Tetrathionate is S4O62-, but can be produced by the oxidation of thiosulphate (S2O32-).
As it turns out, there's a lot of thiosulphate in the guts. Bacteria in
the colon produce significant amounts of hydrogen sulphide (H2S), a very toxic gas, which is converted to the safer thiosulphate in the mucous membranes of the cecum.
