Gram Staining Uses and Procedures
Who developed gram stains? • Hans Christian Gram • Danish physician • 1884 • Original purpose was to discern the difference between two types of pnuemonia.
Gram Stain Procedure • Separates all bacteria into 2 groups: • Gram Positive • Stain purple • Retain crystal violet • Gram Negative • stain pink or red • Retain safranin
Why do bacteria retain certain stains? 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.
Common Gram Positive Bacteria • Staphylococcus • Cause of staph infections • Ex. aureus • Streptococcus • Ex. pneumonia • Micrococcus • Skin, water, dust, soil • Clostridium • Causes diarrhea and colitis • Listeria • Meningitis
Common Gram Negative Bacteria • E. coli • Found in digestive tracts of animals • Neisseriagonorrhoeae • STD • Salmonella • Found in undercooked food • Neisseriameningitidis • meningitis
Gram Stain Procedure • Crystal violet is dropped onto bacteria cells staining gram-positive cells purple • Iodine is added to fix the violet dye into place. • Ethanol (EtOH) is used to wash dye off the unstained cells. • Red dye safranis is used to stain any gram-negative cells present.