Gram's stain

views updated Jun 08 2018

Gram's stain A staining method used to differentiate bacteria. The bacterial sample is smeared on a microscope slide, stained with a violet dye, treated with acetone-alcohol (a decolourizer), and finally counterstained with a red dye. Gram-positive bacteria retain the first dye, appearing blue-black under the microscope; such bacteria have a thick layer of peptidoglycan in their cell walls. In Gram-negative bacteria, the acetone-alcohol washes out the violet dye and the counterstain is taken up, the cells appearing red. The cell walls of these bacteria have an outer layer of lipoprotein overlying a thin layer of peptidoglycan. The stain is named after the Danish bacteriologist H. C. J. Gram (1853–1938), who first described the technique (since modified) in 1884.

Gram's stain

views updated Jun 08 2018

Gram's stain (gramz) n. a method of staining bacterial cells, used as a primary means of identification. The bacterial cells are stained with a violet dye, treated with decolorizer (e.g. alcohol), and then counterstained with red dye. Gram-negative bacteria lose the initial stain but take up the counterstain, so that they appear red microscopically. Gram-positive bacteria retain the initial stain, appearing violet microscopically. [ H. C. J. Gram (1853–1938), Danish physician]

Gram's stain

views updated May 29 2018

Gram's stain The staining procedure used to determine the Gram reaction of bacteria.