Dye / Stain

Dye or stain is defined as an organic compound containing both chromophore and auxochrome group linked to a benzene ring. Stains basically enhances the contrast between the microorganism and background making it easy to view and study microorganism. The coloring property of the dye is due to its chromophore group. Compounds of benzene along with a chromophore radical are known as chromogens. Chromogens are coloured but they are not a stain as they neither have the affinity nor the ability to unite with i.e. they lack the binding property. In order for a chromogen to be a dye, they must have another group that imparts ionic property to stain. Such groups are called auxochrome. Auxochrome group gives the property of electrolytic dissociation to the compound.

For example: Picric acid is a dye. It is basically a benzene molecule in which three hydrogen atoms (3H) are replaced by three nitro groups (NO₂) and one hydrogen atom is replaced by one hydroxyl (OH) group. In this, the color of picric acid is due to its NO₂ group which is a chromophore and its dyeing property is due to the presence of OH group acting as an auxochrome. Also in this, benzene molecule linked with three nitro group (i.e. chromophore) forming yellow color trinitrobenzene is a chromogen.