Gram staining, or Gram method, is used to differentiate bacteria into 2 main groups – those with and those without a thick cell wall. Crystal violet is used to stain the bacteria; those with a thick cell wall retain the dye (positive), whereas those without this type of cell wall do not retain the dye (negative). A counterstain – usually safranin or fuchsine – is also used to colour both bacteria types.
Did you know? Gram staining is named after a Danish physician and bacteriologist, Hans Christian Gram, who discovered, in 1884, that crystal violet stains some bacteria but can be washed out from others.
Use an initial capital when referring to the stain or method, but lower case for adjectival (eponymic) forms:
Gram stain gram-negative bacteria
See also Terms derived from proper nouns.
