Optical or light microscopes are fundamental tools in microscopy that utilize visible or ultraviolet light to produce an image, typically achieving magnifications up to approximately $${}1000\times$$. They come in two primary configurations: a simple microscope passes light through just one lens, a design notably used by Antonie van Leeuwenhoek; whereas a compound microscope is more sophisticated, passing light through two or more sets of lenses in a series to achieve greater magnification. Bacteria, whether living or fixed to a slide, may be viewed under these microscopes to indicate shape, motility, arrangement, and size. However, due to the absence of inherent contrast in standard optical microscopes, finer internal details often cannot be examined without additional techniques. This limitation has led to the use of various advanced light microscope types—such as brightfield, darkfield, phase-contrast, fluorescence, and confocal microscopes—to enhance contrast and resolution for specific applications.

Optical Microscope

The phase contrast microscope allows researchers to differentiate structures within the cell by viewing differences in thickness and refractice index. Retardation of the rays of light that pass through the object produces phase differences, allowing the viewer to compare cellular structures and the surroundings. 

Phase Contrast Microscope

The dark field microscope improves the contrast as reflected light is substituted for transmitted light used in the optical microscope. The resulting contrast provides increased resolution; therefore, smaller organisms and structures, such as spirochetes, can be clearly observed. 

Dark Field/Dark Ground Microscope

Beams of electrons are used in electron microscopes; these electrons are dispersed after hitting the object, producing an image which can be viewed on a fluorescent viewing screen. 

Electron Microscopes

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

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

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 arrangements of bacteria 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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