable bacteria. Our data support the conclusion that cholera phages can influence cholera seasonality and may also play a role in emergence of new V. cholerae pandemic serogroups or clones.

Bacteriophages↗
PubMed2005

Genomic relatedness of the new Matlab variants of Vibrio cholerae O1 to the classical and El Tor biotypes as determined by pulsed-field gel electrophoresis.

The genomes of the recently described Matlab variants of Vibrio cholerae O1 that are hybrids between classical and El Tor biotypes were compared with those of El Tor and classical biotypes by the use of pulsed-field gel electrophoresis. Dendrograms constructed using the unweighted-pair group method using average linkages generated from NotI restriction patterns of whole-chromosomal DNA grouped these strains into two major clusters that were found to be similar but not identical to those of either of the biotypes. Strains that clustered with the classical biotype appear to have been derived from the classical strains, which are thought to be extinct.

Bacterial Typing Techniques↗
PubMed2004

Genetic relatedness of ciprofloxacin-resistant Shigella dysenteriae type 1 strains isolated in south Asia.

OBJECTIVES: The aim of the present study was to determine the clonal relationships of ciprofloxacin-resistant Shigella dysenteriae type 1 strains isolated from south Asia, and S. dysenteriae 1 strains associated with epidemics in 1978, 1984 and 1994. METHODS: The antimicrobial susceptibilities were examined by NCCLS methods. Molecular epidemiological characterization was performed by plasmid profiling, pulsed-field gel electrophoresis (PFGE) and mutation analysis of the quinolone resistance-determining region (QRDR) of gyrA by sequencing. RESULTS: Plasmid patterns of the current ciprofloxacin-resistant strains from India, Nepal and Bangladesh were very similar to those of the 1978, 1984 and 1994 epidemic isolates of S. dysenteriae 1, except for the presence of a new plasmid of approximately 2.6 MDa, which was found in one recent ciprofloxacin-resistant strain isolated in Bangladesh. PFGE analysis showed that the ciprofloxacin-resistant strains isolated in Bangladesh, India and Nepal belonged to a PFGE type (type A), which was possibly related to that of the 1984 and 1994 clone of S. dysenteriae 1, but different from 1978 epidemic strains. The current ciprofloxacin-resistant strains belong to five subtypes (A3-A7), all of which were found in India, but in Bangladesh and Nepal, only A3 existed. Mutation analysis of the QRDR of gyrA revealed that amino acid substitutions at positions 83 and 87 of ciprofloxacin-resistant strains isolated in Bangladesh were similar to those of the strains isolated in Nepal, but different (at position 87) from ciprofloxacin-resistant strains isolated in India. CONCLUSIONS: PFGE and mutation analysis of gyrA showed differences between the current ciprofloxacin-resistant S. dysenteriae 1 strains isolated in south Asia and those associated with epidemics in 1978, 1984 and 1994.

Amino Acid Substitution↗
PubMed2004

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.

Animals↗
PubMed2004

Cholera.

Intestinal infection with Vibrio cholerae results in the loss of large volumes of watery stool, leading to severe and rapidly progressing dehydration and shock. Without adequate and appropriate rehydration therapy, severe cholera kills about half of affected individuals. Cholera toxin, a potent stimulator of adenylate cyclase, causes the intestine to secrete watery fluid rich in sodium, bicarbonate, and potassium, in volumes far exceeding the intestinal absorptive capacity. Cholera has spread from the Indian subcontinent where it is endemic to involve nearly the whole world seven times during the past 185 years. V cholerae serogroup O1, biotype El Tor, has moved from Asia to cause pandemic disease in Africa and South America during the past 35 years. A new serogroup, O139, appeared in south Asia in 1992, has become endemic there, and threatens to start the next pandemic. Research on case management of cholera led to the development of rehydration therapy for dehydrating diarrhoea in general, including the proper use of intravenous and oral rehydration solutions. Appropriate case management has reduced deaths from diarrhoeal disease by an estimated 3 million per year compared with 20 years ago. Vaccination was thought to have no role for cholera, but new oral vaccines are showing great promise.

Africa↗
PubMed2004

Enteroaggregative Escherichia coli diarrhea in travelers: response to rifaximin therapy.

BACKGROUND AND AIMS: We have recently shown that enteroaggregative Escherichia coli (EAEC) strains commonly cause travelers' diarrhea. The study was designed to determine whether U.S. travelers with EAEC diarrhea responded to rifaximin therapy. METHODS: In a multicenter placebo-controlled clinical trial of travelers' diarrhea without non-EAEC pathogens we evaluated 2 doses of rifaximin. EAEC was sought in stool samples in enrolled subjects by HEp-2 cell assay. Response to rifaximin (both groups combined) and placebo were evaluated in EAEC-positive and EAEC-negative patient groups. RESULTS: Compared with placebo, rifaximin shortened the postenrollment illness in travelers with EAEC diarrhea (median, 22 vs. 72 hours; P = 0.03). In subjects with EAEC-negative diarrhea, the median duration of post-treatment diarrhea was shorter with rifaximin (33 hours) than with placebo (52 hours), but this difference was not significantly different (P = 0.14). CONCLUSIONS: Improvement of EAEC-mediated diarrhea with antibiotic treatment supports the pathogenicity of this organism in travelers to developing countries. The study provides information on the value of the poorly absorbed drug rifaximin in therapy of travelers' diarrhea.

Anti-Infective Agents↗
PubMed2004

Diarrhoea in elderly people: aetiology, and clinical characteristics.

We examined the characteristics of elderly people attending the Dhaka Hospital of ICDDR,B. The hospital has a diarrhoeal disease surveillance system that enrols a 2% systematic sample of all patients visiting the hospital. We reviewed data of all patients enrolled into the surveillance system (n = 13,782) over the period 1996-2001 to identify patients aged 60 y and above for inclusion into the current study (4% of all surveillance patients; n = 478). V. cholerae O1 was the most common enteric pathogen isolated from faecal culture of the patients (20%), followed by ETEC (13%), Shigella (11%), V. cholerae O139 (10%), Campylobacter jejuni (5%), Salmonella (3%), EPEC (2%), rotavirus (4%), and E. histolytica (2%). The isolation rate of V. cholerae O139 and Shigella was higher among the elderly compared to adults (15-59 y of age, 10% vs 6%, and 11% vs 7% respectively; p < 0.05 for both comparisons). Compared to 15-59-y-olds, a significantly higher proportion of the elderly had visible blood in stools (8% vs 5%), required short-stay ward admission (86% vs 82%) or referral (1% vs < 1%) to a health facility. Early initiation of oral or i.v. rehydration therapy, prompt referral, and immediate clinical diagnosis for assessment of the need for antibiotic therapy might be beneficial for the elderly.

Adolescent↗
PubMed2004

Use of antibodies in lymphocyte secretions for detection of subclinical tuberculosis infection in asymptomatic contacts.

We have previously demonstrated that Mycobacterium bovis BCG-specific immunoglobulin G antibodies in lymphocyte secretions (ALS) can be employed as a marker for active tuberculosis (TB). We aimed to determine whether the ALS method allows detection of subclinical TB infection in asymptomatic individuals. A prospective study of family contacts (FCs) of patients with active TB and healthy controls was performed. Thirteen of 42 FCs had high ALS responses, including 6 FCs who subsequently developed active TB. No correlation was observed between the tuberculin skin test and the ALS responses in the FCs (r = 0.1, P = 0.23). Among patients with active TB, BCG-specific ALS responses steadily declined from the time of diagnosis through 6 months following antimycobacterial chemotherapy (P = 0.001). The ALS assay enabled detection of infection in exposed symptom-free contacts, who are at greater risk for developing active TB. The method may also allow discrimination between effective treatment of active infection and suboptimal response to therapy.

Antibodies, Bacterial↗
PubMed2004

Pandemic strains of O3:K6 Vibrio parahaemolyticus in the aquatic environment of Bangladesh.

A total of 1500 environmental strains of Vibrio parahaemolyticus, isolated from the aquatic environment of Bangladesh, were screened for the presence of a major V. parahaemolyticus virulence factor, the thermostable direct haemolysin (tdh) gene, by the colony blot hybridization method using a digoxigenin-labeled tdh gene probe. Of 1500 strains, 5 carried the tdh sequence, which was further confirmed by PCR using primers specific for the tdh gene. Examination by PCR confirmed that the 5 strains were V. parahaemolyticus and lacked the thermostable direct haemolysin-related haemolysin (trh) gene, the alternative major virulence gene known to be absent in pandemic strains. All 5 strains gave positive Kanagawa phenomenon reaction with characteristic beta-haemolysis on Wagatsuma agar medium. Southern blot analysis of the HindIII-digested chromosomal DNA demonstrated, in all 5 strains, the presence of 2 tdh genes common to strains positive for Kanagawa phenomenon. However, the 5 strains were found to belong to 3 different serotypes (O3:K29, O4:K37, and O3:K6). The 2 with pandemic serotype O3:K6 gave positive results in group-specific PCR and ORF8 PCR assays, characteristics unique to the pandemic clone. Clonal variations among the 5 isolates were analyzed by comparing RAPD and ribotyping patterns. Results showed different patterns for the 3 serotypes, but the pattern was identical among the O3:K6 strains. This is the first report on the isolation of pandemic O3:K6 strains of V. parahaemolyticus from the aquatic environment of Bangladesh.

Bacterial Toxins↗
PubMed2003

Rapid diagnosis of active tuberculosis by detecting antibodies from lymphocyte secretions.

In the present study, we investigated the tuberculosis (TB) diagnostic performance of an assay on the basis of detection of TB-specific antibodies from peripheral blood mononuclear cells (PBMCs), to determine whether antibodies in lymphocyte secretions obtained from PBMCs would better reflect active disease than antibodies in serum. PBMCs from patients with and without TB cultured in various concentrations for different times were assessed. Immunoglobulin G (IgG) specific for antigen (bacille Calmette-Guérin [BCG] vaccine and purified protein derivative [PPD]) was measured in lymphocyte secretions. Patients with active TB had higher BCG- or PPD-specific IgG antibody responses than patients without TB or healthy subjects (P=.001). This method can be used as a quick diagnostic aid to facilitate rapid detection of TB cases.

Acute Disease↗
PubMed2003

Safety and immunogenicity of an oral, inactivated enterotoxigenic Escherichia coli plus cholera toxin B subunit vaccine in Bangladeshi children 18-36 months of age.

A phase II safety and immunogenicity study of an oral-formalin inactivated enterotoxigenic Escherichia coli (ETEC) vaccine containing six colonization factors (CFA/I, CS1, CS2, CS3, CS4, CS5) and 1mg of recombinant cholera toxin B subunit (the CF-BS-ETEC vaccine) was carried out in an urban slum of Dhaka city in Bangladesh. The study was carried out in a double blinded, placebo controlled design in 158 children, 18-36 months of age. Children were given two doses of the CF-BS-ETEC vaccine or the placebo which consisted of E. coli K12. The vaccine was well tolerated. The immune response was studied in 60 children (30 each in the placebo and vaccine group). Significant vaccine specific IgA antibody-secreting cell (ASC) responses were seen 7 days after ingestion of the first and second dose of the vaccine. The responses to CFA/I (P<or=0.001), CS2 (P=0.021), CS4 (P=0.009) and rCTB (P<or=0.001) were elevated in the vaccines in comparison to the pre-immune values and in comparison to those seen in the placebo recipients (P=0.018 to <0.001). Vaccines but not placebo recipients also showed significantly increased IgM ASC responses to all three CF antigens that were tested (P=0.012 to <0.001) and IgG-ASCs to rCTB (P<0.001). Peak ASC levels were reached after one dose of the vaccine with no further increase or decrease after the second dose. The vaccine recipients also responded with IgA plasma antibodies to CFA/I, CS1, CS2, CS4 and rCTB after one or two doses of the vaccine (P=0.01 to <0.001). Subjects in the placebo group failed to mount responses to any of the antigens. The vaccine also induced responses in mucosal IgA antibodies in feces to CFA/I, CS2 and rCTB (61, 88 and 69% responder frequency, respectively) and the magnitude of the response was elevated in comparison to the pre-immune levels (P=0.031 to <0.001) and to the levels of the control group (P=0.003 to <0.001). This study thus shows that the CF-BS-ETEC vaccine is well tolerated in children, 18-36 months of age and gives rise to significant systemic and mucosal IgA antibody responses.

Antibodies, Bacterial↗
PubMed2003

Adaptive and inflammatory immune responses in patients infected with strains of Vibrio parahaemolyticus.

In patients with diarrhea caused by Vibrio parahaemolyticus, antibody-secreting cell responses to thermostable direct hemolysin (TDH), lipopolysaccharide (LPS), and whole-cell bacteria were seen. TDH- and LPS-specific responses were seen in serum samples, and immunoglobulin A antibody responses were observed in stool. Levels of C-reactive protein and nitric oxide metabolites increased in the systemic circulation at the onset of illness. Tumor necrosis factor-alpha and lactoferrin levels were high during the acute stage in mucosal secretions and in plasma, whereas interleukin-1beta levels were high only in mucosal secretions. Duodenal and rectal biopsy specimens obtained at the onset of illness showed an acute inflammatory response. The lamina propria showed edema, congestion of blood vessels, and hemorrhage, with an increase in levels of polymorphonuclear neutrophils and macrophages. Strains belonging to different serotypes exhibited varying resistance to killing by serum; the O8:K21 strain was most sensitive. Infection with V. parahaemolyticus results in B cell responses and an acute inflammatory response that is self-limiting.

Adult↗
PubMed2003

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.

Bangladesh↗
PubMed2003

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.

Bacteriophages↗