This butt is recommended for determination of hydrogen sulphide production, indole formation & motility of enteric bacilli. It is used to differentiate enteric bacilli particularly Salmonella and Shigella on the basis of sulphide production, indole formation and motility . Determine motility and H2S production prior to determination of indole production. Peptonized iron and sodium thiosulphate are the indicators of H2S production. To detect indole production, add three or four drops of Kovac’s reagent (2) and observe for development of red color (positive reaction).