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

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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.

Identification of Bacteria Questions