Fundamentals of Microbiology

Simple staining

 The objective of simple staining is to increase the contrast between the specimen and the background. In positive staining procedure, the stain is attracted to the cell. The stain is basic (cationic), has positively charged chromophore (coloured portion) that is attracted to the negatively charged outer surface of the bacterial cell.   Eg. Methylene blue, Crystal violet, etc.

In negative staining procedure, the stain is acidic, has a negatively charged chromophore that is repelled by the negatively charged microorganisms. This would occupy the space between the microorganisms so that contrast is increased facilitating observation of the microorganisms in a darkly stained background.

Eg. India ink, Nigrosin