University of Michigan - Ann Arbor

Bacteria are classified based on size, shape and arrangement. 

Morphology & 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

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

Individual cells of a specific prokaryotic organism typically exhibit a characteristic shape, known as cell morphology. While thousands of prokaryotic organisms exist, only a handful of morphologies are commonly seen microscopically. These standard shapes include spherical (coccus), rod-shaped (bacillus), curved comma-shaped (vibrio), short oval-like rods (coccobacillus), long rigid corkscrews (spirillum), and loose, flexible helical spirals (spirochete).

Cell Morphologies

Which of the following characteristics are used to classify bacteria?

Which of the following bacterial shapes is correctly matched with its description?

Which of the following is NOT a common bacterial shape?

In addition to their individual cellular shape, prokaryotic cells of the same species may group together into distinctive arrangements. These characteristic groupings are determined by the plane of cell division. The most commonly observed groupings are the various arrangements of cocci and bacilli.

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