Types of Staining

Based on the process, staining is grouped into three types: Simple staining, Differential staining, and Special staining.

Simple Staining

Simple staining is quick and is generally used for studying the morphology of cells using a single stain. In simple staining only single stain is used. Depending on the targeted component, simple staining can be classified into two types:

• Positive staining: This technique uses simple basic stains like safranin, crystal violet, etc. In this method the cells are stained and hence this technique is particularly useful in highlighting cell morphology, cellular arrangements, and specific organelles to understand the internal structures of microorganisms or cells.
• Negative Staining: This technique mostly uses acidic stains like India ink, nigrosin, etc. Acidic stains bear a negative charge and hence do not stain the cells due to repulsion from the cell wall. These stain color the background and create a contrast between the cell and the background.

Differential Staining

Differential staining involves multiple stains to distinguish between different types of microorganisms or structures within a specimen. Some examples include gram staining, acid fast staining, etc.

• Gram Staining: Gram staining categorizes bacteria into two groups based on their cell wall composition. This method involves staining with crystal violet followed by iodine treatment, alcohol wash, and counterstaining with safranin. Gram-positive bacteria retain the violet stain, appearing purple under a microscope, while Gram-negative bacteria take on the safranin counterstain and appear red.
• Acid-Fast Staining: Acid fast staining is a staining technique that is used to stain bacteria with waxy coating on cell walls. This method involves staining with carbol fuchsin, heat treatment, acid-alcohol wash, and counterstaining with methylene blue.

Special Staining

Some staining techniques utilize specific stains or procedures to highlight particular cellular structures, substances, or microorganisms. These techniques are used for detailed study of particular cellular component. Some of these staining methods are discussed below:

• Capsule Staining: Capsules are protective structure formed by bacteria. These are made up of polysaccaride chain and do not get stained easily. To detect them alternatively acidic and basic stains are applied. Acidic staining stains the background and basic stain stains the cell. The capsule being resistant to stains remain colourless and distinctly appear under microscope.
• Endospore Staining: This technique is applied to study the bacterial endospores. In this technique the bacterial smear is treated with malachite green followed by heat treatment that results in the penetration and binding of the stains to the endospore.
• Flagella Staining: Flagella are extremely thin structure and do not take up stain easily. They are treated with basic stains like carbol fuschin or fuschin. These stain color the thin flagella and help to determine their divisions and numbers on the cell.

Staining in biology is a technique to apply chemical substances or dyes to a biological specimen to enhance the visibility and contrast of its cellular constituents under the microscope. Staining is classified into two types. They are vital and non-vital. Vital staining involves the application of non-toxic water-soluble stains to the living specimen to study them live. Whereas, non-vital staining is typical for dead tissues. Some common examples of stains include methylene blue, orcein, carmine, etc.