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Chapter 19
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Bacteria and Viruses
Summary
19–1 Bacteria
The smallest and most common microorganisms are prokaryotes.
Prokaryotes are unicellular and lack a nucleus. There are two
kingdoms of prokaryotes: eubacteria and archaebacteria.
• Eubacteria live almost everywhere. Most eubacteria have
a cell wall that contains the carbohydrate peptidoglycan.
Inside the cell wall is a cell membrane that surrounds
the cytoplasm.
• Archaebacteria look like eubacteria, but they have some
differences. Archaebacteria do not have peptidoglycan in
their cell walls. They have many different membrane lipids.
Also, the DNA sequences of key archaebacterial genes are
more like those of eukaryotes than those of eubacteria.
Archaebacteria may be the ancestors of eukaryotes.
Prokaryotes are identified by characteristics that include
the following:
• Shape Prokaryotes have three different shapes: rod-shaped
(bacilli), sphere-shaped (cocci), or spiral-shaped (spirilla).
• Chemical makeup of cell walls Prokaryotes have two
types of cell walls. Gram staining is used to tell them apart.
Gram-positive bacteria appear violet when stained. Gramnegative bacteria appear pink.
• Means of movement Prokaryotes move in a variety of ways.
• Means of getting energy Most prokaryotes are heterotrophs,
organisms that get energy by consuming food. Other prokaryotes are autotrophs, organisms that make their own food.
Prokaryotes get energy through cellular respiration and
fermentation.
• Obligate aerobes need a constant oxygen supply in order
to live.
• Obligate anaerobes do not need oxygen, and may be killed
by it.
• Facultative anaerobes can survive with or without oxygen.
Bacteria reproduce asexually by binary fission. When a
bacterium has grown to nearly double its size, it replicates its
DNA and divides in half. Bacteria also reproduce by conjugation.
During conjugation, genetic material is transferred from one
bacterium to another. Many bacteria can form an endospore
when conditions are bad. An endospore is a thick internal
wall that surrounds the DNA and part of the cytoplasm of the
bacterium. The spore can survive harsh conditions that would
kill the bacterium in its active form.
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Bacteria are vital to the living world. Some are producers
that carry out photosynthesis. Others are decomposers that
break down dead matter. Some soil bacteria convert nitrogen
gas into a form that plants can use through a process called nitrogen fixation. Humans use bacteria in industry, food production,
and other ways.
19–2 Viruses
Viruses are particles of nucleic acid and protein. Some contain
lipids, too. A typical virus is made of a core of DNA or RNA
surrounded by a protein coat called a capsid.
To reproduce, a virus must invade, or infect, a living host cell.
Viruses that infect bacteria are called bacteriophages. After a virus
enters a host cell, one of two processes may occur.
• In a lytic infection, a virus attaches itself to a host cell. It
injects its DNA into the cell. The host cell starts making
messenger RNA from the viral DNA. The messenger RNA
takes over the host cell. Copies of viral DNA and the viral
protein coats are made and assembled into new viruses.
Then, the host cell bursts and the new viruses infect
other cells.
• In a lysogenic infection, the virus does not reproduce
immediately after infecting the host cell. Instead, the nucleic
acid of the virus is inserted into the DNA of the host cell.
The viral DNA may stay within the host DNA for quite
some time. However, eventually it may become active,
remove itself from the host DNA, and begin the production
of new viruses.
Some viruses, called retroviruses, contain RNA as their genetic
information. They produce a DNA copy of their RNA genes when
they infect a cell. AIDS is a disease caused by a retrovirus.
Viruses are parasites. They must infect a living cell in order to
reproduce. Because viruses are not made up of cells and cannot
live on their own, viruses are not considered to be living.
19–3 Diseases Caused by Bacteria and Viruses
Some bacteria and viruses can be pathogens. Pathogens are
disease-causing agents.
Bacterial Disease
Bacteria can cause tuberculosis, strep throat, and tetanus. Bacteria
cause disease in two general ways.
1. Bacteria break down cells for food.
2. Bacteria release toxins (poisons) that disrupt normal body
functions in the host.
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To prevent some bacterial diseases, vaccines are used. A vaccine
is a preparation of weakened or killed pathogens. A vaccine works
by prompting the body to form immunity to a disease. Immunity
is the body’s natural way of killing pathogens.
When a bacterial infection occurs, antibiotics may help fight
the disease. Antibiotics are compounds that block the growth and
reproduction of bacteria.
Bacterial growth can be controlled. Sterilization, disinfectants,
and proper food storage and food processing can control bacteria.
Disinfectants include soaps and cleaning solutions. Food storage
includes using a refrigerator.
Not all bacteria are pathogens. Some live in and on the human
body and help it carry out needed functions. For example, bacteria
that live in the intestines make vitamin K.
Viral Disease
Like bacteria, viruses cause disease by disrupting the body’s
normal equilibrium, or balance. In many viral infections, viruses
attack and destroy certain body cells. This causes the symptoms of
the disease. Viral diseases in humans include the common cold,
influenza, AIDS, chickenpox, and measles. Some viral diseases can
be prevented with vaccines. Viruses also cause diseases in animals
and plants.
Two viruslike particles also can cause disease.
• Viroids are single-stranded RNA molecules that have no
surrounding capsids. Viroids cause disease in plants.
• Prions are particles containing only protein—there is
no DNA or RNA. Prions cause disease in animals,
including humans.
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