facultative intracellular
Summary
Facultative intracellular organisms can survive and replicate inside host cells, particularly macrophages, but are also able to grow extracellularly and on artificial media. Their intracellular niche means control depends on cell-mediated immunity rather than antibody.
Detail
Because these organisms hide within cells, antibody and complement are largely ineffective, and elimination requires Th1 activation of macrophages via interferon-gamma and IL-12, with granuloma formation maintained by TNF-alpha. This explains why patients with defects in the IL-12/IFN-gamma axis, with HIV, or on TNF-alpha inhibitors are susceptible to disseminated mycobacterial and salmonella infection, and why treatment must use drugs that penetrate cells, such as macrolides, fluoroquinolones, rifampin, and trimethoprim-sulfamethoxazole, rather than beta-lactams and aminoglycosides. The standard list is Salmonella, Yersinia, Francisella, Brucella, Legionella, Listeria, Mycobacterium, Nocardia, and the fungus Histoplasma. Obligate intracellular organisms — Rickettsia, Chlamydia, and Coxiella — differ in that they cannot be grown on cell-free media at all, because Rickettsia depends on host ATP and Chlamydia cannot synthesize its own ATP. Listeria is distinctive for escaping the phagosome using listeriolysin O and moving between cells by actin rockets, thereby avoiding extracellular exposure entirely.
Sources
- Levinson Medical Microbiology and Immunology
- Janeway's Immunobiology
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