In an introductory microbiology lab, if the only test that turned out positive was for catalase activity, which of the following organisms was most likely present?

• A. Streptococcus pyogenes
• B. Staphylococcus aureus
• C. Clostridium botulinum
• D. Escherichia coli

Answer: (B)

Verified step by step guidance

1. Understand the role of the catalase test in microbiology: it detects the presence of the enzyme catalase, which breaks down hydrogen peroxide (H\_2O\_2) into water and oxygen, producing bubbles.
2. Recall that catalase-positive organisms produce bubbles when exposed to hydrogen peroxide, while catalase-negative organisms do not.
3. Identify the catalase activity of each organism listed: Streptococcus pyogenes is catalase-negative, Staphylococcus aureus is catalase-positive, Clostridium botulinum is catalase-negative (anaerobic), and Escherichia coli is catalase-positive but less commonly tested in this context.
4. Since the only positive test was for catalase activity, focus on organisms known to be catalase-positive and commonly identified by this test in introductory labs.
5. Conclude that the organism most likely present is Staphylococcus aureus, as it is a classic catalase-positive bacterium used to differentiate from Streptococcus species.