What growth characteristic differentiates Shigella from E. coli on XLD agar?

• A. Shigella forms yellow colonies
• B. Shigella forms green colonies
• C. Shigella typically forms red colonies due to lack of H2S production
• D. Shigella produces gas

Answer: (C)

On XLD agar (Xylose Lysine Deoxycholate agar), the differentiation between Shigella and Escherichia coli is primarily based on their ability to ferment xylose and produce hydrogen sulfide (H2S). Shigella does not ferment xylose and does not produce H2S, which results in the formation of red colonies.

In contrast, E. coli typically ferments xylose, which can lead to yellow colonies on XLD agar due to acid production. The key characteristic that sets Shigella apart is its lack of H2S production, which is why it retains the red color in the medium. This biochemical behavior is crucial for distinguishing Shigella from other members of the Enterobacteriaceae family, including E. coli, on this selective agar medium.

Understanding these growth characteristics is essential for microbiological diagnostics, particularly in identifying pathogens responsible for gastrointestinal diseases.