Bacteria and Viruses
I. Bacteria
A. Classifying Prokaryotes
1. Microscopic life covers nearly every square centimeter of Earth.
2. The smallest and most common microorganisms are
prokaryotes—unicellular organisms that lack a nucleus.
3. Biologists have divided prokaryotes into two very different
groups: the eubacteria and the archaebacteria.
a)
Eubacteria
(1)
The larger of the two kingdoms of prokaryotes
(2)
Includes a wide range of organisms with different lifestyles
They can live in freshwater, saltwater, on land, and within the
human body
(3)
They are usually surrounded by a cell wall that protects the cell
from injury and determines its shape.
(4)
(5)
b)
The cell walls contain peptidoglycan, which is a carbohydrate
Archaebacteria
Archaebacteria lack the peptidoglycan of eubacteria and also
have different membrane lipids.
(1)
The DNA sequence of key archaebacterial genes are more like
those of eukaryotes than those of eubacteria.
(2)
Based on this and other data, scientists reason that
archaebacteria may by the ancestors of eukaryotes.
(3)
(4)
Many live in very harsh environments.
(a) Oxygen-free environments, such as the thick mud and the
digestive tracts of animals
(b) Others in extremely salty environments, like the Great Salt Lake
in Utah
(c) Or in hot springs where the temperatures approach the boiling
point of water
B. Identifying Prokaryotes
1. Prokaryotes are identified by characteristics such as shape, the
chemical nature of their cell walls, the way they move, and the way
they obtain energy.
a)
b)
Shapes
(1)
Rod-shaped—bacilli
(2)
Spherical—cocci
(3)
Spiral and corkscrew-shaped—spirilla
Cell Walls
(1)
Two different types of cell walls are found in eukacteria.
(2)
A method called Gram staining is used to tell them apart.
(a) Gram-positive with thick peptidoglycan walls
(b) Gram-negative with much thinner cell walls.
c)
Movement
(1)
Some prokaryotes do not move at all
(2)
Others are propelled by flagella, whiplike structure
(3)
Others lash, snake, or spiral forward.
Some glide slowly along a layer of slimelike material they
secrete
(4)
C. Metabolic Diversity
1. No characteristic illustrates their diversity better than the ways in
which they obtain energy.
2. Heterotrophs
Most must take in organic molecules for both energy and a
supply of carbon. These are called chemoheterotrophs.
a)
A smaller group are called photoheterotrophs. These
organisms are photosynthetic, using sunlight for energy, but they
also need to take in organic compounds as a carbon source.
b)
3. Autotrophs
Some autotrophs, the photoautotrophs, use light energy to
convert carbon dioxide and water to carbon compounds and
oxygen in a process similar to that used by green plants.
a)
These organisms are found in areas that light is plentiful, such
as near the surfaces of lakes, streams, and oceans.
(1)
One group, the cyanobacteria, contains bluish pigment and
chlorophyll a.
(2)
Other prokaryotes can perform chemosynthesis and are called
chemoautotrops.
b)
(1)
They do not require a light source of energy.
Instead, they use energy directly from chemical reactions
involving ammonia, hydrogen sulfide, nitrates, sulfur, or iron.
(2)
4. Releasing Energy
a)
Like all organisms, bacteria need a constant supply of energy.
This energy is released by the process of cellular respiration or
fermentation or both.
b)
Organisms that require a constant supply of oxygen in order to
live are called obligate aerobes. (Obligate means that the
organisms are required by their life process to life only in that
particular way.)
c)
d)
Some do not require oxygen called obligate anaerobes.
A third group of bacteria can survive with or without oxygen
and are known as facultative anaerobes. (facultative means that
the organism are able to function in different ways, depending on
their environments.
e)
D. Growth and Reproduction
1. When conditions are favorable, bacteria can grow and divide at
astonishing rates. Some divide as often as every 20 minutes! If
unlimited space and food were available to a single bacterium and if
all of its offspring divided every 20 minutes, in just 48 hours they
would reach a mass approximately 4000 times the mass of Earth!
Binary Fission—Type of asexual reproduction in which an
organism replicates its DNA and divides in half, production two
identical daughter cells.
a)
b)
Conjugation
(1)
Many bacteria are also able to exchange genetic information.
A hollow bridge forms between two bacterial cells, and genes
move from one cell to the other
(2)
c)
Spore Formation
When growth conditions become unfavorable, many bacteria
form structures called spores.
(1)
One type of spore, called an endospore, is formed when a
bacterium produces a thick internal wall that encloses its DNA and
a portion of its cytoplasm.
(2)
Spores can remain dormant for months or even years while
waiting for more favorable growth conditions.
(3)
E. Importance of Bacteria
1. Bacteria are vital to maintaining the living world. Some are
producers that capture energy by photosynthesis. Others are
decomposes that break down the nutrients in dead matter and the
atmosphere. Still other bacteria have human uses.
a)
Decomposers
Every living thing depends directly or indirectly on a supply of
raw materials.
(1)
If these materials were lost when an organism died, life could
not continue.
(2)
b)
Nitrogen Fixers
(1)
Plants and animals depend on bacteria for nitrogen.
The process of converting nitrogen gas into a form plants can
use is known as nitrogen fixation.
(2)
c)
Human Uses of Bacteria
Bacteria are used in the production of a wide variety of foods
and beverages
(1)
(2)
One can be used to clean up small oil spills
(3)
Some remove waste products and poisons from water
(4)
Other can help to mine minerals
(5)
Some are used to synthesize drugs and chemicals
E. coli is found in the human intestines and make a number of
vitamins that the body cannot produce itself.
(6)
II. Viruses
A. What Is a Virus?
1. Viruses are particles of nucleic acid, protein, and in some cases
lipids.
2. Viruses can reproduce only by infecting living cells.
3. Viruses differ widely in terms of size and structure.
4. They all have one thing in common: They enter living cells and,
once inside, use the machinery of the infected cell to produce more
viruses.
5. A typical virus is composed of a core of DNA or RNA surrounded
by a protein coat.
6. A virus’s protein coat is called its capsid.
7. The capsid proteins of a typical virus bind to receptors on the
surface of a cell and “trick” the cell into allowing it inside.
8. Because viruses must bind precisely to proteins on the cell
surface and then use a host’s genetic system, most viruses are
highly specific to the cells they infect.
9. Viruses that infect bacteria are called bacteriophages.
B. Viral Infection
1. Once the virus is inside the host cell, two different processes may
occur.
a)
Lytic Infection
In a laytic infection, a virus enters a cell, makes copies of itself,
and causes the cell to burst.
(1)
The virus uses the materials of the host cell to make thousands
of copies of its own DNA molecule.
(2)
Before long, the infected cell lyses, or bursts, and releases
hundreds of virus particles that may go on to infect other cells.
(3)
Because the host cell is lysed and destroyed, this process is
called a lytic infection.
(4)
b)
Lysogenic Infection
Other viruses causes lysogenic infections in which a host cell
makes copies of the virus indefinitely.
(1)
In a lysogenic infection, a virus integrates its DNA into the
DNA of the host cell, and the viral genetic information replicates
along with the host cell’s DNA.
(2)
Unlike lytic viruses, lysogenic viruses do not lyse the host cell
right away. Instead, a lysogenic virus remains inactive for a
period of time.
(3)
The viral DNA that is embedded in the host’s DNA is called a
prohpage.
(4)
The prophage may remain part of the DNA of the host cell for
many generations before becoming active.
(5)
2. The steps of lytic and lysogenic infections may be different from
those of other viruses when they attack eukaryotic cells.
3. Most animal viruses, however, show patterns of infection similar
to either the lytic or lysogenic patterns of infection of bacteria.
C. Retroviruses
1. Some viruses contain RNA as their genetic information and are
called retroviruses.
2. When retroviruses infect a cell, they produce a DNA copy of their
RNA.
3. This DNA, much like a prophage, is inserted into the DNA of the
host cell.
4. There the retroviruses may remain dormant for varying lengths of
time before becoming active, directing the production of new viruses,
and causing the death of the host cell.
5. Retroviruses are responsible for some types of cancer in animals,
including humans.
6. The virus that causes acquired immune deficiency syndrome
(AIDS) is a retrovirus.
D. Viruses and Living Cells
1. Viruses must infect a living cell in order to grow and reproduce.
2. They also take advantage of the host’s respiration, nutrition, and
all the other functions that occur in living things.
3. Therefore, viruses can be considered to be parasites.
4. Are viruses alive?
If we require living things be made up of cells and be able to
live independently, then viruses are not alive.
a)
b)
Yet, viruses have many of the characteristics of living things.
c)
Viruses are at the borderline of living and nonliving things.
III. Diseases Caused by Bacteria and Viruses
A. Bacteria and viruses are everywhere in nature, but only a few
cause disease. However, these pathogens, or disease-causing
agents, get all the attention.
B. Bacterial Disease in Humans
1. Many bacteria live on and within our bodies, and some bacteria
even help us to perform essential functions, such as digesting food.
2. Bacteria produce disease in one of two general ways.
Some bacteria damage the cells and tissues of the infected
organism directly by breaking down the cells for food.
a)
The bacterium Mycobacterium tuberculosis, which causes
tuberculosis, is inhaled into the lungs, where it destroys the lung
tissue.
(1)
The bacterium may also enter a blood vessel and travel to new
sites in the body where it destroys more tissue.
(2)
Other bacteria release toxins (poisons) that travel throughout
the body interfering with the normal activity of the host.
b)
(1)
Bacterial toxins can travel throughout the body.
The Streptococcus bacterium that causes strep throat can
release toxins into the bloodstream.
(2)
These toxins can cause scarlet fever, a red rash appears on the
skin.
(3)
c)
Preventing Bacterial Disease
Many bacterial disease can be prevented by stimulating the
body’s immune system with vaccines.
(1)
(2)
A vaccine is a preparation of weakened or killed pathogens.
When injected into the body, a vaccine sometimes prompts the
body to produce immunity to the disease.
(3)
(4)
Immunity is the body’s ability to destroy new pathogens.
If a bacterial infection does occur, a number of drugs can be
used to attach and destroy the invading bacteria.
(5)
These drugs include antibiotics, such as penicillin and
tetracycline.
(6)
Antibiotics are compounds that block the growth and
reproduction of bacteria.
(7)
C. Bacterial Disease in Animals
1. Animals are also affected by bacterial diseases, requiring farmers
and ranchers to take precautions to protect their livestock from
infection.
2. Adding to the danger is the fact that many bacteria can affect both
humans and animals.
One example of such a bacterium is Bacillus anthracis, which
causes the disease known as anthrax.
a)
Anthrax infections are often found in sheep, sometimes
spreading to farmers and wool workers who have contact with the
animals.
b)
c)
Anthrax can be fatal to both humans and animals.
The bacterium produces tough, resistant spores that can last
for years.
d)
These properties have led some groups to develop anthrax as a
biological warfare agent.
e)
D. Controlling Bacteria
1. Although most bacteria are harmless, and many are beneficial, the
risks of bacterial infection are great enough to warrant efforts to
control bacterial growth.
2. There are various methods used to control bacterial growth,
including sterilization by heat, disinfectants, and food storage and
processing.
E. Viral Disease in Humans
1. Like bacteria, viruses produce disease by disrupting the body’s
normal equilibrium.
2. In many viral infections, viruses attack and destroy certain cells in
the body, causing the symptoms of the disease.
3. Other viruses cause infected cells to change their patterns of
growth and development.
4. Viral disease CANNOT be treated by antibiotics.
5. Once a viral disease has been contracted, it may be too late to
control the infection. However, sometimes the symptoms of the
infection can be treated.
F. Viral Disease in Animals
1. Viruses produce serious animal diseases as well.
2. An epidemic of foot-and-mouth disease, caused by a virus that
infects livestock, swept through parts of Europe in the late 1990s.
3. Thousands of cattle were destroyed in efforts to control the
disease.
4. American authorities took special precautions to guard against the
spread of the foot-and-mouth virus to North America.
G. Viral Disease in Plants
1. Unlike animal viruses, most plant viruses have a difficult time
entering the cells they infect.
2. Many plant viruses are spread by insects.
3. Once inside the plant, many viruses spread rapidly, causing
severe tissue damage, mottled leaves, and wilting, and sometimes
killing the infected plant.
4. Plant viruses infect many valuable fruit trees, including apples and
peaches, and have caused serious losses in the potato crop.
H. Viroids and Prions
1. Scientists have discovered two other viruslike particles that also
cause disease: viroids and prions.
2. Viroids cause disease in plants. Prions cause disease in animals
Viroids—single-stranded RNA molecules that have no
surrounding capsids.
a)
b)
Prions—short for “protein infectious particles.”