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Microorganisms
Chapter 20
Sec. 2 Viruses
Are viruses alive?

Viruses are so small they can only be seen with an
electron microscope.

But is a virus alive?
–
–
–
–

Viruses can only reproduce by infecting a host cell.
Viruses do not grow, instead they are assembled.
Viruses are not made of cells.
Viruses do not carry out metabolic processes to maintain
homeostasis.
But, viruses do have genetic material – DNA or RNA
Viral Structure

Basic structure:
– Nucleic acid core (RNA or DNA)
– Protein coat or capsid – a protein sheath that surround the nucleic acid
core
– Some viruses have an envelope – a membrane that surrounds the
capsid
•
has receptors to help the virus bind to host cells
– Nucleocapsid – envelope that covers the nucleic acid
– Tail fibers – some viruses called bacteriophages that infect bacterial
cells have a tail with tail fibers.
Viral Structure Continued

Glycoproteins – proteins on
the outside of the envelope
which attach to carbohydrate
molecules on host cells.

Reverse Transcriptase –
only in RNA viruses –
transcribes (translates) the
viral RNA into viral DNA and
is then inserted into host cell.

Tail fibers – some viruses
called bacteriophages that
infect bacterial cells have a
tail with tail fibers.
Virus Shape

Viruses have many different shapes.
Virus Categories

Viruses are split into two categories based on their nucleic acid
core:
– DNA viruses either insert their DNA into the host cell or the DNA
remains separate, but uses the host’s enzymes and nucleotides to
make new copies of its DNA and more viruses.
• The DNA may be single-stranded or double stranded.
• Examples include: smallpox and Hepatitis B
– RNA viruses use reverse transcriptase to transcribe their RNA into
DNA, insert the DNA into the host cell’s DNA, and then is
transcribed into viral mRNA to make new viruses.
• Most often called retroviruses
• Examples include: HIV, Rhinovirus
Viral Reproduction

Viruses can only reproduce inside a
host cell.

Two cycles by which viruses can
reproduce:
– Lytic life cycle
– Lysogenic life cycle
Lytic Cycle

Virus attaches to host cell’s membrane and injects its
nucleic acid into the host cell.

The viral nucleic acid takes over protein synthesis,
creating new viruses.

The host cell bursts, lyses, and releases the newly
formed viruses and then dies.

Viruses that reproduce only by the lytic cycle are
called virulent.
Lytic Cycle
Lysogenic Cycle

The viral DNA becomes part of its host cell’s DNA.
–


When the host cell replicates its own DNA, it also replicates the provirus.
–
New cells that are produce contain the provirus
–
New viral particles are NOT assembled and the host cell is not destroyed.
After a period of time – days, months, or even years – the provirus may leave
the host’s DNA and enter a lytic cycle.
–

This virus is called a prophage.
Sometimes the virus never enters the lytic cycle and becomes a permanent part of the
host’s DNA
A virus whose reproduction includes the lysogenic cycle is called a temperate
virus.
Lysogenic Cycle
Combined Lytic and Lysogenic
Cycles
Viroids and Prions

Viriods and prions are molecules that are able to reproduce and
cause disease.

Viroids are a single strand of RNA without a capsid.
– The RNA of a viroid is much smaller than that of a viruses.
– Replicate inside a host’s cell to make new viroids
– Disrupt a cell’s regulation of growth
– Some viroids have caused abnormal development and stunted
growth in plants such as cucumbers and potatoes.
Viroids and Prions

Prions are mistake
versions of proteins that
are found in the brain.
– They attach to normal
proteins and cause
them to take on the
shape of the prion.
– The normal protein
then stops functioning.
– This causes a chain
reaction of misfolding
among the other
proteins which
destroys brain tissue.
Prions Continued
Cause a disease in
humans known as
Creutzfeldt-Jakob
disease
You may also be
familiar with Mad
Cow Disease in
cows.
Another disease is
chronic wasting
disease in deer and
elk
Microorganisms
Chapter 20
Sec. 1 Bacteria
Prokaryotes

Let’s recall what a prokaryote is . . . .

Single-celled, no membrane bound organelles, no nucleus

Oldest living things on Earth

Fossils date back 3.5 billion yrs ago

Divided into two major groups
– Archea
– Bacteria
Archea

Found in many places, including extreme environments such as salt
lakes and hot springs.
– methanogens, halophiles and thermoacidophiles

Structurally they are different from Bacteria.

Some Archaean molecules are more similar to those found in
eukaryotes.

Includes cyanobacteria which helped produce oxygen through
photosynthesis to make the current Earth atmosphere

Oldest fossil is 3.5 billion years old and found in Western Australia.
Eubacteria

Most prokaryotes
belong to this domain.

Virtually found
everywhere.

Broken into groups
based on three shapes.
– Spirillum, coccus, and
bacillus.
Bacterial Structure

Bacterial DNA is a single chromosome
clustered in a mass called a nucleoid.

May have extra loops of DNA called
plasmids.
– Bacterial plasmids are used in current medical
research to help find cures for diseases.
Bacterial Structure

Ribosomes and
other enzymes

Cell membrane

Flagella for
movement

Pili to adhere to
surfaces
Bacterial Structure
Bacterial Structure

Rigid cell wall
– Made of peptidoglycan, a protien-carbohydrate compound
– Presence or absence of a cell wall helps biologist to group
bacteria
– Use a technique called gram-staining to identify two groups
of bacteria
• Gram-positive or gram-negative
Gram-Staining

Dies (crystal violet and
safranin) are used to
stain the peptidoglycan
layer of the bacterial
cell.

Gram-staining helps
doctors decide what
type of antibiotics can
be used to treat the
infection.
More on Gram-Staining
Anthrax bacteria (purple rods) in cerebrospinal
fluid sample. The other cells are white blood
cells.
A Gram stain of mixed Staphylococcus aureus
(Gram positive) and Escherichia coli (Gram
negative).
How do Bacteria obtain energy?

Bacteria are grouped based on their energy sources.

3 groups
– Photoautotrophs – energy from photosynthesis
– Chemoautotrophs – energy from inorganic compounds
– Heterotrophs – energy and nutrients from other organisms
Bacterial Reproduction

Bacteria reproduce using 3 different methods.
1) Binary Fission – single cell divides into two
identical cells
2) Endospore Formation – formation of a thickwalled stucture that protects the bacterial DNA
and a small bit of cytoplasm from harsh conditions
• Can survive boiling, radiation, and acid
• Can survive for hundreds of years
Bacterial Reproduction continued
3) Genetic Recombination – ways that bacteria can
form new genetic combinations
•
Conjugation occurs when two bacteria exchange
information.
•
Transformation occurs when bacteria take up DNA
fragments from their environment.
•
Transduction occurs when genetic material (such as
a plasmid) is transferred by a virus.
Microorganisms
Chapter 20
Sec. 3 Bacteria, Viruses and
Humans
Viruses, Bacteria, and Humans

Pathology – the study and the diagnosis of
disease.
– Pathogen – anything that causes a disease.
– Toxin - a poison that is produced by bacterial
pathogens and that damage cells.
• Ex: Botulism occurs when canned food is contaminated
with endospores of the bacterium Clostridium botulinum
Bacterial Disease

Bacteria cause disease by
producing toxins or by
destroying body tissue.
– Ex: Group A streptococcus
bacteria produce strept
throat, but if untreated the
bacteria can get into muscle
or skin tissue and cause
necrotizing fasciitis (when
the flesh dies off).

We treat bacterial diseases
with antibiotics.
Antibiotics

First discovered by the scientist
Alexander Fleming in 1927.
–

penicillin
Antibiotics – a drug that kills or
slows the growth of bacteria.
– Ex: amoxicillin, zithromycin,
ciprofloxacin
– Each antibiotic targets a
specific group or specific
bacteria
Antibiotic Resistance

Antibiotic resistance – as time goes on
many bacteria are becoming resistant to
antibiotics because of overuse of the
antibiotic or mutations for resistance.
– Overuse of hand-sanitizers and not taking
the recommended dose are contributing
factors
Useful Bacteria

Bacteria make oxygen,
decompose organisms, and
make nitrogen available to
other organisms
– Ex: nitrogen fixing bacteria
combine nitrogen and
oxygen and make it
available for plants.

Bacteria help humans digest
food and exist virtually on
every inch of the human
body.
Nodules on roots of this soy
bean plant are full of nitrogen
fixing bacteria.
Useful Bacteria Continued

Bacteria are used in
many industries such as
sewage treatment
facilities and cleaning
up oil spills.

Bacteria are also used
to make a variety of
foods.
– Ex: pickles, yogurt, soy
sauce, and sourdough
bread
Viral Disease

Because viruses enter host cells to
reproduce, it is difficult to develop a drug that
kills the virus without harming the living host.

Viruses can be transmitted by any action that
brings virus particles into contact with a host
cell.
– Ex: common cold, Influenza A and B, and HIV
Vaccines

Vaccine – is a weakened form of a pathogen that prepares the
immune system to recognize and destroy the pathogen.

Most common types:
– Killed virus – uses killed virus to create an immune response
• Ex: influenza and hepatitis
– Live – attenuated virus – uses live, but modified virus to create an
immune response, but the virus does not get the person sick.
• Ex: measles and mumps
Developed Vaccines

Since viruses have the ability to
enter into the host cell they are also
able to mutate quite rapidly.

This makes producing vaccines
sometimes difficult.

Some viruses have been shown to
cause some types of cancer.
–
Ex: Hepatitis B (liver cancer) and
Human papilloma virus (cervical
cancer).
–
Gardasil vaccine – given to teenage
girls to protect against cervical
cancer