Gram's stain

In microbiology, Gram's stain is a differential staining method named after the Danish physician Hans Christian Gram (1853–1938). It aids in the categorizing and identification of bacteria, which are said to be Gram-positive or Gram-negative, depending on whether or not they retain the original violet stain at the end of the process. If Gram-negative, the stain washes out to a red counterstain.