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

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

Answer: (A)

On TSI agar, yellow in both the slant and the butt means the organism is fermenting sugars to acid in both aerobic and anaerobic environments (A/A). That requires fermentation of lactose and/or sucrose (not just glucose), since glucose alone usually causes the butt to turn yellow while the slant reverts to red after glucose is exhausted. Escherichia coli is a strong lactose fermenter, so it acidifies the entire medium, producing a yellow slant and a yellow butt after incubation.

Other choices tend to show red in the slant or a blackening of the butt due to hydrogen sulfide production, or fail to ferment lactose, which would not give the A/A pattern. Shigella sonnei, for example, typically yields a red slant with a yellow butt (K/A) or no gas, not yellow/yellow. Salmonella Typhimurium and Proteus mirabilis often produce H2S, which also disrupts a pure yellow/yellow reading.

So the yellow on both regions after 18 hours best fits a strong lactose fermenter like E. coli.