according to what color
stain they retain. A sample
of bacteria (on a slide) is first stained with a violet dye. The slide is then rinsed with ethanol and (if the violet stain washes off) a red dye is added. Depending on the structure
of the cell
walls, some types of bacteria (such as staphylococcus
and streptococcus) retain the violet stain and are called Gram-positive. Other types (such as pseudomonas and salmonella) retain the red, but not the violet, stain and are called Gram-negative. This technique
is named after the Danish bacteriologist Hans C. J. Gram
(1853-1938) who invented it in 1884.