Chapter 19: Bacteria
and Viruses
Viruses
• Means “poison” in Latin
• Nonliving
• Composed of nucleic acids enclosed in a
protein coat
• Smaller than bacteria (can vary in structure
and size)
• Can only replicate
– Make copies of itself
– Need a host/living cell to reproduce
• No cell respiration, growth, response to stimuli
– Not considered a living cell
1935: W. Stanley
• 1st to identify a virus
• Tobacco Mosaic Virus
Structure of a Virus
• Inner core of nucleic
acid (DNA or RNA)
• Outer protein coat
called a capsid
– Gives virus its shape
• Some viruses have a
membrane (envelope)
over the capsid
– Can have spikes,
hooks, etc.
Bacteriophage
• Special group of viruses
• Infect bacteria
• Inject their nucleic acid into bacteria like a
syringe
Viruses are classifies by the
type of host they infect
• Plant viruses
• Animal viruses
• Bacterial viruses
Viruses are often species-specific
• Some are cell-type
specific
–Ex: cold
viruses affect
respiratory cells
–Ex: Polio virus 
affects nerve
cells
2 Types of Reproduction
• Lytic cycle (short): takes about 30
minutes to produce 200 new
viruses
– Virus enters cell  reproduces
 exits cell
• Kills host cell: causes host
cell to break open
– Steps: ex: cold virus
• attachment
• entry
• replication
• assembly
• lysis and release
Lysogenic Cycle (long)
• Virus has an inactive
(dormant) period
• Steps: ex: herpes, HIV, and
chickenpox
– attachment and entry
– provirus formation
(prophage)
• viral DNA integrated
into host’s
chromosomes
– normal cell activity
– activation of the lytic cycle
Retrovirus
• RNA virus with the
enzyme reverse
transcriptase
• Makes viral DNA
from host cell RNA
• Ex: HIV
Fighting Viruses
• Natural defenses
– Skin, mucus membranes,
white blood cells, antibody
production, interferon
• Vaccines
– Preparation of weakened or
killed virus/viral proteins
– Some produce life-long
specific antibodies
– Antibiotics do not kill viruses
How Viruses are Spread
• Air (measles, cold)
• Food and water (hepatitis)
• Animal/insect bites (rabies,
encephalitis)
• Insects (flies can “carry” viruses)
• Sexual contact (HIV, Herpes)
• 1st line of defense  skin and
membranes
Chapter 19 Review #1
1. A typical virus has a core composed of ___.
a. capsid proteins
b. surface proteins
c. membrane envelopes
d. DNA or RNA
2. The outer layer of a virus is composed of
____.
a. RNA
b. viral genes
c. DNA
d. proteins
3. A(an) _____ is a preparation of weakened or
killed pathogens that can prompt the body
to produce immunity to a disease.
Bacteria
• Prokaryotic:
–Unicellular
organisms
–Have no nucleus
–No membranebound organelles
Divided into 2 Kingdoms
Archaebacteria
• Live in extreme habitats (usually
no oxygen present)
• Cell walls lack peptidoglycan
(starch)
• 3 types:
– Methanogens
• Produce methane gas
• Live in marshes, lake
sediments, and cow digestive
tracts
• Found at sewage disposal
plants to help break down
sewage
Extreme Halophiles
• Lives only in places
where there is a high
salt concentration
• Found in Utah’s Great
Salt Lakes and the
Dead Sea
Thermoacidophiles
• Lives in hot, acidic waters of sulfur
springs
• Found in volcanic vents
Eubacteria
• Live almost everywhere
• Cell walls made of peptidoglycan
• Divided up based on how they get
their food
– Autotrophs
• Make their own food
• 2 types:
– Photosynthetic
» Need sunlight
» Ex: cyanobacteria
(blue-green algae)
– Chemosynthetic
» Use sulfur and
nitrogen to make food
Eubacteria
• Heterotrophs
– Use others/consumes to get
food
– 2 ways:
• Parasitism, mutualism,
commensalism
– Gets food from other
living things
• Saprophytes
– Feed on dead
organisms/wastes
– Decomposers
Structure of Bacteria
• Very small in size
• All have:
Some have:
• DNA
Pili
• Cell membrane Capsule
• Cell wall
Flagella
• Ribosomes
– Genes are in a big circular chromosome (not
paired chromosomes)
• Cell wall keeps bacteria
from bursting
– Most bacteria live in a
hypotonic environment
– 1928: Alexander
Fleming
• Accidentally
discovered Penicillin
–1st antibiotic
–Destroys bacteria
by interfering with
its ability to make a
cell wall
Identifying Bacteria
• Gram Staining
– Based on differences in a
bacterium’s cell wall
– Gram positive: will stain a
blue/purple color
– Gram negative: will stain a
pink color
Shapes of Bacteria
• Bacteria have 3
shapes:
– Cocci: sphereshaped
– Bacilli: rod-shaped
– Spirilla: spiralshaped
Bacteria can grow in patterns
• Diplo-: arranged in pairs
• Staphylo-: cells arranged
like grapes
• Strepto-: arrangement of
chains of cells
Asexual Reproduction
• Binary Fission
• Exponential growth:
12481632
…etc
• Very fast (every 20
minutes)
• Cells produced are
genetically identical
Sexual Reproduction
• Called conjugation
• DNA is exchanged
through structures
called pili that
connect cells
together
Metabolism
• Breaking down food to
release energy
• 3 groups of bacteria based
on way they metabolize:
–Obligate aerobes:
require oxygen
• Ex: Mycobacterium
tuberculosis
• Facultative anaerobes: can live with or
without oxygen
– Ex: E. coli
• Obligate anaerobes: die in the presence
of oxygen
– Ex: Clostridium, Treponema
Importance of Bacteria
• Help fertilize fields
• Recycles nutrients: breakdown dead
materials/waste (called saprophytes)
• Produce foods and medicines/antibiotics
• Nitrogen fixation: makes a usable form of
nitrogen
Harmful Bacteria:
called pathogens
• Can cause disease in plants and
animals
–Interferes with normal body
function
–Affects homeostasis
–Can release a toxin that attacks
the host (like a poison)
Survival Adaptations
• Produce endospores
– Contain bacterial DNA and cytoplasm
– Covered with a tough, outer case that
resists temperature extremes, drying
out, and harsh chemicals
– Dormant: resting state (can be as
long as 1,000 years)
• Does not reproduce during this
time
• Can germinate when environmental
conditions improve
– Must be exposed to high heat under
pressure to be killed
• Called sterilization
Chapter 19 Review #2
1. A ______ is a disease-causing agent.
2. List 4 ways to identify prokaryotes.
3. One way to control bacterial growth is by
subjecting them to great heat, a process
called _____.
4. Bacteria can also cause disease by
releasing ______ that harm the body.