Enzyme-based differentiation: Catalase production is characteristically negative in which of the following bacterial genera?

Introduction / Context:
The catalase test distinguishes catalase-negative streptococci/enterococci from catalase-positive staphylococci and many Gram-negative rods. It is among the quickest bedside bench tests in clinical microbiology.

Given Data / Assumptions:

• Catalase test uses 3% hydrogen peroxide on a colony.
• Immediate bubbling indicates catalase activity (breakdown of H2O2 to water and oxygen).

Concept / Approach:
Streptococcus and Enterococcus are catalase-negative. In contrast, Staphylococcus is catalase-positive. Most Enterobacterales (e.g., Salmonella, Proteus, Klebsiella) are catalase-positive under standard conditions. Therefore, the genus that is classically catalase-negative is Streptococcus.

Step-by-Step Solution:
Pick a fresh colony and add a drop of H2O2. Observe for effervescence within seconds. No bubbles → catalase negative → consistent with Streptococcus.

Verification / Alternative check:
Ensure not to scrape blood agar media into the test, as erythrocytes contain catalase and can cause false positives.

Why Other Options Are Wrong:

• Salmonella / Proteus / Klebsiella: Typically catalase-positive Enterobacterales.
• Staphylococcus: A hallmark catalase-positive genus.

Common Pitfalls:
Using old cultures or mixing agar into the test can confound results; test with minimal media carryover.

Final Answer:
Streptococcus is catalase-negative.