Which enzyme differentiates Listeria monocytogenes from streptococci?

• A. Catalase
• B. Oxidase
• C. Urease
• D. Coagulase

Answer: (A)

The key idea here is how the catalase enzyme distinguishes certain Gram-positive bacteria. Listeria monocytogenes produces catalase, so when a small amount of hydrogen peroxide is added, it rapidly breaks it down into water and oxygen, causing visible bubbles. Streptococci, on the other hand, do not produce catalase, so no bubbling occurs. This simple catalase test reliably separates catalase-positive organisms like Listeria from catalase-negative ones like Streptococcus, which is why catalase is the enzyme that differentiates them.

In the lab, this is particularly useful because Listeria can resemble streptococci in appearance and growth characteristics, but the catalase result provides a quick, definitive clue. Other enzymes mentioned—oxidase, urease, and coagulase—do not specifically differentiate these two groups: oxidase is generally negative for both; urease and coagulase test for different organisms (coagulase for distinguishing Staphylococcus aureus from others, urease for certain yeasts and bacteria).