What type of organism is Chlamydia psittaci?

• A. Virus
• B. Fungus
• C. Gram-positive bacterium
• D. Obligate, intracellular bacterium

Answer: (D)

Chlamydia psittaci is best described as an obligate intracellular bacterium. It is a true bacterium, not a virus, but it can only replicate inside living host cells and cannot be cultured on standard cell-free media. Its life cycle involves an infectious elementary body that enters a host cell and a replicating reticulate body that multiplies within a vacuole. This intracellular lifestyle is what sets it apart from extracellular organisms and explains why the correct descriptor is “obligate, intracellular bacterium.” It is not a fungus, and in fact Chlamydia are Gram-negative bacteria, not Gram-positive, which is why the option describing a Gram-positive bacterium wouldn’t fit.