Triple sugar iron (TSI) agar is a tubed differential medium used in determining carbohydrate fermentation and H2S production. Gas from carbohydrate metabolism can also be detected. Bacteria can metabolize carbohydrates aerobically (with oxygen) or fementatively (without oxygen). TSI differentiates bacteria based on their fermentation of lactose, glucose and sucrose and on the production of hydrogen sulfide. TSI is most frequently used in the identification of the Enterobacteriaceae, although it is useful for other gram-negative bacteria.