Types of Dye

Depending on the nature of the dyes charged group i.e. auxochrome group, they are divided into (I) Acidic dyes (II) Basic dyes and (III) Neutral dyes.

(I)                   Acidic dyes

As the name implies, acidic dyes have negative charge. They ionize to give negative electrical charge to the dye portion of the molecule. They can bind to positively charged molecules like some proteins. They are not commonly used in microbiology except for some background staining for example capsule staining. Examples of acidic dyes are Eosin, Acid fuschin, Rose Bengal, Picric acid etc. – all have negatively charged groups.

(II)                Basic dyes

As the name implies, basic dyes have positive charge. They ionize to give positive electrical charge to the dye portion of the molecule. They can bind to the negatively charged molecules like nucleic acids and many proteins. They are most often used in bacteriology as bacterial cells are also negatively charged. Examples of basic dyes are Safranin, Basic fuschin, Methylene blue, Malachite Green, Crystal violet etc. – all have positively charged groups.

(III)              Neutral dyes

When aqueous acidic and basic stains are combined, neutral dyes are formed from precipitation. They are used to stain nucleic acid and cytoplasm. Examples of some common neutral dyes are Giesma stain, Eosinate of methylene blue etc.

Note:

It is important to note that pH plays an important role while staining with dyes. Since the degree and nature of the charge on the cell components changes with the pH. Thus a basic dye is most effective at higher pH and anionic dyes will stain best under acidic condition i.e. at lower pH.