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Biology
Unit 8a – Viruses & Bacteria
Chapter 18
Viruses
Infections can be caused in several ways
oViruses, bacteria, viroids, and prions
can all cause infection.
Any diseasecausing agent
is called a
pathogen.
oA virus is made of DNA or RNA and a
protein coat.
onon-living pathogen
ocan infect many
organisms
o A viroid is made only
of single-stranded
RNA.
•causes disease in plants
•passed through seeds or pollen
oA prion is made only of proteins.
•causes misfolding of other proteins
•results in diseases of the brain
Viruses differ in shape and in ways of
entering host cells.
o Viruses have a simple structure.
o genetic material
o capsid, a protein shell
o maybe a lipid envelope, a protective outer
coat
enveloped
(influenza)
capsid
nucleic acid
lipid
envelope
polyhedral
(foot-and-mouth
disease)
helical
(rabies)
Surface proteins
capsid
nucleic acid
surface
proteins
lipid envelope
surface
proteins
capsid
nucleic acid
Bacteriophages infect bacteria
capsid
DNA
tail sheath
tail fiber
Viruses enter cells in various ways
o bacteriophages pierce host cells.
o Viruses of eukaryotes enter by endocytosis
or by fusing with the membrane
colored SEM; magnifications:
large photo 25,000; inset 38,000x
Viruses cause two types of infections.
o A lytic infection causes the host cell to burst.
host bacterium
The bacterophage attaches
and injects it DNA into a
host bacterium.
The host bacterium breaks apart,
or lyses. Bacteriophages are
able to infect new host cells.
The viral DNA
forms a circle.
The viral DNA directs the host
cell to produce new viral parts.
The parts assemble into new
bacteriophages.
o A lysogenic infection does no immediate
harm.
The prophage may leave the
host’s DNA and enter the
lytic cycle.
The viral DNA is called a prophage
when it combines with
the host cell’s DNA.
Many cell divisions produce a
colony of bacteria infected
with prophage.
Although the prophage is not
active, it replicates along with
the host cell’s DNA.
Viruses cause many infectious diseases
o There are many examples of viral infections.
o common cold
o influenza
o SARS
o HIV
The body has
natural defenses
against viruses
HIV-infected
white
blood cell
Vaccines are made from weakened
pathogens
o A vaccine stimulates the body’s own
immune response.
o Vaccines prepare the immune system for
a future attack.
o Vaccines are the only way to control the
spread of viral disease.
Bacteria &
Archaea
Bacteria and archaea are both
single-celled prokaryotes.
o Prokaryotes can be grouped by their
need for oxygen.
• obligate
anaerobes
are poisoned by
oxygen
• obligate aerobes
need oxygen
• facultative
aerobes can live
with or without
oxygen
Bacteria commonly come in three forms
o rod-shaped, called bacilli
o spiral, called spirilla or
spirochetes
o spherical, called cocci
Lactobacilli: rod-shaped
Enterococci: spherical
Archaea have many shapes
Spirochaeta: spiral
Bacteria and archaea have similar
structures
o plasmid
pili
plasma
o flagellum
flagellum
membrance
chromosome
o pili
cell
wall
plasmid
This diagram shows the typical
structure of a prokaryote. Archaea and
bacteria look very similar, although
they have important molecular
differences.
o The amount of peptidoglycan within the
cell wall can differ between bacteria
GRAM NEGATIVE
GRAM POSITIVE
Archaea have different lipids entirely
Gram staining identifies bacteria
o gram-positive stains purple, more
peptidoglycan
o gram-negative stains pink, less
peptidoglycan
Gram-negative bacteria have a thin
layer of peptidoglycan and stain red.
Gram-positive bacteria have a thicker
peptidoglycan layer and stain purple.
Bacteria have various strategies for
survival
o Prokaryotes exchange genes during
conjugation
o Bacteria may
conjugation bridge
survive by
forming
endospores
(dormant alternate
life forms)
Clostridium botulinum - Gram-positive, endospore-forming, rod
prokaryote. Vegetative and spore stages: note TEM;
the magnification
flagella. Causes
6000x
food poisoning (botulism), wound infections and infant botulism
Prokaryotes perform important functions
for organisms and ecosystems
o Prokaryotes live in digestive systems of
animals.
o make vitamins
o break down food
o fill niches
o Bacteria help ferment
many foods
o yogurt, cheese
o pickles, sauerkraut
o soy sauce, vinegar
Lactobacillus acidophilus
o Prokaryotes have many functions in
Nitrogen fixing bacteria
ecosystems
o photosynthesize
o recycle carbon, nitrogen,
hydrogen, sulfur
o fix nitrogen
o Bioremediation uses
prokaryotes to break down
pollutants
o oil spills
o biodegradable materials
Bioremediation in soil
Some bacteria cause disease
o Bacteria cause disease by invading
tissues or making toxins.
o A toxin is a poison released by an
organism.
Clostridium botulinum – food poisioning
Normally harmless bacteria can
become destructive
o may colonize new tissues
Streptococcus bacteria on skin
They are fairly harmless unless they come in
contact with other tissue such as muscle or fat
through an open wound.
Antibiotics are used to fight bacterial
disease
oAntibiotics may stop bacterial cell
wall formation
•Antibiotics do not work on
viruses.
•Prevention is best method
to fight bacterial disease
Penicillium chrysogenum
Bacteria are gaining resistance to
antibiotics
oThrough:
o overuse
o Using them too often
and when not ill
o underuse
o Failure to take the
entire course of
medicine
o misuse
o Used in agriculture to
increase rate of growth
but bacteria already in
the food becomes
resistant.
A bacterium carries
genes for antibiotic
resistance on a
plasmid.
A copy of the plasmid
is transferred through
conjugation.
Resistance is quickly
spread through many
bacteria.
That’s all until next week