UNIT 11
CHAPTER 20:
VIRUSES AND
BACTERIA
Viruses


Viruses are segments of nucleic acids contained in a protein
coat.
A virus is not a cell. Viruses are pathogens – agents that can
cause disease.

Most viruses are so small, they can only be seen with an electron
microscope.

Viruses do not grow and do not metabolize, and therefore cannot
reproduce on their own. They can only reproduce in other living
cells.

Viruses were first discovered near the end of the nineteenth
century by scientists trying to find the cause of the tobacco
mosaic virus.
Viruses can only reproduce
inside a living cell called
the host cell.
Outside the host cell, a
virus is “lifeless” and often
exists as a crystal.
A virus has no
means of
locomotion,
and exist in a
variety of
shapes.
Viral Structure

The virus protein coat, or capsid, may contain either RNA
or DNA, but not both.

RNA viruses include the human immune-deficiency virus
(HIV), influenza viruses, and the rabies virus.

DNA viruses include those viruses that cause warts,
chickenpox, and mononucleosis.


Many viruses have a membrane, or envelope, surrounding
the capsid. The envelope helps viruses enter cells.
The virus’ envelope consists of proteins, lipids, and
glycoproteins, which are proteins with attached
carbohydrate molecules.
Human Papilloma Virus - Warts
Rabies Virus
Influenza Virus - Flu
Human Immunodeficiency Virus
AIDS
Viral Reproduction

Viruses lack the enzymes necessary for metabolism, and have
no structures to make proteins.

Viruses must rely on living host cells for replication.

Before a virus can replicate, it must first infect a living cell.

An animal virus enters its host cell by endocytosis.


A bacterial virus, or bacteriophage, punches a hole in the
bacteria cell wall and injects its DNA into the cell.
Viruses reproduce using one of two paths: the lytic cycle
or the lysogenic cycle.
Lytic Cycle

In bacterial viruses, the cycle of viral infection,
replication, and cell destruction is called the lytic
cycle.

After entering the cell, the virus uses the host cell to
replicate viral genes and make viral proteins such as
capsid proteins.

The proteins are then assembled with the replicated
viral genomes to form complex viruses.

The host cell is broken open and releases newly made
viruses.
Lysogenic Cycle





A provirus is viral DNA that has attached to a host cell’s
chromosome and that is replicated with the chromosome’s DNA.
When the cell divides, the provirus also divides, resulting in two
infected host cells.
This cycle is called the lysogenic cycle, and the viral genome
replicates without destroying the host cell.
Some viruses can alternate between the lytic cycle and
the lysogenic cycle based on changes in the environment.
When conditions in the human body become favorable for the
virus, such as when a person is under stress, the virus then begins
to cause tissue damage that is seen as a cold sore or fever blister.
Viral Reproduction
Negative Viral Influences

Viruses can cause infections in both plants and animals.

Some viruses cause tumors and warts.

Viruses also destroy cells, and are responsible for
human diseases such as:
– polio
– measles
– mumps
– influenza
– hepatitis
– colds
– AIDS
Positive Viral Influences

Certain viruses are used in the
control of insect pests.

Others viruses are used in
genetic research, or gene therapy.
Scientists are trying to find ways to
use cold viruses to deliver healthy
genes to a host cell in an attempt to
repair damaged genes.

Scientists have been able to use
viruses to biologically control
caterpillars of the European pine
sawfly and the gypsy moth which
can cause extensive crop damage.
Emerging Viruses




Emerging viruses are newly recognized viruses, or viruses that
have reappeared or spread to new areas.
In 1999, a mosquito-borne virus called the West Nile virus began
to spread across the United States, causing flu-like symptoms that
can lead to inflammation of the brain in some people.
Prions are a newly discovered class of pathogens that are
composed of proteins, but have no nucleic acid. Contact with a
prion will cause a normal version of a protein to misfold and not
allow the protein to function properly. The brain disease called
mad cow disease is associated with prions.
A viroid is a single strand of RNA that has no capsid. Viroids
have infected economically important plants such as potatoes and
oranges.
Bacteria

Bacteria are prokaryotic, microscopic, unicellular
organisms that lack a cell nucleus. Many bacteria lack
internal compartments and membrane systems.

Bacteria can range in size, but all are single cells that may
stick together or may form long strands.

Bacterial chromosomes consist of a single circular piece of
DNA, unlike eukaryotic chromosomes which are linear
pieces of DNA that are associated with proteins.

Bacteria reproduce by binary fission, a process in which
one cell pinches into two cells.
Structure of Bacteria


A bacterium is
made up
of a thick
cell wall
that encloses
and protects the
cellular material.
Outside the cell
wall and
membrane, many
bacteria have a
gel-like layer
called a
capsule.
Bacteria Structure

Bacterial flagella are simple structures composed of
a single fiber of protein that spins like a corkscrew to
move the cell.

Some bacteria also have shorter, thicker outgrowths
called pili that enable bacteria to attach to surfaces or
to other cells.

As prokaryotes, bacteria can perform different kinds of
anaerobic and aerobic processes. In contrast,
eukaryotes are mostly aerobic organisms.

Most bacteria are heterotrophs -- organisms that
cannot make their own food.
Shapes of Bacteria
A bacterial cell is usually one
of three basic shapes:
 coccus - round-shaped
 bacillus – rod-shaped
 spirillum – spiral-shaped



Species that form filaments are
indicated by the prefix strepto-.
Species that form clusters are
indicated by the prefix staphylo-.
LEFT: bacterial flagella
RIGHT: bacterial filaments
Shapes of Eubacteria

Members of the kingdom Eubacteria have a cell wall made of
peptidoglycan, a network of polysaccharide molecules
linked together with chains of amino acids.

Eubacteria can have two types of cell walls, distinguished by
a dye staining techniques called the Gram stain: Gramnegative, or Gram-positive.

Gram staining is important in medicine because the two
groups of eubacteria differ in their susceptibility to different
antibiotics.

Antibiotics are chemicals that interfere with the life
processes in bacteria. Gram staining can determine which
antibiotic would be most useful in fighting an infection.
Endospores



Some bacteria form thick-walled endospores around their
chromosomes and a small bit of cytoplasm when they are
exposed to harsh conditions such as depletion of nutrients,
a drought, or high temperatures.
Endospores can survive environmental stress and may
germinate years after they were formed, releasing new,
active bacteria.
Pili enable bacteria to adhere to the surface of sources of
nutrition.

Some type of pili help bacteria to exchange genetic material
through conjugation – a process in which organisms
exchange genetic material.
LEFT:
conjugation
between
bacterial cells
BELOW: binary
fission produces
a new bacterial
cell
Classification of Bacteria

There are over 4,000 species of bacteria that have been
named, and probably many more that have not been
discovered.

Bacteria can be classified in how they obtain energy, or by
their phylogenetic, or evolutionary, relationships.


Photosynthetic bacteria can be classified into four major
groups based on the photosynthetic pigments they contain:
purple nonsulfur bacteria, green sulfur bacteria, purple sulfur
bacteria, and cyanobacteria.
Green sulfur bacteria and purple sulfur bacteria grow in
anaerobic (oxygen-free) environments.
Classification of Bacteria

Chemoautotrophs are bacteria that obtain energy by
removing electrons from inorganic molecules such as
ammonia (NH3), and hydrogen sulfide, (H2S), or from
organic molecules such as methane (CH4).

Chemoautotrophs that live in the soil play an important
role in the nitrogen cycle called nitrification – the
process in which bacteria oxidize ammonia into nitrate.
Nitrate is the form of nitrogen most commonly used by
plants.

Most bacteria are heterotrophs and, together with fungi,
are the principal decomposers of the living world.
Obtaining Energy



Some bacteria live as parasites. Parasites
absorb food materials from other living
organisms.
Other bacteria digest and absorb food
materials from dead organisms. These bacteria
are called saprophytes.
Heterotrophic bacteria obtain nutrients by
secreting enzymes that break down complex
organic structures in their environment, and
then absorbing them.
Beneficial Bacteria


Many bacteria are used to
produce food and life-saving
drugs.
Some types of bacteria are used
to make yogurt, cheese, pickles,
sauerkraut, and vinegar.
Beneficial Bacteria
 By
using methods of gene transplanting,
bacteria have been encouraged to produce
substances such as insulin.
Beneficial Bacteria
 The
human intestinal
tract contains
millions of bacteria
that aid in the
digestion of food.
 Many
of these
bacteria also help
produce vitamins.
Beneficial Bacteria
 Bacteria
are also
used in the
tanning of
leather, the
curing of
tobacco, and the
production of
food for feeding
cows.
Harmful Bacteria

Some bacteria are not
beneficial to man.

Bacteria spoil food by
secreting enzymes into
the food causing it to rot.

Some bacteria can also secrete chemical
compounds into their environment
called toxins that are poisonous to
eukaryotic cells. When bacteria grow in
food and produce toxins, the toxins can
cause food poisoning.
Harmful Bacteria
 Bacteria
in large
numbers can
pollute, or dirty,
lakes, streams,
and drinking water.
 During
cellular respiration, bacteria reduce
the dissolved oxygen content in
the water supply.
Harmful Bacteria



Many bacteria are
pathogenic.
A pathogen is an
organism that causes
disease or infections.
Some bacterial diseases
that occur in humans
are tuberculosis (TB),
tetanus, E. coli, staph
infections, and strep
throat.
Controlling Harmful Bacteria


Antiseptics, disinfectants, and
antibiotics are used to control
pathogenic bacteria.
An antibiotic is a chemical that
can stop the growth of some
bacteria.

Some bacteria are able to produce
types that are resistant to certain
antibiotics.

When this happens, new
antibiotics must be developed.
Controlling Harmful Bacteria
 Bacteria
can also be killed in foods by:
– pasteurization
– canning
– chemical preservatives
– radiation
– steam or pressure
– salt curing
– dehydration (drying)
Harmful Anaerobic Bacteria

Some bacteria can live anaerobically and cause botulism.

Botulism is a dangerous type of food poisoning that occurs in foods that
have not been properly canned. When canned foods are not heated
enough to kill endospore-forming bacteria, the bacteria can grow and
produce a deadly toxin that affects the nervous system.

A person who eats food that contains this toxin can become ill and die.