A child comes into your clinic with impetigo. The lab cultures a sample for further analysis. If the sample is S. aureus, which lab results would you expect?
a. Gram-positive cocci in clusters, catalase and coagulase positive
b. Gram-negative diplococci, catalase positive, and coagulase negative
c. Gram-positive cocci in clusters, catalase negative, and coagulase positive
d. Gram-positive cocci in chains, catalase positive, and coagulase negative
e. Gram-positive cocci in chains, catalase negative, and coagulase positive

Verified step by step guidance

1. Step 1: Identify the morphology of Staphylococcus aureus under the microscope. S. aureus appears as Gram-positive cocci arranged in clusters, which is a key distinguishing feature from other bacteria.
2. Step 2: Understand the catalase test result. Staphylococcus species, including S. aureus, are catalase positive, meaning they produce the enzyme catalase that breaks down hydrogen peroxide into water and oxygen.
3. Step 3: Consider the coagulase test. S. aureus is coagulase positive, which differentiates it from other Staphylococcus species like S. epidermidis that are coagulase negative.
4. Step 4: Compare these characteristics with the options given: look for Gram-positive cocci in clusters, catalase positive, and coagulase positive to match S. aureus.
5. Step 5: Eliminate options that describe Gram-negative bacteria, cocci in chains (typical of Streptococcus), or incorrect catalase/coagulase results to find the correct lab profile for S. aureus.

Key Concepts

Gram Staining and Morphology of Staphylococcus aureus

Staphylococcus aureus is a Gram-positive bacterium that appears as cocci (spherical cells) arranged in clusters resembling grape bunches. Gram staining differentiates bacteria by cell wall properties, with S. aureus retaining the crystal violet stain, appearing purple under a microscope.

Catalase Test

The catalase test detects the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen. Staphylococci, including S. aureus, are catalase positive, producing bubbles when exposed to hydrogen peroxide, distinguishing them from catalase-negative streptococci.

Coagulase Test

The coagulase test identifies the ability of bacteria to clot plasma by producing the enzyme coagulase. S. aureus is coagulase positive, which helps differentiate it from other Staphylococcus species that are coagulase negative, making this test critical for accurate identification.