Which of the following reactions is typical for Escherichia coli?

• A. Beta-hemolytic on sheep blood agar
• B. Colorless colonies on MacConkey agar
• C. Colorless colonies on xylose-lysinedesoxycholate agar
• D. Green colonies with black centers on Hektoen enteric agar

Answer: (A)

Hemolysis patterns on blood agar are a quick clue to virulence traits of bacteria. Escherichia coli is a lactose-fermenter and is usually non-hemolytic, but some strains produce a hemolysin that can cause beta-hemolysis, producing a clear zone around the colonies on sheep blood agar. That visible hemolysis is what this option describes and can be seen with certain pathogenic E. coli isolates, making it a notable reaction to associate with the organism in some contexts.

In contrast, the other media reactions reflect carbohydrate fermentation or sulfur production rather than hemolysis. MacConkey agar shows lactose fermentation as pink colonies for E. coli, not colorless colonies. XLD agar interactions with xylose, lysine, and sulfur sources produce patterns that typically don’t present as colorless colonies for E. coli, and Hektoen enteric agar highlights lactose fermentation and hydrogen sulfide production, where E. coli usually appears as a non-green color with no black center, unlike green colonies with a black center that point toward Salmonella.