The immission fallout increases the sulphur content of spruce forests. This leads to an increase of the concentration of thiobacilli, occurrence of heterotrophic bacteria oxidizing So to SO4 2-, as well as of the incidence of bacteria resistant to sulphite at acid pH. Sulphite acts as an inhibitor or as a stimulator of bacterial growth depending on the pH of the environment. Sulphite in soil is rapidly oxidized to sulphate; various soil heterotrophic bacteria produce sulphite oxidase. - from Author