Reservoirs of faecal indicator bacteria in well-head hand pumps in Bangladesh.

The majority of the population of Bangladesh (90%) rely on untreated groundwater for drinking and domestic use. At the point of collection, 40% of these supplies are contaminated with faecal indicator bacteria (FIB). Recent studies have disproved the theory that latrines discharging to shallow aquifers are the major contributor to this contamination. In this study, we tested the hypothesis that hand pumps are a reservoir of FIB. We sampled the handle, spout, piston and seal from 19 wells in Araihazar Upazila, Bangladesh and identified that the spout and seal were reservoirs of FIB. These findings led to our recommendation that well spouts be regularly cleaned, including the removal of precipitated deposits, and that the seals be regularly changed. It is envisaged that one or both of these interventions will reduce the numbers of FIB in drinking water, thereby reducing the burden of diarrhoeal disease in Bangladesh.

Microbial oxidation of arsenite in a subarctic environment: diversity of arsenite oxidase genes and identification of a psychrotolerant arsenite oxidiser.

BACKGROUND: Arsenic is toxic to most living cells. The two soluble inorganic forms of arsenic are arsenite (+3) and arsenate (+5), with arsenite the more toxic. Prokaryotic metabolism of arsenic has been reported in both thermal and moderate environments and has been shown to be involved in the redox cycling of arsenic. No arsenic metabolism (either dissimilatory arsenate reduction or arsenite oxidation) has ever been reported in cold environments (i.e. < 10 degrees C). RESULTS: Our study site is located 512 kilometres south of the Arctic Circle in the Northwest Territories, Canada in an inactive gold mine which contains mine waste water in excess of 50 mM arsenic. Several thousand tonnes of arsenic trioxide dust are stored in underground chambers and microbial biofilms grow on the chamber walls below seepage points rich in arsenite-containing solutions. We compared the arsenite oxidisers in two subsamples (which differed in arsenite concentration) collected from one biofilm. 'Species' (sequence) richness did not differ between subsamples, but the relative importance of the three identifiable clades did. An arsenite-oxidising bacterium (designated GM1) was isolated, and was shown to oxidise arsenite in the early exponential growth phase and to grow at a broad range of temperatures (4-25 degrees C). Its arsenite oxidase was constitutively expressed and functioned over a broad temperature range. CONCLUSIONS: The diversity of arsenite oxidisers does not significantly differ from two subsamples of a microbial biofilm that vary in arsenite concentrations. GM1 is the first psychrotolerant arsenite oxidiser to be isolated with the ability to grow below 10 degrees C. This ability to grow at low temperatures could be harnessed for arsenic bioremediation in moderate to cold climates.

Detection, diversity and expression of aerobic bacterial arsenite oxidase genes.

The arsenic (As) drinking water crisis in south and south-east Asia has stimulated intense study of the microbial processes controlling the redox cycling of As in soil-water systems. Microbial oxidation of arsenite is a critical link in the global As cycle, and phylogenetically diverse arsenite-oxidizing microorganisms have been isolated from various aquatic and soil environments. However, despite progress characterizing the metabolism of As in various pure cultures, no functional gene approaches have been developed to determine the importance and distribution of arsenite-oxidizing genes in soil-water-sediment systems. Here we report for the first time the successful amplification of arsenite oxidase-like genes (aroA/asoA/aoxB) from a variety of soil-sediment and geothermal environments where arsenite is known to be oxidized. Prior to the current work, only 16 aroA/asoA/aoxB-like gene sequences were available in GenBank, most of these being putative assignments from homology searches of whole genomes. Although aroA/asoA/aoxB gene sequences are not highly conserved across disparate phyla, degenerate primers were used successfully to characterize over 160 diverse aroA-like sequences from 10 geographically isolated, arsenic-contaminated sites and from 13 arsenite-oxidizing organisms. The primer sets were also useful for confirming the expression of aroA-like genes in an arsenite-oxidizing organism and in geothermal environments where arsenite is oxidized to arsenate. The phylogenetic and ecological diversity of aroA-like sequences obtained from this study suggests that genes for aerobic arsenite oxidation are widely distributed in the bacterial domain, are widespread in soil-water systems containing As, and play a critical role in the biogeochemical cycling of As.

The NT-26 cytochrome c552 and its role in arsenite oxidation.

Arsenite oxidation by the facultative chemolithoautotroph NT-26 involves a periplasmic arsenite oxidase. This enzyme is the first component of an electron transport chain which leads to reduction of oxygen to water and the generation of ATP. Involved in this pathway is a periplasmic c-type cytochrome that can act as an electron acceptor to the arsenite oxidase. We identified the gene that encodes this protein downstream of the arsenite oxidase genes (aroBA). This protein, a cytochrome c(552), is similar to a number of c-type cytochromes from the alpha-Proteobacteria and mitochondria. It was therefore not surprising that horse heart cytochrome c could also serve, in vitro, as an alternative electron acceptor for the arsenite oxidase. Purification and characterisation of the c(552) revealed the presence of a single heme per protein and that the heme redox potential is similar to that of mitochondrial c-type cytochromes. Expression studies revealed that synthesis of the cytochrome c gene was not dependent on arsenite as was found to be the case for expression of aroBA.

The evolution of parasites from their hosts: intra- and interspecific parasitism and Emery's rule.

In some taxa of Hymenoptera, fungi, red algae and mistletoe, parasites and their hosts are either sibling species or at least closely related (Emery's rule). Three evolutionary mechanisms have been proposed for this phenomenon: (i) intraspecific parasitism is followed by sympatric speciation; (ii) allopatric speciation is followed by secondary sympatry and the subsequent parasitism of one sibling species by the other; and (iii) allopatric speciation of a species with intraspecific parasitism is followed by secondary sympatry, in which one species becomes an obligate parasite of the other. Mechanisms (i) and (ii) are problematic, while mechanism (iii) has not, to our knowledge, been analysed quantitatively. In this paper, we develop a model for single- and two-species evolutionary stable strategies (ESSs) to examine the basis for Emery's rule and to determine whether mechanism (iii) is consistent with ESS reasoning. In secondary sympatry after allopatric speciation, the system's evolution depends on the relative abundances of the two sibling species and on the proportional damage wrought by parasites of each species on non-parasitic members of the other. Depending on these interspecific effects, either the rarer or the commoner species may become the parasite and the levels of within-species parasitism need not determine which evolves to obligate parasitism.