- Obligate Intracellular Pathogens
- These organisms must live inside a host cell to replicate. Examples include Chlamydia, Rickettsia, Ehrlichia, and Coxiella burnetii.
- They do not Gram stain well for two reasons:
- They reside within the host cell, and the stain cannot easily penetrate the host cell membrane.
- They often have atypical cell walls or are simply too small.
- These are the organisms that require special stains (e.g., Giemsa, Gimenez).
- Facultative Intracellular Pathogens
- These organisms can survive and replicate inside host cells (often macrophages) but can also grow on their own in the lab (e.g., on agar plates).
- Because they can exist outside of cells, they are readily identified on Gram stain from cultures or from clinical samples containing extracellular bacteria.
- High-yield examples that DO Gram stain well:
- Salmonella: Gram-negative rod
- Listeria monocytogenes: Gram-positive rod
- Neisseria: Gram-negative diplococcus
- Yersinia: Gram-negative rod (often with bipolar “safety pin” staining)
- Francisella: Faintly staining Gram-negative coccobacillus
- Brucella: Faintly staining Gram-negative coccobacillus