Stains

Overview

• Gram stain 
  • Gram-positive bacteria have a thick peptidoglycan layer in their cell walls that holds the crystal violet stain
  • Gram-negative bacteria cannot hold the crystal violet stain and are counterstained by safranin
• Giemsa stain
  • Borrelia
  • Plasmodium
  • trypanosomes
  • Chlamydia
• Periodic acid-Schiff stain (PAS)
  • glycogen
  • mucopolysaccharides
  • used to diagnose Whipple’s disease (Tropheryma whippelii)
• Carbolfuscin
  • acid-fast stain
• Ziehl-Neelsen
  • acid-fast organisms
    • Mycobacterium
    • Nocardia (partially acid fast)
    • Cryptosporidum oocysts
• India ink
  • Cryptococcus neoformans 
    • stain is not taken up by polysaccharide capsule, creating a transparent halo
    • mucicarmine can also be used to stain capsule red
• Silver stain 
  • fungi
• e.g. Pneumocystis, Legionella

Bugs that Gram stain poorly

• Treponema
  • to thin to be visualized
  • use darkfield microscopy or fluorescent antibodies
• Rickettsia
• Mycobacteria
  • high-lipid-content cell wall
  • use acid-fast stain 
• Legionella pneumophila
  • intracellular parasite
  • use silver stain
• Mycoplasma
  • no cell wall 
• Ureaplasma 
  • no cell wall
• Chlamydia
  • intracellular parasite
  • has a cell wall but the cell wall does not contain peptidoglycan