**Question:** The "String of pearl" colonies on Nutrient agar is produced by -

A. Staphylococcus aureus
B. Streptococcus viridans
C. Bacillus subtilis
D. Klebsiella pneumoniae

**Core Concept:**
The term "String of pearl" refers to the unique colony morphology observed in certain bacteria when grown on Nutrient agar. This characteristic is due to the arrangement and growth pattern of the bacteria, which results in a string-like or pearl-like appearance. Nutrient agar is a medium used for the growth and identification of microorganisms in microbiology.

**Why the Correct Answer is Right:**
The "String of pearl" colonies are characteristic of certain species of bacteria, particularly those belonging to the Streptococcus and Staphylococcus genera. Out of the given options, Streptococcus viridans is the correct answer. Streptococcus viridans is a group of streptococci that can cause infections in humans, including throat infections, endocarditis, and endocarditis. The colonies on Nutrient agar appear as "String of pearl" due to the unique arrangement and growth pattern of the bacteria, which results in a string-like or pearl-like appearance.

**Why Each Wrong Option is Incorrect:**

A. Staphylococcus aureus: Staphylococcus aureus is a different genus of bacteria and does not produce the "String of pearl" morphology on Nutrient agar. Its colonies typically appear round, convex, and yellow-white in colour.

B. Streptococcus pneumoniae: Streptococcus pneumoniae is a different species of Streptococcus and does not produce the "String of pearl" morphology on Nutrient agar. Its colonies have a smooth, convex, and white appearance.

C. Bacillus subtilis: Bacillus subtilis is a different species of bacteria and does not produce the "String of pearl" morphology on Nutrient agar. Its colonies are typically small, convex, and yellow-white in colour.

D. Klebsiella pneumoniae: Klebsiella pneumoniae is a different species of Klebsiella and does not produce the "String of pearl" morphology on Nutrient agar. Its colonies have a smooth, convex, and yellow-white appearance.

**Clinical Pearl:**

The "String of pearl" morphology observed in Streptococcus viridans is a valuable clue for identifying these bacteria in microbiological cultures. This distinctive growth pattern can help differentiate Streptococcus viridans from other closely related species and genera, facilitating accurate identification and appropriate antimicrobial therapy.