 Hans Christian Gram › Danish physician › 1884  Original purpose was to discern the difference between two types of pnuemonia.

 Separates all bacteria into 2 groups: › Gram Positive  Stain purple  Retain crystal violet › Gram Negative  stain pink or red  Retain safranin

Gram positive bacteria have a thick peptidoglycan layer on the outside of the cell membrane. Crystal violet adheres to this layer. Gram negative bacteria have the peptidoglycan layer, but it is located near the inside of the cell, therefore, the safranin stain adheres to the outer membrane rather than the crystal violet.

 Staphylococcus › Cause of staph infections › Ex. aureus  Streptococcus › Ex. pneumonia  Micrococcus › Skin, water, dust, soil  Clostridium › Causes diarrhea and colitis  Listeria › Meningitis

 E. coli › Found in digestive tracts of animals  Neisseria gonorrhoeae › STD  Salmonella › Found in undercooked food  Neisseria meningitidis › meningitis

1. Crystal violet is dropped onto bacteria cells staining gram-positive cells purple 2. Iodine is added to fix the violet dye into place. 3. Ethanol (EtOH) is used to wash dye off the unstained cells. 4. Red dye safranis is used to stain any gram-negative cells present.