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

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.

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.