triple sugar–iron agar

an agar medium containing peptone, lactose, sucrose, glucose, ferrous ammonium sulfate, sodium thiosulfate, sodium chloride, and phenol red, used for the preliminary screening of Enterobacteriaceae. Production of hydrogen sulfide causes the formation of black ferrous sulfide along the stab line, gas production causes bubbles in the agar, and fermentation of the sugars is indicated by the amount of acid produced. Called also TSI a.