<P> Some bacteria utilize thiosulfate anion as a terminal electron acceptor, reducing it to sulfide . If this occurs, the newly formed hydrogen sulfide (H 2S) reacts with ferrous sulfate in the medium to form ferrous sulfide, which is visible as a black precipitate . Examples of sulfide - producing bacteria include Salmonella, Proteus, Citrobacter and Edwardsiella species . The blackening of the medium is almost always observed in the butt (bottom) of the medium . </P> <P> A bacterium that is a non-lactose fermenter and ferments glucose, initially causes a yellow slant / yellow bottom (acid / acid reaction) after 8 hours but then converts to a red slant / yellow bottom after 24 hours (alkali / acid reaction). Whereas if it ferments both lactose and glucose, it results in a yellow / yellow tube and remains that way due to the large amount of acid produced in the reaction . Blackening of the bottom due to H S production may mask the acid reaction (yellow) in the bottom of the tube . Salmonella enterica serovar Typhi may result in blackening of the medium at the interface of bottom and the slant . </P> <P> Under anaerobic conditions (as occur toward the bottom of the tube) some bacteria use thiosulfate as an electron acceptor and reduce it to hydrogen gas . This is not very soluble and may accumulate as bubbles along the inoculation track, between the agar and the glass, or in the fluid which accumulates at the bottom of the slant . Hydrogen production may lift the agar from the butt of the tube or fracture the agar . Carbon dioxide, if produced, may not show as bubbles because it is far more soluble in the medium . </P>

What is the purpose of the sodium thiosulfate in tsi media