Successive one-electron reductions of molecular oxygen yield the superoxide radical (O2-) H2O2, the hydroxyl radical (OH) and water. Redox potentials at pH 7 for one-, two- and four-electron couples involving these states are presented as a potential diagram. The significance of each of these potentials is explained. The complete potential diagram enables complex systems to be rationalized, such as production of OH by H2O2 plus Fe3+.

In animals, electron transfer from NADH to molecular oxygen proceeds via large respiratory complexes in a linear respiratory chain. In contrast, most fungi utilise branched respiratory chains. These consist of alternative NADH dehydrogenases, which catalyse rotenone insensitive oxidation of matrix NADH or enable cytoplasmic NADH to be used directly. Many also contain an alternative oxidase that probably accepts electrons directly from ubiquinol. A few fungi lack Complex I. Although the alternative components are non-energy conserving, their organisation within the fungal electron transfer chain ensures that the transfer of electrons from NADH to molecular oxygen is generally coupled to proton translocation through at least one site. The alternative oxidase enables respiration to continue in the presence of inhibitors for ubiquinol:cytochrome c oxidoreductase and cytochrome c oxidase. This may be particularly important for fungal pathogens, since host defence mechanisms often involve nitric oxide, which, whilst being a potent inhibitor of cytochrome c oxidase, has no inhibitory effect on alternative oxidase. Alternative NADH dehydrogenases may avoid the active oxygen production associated with Complex I. The expression and activity regulation of alternative components responds to factors ranging from oxidative stress to the stage of fungal development.

Acetylene brings about a progressive inactivation of ammonia mono-oxygenase, the ammonia-oxidizing enzyme in Nitrosomonas europaea. High NH4+ ion concentrations were protective. The inactivation followed first-order kinetics, with a rate constant of 1.5 min-1 at saturating concentrations of acetylene. If acetylene was added in the absence of O2, the cells remained active until O2 was re-introduced. A protective effect was also demonstrated with thiourea, a reversible non-competitive inhibitor of ammonia oxidation. Incubation of cells with [14C]acetylene was found to cause labelling of a single membrane polypeptide. This ran on dodecyl sulphate/polyacrylamide-gel electrophoresis with an Mr value of 28 000. It is concluded that acetylene is a suicide substrate for the mono-oxygenase. The labelling experiment provides the first identification of a constituent polypeptide of ammonia mono-oxygenase.

The increasing prevalence of E. coli bloodstream infections is a serious public health problem. We used genomic epidemiology in a One Health study conducted in the East of England to examine putative sources of E. coli associated with serious human disease. E. coli from 1,517 patients with bloodstream infections were compared with 431 isolates from livestock farms and meat. Livestock-associated and bloodstream isolates were genetically distinct populations based on core genome and accessory genome analyses. Identical antimicrobial resistance genes were found in livestock and human isolates, but there was limited overlap in the mobile elements carrying these genes. Within the limitations of sampling, our findings do not support the idea that E. coli causing invasive disease or their resistance genes are commonly acquired from livestock in our region.

In timber infested by brown-rot fungi, a rapid loss of strength is attributed to production of hydroxyl radicals (HO.). The hydroxyl radicals are produced by the Fenton reaction [Fe(II)/H2O2], but the pathways leading to Fe(II) and H2O2 have remained unclear. Cellobiose dehydrogenase, purified from cultures of Coniophora puteana, has been shown to couple oxidation of cellodextrins to conversion of Fe(III) to Fe(II). Two characteristics of brown rot are release of oxalic acid and lowering of the local pH, often to about pH 2. Modelling of Fe(II) speciation in the presence of oxalate has revealed that Fe(II)-oxalate complexes are important at pH 4-5, but at pH 2 almost all Fe(II) is in an uncomplexed state which reacts very slowly with dioxygen. Diffusion of Fe(II) away from the hyphae will promote conversion to Fe(II)-oxalate and autoxidation with H2O2 as product. Thus the critical Fe(II)/H2O2 combination will be generated at a distance, enabling hydroxyl radicals to be formed without damage to the hyphae.