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

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.

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.

Impregnation Methods

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

Tulane University

Bacteria are classified based on size, shape and arrangement. 

Morphology & Classification of Bacteria

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

Morphology and Classification of Bacteria

Studying the morphology of bacteria requires using microscopes. The types of microscopes are:
- Optical microscope
- Phase contrast microscope
- Dark ground microscope
- Electron microscope

Microscopy

Stained Preparations

Bacteria can be classified based on their different morphologies.
- Coccus
- Bacillus
- Vibrio
- Coccobacillus
- Spirillum
- Spirochete

Cell Morphologies

- Arrangements of Cocci
- Arrangements of Bacilli

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