Environmental surveillance of commonly-grown vegetables for investigating potential lead and chromium contamination intensification in Bangladesh.

With regard to previously reported Lead (Pb) and Chromium (Cr) contamination in egg, a semi-quantitative assessment of the general environment of Bangladesh is carried out through nation-wide sampling of commonly grown and consumed vegetables. Five vegetables, namely, White Potato (Solanum tuberosum), Green Cabbage (Brassica oleracea capitata var. alba L.), Red Spinach (Amaranthus dubius), White Radish (Raphanus sativus var. longipinnatus), and Green Bean (Phaseolus vulgaris) were selected for sampling from all 64 Districts of the country as per their availability during the sampling season. This results in a collection of 292 samples. Due to the necessity of incorporating a wide spectrum of vegetable types (e.g. tuber, brassica, leafy, root, and fruiting vegetables) as well as the need for including the geographic expanse of the whole country, replicates were not accommodated in the study, hence, the study being semi-quantitative in nature. The results do not substantiate evidence of public health risk due to Cr yet, with only 0.69 % of the samples found contaminated with Cr. Pb contamination (concentrations above a health-based guidance value) is found in 29.47 % of the samples; and together with the insight of such contamination likely being non-point origin, communicates on potential public health risk due to Pb as residual effects of extensive ambient atmospheric Pb pollution in recent history of the country. For the purpose of comparison, Cadmium (Cd) contents of the samples are also analyzed. Although there is no extensive Cd pollution history/source in the country, the likely point-source nature of Cd contamination found in 17.83 % of the samples indicates the scope of managing any existing contamination source by directing efforts into the potential point-sources.

Distribution of genes for virulence and ecological fitness among diverse Vibrio cholerae population in a cholera endemic area: tracking the evolution of pathogenic strains.

The pathogenic strains of Vibrio cholerae that cause acute enteric infections in humans are derived from environmental nonpathogenic strains. To track the evolution of pathogenic V. cholerae and identify potential precursors of new pathogenic strains, we analyzed 324 environmental or clinical V. cholerae isolates for the presence of diverse genes involved in virulence or ecological fitness. Of 251 environmental non-O1, non-O139 strains tested, 10 (3.9%) carried the toxin coregulated pilus (TCP) pathogenicity island encoding TCPs, and the CTX prophage encoding cholera toxin, whereas another 10 isolates carried the TCP island alone, and were susceptible to transduction with CTX phage. Most V. cholerae O1 and O139 strains carried these two major virulence determinants, as well as the Vibrio seventh pandemic islands (VSP-1 and VSP-2), whereas 23 (9.1%) non-O1, non-O139 strains carried several VSP island genes, but none carried a complete VSP island. Conversely, 30 (11.9%) non-O1, non-O139 strains carried type III secretion system (TTSS) genes, but none of 63 V. cholerae O1 or O139 strains tested were positive for TTSS. Thus, the distribution of major virulence genes in the non-O1, non-O139 serogroups of V. cholerae is largely different from that of the O1 or O139 serogroups. However, the prevalence of putative accessory virulence genes (mshA, hlyA, and RTX) was similar in all strains, with the mshA being most prevalent (98.8%) followed by RTX genes (96.2%) and hlyA (94.6%), supporting more recent assumptions that these genes imparts increased environmental fitness. Since all pathogenic strains retain these genes, the epidemiological success of the strains presumably depends on their environmental persistence in addition to the ability to produce major virulence factors. Potential precursors of new pathogenic strains would thus require to assemble a combination of genes for both ecological fitness and virulence to attain epidemiological predominance.

Genomic characterization of non-O1, non-O139 Vibrio cholerae reveals genes for a type III secretion system.

Non-O1, non-O139 Vibrio cholerae can cause gastroenteritis and extraintestinal infections, but, unlike O1 and O139 strains of V. cholerae, little is known about the virulence gene content of non-O1, non-O139 strains and their phylogenetic relationship to other pathogenic V. cholerae. Comparative genomic microarray analysis of four pathogenic non-O1, non-O139 strains indicates that these strains are quite divergent from O1 and O139 strains. Genomic sequence analysis of a non-O1, non-O139 strain (AM-19226) that appeared particularly pathogenic in experimental animals suggests that this strain carries a type III secretion system (TTSS) that is related to the TTSS2 gene cluster found in a pandemic clone of Vibrio parahaemolyticus. The genes for this V. cholerae TTSS system appear to be present in many clinical and environmental non-O1, non-O139 strains, including at least one clone that is globally distributed. We hypothesize that the TTSS present in some pathogenic strains of non-O1, non-O139 V. cholerae may be involved in the virulence and environmental fitness of these strains.

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.