Final answer:
Gram-negative bacilli that produce an alkaline slant and acid butt in a TSI test have only fermented glucose, as the fermentation of lactose or sucrose would cause the entire medium to turn acidic.
Step-by-step explanation:
The gram-negative bacillus producing an alkaline slant and an acid butt on triple sugar agar has fermented glucose. A triple sugar iron (TSI) agar test is used to differentiate bacteria based on their ability to reduce sulfur and ferment carbohydrates. If the organism ferments glucose only, the small amount of glucose in the medium is exhausted quickly, resulting in an initial acid reaction (yellow butt) followed by an alkaline reaction (red slant) as the organism moves on to oxidize amino acids. Fermentation of lactose or sucrose would lead to acid conditions throughout the medium. Therefore, when an acid butt and alkaline slant are observed, it indicates that the bacterium has fermented glucose, but not lactose or sucrose.