obligate intracellular
Summary
Obligate intracellular organisms are pathogens (mainly certain bacteria and all viruses) that cannot replicate outside a host cell, as they depend on the host's ATP and/or metabolic machinery. Classic examples include Rickettsia, Chlamydia, and Coxiella burnetii among bacteria, and all viruses by definition.
Detail
Obligate intracellular organisms lack the ability to generate their own ATP (energy parasites) or lack certain metabolic pathways, forcing them to live and replicate within host cells. This is distinct from facultative intracellular organisms (e.g., Listeria, Salmonella, Mycobacterium, Legionella, Brucella, Francisella) which can survive both inside and outside cells.
Key obligate intracellular bacteria (high-yield for boards): - Rickettsia species (R. rickettsii - Rocky Mountain spotted fever, R. typhi - typhus) - require host cell for ATP, transmitted by arthropod vectors, infect endothelial cells causing vasculitis. - Chlamydia species (C. trachomatis, C. pneumoniae, C. psittaci) - have unique biphasic life cycle (elementary body - infectious, metabolically inactive; reticulate body - replicating form), lack peptidoglycan wall (thus resistant to penicillins), and depend on host ATP. - Coxiella burnetii - causes Q fever, unique among rickettsiae for being transmitted via aerosol (not arthropod), survives in phagolysosome.
All viruses are obligate intracellular parasites by definition since they lack ribosomes and metabolic machinery, requiring host cell machinery for protein synthesis and replication.
Clinical relevance: Because these organisms live inside cells, this affects: (1) diagnostic testing - often requires serology, PCR, or special culture techniques (cell culture) rather than standard bacterial culture; (2) antibiotic selection - must use antibiotics that penetrate cell membranes (e.g., tetracyclines, macrolides for Chlamydia and Rickettsia) rather than cell-wall active agents like penicillins.
Mnemonic: 'Stay inside (cell) when it is Really Cold outside' - Rickettsia and Chlamydia are obligate intracellular.
Sources
- First Aid for the USMLE Step 1
- Sherris Medical Microbiology
- Levinson's Review of Medical Microbiology and Immunology
Reviewed by AnkiBoss editorial — medical student review. Information here is for study reference only and is not medical advice. Spotted an error? Let us know.