Which of the following is correct for Aeromonas hydrophila complex?

• A. Beta-hemolytic
• B. Forms clear colonies on CIN medium
• C. Member of the Enterobacteriaceae
• D. Oxidase negative

Answer: (A)

Beta-hemolysis on blood agar reflects Aeromonas hydrophila’s ability to produce hemolysins that completely lyse red blood cells around the colony, creating a clear zone. This β-hemolysis is a common and useful clue for identifying Aeromonas species in culture and helps differentiate them from non-hemolytic Gram-negative rods or organisms that don't lyse red cells as readily. In addition, Aeromonas hydrophila is oxidase positive, so a negative oxidase result would not be consistent with this organism. It is not a member of the Enterobacteriaceae family, which explains why that label would be incorrect. CIN medium is tailored to highlight Yersinia enterocolitica with distinctive bull’s-eye colonies; Aeromonas species do not produce that characteristic appearance, so claiming they form clear CIN colonies isn’t a defining trait.