What are stains?

Microorganisms are difficult to be seen in living state because of their minute, colourless and transparent nature. Moreover, there is limitation of insufficient contrast to bright field microscopy. Therefore, staining is of prime importance to visualize microbial cells and increase their contrast. Different stains can be used to see these tiny creatures. But first, what are stains? Stains are dyes used to increase contrast by their binding to certain parts of cells or cells themselves. Chemically, stains are coloured organic compounds consisting of three portions.

1. Benzene - colourless organic solvent

2. Chromophore - group with conjugated double bonds that impart colour to benzene or to dye

3. Auxochrome - gives property of ionization to the chromogen (Benzene + Chromophore)

Dyes can bind with cells by ionic, covalent or hydrophobic bonding. Though ionic interaction is the most common means of attachment, dyes can also bind through covalent interactions or due to their solubility characteristic, e.g. Schiff's reagent in Feulgen procedure for DNA staining binds covalently to deoxyribose sugars after hydrochloric acid treatments. Similarly, Sudan III stains lipids because of its solubility in lipid. We shall learn about these dyes/ stains in this practical. Let us begin by studying about the types of stains.