Gram stain

• 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:
    1. They reside within the host cell, and the stain cannot easily penetrate the host cell membrane.
    2. 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

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Algorithm

Gram (+)

1. Does it end in -coccus? → Cocci (Sphere).
2. Is it Nocardia/Actinomyces? → Branching.
3. Is it L.C.C.B.? → Bacilli (Rod).

Gram (-)

1. Is it a “Double N”? (Neisseria) → Diplococci
2. Is it a “C.V.H.”? (Campy, Vibrio, H. pylori) → Curved
3. Is it a Respiratory/Zoonotic oddball? (Haemophilus, Pasteurella, etc.) → Coccobacilli
4. None of the above? → It’s a Bacillus (Straight Rod).