Bacterial reduction of tetrathionate

Abstract

It was reported by the authors elsewhere (Nature, 18, 325) that certain intestinal micro-organisms have the power to reduce Na tetrathionate rapidly and quantitatively to thiosulfate. Here the exact method of following the reaction by direct iodine titration, using washed suspensions of Bacterium paratyphosum B, is described, and evidence given showing that thiosulfate is the main product of reduction, only traces of other iodine-reducing substances being formed. The subsequent further reduction of thiosulfate to H2S is slow. The reducing system appears to be similar to the bacterial reduction of nitrate, needing an added H-donator and being inhibited by O2. The transfer of H2 by this means is nearly as rapid as when O2 itself is the H-acceptor. The enzyme is 50% destroyed by 10 min. treatment at 56[degree]C. The pH optimum is about 7.5, and the affinity for tetrathionate high. Neither tetrathionate nor thiosulfate have any inhibitory effect up to a conc, of 0.05M. "Tetrathionase" is possessed by most Salmonellas (excluding Bact. paratyphosum A), Bact-typhosum, Proteus vulgaris and Pr. morganii and a few organisms of the intermediate and paracolon groups. Bact. coli I, most intestinal non-pathogens and dysentery bacilli do not have the enzyme. The possible role of tetrathionase in the selective action of tetrathionate in enrichment media, by acting as an alternative means of substrate oxidation in the absence of O2, is pointed out; for, with few exceptions, tetrathionate selects only those organisms which can reduce it.