Triple Sugar Iron Agar (TSI Agar) is used for the differentiation of gram-negative enteric bacilli based on carbohydrate fermentation and the production of hydrogen sulfide.

• Contains three sugars: dextrose, lactose, and sucrose
• Phenol red allows indication of carbohydrate fermentation
• Ferrous ammonium sulfate detects hydrogen sulfide production

To enhance the alkaline condition of the slant, free exchange of air must be permitted by closing the tube cap loosely. If the tube is tightly closed, an acid reaction (caused solely by dextrose fermentation) will also involve the slant.