Skip Navigation


Author(s): Nancy Moreno, PhD, Barbara Tharp, MS, Deanne Erdmann, MS, Sonia Rahmati Clayton, PhD, and James Denk, MA.

Comparing Sizes of Microorganisms

Microbes are organisms too small to be seen with the naked eye. Even so, there are enormous variations in size and type among microbes. This activity allows students to compare the sizes of various microorganisms, relative to an object with a standard size (0.5 mm) that is visible without magnification. Students will compare microbes listed on the “Microbe Scaling Chart,” which range from an amoeba—measuring 300 micrometers (equivalent to 0.3 millimeters) in diameter or larger—to the polio virus, which is only 0.03 micrometers in length. 

Students will use metric measurements for their calculations. 


1. Call students’ attention to the prepared strips of paper.

2.  Ask students to examine the periods at the end of the phrase, first with their eyes only, and then with a hand lens. Tell students to draw what the period looked like in each case. Discuss their observations. Ask, Did the period appear the same when it was magnified as when you observed it with a naked eye? (When magnified, the periods are square.)

3. Have students record their observations in science notebooks or on sheets of paper. 

4. Ask, What can you say about the size of the period? Tell students the period is about 0.5 millimeters (mm), or 500 micrometers (µm), in length and width. Have students identify the centimeter and millimeter markings on a centimeter ruler. Ask, How many periods could be lined up, end-to-end, within a meter? (2,000) 

5. Before continuing, you may wish to review the metric system. Explain that the meter is the fundamental unit of length in the metric system. At 39.37 inches, a meter is slightly longer than a yard (36 inches). A centimeter is approximately the width of an average fingernail (0.3937 inches). Ask students, How many centimeters make a meter? Hopefully, they will say “100” (the prefix, “centi,” is Latin for one hundred). Ask, How many millimeters make a meter? (1,000; the prefix, “milli,” signifies one thousand.) Thus, one centi­meter (cm) is equivalent to 10 millimeters (mm). 

6. Introduce students to an even smaller measure, the micrometer (µm), or micron, which is one millionth (or 10-6) of a meter. Mention that a micrometer is a measure too small for the naked eye to see, and that one centimeter contains 10,000 micrometers. Ask, What is the size, in micrometers, of the period you observed? (500 µm) Follow by asking, Why is the ruler not divided into micrometers? (markings would be too small)

7. Ask students, What do you know about scale models? For example, you might mention a road map or a model of the solar system. Ask, Why do we make scale models? (to understand the relative position, size or distance of objects)

8. Next, tell students that they are going to make a scale model of microbes, called a “Microbial Mural,” using the size of the period as the scale standard. Ask, If I increased the length and width of the period by 5,000 times, what shape would it have? (square) How large do you think it would be? (If the size of the period is 0.5 mm, multiply 0.5 mm x 5,000. Answer: 2,500 mm x 2,500 mm, which is equivalent to 2.5 m x 2.5 m.)

9. Bring out the prepared square of paper (period model) and display it on the wall. Explain that the sheet represents the size of the period enlarged 5,000 times. Ask, If we enlarged most microbes 5,000 times, do you think they would be larger or smaller than the period? (Even when enlarged 5,000 times, each of the microbe models will fit on the period.)

10. Distribute the student sheets and assign each group several microbes. Instruct students to make scale drawings or artwork of each of their assigned microbes, based on the line drawings and sizes provided on the chart. Depending on students’ ages and experience, you may want to give them only the information from one or both of the “approximate actual size” columns, and have each group calculate the scale sizes of their organism models.

Note: Make certain every group is assigned at least one microbe large enough to draw (organisms 1–7). The remaining microbes are so small that some will be represented only by a tiny dot on the mural. 

The microbe sizes described on the “Microbe Scaling Chart” represent typical measurements within the normal size range for each kind of organism. Students may find references to different sizes if they are conducting additional research about the organisms. In addition, some organisms named on the chart actually represent relatively large groups of related species or forms. For example, there are approximately 150 different known species of Euglena

11. Have students place their models on the large paper square. This is an effective way for students to self-check. 

12. Discuss the mural with students. Ask students if they could use the names of the any of the microbes on the mural to complete the sentence on their sentence strips. Revisit the concept maps and have students add information from this activity. 

Funded by the following grant(s)

Science Education Partnership Award, NIH

Grant Number: 5R25RR018605