Fundamentals of Microbiology (2+2)

Background

The aim of simple staining is to increase the contrast between the specimen and the background while observing under a microscope. The use of a single stain to colour a bacterial organism is commonly referred to as simple staining. Some of the most commonly used dyes for simple staining are the methylene blue, crystal violet and basic fuchsin. These dyes have colour-bearing ions (chromophores) that are positively charged (cationic). As the bacteria are slightly negatively charged, they produce an attraction with these cationic chromophores and therefore, these dyes work well on bacteria. These cationic dyes are called basic dyes. There are other dyes with anionic chromophores which are called acidic dyes. Eosin is such a dye (Sodium+ eosinate-). The anionic eosin ate will not stain bacteria because of the electrostatic repelling forces involved.

The staining duration for most simple stains is relatively short, from 30 sec. to 2 minutes depending on dye affinity. Simple staining is useful in determining basic morphology.