Viruses and Bacteria
Unit 6
Chapter 18
Virus characteristics
 nucleic acids enclosed in a protein coat
 smaller than the smallest bacterium
 nonliving particles
 shape determines the cell the virus attacks
This picture
shows the three
major virus
shapes.
Naming a virus
 named after diseases that they cause or
organs that they attack

Ex: poliovirus, small pox virus
A virus that infects a bacterium is called a
bacteriophage or phage for short.
Viral metabolism
 Viruses cannot grow, reproduce, or develop
without a host cell.
 Viruses share parasitic relationships with
their hosts.
Capsid
Viral structure
 Nucleic acid (DNA or
RNA)
 Envelope: layer that
surrounds the capsid,
found mostly in larger
viruses (such as those
that affect humans)
Viral replication (lytic)
1. Attach to a host cell with matching
2.
3.
4.
5.
receptors.
The viral genetic material enters the cell
either through injection or through
endocytosis that forms a vesicle.
The host cell copies the viral material.
The viral DNA is transcribed, translated,
and reassembled into new virus.
The host cell bursts open, releasing new
viruses.
Lytic cycle: rapid replication and
assembly, followed with lysis (bursting)
Lytic cycle video
Click on image to play video.
Viral replication (lysogenic)
 Some viruses go through a lysogenic cycle,
a replication cycle in which the virus’s
nucleic acid is integrated into the host cell’s
chromosome, followed by a lytic cycle.
1. Attachment
2. Infection
3. Genetic material inserted into host DNA
(now called provirus)
4. Replication
Lysogenic cycle video
Click on image to play video.
Lysogenic cycle
B. Provirus Formation
A. Attachment and Entry
Provirus
C. Cell Division
Bacterial host
chromosome
A lysogenic virus
injects its nucleic
acid into a bacterium.
The viral nucleic acid is called
a provirus when it becomes
part of the host’s chromosome.
LYSOGENIC CYCLE
LYTIC CYCLE
The cell breaks
open releasing
viruses.
Viral nucleic acid and
proteins are made.
The provirus leaves
the chromosome.
Although
the provirus
is inactive,
it replicates
along with
the host cell’s
chromosome.
Lysogenic viral diseases
 herpes simplex I
 herpes simplex II
that causes genital
herpes
 hepatitis B virus
that causes
hepatitis B
 Chickenpox virus
but may become
lytic and cause
shingles
Herpes virus
HIV
 Human immunodeficiency virus (HIV) causes
a condition called AIDS.
 HIV destroys the T cells that are part of the
human immune system.
 When the T cell counts drop to a particular
level, a person is said to have AIDS.
HIV
HIV is a retrovirus.
 Retroviruses are viruses that have RNA
instead of DNA as the genetic material.
 HIV injects its RNA into the host cell.
 An enzyme (reverse transcriptase) makes a
DNA complement of the RNA.
 The viral DNA is embedded into the host cell,
which the virus then goes through a lysogenic
and lytic cycle.
HIV life cycle
RNA
Retrovirus
DNA is made from
the viral RNA.
RNA
DNA
Reverse
transcriptase
Entering
cell
Provirus in
host chromosome
mRNA
Retrovirus Cycle
New virus parts
Exiting
cell
New virus
forming
Cancer and viruses
 Some viruses cause cancers.
 These viruses cause the cells to divide
abnormally, creating tumors.
Prions and viroids
 Prions are composed of proteins but have no
nucleic acid to carry genetic information


They cause other proteins to fold incorrectly,
resulting in improper functioning proteins.
Prions cause mad cow disease.
 Viroids are composed of a single circular
strand of RNA with no protein coat.

Viroids have been shown to cause infectious
diseases in several plants.
Plant virus
 The first virus to be identified was a plant
virus, called tobacco mosaic virus, that
causes disease in tobacco plants.
 Plant viruses enter the plant through wounds
or insect bites.
Tobacco mosaic virus
causes yellow spots on
tobacco leaves, making
them unmarketable.
Neutral plant viruses
 Some mosaic viruses cause striking patterns
of color in the flowers of plants.
Rembrandt
tulips
Where do viruses come from?
 Some scientists suggest that viruses are
nucleic acids that break free from their host
cells while maintaining an ability to replicate
parasitically within the host cells.
Archaebacteria and
Eubacteria
Bacteria characteristics
 Unicellular
 Prokaryotic: lacks nucleus and organelles
 Cell wall
 Diverse metabolism
 Archaebacteria and Eubacteria may have
shared a common ancestor billions of years
ago.
Three main types of Archaebacteria
 Methanogens: live on anaerobic environment,
produces methane gas
 Habitat: marshes, lake sediments, the
digestive tracts of some mammals, such as
cows, sewage disposal
Three main types of Archaebacteria
 Halophiles: lives only in water with high
concentrations of salt
Three main types of Archaebacteria
 Thermophiles: live in
the hot, acidic
waters of sulfur
springs and near
cracks deep in the
ocean floor, where it
is the autotrophic
producer for a
unique animal
community’s food
chain.
Eubacteria characteristics
 Small
 Unicellular
 Ribosomes for protein synthesis
 Single circular DNA chromosome instead of
linear chromosome
 May have plasmids (small circular DNA
outside of the main chromosome)
Eubacteria anatomy
Ribosome
Cytoplasm
Chromosome
Flagellum
Gelatin-like
capsule
Cell
Wall
Cell
Membrane
E. coli – What are the parts of this
cell?
Eubacteria characteristics
 Lives in more hospitable environments than
archaebacteria
 Diverse nutritional needs



Heterotroph: consume organic matter
Saprobe: digests dead or decaying matter
Autotroph: produces own organic matter
through photosynthesis
Photosynthetic bacteria
 Most cyanobacteria are blue-green and some
are red or yellow in color.
 Made up of chains of independent cells.
 Habitat: rivers and streams
Cell wall function
 A bacterial cell remains intact as long as its
cell wall is intact.
 If the cell wall is damaged, water will enter
the cell by osmosis, causing the cell to burst.
 Penicillin can block cell wall production,
which can cause bacteria to be destroyed.
Alexander Fleming, discoverer of
penicillin in 1928
Gram stain
 a technique that determines the differences in
the composition of bacterial cell walls (thick or
thin)
Gram-positive bacteria
Gram-negative bacteria
Bacterial shapes
 Bacterial cell walls
also give bacteria
different shapes
 Coccus: spheres
 Bacillus: rods
 Spirillum: spirals
How bacteria grow
 Diplo: a paired arrangement of cell growth
 Staphylo: an arrangement of cells that
resemble grapes
 Strepto: an arrangement of chains of cells
 Bacteria reproduce asexually by a process
known as binary fission.
Binary fission
 A bacterium first copies
its chromosome.
 The original
chromosome and the
copy become attached
to the cell’s plasma
membrane for a while.
• The cell grows larger, and eventually the two
chromosomes separate and move to opposite ends of
the cell.
Binary fission
 Then, a partition forms between the
chromosomes. This partition separates the
cell into two similar cells.
Binary fission
• Because each new cell has either the
original or the copy of the chromosome,
the resulting cells are genetically identical.
Conjugation
 one bacterium transfers all or part of its
chromosome to another cell through or on a
structure called a pilus that connects the two
cells
 Conjugation results in a bacterium with a new
genetic composition.
 This is the closest form of bacterial sexual
recombination.
Metabolic diversity
 Requires oxygen (obligate aerobe)
 Requires oxygen-free environment (obligate
anaerobe)
 Can live with or without oxygen, able to
perform cellular respiration or fermentation
Endospores
 Structure that contains a bacterium’s DNA
and a small amount of its cytoplasm
 Encased by a tough outer covering
 Resistant to drought and extreme conditions
 May survive thousands of years
 Bacillus anthracis, the bacteria that causes
anthrax is able to form spores in harsh
conditions and become active under
favorable conditions. Spores can spread
through inhalation, digestion, or contact.
How to kill endospores
 Sterilization: heating under high pressure in
either a pressure cooker or an autoclave
 Canned foods must be sterilized and acidified
to prevent spoiling.
Botulism
 C. botulinum can thrive in canned foods
where they produce toxins.
 When eaten, a person develops severe food
poisoning.
 Sterilization can kill off C. botulinum.
Helpful bacteria
 More bacteria are helpful than cause
diseases.
 Ex: nitrogen fixing bacteria
Nitrogen fixation
• Several species of bacteria have enzymes that
convert N2 into ammonia (NH3) in a process
known as nitrogen fixation.
• Other bacteria then convert the ammonia into
nitrite (NO2–) and nitrate (NO3–),which plants
can use.
• Bacteria are the only organisms that can
perform these chemical changes.
Nitrogen fixation
• Some nitrogenfixing bacteria live
symbiotically within
the roots of some
trees and legumes.
• Farmers grow legume crops after the
harvesting of crops such as corn,
which depletes the soil of nitrogen.
Food and medicine
• Some foods that you eat—mellow Swiss
cheese, crispy pickles, tangy yogurt—
would not exist without bacteria.
Harmful bacteria
 Disease-causing bacteria can enter bodies
through openings, such as the mouth.
 How bacteria harm host bodies


Bacterial growth can interfere with the normal
function of body tissue.
Bacteria can release a toxin that directly
attacks the host.
Diseases caused by bacteria
Diseases Caused by Bacteria
Disease
Transmission Symptoms
Fever, sore throat,
Inhale or
Strep throat
ingest through swollen neck glands
(Streptococcus)
mouth
Inhale
Fatigue, fever, night
Tuberculosis
sweats, cough, weight
loss, chest pain
Puncture
Stiff jaw, muscle
Tetanus
wound
spasms, paralysis
Rash at site of bite,
Lyme disease
Bite of
infected tick chills, body aches,
joint swelling
Bacteria
Destruction of tooth
Dental
cavities (caries) in mouth
enamel, toothache
Sore throat, fever,
Inhale or
Diptheria
close contact heart or breathing
failure
Treatment
Antibiotic
Antibiotic
Open and clean wound,
antibiotic; give antitoxin
Antibiotic
Remove and fill the
destroyed area of tooth
Vaccination to
prevent, antibiotics