45 The World of Microbes

readi
n
H ave you had a cold, flu, or other infectious disease recently? Do you
know what caused your illness? Microbes cause most infectious
­diseases. Microbes include the protists, bacteria, and viruses that you clas-
sified in Activity 44, “Who’s Who?” They also include some fungi, such as
yeast and the fungi that cause athlete’s foot.

By now you know that germ is simply another word for a microbe that
causes disease. But you may have also heard the word microorganism used.
Why, then, do we keep referring to microbes? To find out, you need to know
a little more about the differences among the microbes you’ve studied so
far (protists, bacteria, and viruses).

CHALLENGE How do microbes fit into the classification of organisms?

Materials

For each student

1 Student Sheet 45.1, “Anticipation Guide: ­Diseases
and Prevention”

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 The World of Microbes  •  Activity 45

Reading
Complete the “Before” column on Student Sheet 45.1, “Anticipation Guide:
­Diseases and Prevention.”

Classifying Organisms
People have invented classification systems to organize and make sense
of the many types of living things on earth. As information changes, new
classification systems evolve. In Activity 44, “Who’s Who?” you grouped
the Micro-Life cards based on the physical appearance of the microbes.
Until recently, scientists classified organisms based on their appearance
into five groups called kingdoms. You are most familiar with the animal
kingdom and the plant kingdom. A third kingdom is made up of fungi. The
fungi include yeasts (like the one you used in Activity 39, “Cells Alive!”),
molds, and mushrooms. Protists and bacteria, like the ones you observed in
Activity 43, “Microbes Under View,” belong to two additional kingdoms.

New evidence led later scientists to classify organisms into six groups, divid-
ing the bacteria kingdom into two groups, called bacteria and archaea
(are-KAY-uh). Although archaea are made up of a single cell and look like
the bacteria you saw under the microscope, they are genetically distinct
from bacteria. They are believed to be the oldest form of life, and most of
them live in environments that bacteria could not survive in. Archaea can
live in extremely salty conditions, like the Great Salt Lake in Utah. They
can live in volcanoes and deep sea vents where the conditions are very
acidic and extremely hot.

Because archaea are so different from other living organisms, in 1990 many
scientists proposed the three-domain system. The three-domain system
divides all living things into three groups—archaea, bacteria, and eukaryote
(you-CARE-ee-ott) domains. The Eukaryote Domain is made up of all living
things that have cells with a nucleus. The Eukaryote Domain includes ani-
mals, plants, fungi and protists. Because archaea and bacteria do not have
a nucleus, they are considered to be prokaryotes (pro-CARE-ee-otts). Both
the five- and six-kingdom classification systems grouped organisms by their
physical characteristics, while the three-domain system groups organisms by
their genetic similarity. As scientists have continued to collect evidence, the
three-domain system has been widely accepted. The three-domain system
and its relationship to the older five-kingdom system are shown in the dia-
gram on the next page.

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Activity 45  •  The World of Microbes 

Classification Systems:
The Three-Domain Classification System

Eukaryotes Prokaryotes

Archaea
Bacteria
Eukarya (organisms with cells that have a nucleus)

The Five-Kingdom Classification System

Animals Plants Fungi Protists Bacteria (Monera)

Stopping to Think 1
a. Think about all of the slides you have observed. Have you observed
cells from every kingdom? List all of the cells you have observed
and the kingdom they are from.

b. Think about all of the slides you have observed. Have you observed
cells from every domain? List all of the cells you have observed and
the domain they are from.

c. Why do you think the three-domain system was not accepted by

scientists right away?

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 The World of Microbes  •  Activity 45

Protists
Protists are single-celled microbes that have a nucleus. While some protists
cause illness, many others are harmless. The Trypanosoma that you observed
in Activity 43 is closely related to another type of Trypanosoma that causes
sleeping sickness in people. Species of Paramecium are often harmless, living
in fresh and salt water, where they feed on bacteria, algae, and other protists.
Many types of Amoeba are harmless, while others cause illnesses of the
digestive system.

Bacteria
Bacteria are single-celled microbes that do not have a nucleus. Bacteria are
also the most common microbes and can be found everywhere—in snow,
deserts, lakes, the ocean, and the human body. As you may recall, bacteria
are extremely tiny; a thousand bacteria could fit in a cluster on the dot of
an “i.” There are more bacterial cells in your digestive system and on your
skin than the number of cells that make up your entire body!

While some bacteria, such as Mycobacterium tuberculosis, cause diseases,

other species of bacteria are helpful. In fact, without bacteria, nothing
would ever decompose; the world would be full of dead organisms, from the
tiniest microbes to large plants and animals! Bacteria also are important in
the preparation of foods and beverages. You may have noticed a statement
on some yogurt containers: “contains live and active yogurt cultures.” That’s
because yogurt is produced by the fermentation of milk by bacteria! The
table on the next page shows the shapes and some information about differ-
ent kinds of bacteria.

Stopping to Think 2
Would you describe bacteria as being helpful or harmful to people?
Explain.

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Activity 45  •  The World of Microbes 

Some Common Types of Bacteria

Shape Examples Ecological roles

Diplococci cause pneumonia

(pairs of cocci)

sphere Staphylococci are normally present on human skin;

(clusters of cocci) some cause boils and infections

Streptococci are used to make yogurt and cheese; cause

(chains of cocci) strep throat

Bacilli (rods) decompose hay; are used to make cheese,

yogurt, pickles, and sauerkraut; are
rod normally present in the human digestive
tract; cause diarrhea; cause anthrax in
cattle and sheep

Mycobacteria cause tuberculosis; are found normally in

(chains of bacilli) soil and water.

curved rod Vibrio cause cholera; help break down sewage

short spirals Spirilla are decomposers in both fresh and

salt water

long spirals Spirochete cause syphilis; are decomposers

branched c­­­hain Actinomyces produce several antibiotics; were once

classified as fungi

Cocci are spherical bacteria; the singular of cocci is coccus.

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 The World of Microbes  •  Activity 45

Viruses: A Group Apart

Viruses are not made of cells. They don’t perform most of the functions, such
as respiration, that cells do. Viruses cannot grow or reproduce by themselves.
Instead, they must invade living cells to multiply. For this ­reason, many biol­
ogists think of viruses as nonliving. However, viruses contain genes and they
evolve over time. Some scientists think this means that they are alive. Debate
about how to classify viruses is likely to continue for some time. It is because
of this debate that we say microbes, and not microorganisms, cause infec-
tious diseases.

virus

bacterium

protist

Comparing Average Sizes of Microbes

These are relative, not actual, sizes of microbes. An average bacterium is actually much
smaller than the virus shown here.

Stopping to Think 3
a. Why are viruses not considered to be microorganisms?

b. L
 ook at the figure above, “Comparing Average Sizes of Microbes.”
How do the sizes of protists, bacteria, and viruses compare?

c. Which do you think cannot be seen with a classroom microscope?

How do we know viruses exist? The existence of viruses was first suggested
in 1898, nearly 45 years before they were first seen. In 1895, Dutch scien-
tist Martinus Beijerinck (BY-er-ink) began experimenting with the tobacco
plant. He was studying a plant disease that he believed to be infectious. By
this time, scientists were familiar with protists and bacteria, so Beijerinck
began searching for a bacterium that might be causing this disease. But he
could not find one. Yet his experiments demonstrated that the disease could
be passed from plant to plant, so he concluded that the disease was caused
by a microbe. Since it wasn’t a protist or a bacterium, he called it a virus,
which means “poison” in Latin.

Viruses are so small that you need an electron microscope to see one. The
electron microscope was not invented until the 1930s. As a result, viruses
were first seen in 1939. Today, we know that viruses cause many diseases,
including the flu, colds, chickenpox, and AIDS.

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Activity 45  •  The World of Microbes 

The chart below shows the five-kingdom classification plus viruses. Note
the examples of diseases caused by members of each group. What do you
think the dotted lines mean?

Cause Disease

multicellular unicellular not a cell

Animals Plants Fungi Protists Bacteria Archaea Viruses

single single single
cell cell cell
have no have no
nucleus nucleus nucleus nucleus

Disease example: tapeworm athlete's foot malaria strep throat flu

Classifying Disease-Causing Organisms and Viruses

3030 LabAids SEPUP IALS SE

Figure: IALS SE2.45.04 Analysis
LegacySansMedium 10/11.5
1. Fill in the “After” column for Statements 1–4 only on Sheet 45.1,
“Anticipation Guide: Diseases and Prevention.” Did your thinking
change?

2. Copy the lists of words shown below:

List 1 List 2 List 3

archaea animal cell protists
cellular cell wall bacteria
viruses plant cell viruses
bacteria chloroplasts plants
fungi mitochondria can cause disease
protists nucleus animals

a. In each list, look for a relationship among the words. Cross out the
word or phrase that does not belong.
b. In each list, circle the word or phrase that includes the others.
c. Explain how the word or phrase you circled is related to the other
words on the list.

3. You have read how microbes can be both helpful and harmful to
humans. Do you think a microbe can be neither helpful nor harmful?
Explain.

growth
plate
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 The World of Microbes  •  Activity 45

4. You decide to examine some pond water under a microscope. With a

magnification of 40 (using the 4x objective), you observe a long, cylin-
drical organism moving across your field of view (see left). As you look
more closely, you notice what appears to be a round structure inside of
it. Is this organism most likely a protist, bacterium, or virus? Explain
how you arrived at your conclusion.

5. Suppose your school’s microscopes did not have 40x objectives, but only
10x objectives. Your friend, who is in high school, uses a 40x objective.
Explain what group of microbes he or she can study that you cannot.

6. What are the advantages of using the highest power objective on a

microscope? What are the advantages of using the lowest power objec-
tive on a microscope? Explain.

7. In your science notebook, draw a larger version of the Venn diagram

shown below. Record unique features of each group of microbes in
the appropriate space. Record common features among groups in the
spaces that overlap. Hint: Think about what you have learned about
cells in the last few activities.

Protists bacteria

Viruses

8. Reflection: On a field trip, you visit a laboratory that has an electron

microscope. The microscopist (the person who runs the microscope)
offers to set up a microbe for you to view. What microbe, or group of
microbes, would you choose to view? Why?

EXTENSION
For links to more information about microbes, go the Issues and Life Science
page of the SEPUP website.

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