CHAPTER PREVIEW 17-1 Viruses

Guide For Reading

Main Ideas • What is a virus?
In this chapter, you will learn • How do viral life cycles
differ?
and
about the structure of viruses • What is the relationship between
viruses and
how they replicate inside cells. their hosts?
You will also explore the world of Imagine for a moment that you have been presented with
bacteria, including their a great challenge. A disease has begun to destroy certain
dassification, growth and crops. The leaves of diseased plants are covered with large
reproduction, and importance to bleached spots that form a pattern that farmers call a mosaic.
other organisms. You will learn As the disease progresses, the leaves turn yellow, wither, and
about several diseases caused by fall off, killing the plants.
vinises and bacteria. To determine what is causing the disease,
you take some
leaves from a diseased plant and crush them until a juice is ex-
tracted. You then place a few drops of the juice on the surfaces
Reading Strategies of the leaves of healthy plants. A few days later, you discover
Sequencing Events As you read that, wherever you have placed the juice on the healthy leaves,
about viruses, list the steps of a mosaic pattern has appeared. You reason that the cause of
viral replication. Write a few the disease must be in the juice of the infected plant.
sentences to describe each stage. You then search for a microorganism that might be respon-
Outlining Information As you sible for the disease, but none can be found. In fact, even when
read about bacteria, write down the juice is passed through a filter with pores so fine that not
all the titles of the blue headings. even cells can pass through, the juice still causes the disease.
Then, under each heading, write When you look at a small amount of the filtered juice under the
down the key points. light microscope, you see no evidence of cells. The juice,
which is capable of transferring the disease from one
plant to another, must contain disease-causing particles so
Journal Activity
Biology and Your World Chances
are you have been sick at least
once in your life, thanks to the
elects of viruses or bacteria!
What was the most memorable
lime you were sick, and
why?
What were your symptoms?
mas your illness treated? How
Describe
your recollections in your journal.

Figure17-1Tobacco
MVcausesthe mosaicvirus
Mentstodevelopaleavesoftobacco
calledamosaic patternofspots
ide, (left).ATMV
magnified
1309times approximately
atunpurple(right),
elanced tubeinthe appearsas
mierograph. color-
355
 small that they are not visible under aus ing i microscope, u
though you eahert see the ls varuses, fing articles, you de
cide to give them the name viruses, from the Latin word
meaning poison.
Figure 17-2 A bacteriophage
Compare
is a With a few exceptions, most of these events actually took
bacteria.
virus that infects
the structures shown in the
place.
ic About 100
of tobacco mosaic ago in hauls ed t ukraine, an epiderm.
yearsdisease
diagram of the bacteriophage to
occurred that seriously threat.
those in an actual bacteriophage. ened the tobacco crop. The disease causing nature of the juice
from infected tobacco leaves was discovered by the Russian
Capsid biologist Dimitri wanowski. A few years later, the Dutch scientis
Martinus Beijerinck determined that tiny particles in the juice
caused the disease. He named these particles viruses.

- DNA
Head—
What Is a Virus?
You have just read how scientists hypothesized the exis-
tence of viruses, which they thought were cells even smaller
than one-celled bacteria. This idea persisted until 1935 when
the nature of a virus was discovered by the American biochem-
ist Wendell Stanley. He had set out to chemically isolate the
Tail
particle responsible for the tobacco mosaic disease. Stanley
identified the particle as the tobacco mosaic virus (TMV)

Tail fiber
ruses have distinct structures that are complex and fascinating
A virus is a noncellular particle made up of genetic material
and protein that can invade living cells.

STRUCTURE OF A VIRUS A typical virus is composed of

a core of nucleic acid surrounded by a protein coat called a
capsid. The capsid protects the nucleic acid core. Depending
on the virus, the nucleic acid core is either DNA or RNA but
never both. The core may contain several genes to several hun-
dred genes.
A more complex structure occurs in certain viruses known
as bacteriophages. You may recall from Chapter 7 that bacter-
iophages are viruses that invade bacteria. A bacteriophage has
a head region, composed of a capsid (protein coat), a nucleic
true ors to sual Becteriopheres are interestia) and relat
quickly.
One well-studied bacteriophage, known as T4, has a core of
DNA contained within a protein coat. A number of other pro-
teins (about 30 in all) form the other parts of the virus, includ-
ing the tail fibers. The tail fibers are the structures by which
the virus attaches itself to a bacterium.
Viruses come in a variety of shapes. Some, such as the lo
bacco mosaic virus, are rod-shaped. Others, such as the bac-
teriophages, are tadpole-shaped. Still others are many-sided
helical, or cubelike. Figure 17-3 shows some of these shapes.
356
 Viruses vary in size from approximately 10 to 400 nano-
meters. A nanometer Is one billionth of a meter. The tobacco Sizes Shapes
mosaic virus is about 300 nanometers long, whereas the virus
that causes polio is about 20 nanometers in diameter. Protein
coat
Nueleic
acid

SPECIFICITY OF A VIRUS Usually, specific viruses will

infect specific organisms. For example, a plant virus cannot in-
lect an animal. There are some viruses that will infect only Vaccinia (cowpox)
Variola (smallpox)
humans. Others, such as the virus that causes rabies, infect all 250 nm
mammals and some birds. Still others infect only coldblooded Hooked Coiled
animals (animals with body temperatures that change with the spike nucleic
surrounding air). There are even some viruses that will infect acid
species of animals that are closely related. For example, viruses Influenza, mumps
that infect mice may infect rats. So you can see that viruses are 100 nm

capable of infecting virtually every kind of organism, including 0-t

mammals, birds, insects, and plants. Bacteriophage
65 x 95 nm

Tobacco mosaic virus

Life Cycle of a Lytic Virus 300 x 15 nm

In order to reproduce, viruses must invade, or infect, a

Iving host cell. However, not all viruses invade living cells in Yellow fever virus
22 nm
exactly the same way. When T4 bacteriophages invade living
cells, they cause the cells to lyse, or burst. Thus T4 viruses are
known as lytic (LIHT-ihk) viruses. Poliomyelitis virus
20 nm

INFECTION A virus is activated by chance contact with Foot-and-mouth

the right kind of host cell. In the case of T4, molecules on its virus
tail fibers attach to the surface of a bacterium. The virus then 10 nm

injects its DNA into the cell. In most cases, the complete virus Escherichia coli
particle itself never enters the cell. 2000-2500 nm

GROWTH Soon after entering the host cell, the DNA of

the virus goes into action. In most cases, the host cell cannot Figure 17-3 Viruses come in a
variety of sizes and shapes. Notice
tell the difference between its own DNA and the DNA of the the size of the bacterium E. coli as
virus. Consequently, the very same enzyme RNA polymerase compared to the sizes of the
that makes messenger RNA from the cell's own DNA begins to viruses.
make messenger RNA from the genes of the virus. This viral
messenger RNA now acts like a molecular wrecking crew, shut-
It-

ting down and taking over the infected host cell. Some of these
viral genes turn off the synthesis of molecules that are impor-
lant to the infected cell. One viral gene actually produces an
does not
enge that destroys the host cell's own DNA but
harmtheviral
DNA!
REPLICATION As the virus takes over, it uses the mate-
is of the host cell to make thousands of copies of its own
protein coat and DNA. Soon the host cell becomes filled with
andreds of viral DNA molecules. When Escherichia coli, or E
try the bacterium found in the human intestine, is infected by
« heterophage, this sequence of infection, growth, and rep-
acon can happen in as briet a time as 25 minutes! 357
Bacteriophage- Bacteriophage
protein coal genetic material

Bacterium
Bacteriophage takes over
Bacteriophage injects genetic bacterium's metabolism,
Bacteriophage attaches material into bacterium causing synthesis of new
to bacterium's cell wall
bacteriophage proteins
and nucleic acids

Bacteriophage enzyme
Bacteriophage proteins and
nucleic acids assemble into
lyses bacterium's cell wall,
releasing bacteriophages
complete bacteriophages

Bacteriophage (infective virus)

§ Viral nucleic acid V/ Viral protein

Figure 17-4 In the life cycleaof a the DNA molecules

lytic virus, the virus invades During the final stage of reproduction,
new virus particles
bacterium, reproduces, and is serve as the starting points around which
cell lyses (bursts) and
scattered when the bacterium lyses, are assembled. Before long, the infected
infect other
or breaks.
releases hundreds of virus particles that may nowthis process
cells. Because the host cell is lysed and destroyed,
one way in which
is called a lytic infection. Lytic infections are
viruses can infect host cells.
T4 consists of re-
The life cycle of a lytic virus such as
peated acts of infection, growth, and cell lysis. We may imagine
West.
the virus as a desperado moving into a town in the Old authority
First, the desperado eliminates the town's existing
(host cell DNA). Then the desperado demands to be outfitted
Figure 17-5 This electron with new weapons, horses, and riding equipment by terrorizing
micrograph shows bacteriophages the local merchants and businesspeople (using the machinery
attacking the bacterium E. coli. of the host cell to make proteins). Finally, the desperado re-
How do viruses attach themselves that leaves the town and
cruits more outlaws and forms a gang
to the bacterium?
attacks new communities (the host cell bursts, releasing
hundreds of virus particles).

Lysogenic Infection
Another way in which a virus infects a cell is known as a
lysogenic (ligh-soh-JEHN-ihk) infection. In a lysogenic infec-
tion, the virus does not reproduce and lyse its host cell-al
least not right away! Instead, the DNA of the virus enters the
cell and is inserted into the DNA DNA hostell. Once inserted
into the host cell's DNA, the viral DNA is known as a prophage
358
Microbiology
Viruses
- Viruses are an anomaly of Biology, as scientists are unsure if they should be considered
“alive” or not
- Viruses have the same property of living things but have features that no living thing has
Properties indicating life:
- reproduce
- contain RNA or DNA
- affect/impact living cells
Properties NOT indicating life:
- Non-cellular
- Do not respire
- Do not grow
- Do not eat
- Do not produce waste
Structure of Virus:
- Consist of strands of DNA or RNA coated in a protein sheath (capsid)
- Viruses are too small to be seen with the naked or even a compound light microscope
- They can vary in shape: rod shaped, adenovirus, bacteriophage
Viral Diseases
- Viral diseases can be prevented by the use of vaccines
- Vaccine is a material containing a protein from the pathogen
- By injecting this into a person it causes the person to recognize it and produce
antibodies for
3 main types of Viruses
Reproduction
Bacteria reproduce asexually by binary fission, which is essentially mitosis
- It is the splitting of a parent cell into two equal sized daughter cells

Bacteria do not reproduce sexually by the creation of gametes but they can combine their
genetic material in three ways:
1. Conjunction:
- occurs when a bacterium passes DNA to another bacterium
- most often a plasmid is transferred through a tube called sex pious that
temporarily joins two cells
- this occurs only between closely related species or the same species
- both bacteria’s will separate, and then reproduce asexually by binary fission
2. Transformation
- involves bacteria taking up free pieces of DNA
- results in new genetic traits in bacteria
- natural transformations occurs when DNA is acquired from the new environment
- Artificial transformation involves specific DNA being introduced into a bacterium
 The prophage may remain part of the DNA of the host cell for
many generations. An example of a lysogenic virus is the bac-
infects E. coli. Lysogenic Bacteriophage
teriophage lambda, which bacteriophage genetic material
Bacterial
PROPHAGE ACTIVITY The presence of the prophage can genetic material
block the entry of other viruses into the cell and may even add
useful DNA to the host cell's DNA. For example, a lambda virus
can insert the DNA necessary for the synthesis of important
amino acids into the DNA of E. coli. As long as the lambda virus Bacterium
remains in the prophage state, E. coli can use the viral genes to Lysogenic bacteriophage injects
make these amino acids.
its genetic material into
bacterium's DNA
A virus may not stay in the prophage form indefinitely.
Eventually, the DNA of the prophage will become active, re-
move itself from the DNA of the host cell, and direct the syn-
thesis of new virus particles. A series of genes in the prophage
itself maintains the lysogenic state. Factors such as sudden
changes in temperature and availability of nutrients can turn Bacteriophage genetic material
on these genes and activate the virus. incorporated into
bacterium's DNA
RETROVIRUSES One important class of viruses are the Bacteriophage
genetic material
retroviruses. Retroviruses contain RNA as their genetic infor- may replicate with
mation. When retroviruses infect a cell, they produce a DNA bacterium for many
generations
copy of their RNA genes. This DNA, much like a prophage, is
inserted into the DNA of the host cell. Retroviruses received
their name from the fact that their genetic information is cop-
ied backward-that is, from RNA to DNA instead of from DNA
are re-
to RNA. The prefix retro- means backward. Retroviruses
sponsible for some types of cancer in animals and humans. Conditions cause
bacteriophage to
One type of retrovirus produces a disease called AIDS. enter lytic cycle

Bacteriophage protein
Viruses and Living Cells
cells in
As you have just learned, viruses must infect living
order to carry out their functions of growth and reproduction.
They also depend upon their hosts for respiration, nutrition,
and all of the other functions that occur in living things. Thus
Many copies of bacteriophage
protein and genetic material
viruses are parasites. A parasite is an organism that depends produced
entirely upon another living organism for its existence in such
i way that it harms that organism.
Are viruses alive? If we require that living things be made
up ol cells and be able to live independently, then viruses are
i alive. However, when they are able to infect living cells, vi-
Ales can grow, reproduce, regulate gene expression, and even Mature bacteriophage particles
erive Viruses have so many of the characteristics of living assemble; released when
angs that it seems only fair to consider them as part of the
*emof
bacteriophage enzyme lyses
lifeonEarth. bacterium's cell wall
Because it is possible to study the genes that viruses bring
cels when they infect them, viruses have been extremely Figure 17-6 In a lysogenic
evale in genetic research. And, as we saw in Chapter 12, infection, the DNA of the
host cell
bacteriophage entersitsthe
she viruses are now being used in gene therapy. It is possible and is inserted into
DNA.
a modified viruses may one day be routine medical tools. 359
 Origin of Viruses
Although viruses are smaller and simpler than the smalles
cells, they could not have beed much in the first living things
Viruses are completely dependent upen living cells for growth
and reproduction, and they cannot live outside their host cello
Thus it seems more likely that viruses developed after living
cells. In fact, the first viruses may have evolved from the ge
netic material of living cells and have continued to evolve
of years.
along with the cells they infect, over billions

SECTION
17-1 REVIEW
1. What is a virus?
2. List and describe the parts of a bacteriophage.
of viral infection.
3. Describe two methods
can a virus
4. Critical Thinking-Applying Concepts How
be helpful to its host?

17-2 Bacteria-Prokaryotic
Guide For Reading
• How are prokaryotes classified? Cells
How do bacteria obtain energy?
How do bacteria grow and Imagine living all your life as the only family on your street.
and
Then, on a morning like any other, you open the front door
reproduce?
• How do bacteria affect other living there are houses all around you, cars and bicycles on the
to
street, neighbors tending their gardens, children walking
things?
school. Where did they come from? What if the answer turned
out to be that they were always there-you just couldn't see
them? In fact, they lived on your street for years and years be-
fore your house was even built. How would your view of the
Figure 17-7 With a nutrient-rich world change? What would it be like to go, almost overnight,
culture medium on which to grow,
from being the only family on the block to just one family in a
these bacteria have produced crowded community? A bit of a shock?
thousands of colonies. Because of Robert Hooke and Anton van Leeuwenhoek, the
human species had just such a shock. The invention of the light
microscope opened our eyes to what the world around us is
really like. And it opened our eyes almost overnight. Suddenly
we saw that the block is very crowded!
Microscopic life covers nearly every square centimeter of
planet Earth. What form does that microscopic life take? As you
learned in Chapter 5, there are cells of every size and shape
imaginable, even in a drop of pond water. The smallest and
most common
are cells of these cells are the prokaryotes. Prokaryotes
that do not have a
nucleus.
Where do we find prokaryotes? Everywhere! Prokaryotes
exist in almost every place on Earth. They grow in numbers so
360
great that they form colonies you can see with the unaided eye.
 Classification of Prokaryotes
All prokaryotes are placed in one of two kingdoms: the
Eubacteria or the Archaebacteria. These kingdoms are the
frst large groups of organisms we shall consider as we examine
each of the six kingdoms of living things. The bacteria, or one-
celled prokaryotes, in these two kingdoms include a wide range
ol organisms that live in every imaginable habitat on Earth.
Bacteria range in size from 1 to 10 micrometers (one mi-
crometer is equal to one thousandth of a millimeter). Bacteria
are much smaller than eukaryotic cells, or cells with a nucleus,
which generally range from 10 to 100 micrometers in diameter.
The reason for the difference in size is that bacteria do not con-
tain the complex range of membrane-enclosed organelles that
are found in most eukaryotic cells.

EUBACTERIA The Eubacteria (yoo-bak-TEER-ee-uh) make

up the larger of the two prokaryote kingdoms. Eubacteria are
generally surrounded by a cell wall composed of complex car-
bohydrates that protects the cell from injury. Within the cell
wall is a cell membrane that surrounds the cytoplasm. Some
eubacteria are surrounded by two cell membranes, making
them especially resistant to damage. In some organisms, long
whiplike flagella protrude from the membrane through the cell
wall. Flagella are used for movement.
Within the kingdom Eubacteria there is a wide range of or-
ganisms that have many different lifestyles. The variety is so Figure 17-8 A bacterium such as
great, in fact, that biologists have not been able to agree on ex- E. coli (right) has the basic
actly how many phyla to divide the kingdom into. Some eubac- structure typical of most bacteria:
teria live in the soil. Others infect larger organisms and pro- cell wall, cell membrane, region of
duce disease. Some eubacteria are simple, and contain few genetic material, and cytoplasm.
Note the flagella projecting from the
internal structures. Other eubacteria contain elaborate systems cell surface.
d/ internal membranes and compartments.

Cell wall

Cellmembrane

Ribosomes.
Cytoplasm

Genetic
material

361
 Some of the most important eula kera are the cyanobates
ria (sigh-uh-noh-bak-TEER-ee-uh), asynthetic ms blue green bit
reala. These organisms are photos there, meaning that hug
use the energy of sunlight to make the i owen food. At one time
cyanobacteria were called blue green alta. but today we we
the word algae only for eukaryotes on fact, only a lew blue.
green bacteria are blue-green in color. Those cyanobacteria
that are blue-green in color contain a pigment called phyca.
cyanin. They also contain chlorophyll a, which you will recal
from Chapter 6 is green. The presence of these two piments
gives the name blue-green to the entire group of cyanobacte
ria. The presence of other pigments, however, may change the
even red.
color of these bacteria to yellow, brown, or
Cyanobacteria contain membranes that carry out the light
reactions of photosynthesis. These membranes contain the
photosynthetic pigments and are quite different from and sim-
Figure 17-9 Some archaebacteria pler than the chloroplasts (organelles that trap light energy and
can survive in many environments in plant cells.
convert it to chemical energy)
that support no other forms of life, Cyanobacteria are found throughout the world-in fresh
such as in a near-boiling hot spring ex-
called Morning Glory Pool in and salt water and on land. A few species can survive in
Others can sur-
Yellowstone National Park, tremely hot water, such as that in hot springs.
Wyoming. vive in the Arctic, where they can even grow on snow. In fact,
the
cyanobacteria are often the very first species to recolonize
site of a natural disaster, such as a volcanic eruption.
are a
The prochlorobacteria (proh-klor-oh-bak-TEER-ee-uh)
newly discovered group of organisms that contain chlorophyll
a and b. The presence of these pigments makes prochlorobac-
to
teria more similar to chloroplasts of green plants than
cyanobacteria. For this reason, prochlorobacteria are some-
empha-
times called Prochlorophyta (-phyta means plants) to
size this similarity. To date, only two species of prochlorobac-
teria have been discovered.

ARCHAEBACTERIA Recent studies of prokaryotes have

confirmed that one group of organisms is so different from
those organisms in the Eubacteria kingdom that it should be
considered as a separate kingdom. The organisms in this king-
dom, called the Archaebacteria (ahr-kee-bak-TEER-ee-uh), lack
an important carbohydrate found in the cell walls of nearly all
eubacteria. They also have different types of lipids in their cell
membranes, different types of ribosomes, and some very differ-
ent gene sequences. The archaebacteria include organisms that
live in extremely harsh environments. For example, one group
of archaebacteria lives in oxygen-free environments such as
thick mud and the digestive tracts of animals. These archaebac-
teria are called methanogens because they produce methane
gas. Other archaebacteria live in extremely salty environments,
such as the Great Salt Lake in Utah, or in extremely hot environ-
mints, such as hot springs where temperatures approach the
boilingpoint of water.

362
 Identifying Prokaryotes
Identifying living organisms can be a simple task. If we
were given an unknown plant or animal, we would search

a method works for organisms that we can identify by appear-

ance. But what about bacteria? How can they be identified?
CELL SHAPE One way in which bacteria can be identified
is by their shape. Bacteria have three basic shapes: rod,
sphere, and spiral. Rod-shaped bacteria are called bacilli

Biology Update Heat-stable Enzymes

Prospecting the New Wild West

Established in 1872, Yellowstone National
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treasure may be Yellowstone's unique forms of
life.

Organisms living in geysers such as Old Faithful

"Digging" for Enzymes
in Yellowstone Park may be the source of
More than 60 percent of all the hot springs
heat-stable enzymes that have a variety of
and geysers on Earth are found in Yellowstone
applications in biochemistry and medicine.
Park's 10,000 geothermal sites. These regions
contain bacteria and other microorganisms
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traction of these sites have been explored advance industry but also help preserve the
biologically, a mining corporation signed a "bio- features that made the park so special in the
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Figure 17-10 Bacteria have three
basic shapes. Rod-shaped bacteria (buh-SIHL-igh; singular: bacillus). Spherical bacteria are called
are called bacilli (left), spherical
bacteria are called cocci (center), cocci (KAHK-sigh; singular: coccus). And spiral-shaped bacteria
and spiral-shaped bacteria are are called spirilla (spigh-RIHL-uh; singular: spirillum). See Fig.
called spirilla (right). ure 17-10.
Individual bacterial cells can also arrange themselves in a
number of different ways. For example, cocci sometimes grow
in colonies containing two cells. Many cocci, including the
disease-causing bacteria Streptococcus and Pneumococcus, may
Figure 17-11 Some spherical form long chains. A few others, such as Staphylococcus, form
bacteria like these streptococci
large clumps or clusters. These differences are very helpful in
form long chains.
distinguishing one kind of bacteria from another.
Unfortunately, many bacteria look the same under the mi-
croscope. So we need to find another characteristic by which to
distinguish one type from another. Fortunately, there are three
other characteristics of bacteria that improve our ability to
identify them: their cell walls, the kind of movement they are
capable of, and how they obtain energy.

CELL WALL The chemical nature of bacterial cell walls

can be studied by means of a method called Gram staining-
which is named after its inventor, the Danish physician Hans
Christian Gram. Gram's stain consists of two dyes-crystal vio-
let (purple) and safranine (red).
When Gram added his stain to bacteria, he noticed
that the
bacteria took up either the purple dye or the red dye. The bac-
terial cells with only one thick layer of carbohydrate and pro-
tein molecules outside the cell membrane took up the crystal
violet. They appeared purple under the light microscope
These bacteria are called Gram-positive bacteria. The bacterial
cells with a second, outer layer of lipid and carbohydrate mole-
cules took up the safranine. They appeared red under the mi-
364
croscope. These bacteria are called Gram-negative bacteria
 BACTERIAL MOVEMEN Some can also identity bacteria by
studying how they move. Some bacteria are propelled by
means of one or more flagella. Others lash, snake, or spiral for-
ward. Still others glide slowly along a layer of slimelike mate-
themselves. And there are some bacteria
nial that they secreteall.
that do not move at

How Bacteria Obtain Energy

Although the structure of bacteria is rather simple, their
Ilestyles are remarkably complex. No characteristic of bacteria
illustrates this point better than the ways in which they obtain
energy-

AUTOTROPHS Bacteria that trap the energy of sunlight

in a manner similar to green plants are called phototrophic
autotrophs. Examples of phototrophic autotrophs include
some photosynthetic eubacteria.
Bacteria that live in harsh environments and obtain
energy from inorganic molecules are called chemotrophic
autotrophs. The inorganic molecules that are used by chemo-
trophic autotrophs include hydrogen sulfide, nitrites, sulfur,
and iron. Nitrosomonas is an example of a chemotrophic auto-
Figure 17-12 Many types of
troph that uses ammonia and oxygen to produce energy. bacteria, such as the bacterium that
causes Legionnaires' disease, move
HETEROTROPHS Many bacteria obtain energy by taking by means of a whiplike flagellum.
in organic molecules and then breaking them down and
absorbing them. These bacteria are called chemotrophic
heterotrophs. Most bacteria, as well as most animals, are che-
motrophic heterotrophs.
Because we are chemotrophic heterotrophs ourselves,
many bacteria compete with us for food sources. For example,
Salmonella is a bacteria that grows in foods such as raw meat,
poultry, and eggs. If these foods are not properly cooked, Sal-
there,
monella will get to your dinner table before you do! Once
these bacteria will not only "eat" some of the food ahead of
time, but they will release poisons into the food. Food poison-
range from an
ing can result. The symptoms of food poisoning
upset stomach to serious illness.
There is another group of heterotrophic bacteria that has a
most unusual means of obtaining energy. These bacteria are
photosynthetic-they are able to use sunlight for energy. But
they also need organic compounds for nutrition. These bacte-
ria are called phototrophic heterotrophs. There is nothing
quite like these organisms in the rest of the living world.

Bacterial Respiration
Like all organisms, bacteria need a constant supply of en-
ery to perform all their life activities. This energy is supplied
by the processes of respiration and fermentation. Respiration 365
 is the process that involves oxygen and breaks down food mol-
ecules to release energy. Fermentation, on the other hand, en-
ables cells to carry out energy production without oxygen.
Organisms that require a constant supply of oxygen in
order to live are called obligate aerobes. We, and many species
of bacteria, are obligate aerobes. Some bacteria, however, do
not require oxygen, and in fact may be poisoned by it! These
bacteria are called obligate anaerobes. Obligate anaerobes
must live in the absence of oxygen.
An example of an obligate anaerobe is the bacterium Clos-
tridium botulinum, which is often found in soil. Because Clostri-
dium is unable to grow in the presence of oxygen, it normally
causes very few problems. However, if these bacteria find their
way into a place that is free of air (air contains oxygen) and
filled with food material, they will grow very quickly. As they
grow, the bacteria produce toxins, or poisons, that cause botu-
lism. Botulism is a rare form of food poisoning that interferes
with nerve activity and can cause paralysis and, if the breath-
ing muscles are paralyzed, death. A perfect place for these bac-
teria to grow is in the space inside a can of food. Most
commercially prepared canned foods are safe because the bac-
teria and their toxins have been destroyed by heating the foods
for a long time before the cans are sealed. However, botulism is
always a danger when food is canned at home. Thus experi- Figure 17-13 Botulism, a kind of
enced canners thoroughly heat food before sealing it in jars. food poisoning, is caused by the
A third group of bacteria are those that can survive with or bacterium Clostridium botulinum.
without oxygen. They are known as facultative anaerobes. Fa- The small round structures on some
cultative anaerobes do not require oxygen, but neither are they of the bacteria are endospores.

poisoned by its presence. What does such diversity imply? It

means that bacteria can live in virtually every place on the sur-
face of planet Earth. And indeed they do! Bacteria are found in
freshwater lakes and ponds, at the bottom of the ocean, at the
tops of the highest mountains, in the most sterile hospital
rooms, and even in our own digestive systems!

Bacterial Growth and Reproduction

When conditions are favorable, bacteria can grow and re-
produce at astonishing rates. Some types of bacteria can repro-
duce as often as every 20 minutes! If unlimited space and food
were available to a single bacterium and if all of its offspring di-
vided every 20 minutes, then in just 48 hours (2 days) they
would reach a mass approximately 4000 times the mass of the
Garh! Fortunately for us, this does not happen. In nature, the
Broth of bacteria is held in check by the availability of food
and the production of waste products. However, bacteria do re-
produce, and they do so in a number of ways.
BINARY FISSION When a bacterium has grown so that it
alarly doubled in size, it replicates its DNA and divides
i hall, producing to identical daughter cells. This type of 367
 fis-
binary
ofgenetic
BecauseThe bacter-
binary recombi
fission. n ati
reproduction.
or
on
17-14.
isknown asexchange
the form See of Figure only
not involve
reproductioitn isan binary
asexual fission. reproduceform of
bacteria siondoes undergoes bacteriainsome theex-
17-14
Mostfission,
binary cells
by identicalbacteria
information,
coli Although
part
manytake involves
others sexualreproduction
fission,reproduction
Figure
reproducetwo some orthe ium E. See Fig-
producing
(right). by conjugation,
However,
oftheir
genetic
to CONJUGATION
asexual
binarySexualOneform as
conjugation.between
of
reproduce throughreproduction.
information.
cell
ofparts bridge isknown
one forms
protein information
transfer from protein
information
througha sexual genetic
of insome bacteria of genetic other cell,
another change
occurs bridge the tothe of
(left). that along Partof process
ure17-14. conjugation, cells.istransferred
When the setof
bacterial
During two thedonor, bridge. hasadifferent
connects this Thenew
and
fromone
cell,calledthroughrecipient cell occurred.increase
recipient,
calledthe iscomplete,
the
before
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Genetic di-
conjugation it had
thosegenes result ofbacteria. changes,
thatpopulation
a
genesfrom
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ifthe
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to

thegeneticdiversity
helpstoensure
thateven
right combinations
versity mayhavethe
lew bacteria become
survive.
conditions
Whenstructurescalledspores.One
growth
SPOREFORMATION
many bacteria form
formed whenabacter-
unfavorable, an endospore, is
encloses itsDNAanda
called
type ofspore,a thick internal wallthat
ium produces
portion of its cytoplasm.
remain dormant for months or even
The endospore can favorable growth conditions. When
years, waiting for more
improve, the endospore will open and the bacter-
conditions again. Strictly speaking, spore formation
inm will begin to grow because it does not
in bacteria is not a form of reproduction
368
sult in the formation of new bacterial cells. However, thetoabil-
possible for some bacteria sur-
y to form spores makes itwould
ve harsh conditions that otherwise kill them.

mportance of Bacteria
Many of the remarkable properties of bacteria provide us
ith products upon which we depend every day. For example,
acteria are used in the production of a wide variety of foods
nd beverages, such as cheese, yogurt, buttermilk, and sour
ream. Some bacteria are used to make pickles and sauerkraut,
nd some make vinegar from wine.
Bacteria are also used in industry. One type of bacteria can
igest petroleum, which makes them helpful in cleaning up
mall oil spills. Some bacteria remove waste products and poi-
ons from water. Others can even help to mine minerals from
he ground. Still others have been useful in synthesizing drugs
nd chemicals through techniques of genetic engineering.
Many kinds of bacteria develop a close relationship with
ther organisms in which the bacteria or the other organism or
oth benefit. Such a relationship is called symbiosis (sihm-
sigh-OH-sihs). The symbiotic relationships that bacteria de-
Figure 17-15 The round circle at
velop with other organisms are particularly important. the bottom of this electron
Bacteria form symbiotic relationships with organisms from
micrograph of a bacterium is an
all of the eukaryote kingdoms. endospore. Endospores enable
Our intestines are inhabited by large numbers of bacteria, bacteria to survive unfavorable
including E. coli. Indeed, the species name coli was derived conditions, such as high
trom the fact that these bacteria were discovered in the human temperature.

colon, or large intestine. In the intestines, the bacteria are pro-

vided with a warm safe home, plenty of food, and free transpor-
tation. We, in turn, get help in digesting our food. These
bacteria also make a number of vitamins that we cannot pro-
duce on our own. So both we and the bacteria benefit from this
symbiotic relationship.

Figure 17-16 The large round

structures in this electron
micrograph are cells that form the
intestinal wall of the human large
intestine, or colon. The smaller
rod-shaped cells are the bacteria E.
coli, which inhabit the large
intestine.
369
 Animals such as cattle are also dependent upon the symb.
otic relationship they with the bacteria in their intestines
dou see, no vertebratehave
(animal with a backbone) can produce
the enzymes necessary to break down cellulose, the principal
carbohydrate in grass and hay, Bacteria iving in the digestive
systems of such animals can make these enzymes, thus allow.
ing the animals to digest their food properly.

Bacteria in the Environment

Sometimes we are bold enough to consider ourselves the
principal actors on the stage of life. We tend to place other or.
ganisms in supporting roles, like the minor actors in a play. But
no drama can begin without the dozens of workers who are
never seen on stage. The bacteria are like these unseen stage-
hands. We seldom think about them, but they are absolutely
vital to maintaining the kind of living world we see about us.

NUTRIENT FLOW Every living thing depends on a supply

of raw materials for growth. If these materials were lost forever
when an organism died, then life could not continue. Before
long, plants would drain the soil of the minerals they need
plant growth would stop, and the animals that depend on
plants for food would starve.
Bacteria recycle and decompose, or break down, dead
material. When a tree dies and falls to the forest floor, it begins
to undergo many changes. Over the course of a few summers,
the bark peels off and the wood begins to weaken because it
Figure 17-17 Most bacteria are becomes infested with insects. Then the tree crumbles into the
heterotrophs, or organisms that soil. Over time, the whole substance of the tree disappears
obtain food from the organic What happens to all of the material that made up the tree?
compounds of other organisms From the moment the tree dies, armies of bacteria attack
Many of these heterotrophs live as
and digest the dead wood, breaking it down into simpler sub-
saprophytes, decomposing dead
organisms such as this tree. stances. These bacteria are called saprophytes (SAP-ruh-
fights). Saprophytes are organisms that use the complex
molecules of a once-living organism as their source of energy
and nutrition. Gradually, the material of the tree is recycled,
enriching the soil in which it grew. Although bacteria play a
major role in this process, some eukaryotic organisms, such as
insects and fungi, have a supporting role.

SEWAGE DECOMPOSITION Humans take advantage of

the ability of bacteria to decompose material in the treatment
ol sewage. One of the critical steps in sewage treatment is car-
ried out by a diverse mixture of bacteria that is added directly
to the waste water. Waste water contains human waste, dis-
carded food, organic garbage, and even chemical waste. Bacte-
ria grow rapidly in this mixture. As they grow, they break down
the complex compounds in the sewage into simpler com-
pounds. This process produces purifed water, nitrogen gas
as craphen dioxide gas, and leftover products that can be used
as crop fertilizers.
370
 Biology Update Bacterial Biomass

Who Really Rules the Earth? Thermophilus aquaticus

is one of many
What kind of organism makes up most of species
of bacteria that live in
the living material—the biomass—of the Earth? unusual environments
For many years, most biologists thought that
beneath the Earth's
the answer was obvious: Plants do. In fact, surface.
given the size and density of forests around the
globe, many biologists would have been more
specific: They would have said that most of the
Earth's biomass is in the wood of trees.
This easy conclusion, however, may be
wrong. After discovering microbes and other
surface at the same time, the results would be
organisms thriving at the extreme conditions stunning
found in deep-ocean vents, biologists began to Gold calculates that the total biomass of
wonder if bacteria might be able to thrive in these bacteria is approximately 2 X 10'' tons.
other unexpected places. That's enough to form a layer of bacteria more
than 1.5 meters thick over the entire land mass
Looking Beneath the Earth's Surface of the planet!
In the early 1990s, several groups of scien- The real rulers of this planet may not be the
tists found bacteria thriving deep within the plants and animals at all, but the bacteria and
Earth's interior, at the bottoms of deep wells and other microorganisms that thrive beneath our
oil reservoirs, and buried beneath thousands of feet!
meters of sediment. It now seems clear that an
enormous number of bacteria live deep within
Get an update on bacteria at our
the Earth—so many, in fact, that Thomas Gold of Internet site:
Cornell University has proposed that if all these
deep-dwelling bacteria were brought to the

NITROGEN FIXATION All organisms on our planet are to-

lally dependent on bacteria for nitrogen. All green plants need
nitrogen to make amino acids, which are the building blocks of
proteins. And because animals eat plants, plant proteins are
ultimately the source of proteins for animals.
Although our atmosphere is made up of approximately 80
percent nitrogen gas (N,), plants are not able to use the nitro-
Ben gas. Neither can most other organisms. Living organisms
generally require that nitrogen be "fixed" chemically in the
form ol ammonia (NH,) and related nitrogen compounds.
Chemists can make synthetic nitrogen-containing fertilizers by
coxing nitrogen gas and hydrogen gas, heating the mixture to
SeC, and then squeezing it to 300 times the normal atmos-
plate pressure. The process is expensive, time-consuming,
andsometimes
dangerous. 371
 In contrast to chis aif nilopen from me
air ayanoates
to a form that plants an eria are the ensly graces is on
as nitrogen nationen
of performing
xaion re the only organisms coul
nitrogen fixation.

AxingMany plantsThehase
bacteria. which fatson ships with nitoes
bean, whilizobits
soybean,
fum, is among the best known. hie sunthe bacterium phons
grows in nodules co
knobs, that form on the roots of t so soybean plant. The so,
bean plant provides a home and a storice of nutrients for the
nitrogen-fixing Rhizobium, the bacterium fixes nitrogen di
rectly from the air into ammonia for the plant. All plants benes.
from the nitrogen-fixing ability of monerans, but soybeans ate
a step ahead. With a little help from their "friends,"soybeans
have their own fertilizer factories built right into their rots
As you have learned, eukaryotes are dependent upon bat-
teria to fix nitrogen and release it into the environment. And i
is because of the nitrogen-fixing ability of these organisms that
more than 170 million metric tons of nitrogen are released into
the environment every year.

SECTION
17-2 REVIEW

of prokaryotes.
1. Describe the major groups
of bacteria.
2. Compare the three basic shapes
Figure 17-18 The knoblike and heterotrophic
roots of 3. Distinguish between autotrophic anaerobes,
structures growing on the obligate
bacteria. Between obligate aerobes,
this soybean plant are called and facultative anaerobes.
nodules (top). Within these nodules
are the nitrogen-fixing bacteria 4. Describe binary fission and conjugation.
Rhizobium, which have a
characteristic rod-shaped
5. List some ways in which bacteria are important.
6. Critical Thinking-Making Predictions
Suppose
appearance (bottom). this
bacteria lost the ability to fix nitrogen. How would
affect other organisms?

Guide For Reading

17-3 Diseases Caused by
What are some diseases caused by
viruses and bacteria? How are these Viruses and Bacteria
diseases prevented and cured?
• How is the growth of bacteria
controlled?
a Contrary are capare it produce disease in unt,
Despite their small numbers,these pathogens, or disease-
producing agents, are responsible for much human suffering.
e rom the point of view of a microorganism,Alhowever, Che
than a conflict
more disease
hing, and of lifestyles. viruses li.
fearsing
ees living cells, results where she infection cause
disease the host. Al bacteria require heriches and energ, one
disease results
obtaining them.only when bacteria interfere with the host lo
372
 the cause
Figure 17-19 Viruses are
iruses and Disease
bullet-
of many human diseases. The
shaped rabies virus particles are
diseases as small-
Viruses are the cause of such human about 180 nm in length (left). The
yellow fever, spherical influenza virus is about
ox, polio, measles, AIDS, mumps, influenza,
infections, viruses 100 nm in diameter (right).
abies, and the common cold. In most viral
the T4 bacterio-
ttack cells of the body in the same way that
it destroys the
hage attacks E. coli. As the virus reproduces,
of the disease.
ells that it infects, causing the symptoms are cur-
viral diseases
Although we tend to think that some
against most of them lies in
ble, the only successful protection this, the body's own
reventing their infection. In order to do
to prevent the infection. A
mmune system must be stimulated
the weakened or killed dis-
accine is a substance that contains vaccine
injected into the body, the contain
ase-causing virus. Whenthe Diseases such as small- Figure 17-20 These flasks
disease. interferons, which were produced by
rovides an immunity to vaccines
been eliminated because genetically engineered bacteria.
ox and polio have all but
powerful as they are, Interferons, proteins made by virus-
vere developed to stop their spread. As
protection if they are used infected cells, inhibit the growth of
nowever, vaccines can only provide infection starts, there is
before an infection begins. Once a viral
viruses.
progress of
often little that medical science can do to stop the
of the infec-
he disease. However, sometimes the symptoms
lion can be treated.

the treatment
INTERFERONS One possible approach ininterferons.
ol viral diseases is the use of substances called In-
telerons are small proteins that are produced by the
body's
cells when the cells are infected by viruses. When interferons
are released from virus-infected cells, they seem to make it
more difficult for the viruses to infect other cells. The word in-
tereron is derived from the fact that these proteins interfere
with the growth of the virus. The specific way in which these
woteins work is not yet entirely understood. Until recently, in-
efferons cost millions of dollars a milliliter to isolate and pu-
Ty. But new techniques using genetically engineered bacteria
have made the production of interferons less expensive and
moreplentiful.
373