Cystine promotes the formation of cystine-dependent dwarf colonies. Bromothymol blue is the indicator used in the agar, it changes to yellow in case of acid production during fermentation of lactose or changes to deep blue in case of alkalinization.
Cystine stimulates the growth of cystine-dependent dwarf colonies. Methylthymol blue, the indicator utilized in the agar medium, transforms to yellow when acidic byproducts are produced from lactose fermentation and to a deep blue color upon alkaline conditions.