Bacterial specimens interact with dyes, such as Indian ink or nigrosin, which produce a colored background where the unstained bacteria are easily observed. Slender structures, such as spirochetes, can be viewed with negative staining.

Tulane University

Due to the lack of contrast in light microscopes, observation of structural details in live bacteria is not possible. Staining specimens adds contrast to the image. This involves dying and fixing the specimen, subsequently killing the bacteria. Commonly used staining techniques are:
- Simple Stains
- Negative Staining
- Impregnation Methods
- Differential Stains

Stained Preparations

Al-mohanna, Moshtaq & H., quine. (2016). MORPHOLOGY AND CLASSIFICATION OF BACTERIA. 

Morphology and Classification of Bacteria

Differential stains add different colors to bacterial structures. 
Two most common differential stains are the gram stain, developed by Christian Gram as a method of staining bacteria in tissues, and the acid fast stain 

Differential Stains

Methylene blue or fuchsin dyes are required for simple staining procedures. While adding the same color to bacterial specimens, they provide color contrast. 

Simple Stains

Negative Staining

The addition of silver to the surface of bacteria may allow researchers to view thinner cellular structures that cannot be observed with ordinary microscopy. Impregnation methods are used to demonstrate spirochetes and bacterial flagella.

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