What Do Bacteria Look Like Under an Electron Microscope?

An electron microscope is an instrument that uses a beam of accelerated electrons to view objects at a much higher resolution than a conventional light microscope. The lower wavelength of electrons allows these microscopes to resolve objects down to the atomic level. This capability is necessary for visualizing the intricate details of bacteria. These microorganisms appear as mere dots or simple shapes under a light microscope, so the power of an electron microscope is needed to understand their structure and function.

The Need for High Magnification

A light microscope’s resolution is restricted by the wavelength of visible light, limiting its maximum useful magnification to around 1500x, with a resolution of about 200 nanometers. This is insufficient for observing the detailed internal and external features of a bacterial cell, which often measure in the tens of nanometers. While a light microscope can reveal a bacterium’s basic shape, it cannot resolve its finer details.

Electron microscopes overcome this by using electron beams, which have wavelengths thousands of times shorter than visible light. This allows for magnifications exceeding 500,000x and resolutions down to 0.2 nanometers, making it possible to see the complex ultrastructure of a bacterial cell.

Key Electron Microscopy Techniques for Bacteria

Two main types of electron microscopy are used to study bacteria. Transmission Electron Microscopy (TEM) is used for viewing the internal structures of a bacterium. In TEM, a high-voltage electron beam is passed through an extremely thin slice of the specimen, and magnetic lenses focus the transmitted electrons into a highly magnified image revealing the cell’s inner architecture.

TEM allows microbiologists to see the layers of the cell envelope, including the distinct differences between Gram-positive and Gram-negative cell walls. Inside the cell, it can resolve the granular cytoplasm, ribosomes, and the nucleoid region where the cell’s genetic material is concentrated. These detailed cross-sectional views are important for understanding cellular processes and organization.

For visualizing the exterior of a bacterium, scientists use Scanning Electron Microscopy (SEM). Instead of passing electrons through the sample, SEM scans a focused electron beam across the surface of an intact specimen. A detector collects secondary electrons knocked from the surface to build a three-dimensional image of the topography. This technique provides excellent depth of field, giving images a distinct 3D appearance, and is ideal for observing the overall shape, arrangement, and surface features of bacteria, such as flagella or pili.

Preparing Bacterial Samples for Viewing

Biological specimens like bacteria are mostly water and cannot be placed directly into an electron microscope, as they cannot withstand the vacuum inside. They must undergo a multi-step preparation process to preserve their structure. The first step is fixation, where chemicals like glutaraldehyde are used to stabilize cellular components and prevent decay. This is often followed by a secondary fixation with osmium tetroxide, which also increases image contrast.

After fixation, the water is removed from the sample through dehydration, which is achieved by soaking the specimen in a series of ethanol or acetone solutions of increasing concentration. For TEM, the dehydrated bacteria are then infiltrated with a liquid resin that hardens to form a solid block. This block is sliced into ultra-thin sections, just 40 to 100 nanometers thick, using an ultramicrotome. These sections are placed on a copper grid and stained with heavy metal salts to enhance contrast.

For SEM, after dehydration, the sample undergoes a critical-point drying procedure to remove the ethanol without causing structural collapse. Because biological specimens are not naturally conductive, the dried bacteria must then be coated with an ultra-thin layer of a conductive metal, like gold or palladium. This metallic coating prevents the buildup of electrical charge on the specimen’s surface, ensuring a clear and stable image.

What We See: Bacterial Details Revealed

The images produced by electron microscopy reveal a world of intricate bacterial detail. With Transmission Electron Microscopy (TEM), scientists can distinguish the complex layers of the cell wall, differentiating between Gram-positive and Gram-negative bacteria. Within the cytoplasm, individual ribosomes and the tangled DNA of the nucleoid region become visible.

Other internal features like storage granules, endospores, and even invading viral particles can be identified. This internal view helps researchers understand how bacteria function, store nutrients, and respond to environmental stress. The ability to see these structures provides a direct link between a bacterium’s genetics and its physical form.

Scanning Electron Microscopy (SEM) offers a stunning view of the bacterial surface and its interactions with the environment. SEM images showcase the diversity of bacterial shapes and how they arrange themselves in pairs, chains, or clusters. It is particularly powerful for studying biofilms, revealing the complex three-dimensional architecture of these microbial communities.