Which reaction is typical for Escherichia coli?

Which reaction is typical for Escherichia coli?

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

Answer: (A)

Escherichia coli is a lactose-fermenting, nonhemolytic Gram-negative rod. This means it typically forms pink, lactose-fermenting colonies on MacConkey agar and shows no hemolysis on sheep blood agar. The other patterns described—beta-hemolysis on blood agar, colorless colonies on MacConkey or on XLD, or green colonies with black centers on Hektoen-enteric agar—are not characteristic of E. coli (beta-hemolysis is more typical of some other organisms; black centers on Hektoen indicate hydrogen sulfide production seen with Salmonella, and colorless colonies on MacConkey or XLD indicate non-lactose fermenters). So the standard description for E. coli among common media is lactose fermentation with pink MacConkey colonies and no hemolysis on blood agar.

Escherichia coli is a lactose-fermenting, nonhemolytic Gram-negative rod. This means it typically forms pink, lactose-fermenting colonies on MacConkey agar and shows no hemolysis on sheep blood agar. The other patterns described—beta-hemolysis on blood agar, colorless colonies on MacConkey or on XLD, or green colonies with black centers on Hektoen-enteric agar—are not characteristic of E. coli (beta-hemolysis is more typical of some other organisms; black centers on Hektoen indicate hydrogen sulfide production seen with Salmonella, and colorless colonies on MacConkey or XLD indicate non-lactose fermenters). So the standard description for E. coli among common media is lactose fermentation with pink MacConkey colonies and no hemolysis on blood agar.