Bench identification: Which bacterium is oxidase-negative yet catalase-positive—a combination characteristic that helps differentiate Gram-positive cocci in routine clinical labs?

Introduction / Context: Simple spot tests—catalase and oxidase—are often the first-line tools in bacterial identification. Recognizing typical result patterns streamlines workflows before more specific assays are performed.

Given Data / Assumptions:

• Catalase test distinguishes staphylococci (positive) from streptococci (negative) using hydrogen peroxide bubbling.
• Oxidase test detects cytochrome c oxidase activity (positive in many Gram-negative aerobes).
• We compare common clinical genera.

Concept / Approach: Staphylococcus species are catalase-positive and oxidase-negative. Streptococcus are catalase-negative. Neisseria and Pseudomonas are oxidase-positive Gram-negative organisms. Thus, the oxidase-negative/catalase-positive signature points to staphylococci.

Step-by-Step Solution: Recall catalase: staphylococci bubble; streptococci do not. Recall oxidase: many Gram-negative aerobes (e.g., Neisseria, Pseudomonas) are positive; staphylococci are negative. Combine results: oxidase-negative + catalase-positive = Staphylococcus. Select “Staphylococcus.”

Verification / Alternative check: Follow-up tests (coagulase, mannitol fermentation, novobiocin susceptibility) further speciate within the staphylococci, confirming the initial genus-level call.

Why Other Options Are Wrong:

• Streptococcus: Catalase-negative.
• Neisseria/Pseudomonas: Oxidase-positive; also Gram-negative rods (Pseudomonas) or diplococci (Neisseria).

Common Pitfalls: Using metal loops with catalase tests (false positives due to catalase-like metal activity). Use wooden sticks or plastic loops.

Final Answer: Staphylococcus