Microbiology fundamentals: Which of the following organisms are obligate intracellular parasites (i.e., must live and reproduce within host cells)?

Introduction / Context:
Obligate intracellular parasites cannot complete their life cycles outside host cells. Recognizing these organisms is crucial for diagnostics, therapeutic choices, and understanding transmission and pathogenesis.

Given Data / Assumptions:

• Viruses lack cellular machinery and rely entirely on host cells for replication.
• Chlamydia spp. require host-derived ATP and exist as elementary/reticulate bodies.
• Rickettsia spp. are small bacteria that replicate within host cytoplasm and require intracellular environments.

Concept / Approach:
Evaluate each organism’s biology and dependence on host cells. Viruses, by definition, are intracellular obligates. Chlamydia and Rickettsia, although bacteria, also have strict intracellular requirements for replication and survival, classifying them as obligate intracellular pathogens.

Step-by-Step Solution:

Verification / Alternative check:
Culturing requirements (cell culture, eggs) and clinical treatments reflect intracellular lifestyles (e.g., tetracyclines for rickettsial diseases).

Why Other Options Are Wrong:

• Each single-organism choice is incomplete; the comprehensive, correct choice is “All of these.”

Common Pitfalls:
Assuming all bacteria are free-living; several clinically important bacteria are obligate intracellular.

Final Answer:
All of these