Which organism would yield a yellow slant and a yellow butt on TSI agar after 18 hours?

• A. Escherichia coli
• B. Proteus mirabilis
• C. Salmonella Typhimurium
• D. Shigella sonnei

Answer: (A)

On triple sugar iron agar, yellow coloring indicates acid production from carbohydrate fermentation, while red indicates alkalinity. The slant represents the aerobic-facing surface and the butt the anaerobic interior; seeing yellow in both parts means the organism fermented sugars in both zones, producing acid throughout the tube (an A/A pattern).

Escherichia coli rapidly ferments glucose and also lactose (and often sucrose), so by about 18 hours it typically drives the entire medium to yellow, giving yellow slant and yellow butt. This strong, widespread fermentation is characteristic of E. coli on TSI.

Other organisms in this set usually don’t produce acid in both zones by 18 hours. Those that don’t ferment lactose (like many Shigella and Salmonella) or do so more slowly often show a red slant with a yellow butt (K/A) or may produce blackening from H2S. Proteus can produce gas and may show a K/A pattern with H2S, not a uniform yellow across both zones.

So the yellow-yellow pattern after 18 hours best fits an organism that ferments all available sugars rapidly, with E. coli being the classic example.