Gram stain

General: Used to differentiate Gram+ from Gram- organisms. Initially developed by Danish physician Hans Christian Gram to differentiate Pneumococcus from Klebsiella. Stained with crystal violet (or methylene blue), iodine is used as a mordant, decolorized to remove lipid, then counterstained with safranin (or basic fuschin). Gram+ bacteria appear dark blue or purple due to high peptidoglycan and low lipid content, while Gram- bacteria appear pink to faint red due to loss of purple stained lipid in the decolorization step and subsequent counterstaining.

Standard/common uses: Infectious suspicion.