Using the Metric System to Express the Sizes of Microbes

Microbes are very tiny. But how tiny are they? In most cases, some type of microscope is required to see them; thus, microbes are said to be microscopic. Various types of microscopes are discussed in this chapter. The metric system will be discussed first, however, because metric system units of length are used to express the sizes of microbes and the resolving power of optical instruments.

In microbiology, metric units (primarily micrometers and nanometers) are used to express the sizes of microbes. The basic unit of length in the metric system, the meter (m), is equivalent to approximately 39.4 inches (in) and is, therefore, about 3.4 in longer than a yard. A meter may be divided into 10 (101) equally spaced units called decimeters; or 100 (102) equally spaced units called centimeters; or 1,000 (103) equally spaced units called millimeters; or 1 million (106) equally spaced units called micrometers; or 1 billion (109) equally spaced units called nanometers. Interrelationships among these units are shown in Figure 1.
Representations of metric units of measure and numbers.
Fig.1: Representations of metric units of measure and numbers.

It should be noted that the old terms micron (μ) and millimicron (mμ) have been replaced by the terms micrometer (μm) and nanometer (nm), respectively. An angstrom (Å) is 0.1 nm. Using this scale, human red blood cells are about 7 μm in diameter.

About seven cocci could fit side by side across a human red blood cell. If the head of a pin was 1 mm (1,000 μm) in diameter, then 1,000 cocci could be placed side by side on the pinhead. A typical rod-shaped bacterium (bacillus; pl., bacilli) is about 1  μm wide x 3 μm long, although some bacilli are shorter, and some form very long filaments. The sizes of viruses are expressed in terms of nanometers. Most of the viruses that cause human disease range in size from about 10 to 300 nm, although some (e.g., Ebola virus, a cause of hemorrhagic fever) can be as long as 1,000 nm (1 μm). Some very large protozoa reach a length of 2,000 μm (2 mm).

In the microbiology laboratory, the sizes of cellular microbes are measured using an ocular micrometer, a tiny ruler within the eyepiece (ocular) of the compound light microscope (described later). Before it can be used to measure objects, however, the ocular micrometer must first be calibrated, using a measuring device called a stage micrometer. Calibration must be performed for each of the objective lenses to determine the distance between the marks on the ocular micrometer. The ocular micrometer can then be used to measure lengths and widths of microbes and other objects on the specimen slide. The sizes of some microbes are shown in Figure 2 and Table 2-1.
Fig.2: The relative sizes of Staphylococcus and Chlamydia bacteria and several viruses. Poliovirus is one of the smallest viruses that infect humans. (Redrawn from Winn WC Jr, Allen S, Janda W, et al. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.)
Fig.2: The relative sizes of Staphylococcus and Chlamydia bacteria and several viruses. Poliovirus is one of the smallest viruses that infect humans. (Redrawn from Winn WC Jr, Allen S, Janda W, et al. Koneman’s Color Atlas and Textbook of Diagnostic Microbiology. 6th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006.)

Relative Sizes of Microbes

The sizes of bacteria are expressed in micrometers, whereas the sizes of viruses are expressed in nanometers. An ocular micrometer is used to measure the dimensions of objects being viewed with a compound light microscope.

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