2: How We See the Invisible World

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Over the past several centuries, we have learned to manipulate light to peer into previously invisible worlds—those too small or too far away to be seen by the naked eye. Through a microscope, we can examine microbial cells and colonies, using various techniques to manipulate color, size, and contrast in ways that help us identify species and diagnose disease. This chapter explores how various types of microscopes manipulate light in order to provide a window into the world of microorganisms. By understanding how various kinds of microscopes work, we can produce highly detailed images of microbes that can be useful for both research and clinical applications.

2.E: How We See the Invisible World (Exercises)
2.0: The Properties of Light
Visible light consists of electromagnetic waves that behave like other waves. Hence, many of the properties of light that are relevant to microscopy can be understood in terms of light’s behavior as a wave. An important property of light waves is thewavelength, or the distance between one peak of a wave and the next peak. The height of each peak (or depth of each trough) is called the amplitude.
2.1: Peering into the Invisible World
Italian scholar Girolamo Fracastoro is regarded as the first person to formally postulate that disease was spread by tiny invisible seminaria. He proposed that these seeds could attach themselves to certain objects that supported their transfer from person to person. However, since the technology for seeing such tiny objects did not yet exist, the existence of the seminaria remained hypothetical for a little over a century—an invisible world waiting to be revealed.
2.2: Instruments of Microscopy
The 20th century saw the development of microscopes that leveraged nonvisible light, such as fluorescence microscopy, which uses an ultraviolet light source, and electron microscopy, which uses short-wavelength electron beams. These advances led to major improvements in magnification, resolution, and contrast. In this section, we will survey the broad range of modern microscopic technology and common applications for each type of microscope.
2.3: Staining Microscopic Specimens
In their natural state, most of the cells and microorganisms that we observe under the microscope lack color and contrast. This makes it difficult, if not impossible, to detect important cellular structures and their distinguishing characteristics without artificially treating specimens. Here, we will focus on the most clinically relevant techniques developed to identify specific microbes, cellular structures, DNA sequences, or indicators of infection in tissue samples, under the microscope.

Thumbnail: A compound microscope in a Biology lab. Image used with permission (CC -BY-SA 4.0; Acagastya).