**Question:** All the following are true of streptococcus except

A. It is a Gram-positive coccus
B. It is a Gram-negative bacillus
C. It is a facultatively anaerobic organism
D. It is a parasitic organism

**Core Concept:** Streptococcus is a genus of Gram-positive, facultatively anaerobic, spherical bacteria belonging to the family Streptococcaceae. There are numerous species within the Streptococcus genus, each with unique characteristics and clinical relevance.

**Why the Correct Answer is Right:** The correct answer, **D** (Streptococcus is a parasitic organism), is incorrect because streptococci are not considered parasites. Instead, they are saprophytic bacteria, meaning they obtain nutrients from dead or decaying organic matter. Streptococci are found in various environments, including the human body, where they can be both commensal (harmless) or pathogenic (causing disease).

**Why Each Wrong Option is Incorrect:**

**A.** **Gram-negative bacillus**: Streptococcus is a Gram-positive coccus, not a Gram-negative bacillus, making option A incorrect.

**B.** **Facultative anaerobe**: Although some streptococci can grow under anaerobic conditions, they are generally considered obligate aerobes, meaning they require oxygen for growth. This makes option B incorrect.

**C.** **Parasitic organism**: As explained earlier, streptococci are not parasites but saprophytic bacteria, making option C incorrect.

**Clinical Pearl:** Understanding the characteristics of Streptococcus and its role in the human microbiome is essential for diagnosing and treating infections caused by this genus of bacteria. Some streptococcal infections include strep throat, skin and soft tissue infections, and meningitis. Knowledge of the genus's properties aids in differentiating between commensal and pathogenic streptococci, facilitating proper diagnosis and treatment.

**Correct Answer:** **D.** Streptococcus is a Gram-positive coccus and an obligate aerobe, making it distinct from the other options. Streptococci are commonly found in the human microbiome, but they are not parasites. Instead, they are saprophytic bacteria that obtain nutrients from dead or decaying organic matter.