Gram-negative

(about bacteria) not retaining the primary violet stain after being stained by Gram's method

Gram-negative bacteria are those that do not retain crystal violet dye in the Gram staining protocol. Gram-positive bacteria will retain the dark blue dye after an alcohol wash. In a Gram stain test, a counterstain (commonly Safranin) is added after the crystal violet, colouring all Gram-negative bacteria a red or pink colour. The test itself is useful in classifying two distinctly different types of bacteria based on structural differences in their cell walls.
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Adjective
1. (of bacteria) being or relating to a bacterium that does not retain the violet stain used in Gram's method
(antonym) gram-positive
(classification) bacteria, bacterium

gram-negative /'græm'negətiv/
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GRAM-NEGATIVO