Gram Staining & Selective and Differential Media

Exam

Crystal violet

This is the primary / main stain, it colors all cells purple. The peptidoglycan is what the crystal violet is staining.

Gram’s iodine

This is added after crystal violet, it acts as a mordant (responsible for fixing the substance into the solution), creating an insoluble complex with the crystal violet within the cell walls.

Decolorizer

This is a mixture of acetone and alcohol that removes the crystal violet from Gram-negative cells but not from Gram-positive cells, which retain the purple stain

Safranin

This is the counterstain, it stains the decolorized Gram-negative cells pink, while the Gram-positive cells remain purple (Gram-negative will only be colored since no color will be decolorized in Gram-positive).

Selective media

• This media is designed to favor the growth of specific microorganisms while inhibiting the growth of others

• utilize various mechanisms, such as chemical inhibitors or altered pH levels, to create an environment conducive to certain microbes

e.g. Mannitol Salt Agar Plate

Differential media

• Distinguishing microbes through biochemical reactions

• enable scientists to identify and classify microbes based on visible changes in the medium, revealing essential biochemical reactions that assist in accurate microbial diagnostics

e.g. EMB Plate