**Question:** Which of the following is catalase positive and novobiocin susceptibility negative?

A. Staphylococcus aureus
B. Escherichia coli
C. Bacillus subtilis
D. Streptococcus pneumoniae

**Core Concept:** Microorganisms can be classified based on their catalase production and novobiocin susceptibility. Catalase is an enzyme that helps bacteria to detoxify hydrogen peroxide, while novobiocin is a bacteriostatic agent that inhibits DNA gyrase.

**Why the Correct Answer is Right:**

Catalase positive and novobiocin susceptibility negative refers to bacteria that can produce catalase but are resistant to novobiocin. In this case, the correct answer is **C. Bacillus subtilis**.

Bacillus subtilis is a catalase-positive bacterium, which means it possesses the catalase enzyme. However, it is also resistant to novobiocin, a bacteriostatic agent that inhibits DNA gyrase, a key enzyme involved in bacterial DNA replication and repair. Resistance to novobiocin indicates that B. subtilis has developed an effective mechanism to counteract the drug's effect, allowing it to survive and grow in the presence of novobiocin.

**Why Each Wrong Option is Incorrect:**

1. **Staphylococcus aureus (A)** is a catalase-positive bacterium like B. subtilis. However, it is susceptible to novobiocin, indicating it does not have the effective resistance mechanisms that B. subtilis possesses.

2. **Escherichia coli (B)** is a gram-negative bacterium, which generally has a more complex cell wall structure and may have different resistance mechanisms compared to gram-positive bacteria like B. subtilis. E. coli is also susceptible to novobiocin.

3. **Bacillus anthracis (D)** is another gram-positive bacterium like B. subtilis, but it is not mentioned in the question as an option. However, it is known to be susceptible to novobiocin, similar to S. aureus.

4. **Streptococcus pneumoniae (D)** is a catalase-positive bacterium, but it is also susceptible to novobiocin, indicating that it lacks the resistance mechanisms present in B. subtilis.

**Clinical Pearl:** The ability to identify and differentiate between catalase-positive bacteria with respect to their novobiocin susceptibility can be clinically valuable in microbiology lab diagnosis, as it helps differentiate between gram-positive bacteria (like B. subtilis) and gram-negative bacteria (like E. coli and S. pneumoniae) that generally have different resistance mechanisms and may be treated differently. Novobiocin resistance in gram-positive bacteria like S. aureus can indicate the presence of an acquired resistance mechanism, such as the presence of the cfr gene, which confers resistance to multiple antibiotic classes, including novobiocin.

In summary, the correct answer is **C. Bacillus subtilis** because it is a catalase-positive bacterium