Meet the Microbes
Microbeworld Radio
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Anton van Leeuwenhoek and his microscope, circa late 1600s
A. van Leeuwenhoek’s Microscope: Courtesy of Michael Davidson, Molecular Expressions. Van Leeuwenhoek courtesy of National Library of Medicine, NIH.

Microscopes are to microbiology what telescopes are to astronomy.

The earliest microscopes were simple instruments consisting of one or more crude glass lenses similar to those used to make early spectacles. The invention of the first true microscope is credited to the Jansen family of Middleburg, Holland, around 1595.

Later, in the 17th century, Dutch cloth merchant and amateur scientist Anton van Leeuwenhoek enlightened the world to what he dubbed “animacules” such as protozoa found in standing water. Using microscopes he made himself, Leeuwenhoek wrote up what he viewed in pond water, plant material, even gunk scraped off his teeth. He was the first to identify sperm and red blood cells.

There are two basic types of microscopes: light microscopes and electron microscopes.

Light microscopes

light microscope
You may have used a light microscope like the one above in a biology class. Microbiologists use more sophisticated versions of them, such as the one shown below.
a scientist using a light microscope
Electron microscopes, such as the scanning electron microscope shown below, use electrons rather than light, to create images. Courtesy of the Agricultural Research Service of the USDA.
a scientist using a scanning electron microscope

Light microscopes may be familiar to you from biology classes.

Light microscopes can magnify an object up to 1,000 times. Good light microscopes are powerful enough to view most algae, fungi, and protozoa.

High-quality light microscopes generally allow viewing of bacterial cells, too. They can’t view viruses, however, as these tiny objects are smaller than a wavelength of visible light (about 0.2 microns). Nor can they readily allow scientists to examine individual tiny parts of cells in detail.

To view extremely tiny objects, scientist use electron microscopes.

Electron microscopes

Electron microscopes use streams of electrons instead of light to create images. Scientists don’t see the images directly through lenses as they do with light microscopes. Instead, the machinery of the electron microscope generates a picture on a TV or computer screen.

There are three types of electron microscopes: transmission electron microscopes (TEM), scanning electron microscopes (SEM) and scanning-tunneling electron microscopes (STM).

TEMs transmit electron beams through a thin section or slice of a specimen to create an image. TEMs are particularly useful for studying the insides of cells.

With SEMs, the specimen is usually coated with an ultra-thin layer of gold atoms. The electron beam scans over the surface of the specimen, exciting electrons on the surface. When these surface electrons are emitted (as secondary electrons), they are collected by special devices that create an image out of them.

SEMs are especially useful for studying the surfaces and structures of cells. With their great depth of field, SEMs produce 3-D images.

STMs can display things as infinitesimal as the individual atoms on an object’s surface. They scan specimen surfaces in the same way as SEMs, but they use an electrically charged tip that is placed within nanometers of the surface of the specimen. Electrons “jump” between the tip and the specimen surface in what’s called the tunneling current, hence the name of this kind of microscope.

As the tip is moved back and forth across the specimen, the current varies according to whether the tip is right over an atom or over the space or trough between atoms. A computer creates an image based on these differences in current.