Resolving the contributions of the membrane-bound and periplasmic nitrate reductase systems to nitric oxide and nitrous oxide production in Salmonella enterica serovar Typhimurium

The production of cytotoxic nitric oxide (NO) and conversion into the neuropharmacological agent and potent greenhouse gas nitrous oxide (N2O) is linked with anoxic nitrate catabolism by Salmonella enterica serovar Typhimurium. Salmonella can synthesize two types of nitrate reductase: a membrane-bound form (Nar) and a periplasmic form (Nap). Nitrate catabolism was studied under nitrate-rich and nitrate-limited conditions in chemostat cultures following transition from oxic to anoxic conditions. Intracellular NO production was reported qualitatively by assessing transcription of the NO-regulated genes encoding flavohaemoglobin (Hmp), flavorubredoxin (NorV) and hybrid cluster protein (Hcp). A more quantitative analysis of the extent of NO formation was gained by measuring production of N2O, the end-product of anoxic NO-detoxification. Under nitrate-rich conditions, the nar, nap, hmp, norV and hcp genes were all induced following transition from the oxic to anoxic state, and 20% of nitrate consumed in steady-state was released as N2O when nitrite had accumulated to millimolar levels. The kinetics of nitrate consumption, nitrite accumulation and N2O production were similar to those of wild-type in nitrate-sufficient cultures of a nap mutant. In contrast, in a narG mutant, the steady-state rate of N2O production was ~30-fold lower than that of the wild-type. Under nitrate-limited conditions, nap, but not nar, was up-regulated following transition from oxic to anoxic metabolism and very little N2O production was observed. Thus a combination of nitrate-sufficiency, nitrite accumulation and an active Nar-type nitrate reductase leads to NO and thence N2O production, and this can account for up to 20% of the nitrate catabolized

Nitrous oxide production in soil isolates of nitrate-ammonifying bacteria

Here we provide the first demonstration of the potential for N2O production by soil-isolated nitrate-ammonifying bacteria under different C and N availabilities, building on characterizations informed from model strains. The potential for soil-isolated Bacillus sp. and Citrobacter sp. to reduce NO3-, and produce NH4+, NO2- and N2O was examined in batch and continuous (chemostat) cultures under different C-to-NO3- ratios, NO3--limiting (5 mM) and NO3--sufficient (22 mM) conditions. C-to-NO3- ratio had a major influence on the products of nitrate ammonification, with NO2-, rather than NH4+, being the major product at low C-to-NO3- ratios in batch cultures. N2O production was maximum and accompanied by high NO2- production under C-limitation/NO3-sufficiency conditions in chemostat cultures. In media with lower C-to-NO3-N ratios (5- and 10-to-1) up to 2.7% or 5.0% of NO3- was reduced to N2O by Bacillus sp. and Citrobacter sp., respectively, but these reduction efficiencies were only 0.1% or 0.7% at higher C-to-NO3- ratios (25- and 50-to-1). As the highest N2O production did not occur under the same C-to-NO3- conditions as highest NH4+ production we suggest that a re-evaluation may be necessary of the environmental conditions under which nitrate ammonification contributes to N2O emission from soil

The production and detoxification of a potent cytotoxin, nitric oxide, by pathogenic enteric bacteria

The nitrogen cycle is based on several redox reactions that are mainly accomplished by prokaryotic organisms, some archaea and a few eukaryotes, which use these reactions for assimilatory, dissimilatory or respiratory purposes. One group is the Enterobacteriaceae family of Gammaproteobacteria, which have their natural habitats in soil, marine environments or the intestines of humans and other warm-blooded animals. Some of the genera are pathogenic and usually associated with intestinal infections. Our body possesses several physical and chemical defence mechanisms to prevent pathogenic enteric bacteria from invading the gastrointestinal tract. One response of the innate immune system is to activate macrophages, which produce the potent cytotoxin nitric oxide (NO). However, some pathogens have evolved the ability to detoxify NO to less toxic compounds, such as the neuropharmacological agent and greenhouse gas nitrous oxide (N2O), which enables them to overcome the host's attack. The same mechanisms may be used by bacteria producing NO endogenously as a by-product of anaerobic nitrate respiration. In the present review, we provide a brief introduction into the NO detoxification mechanisms of two members of the Enterobacteriaceae family: Escherichia coli and Salmonella enterica serovar Typhimurium. These are discussed as comparative non-pathogenic and pathogenic model systems in order to investigate the importance of detoxifying NO and producing N2O for the pathogenicity of enteric bacteria

‘Sustainable Mining’? Corporate Social Responsibility, Migration and Livelihood Choices in Zambia

Whilst Corporate Social Responsibility is now part and parcel of many multinational mining operations, and a ‘sustainable mining’ narrative a fundamental part of their public persona, companies still struggle to provide secure, long-term livelihoods for either locals or the swathe of migrants mining attracts. Minimal opportunities in the formal sector leave migrants in particular engaging in informal and illegal activities that offer poor livelihood security. In this paper we examine these activities in Northern Zambia’s emerald mines to highlight some of the issues and barriers to sustainable development that exist across mining zones. We conclude that livelihood choices are not augmented by a so-called ‘sustainable mining’ approach that fails to engage all sectors of the population. We show the numerous challenges faced by migrants in this part of Zambia to accentuate the factors that need to be addressed before favourable environments for fostering sustainable mining might be achieved

Rethinking access: key methodological challenges in studying energy companies

Understanding the role of large energy corporations in society is a crucial, yet challenging task for the social science of energy. Ethnographic methods hold potential for plying into corporations’ own self-representations, to reveal the relations of power and politics that determine flows of energy and extractive capital at the global and local level. Ethnography help us move beyond structural analyses, to locate the agents and processes at work within economies of energy production, and identify tensions and dynamics both within the corporation and at the interface with society. We argue that a multi-method and reflexive approach can help social scientists reflect on frictions in corporate encounters, and more importantly that attention to frictions is in fact a gateway to gain new insights about the field. In our research project about Norwegian energy companies and their corporate social responsibility work when ‘going global’, applying a multi-method made us question dominant assumptions within anthropology of what constitutes “access”. We discuss how multiple approaches to “access”, which takes into account the positionality of the researcher, fluidity of research fields along with attention to power dynamics can shape the sort of knowledge that is produced when studying energy companies

An integrated biochemical system for nitrate assimilation and nitric oxide detoxification in Bradyrhizobium japonicum

Rhizobia are recognized to establish N(2)-fixing symbiotic interactions with legume plants. Bradyrhizobium japonicum, the symbiont of soybeans, can denitrify and grow under free-living conditions with nitrate (NO(3)(−)) or nitrite (NO(2)(−)) as sole nitrogen source. Unlike related bacteria that assimilate NO(3)(−), genes encoding the assimilatory NO(3)(−) reductase (nasC) and NO(2)(−) reductase (nirA) in B. japonicum are located at distinct chromosomal loci. The nasC gene is located with genes encoding an ABC-type NO(3)(−) transporter, a major facilitator family NO(3)(−)/NO(2)(−) transporter (NarK), flavoprotein (Flp) and single-domain haemoglobin (termed Bjgb). However, nirA clusters with genes for a NO(3)(−)/NO(2)(−)-responsive regulator (NasS-NasT). In the present study, we demonstrate NasC and NirA are both key for NO(3)(−) assimilation and that growth with NO(3)(−), but not NO(2)(−) requires flp, implying Flp may function as electron donor to NasC. In addition, bjgb and flp encode a nitric oxide (NO) detoxification system that functions to mitigate cytotoxic NO formed as a by-product of NO(3)(−) assimilation. Additional experiments reveal NasT is required for NO(3)(−)-responsive expression of the narK-bjgb-flp-nasC transcriptional unit and the nirA gene and that NasS is also involved in the regulatory control of this novel bipartite assimilatory NO(3)(−)/NO(2)(−) reductase pathway