Learning Objective: Differentiate obligate and facultative intracellular bacteria, explain their survival mechanisms, and recognize their clinical importance.
Concept Overview
| Type |
Definition |
Dependence on Host |
Examples |
Mnemonic |
| Obligate Intracellular |
Bacteria that cannot replicate outside host cells |
Require host ATP and metabolic machinery |
Rickettsia, Chlamydia, Coxiella burnetii |
“Stay inside when it’s Really Chilly and Cold.” |
| Facultative Intracellular |
Bacteria that can live inside or outside host cells |
Use the intracellular location to evade host defenses |
Salmonella, Neisseria, Brucella, Mycobacterium, Listeria, Francisella, Legionella, Yersinia pestis |
“Some Nasty Bugs May Live FacultativeLY” |
Mechanisms of Intracellular Survival
| Strategy |
Example Organism |
Mechanism |
| Avoid phagolysosome fusion |
Mycobacterium tuberculosis |
Inhibits phagosome–lysosome fusion inside macrophages |
| Escape into cytoplasm |
Listeria monocytogenes |
Uses listeriolysin O to break the phagosomal membrane |
| Survive within a vacuole |
Legionella pneumophila |
Replicates inside specialized endosomal compartments |
| Rely on host ATP |
Rickettsia, Chlamydia |
“Energy parasites” using host ATP |
Activity:
Clinical Relevance
| Organism |
Disease/Condition |
Key Feature |
| Rickettsia rickettsii |
Rocky Mountain spotted fever |
Rash starting on wrists/ankles → spreads to trunk |
| Chlamydia trachomatis |
Nongonococcal urethritis, PID |
Elementary & reticulate body forms |
| Coxiella burnetii |
Q fever |
Forms spore-like structures, no arthropod vector |
| Brucella melitensis |
Brucellosis |
Undulant fever, granulomatous infection |
| Listeria monocytogenes |
Neonatal meningitis, sepsis |
Actin-based motility (“rocket tails”) |
| Legionella pneumophila |
Legionnaires’ disease |
Grows inside macrophages; waterborne infection |
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