Chapter 19
Bacteria and Viruses
19-1: Bacteria
Bacteria are prokaryotes
Recently prokaryotes were split
into two different groups;
eubacteria and archaebacteria
Eubacteria: a very wide range of
prokaryotes that can live almost anywhere
They have a cell wall for
protection – the wall contains a
carbohydrate for extra support
Some even have another
membrane that sits outside the
cell membrane
Archaebacteria: many live in
very harsh environments
They lack the carb in their cell
wall that eubacteria have
Their DNA is also more like eukaryotes
than the DNA of eubacteria
Prokaryotes are identified by
things like shape, their cell
wall, movement, and they
way they get energy
There are 3 main bacteria
shapes; bacilli (rod-shaped),
cocci (spherical), and spirilla
(corkscrew-shaped)
There are 2 different types of
cell walls; a Gram stain can be A Gram stain test uses 2
different dyes to tell if the cell
used to tell them apart
wall contains a specific
carbohydrate
Gram positive appears purple
and means the cell wall has
the carb
Gram negative appears pink
and means the cell wall
doesn’t have the carb
Some prokaryotes do not
Most prokaryotes are
move, some are propelled by
heterotrophs, but some are
flagella, some spiral forward,
autotrophs
some slide along in slime they
secrete
Photoautotrophs use light energy
to make they energy the need
(almost like plants)
Chemoautotrophs use carbon
molecules from carbon dioxide to
make the energy they need
Some bacteria need oxygen,
while others are killed by it
Obligate aerobes must have oxygen
Obligate anaerobes cannot live with oxygen
Facultative anaerobes can live
with or without oxygen
Bacteria can grow and reproduce in different ways
Binary fission = when a bacterium
has grown large enough that it can
replicate its DNA and split in two
Conjugation = when a hollow
bridge forms between 2 bacteria
so they can exchange genetic info
Bacteria can form spores  bacteria DNA
gets surrounded by a thick cell wall
Spores can lay dormant until conditions
are favorable for more growth – this lets
bacteria survive harsh conditions
Anthrax
While we tend to think of bacteria as a bad thing, many
bacteria are very useful
Uses include decomposition,
nitrogen fixation, food production,
helping humans produce vitamins,
digestion of oil and poison, etc…
19-2:
Viruses
Viruses are particles of nucleic acid,
protein, and sometimes lipids
Viruses can reproduce only by
infecting living cells
They vary in terms of
shape and size
A typical virus is composed of a core of DNA and RNA
surrounded by a protein coat
Capsid = the protein coat
The proteins bind to the host
cell and trick the cell into
letting it in
Once the viral DNA is inside the cell
it uses transcription and translation
to create more proteins
Bacteriophages only infect bacteria
Most viruses are very specific to
the type of cell they will infect
Once a virus is inside a cell, two things can happen
In a lytic infection, a virus
enters a cell, makes copies of
itself, and causes the cell to
burst
In a lysogenic infection, a
virus integrates its DNA into
the DNA of the host cell, and
the viral genetic info
replicates along with the host
cell DNA
A retrovirus contains RNA as its genetic code
Once a retrovirus enters a cell, it
makes a DNA copy of its RNA
The new DNA gets inserted into
the DNA of the host cell and can
stay dormant for some time
Retroviruses can cause some
types of cancer in animals
The virus that causes AIDS is
a retrovirus
Comparison of viruses and bacteria
Characteristic
Structure
Reproduction
Genetic Code
Growth and
development
Obtain and use
energy
Response to
environment
Change over time
Virus
Cells (bacteria and
others)
DNA or RNA core, Cell membrane,
capsid
cytoplasm
Only within a host Independent cell
cell
division
DNA or RNA
DNA
No
Yes
No
Yes
No
Yes
Yes
Yes
Viruses are not considered living even though
they have many characteristics of living things
Viruses are considered parasites since they must
infect a living cell to grow and reproduce
19-3:
Diseases caused by
Bacteria and Viruses
Bacteria produce disease in
one of two general ways
Some bacteria damage cells and the tissues of
the infected organism directly by breaking
down the cells for food
Ex. The bacterium that causes tuberculosis is
inhaled into the lungs and destroys lung
tissue
Other bacteria release toxins that travel throughout the body
interfering with the normal activity of the host
Ex. The bacterium that causes strep throat releases toxins in
the bloodstream  these can even cause scarlet fever.
Other examples of bacterial infections
are lyme disease, tetanus, bacterial
meningitus, and tooth decay Many bacterial diseases can
be prevented with vaccines
Antibiotics can be used to
treat bacterial infections that
have already occurred
Bacterial diseases can be seen in animals – many bacteria can
affect both humans and animals
Ex. Anthrax
There are various methods used to
control bacterial growth, including
sterilization, disinfectants, and
food processing
Like bacteria, viruses produce disease by
disrupting the body’s normal equilibrium
In many viral infections, viruses
attack and destroy certain cells in
the body
Examples include the common
cold, influenza, smallpox, AIDS,
chickenpox, measles, and west nile
Viral diseases CANNOT be treated
with antibiotics
The best way to protect against viral
infections is with vaccines
Viruses can produce serious
animal diseases also
Many viruses infect plants and cause serious threats to
agricultural crops - ex. Tobacco mosaic virus and potato
yellow dwarf virus