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Chapter
19
Bacteria and Viruses
19-1 Bacteria
• The invention of the microscope
opened our eyes to the hidden,
living world around us
• Microscopic life covers nearly
every square centimeter of Earth
Prokaryotes
• The smallest and most common
microorganisms
• Unicellular organisms that lack a
nucleusand membrane bound
organelles
• bacteria
Classifying Prokaryotes
• Until recently, all prokaryotes
were placed in a single kingdom monera
• More recently, biologists have
begun to appreciate that
prokaryotes can be divided into
two very different kingdoms: the
eubacteriaand the archaebacteria
Eubacteria
• Eubacteria include a wide range
of organisms with different
lifestyles
• Eubacteria live almost
everywhere
Archaebacteria
• Lack the same carbohydrates of
eubacteria and also have different
membrane lipids
• Also, the DNA sequences of key
archaebacterial genes are more like
those of eukaryotes than those of
eubacteria
• Many archaebacteria live in extremely
harsh environments
Identifying Prokaryotes
• Prokaryotes are identified by
characteristics such as shape, the
chemical nature of their cell walls,
the way they move and the way
they obtain energy
Shapes
• Bacilli – rod shaped
• Cocci – spherical shaped
• Spirilla – spiral and corkscrew
shaped
Cell Walls
• Eubacteria – have peptigoglycan
• Archaebacteia – don’t have
peptigoglycan
Movement
• Some bacteria move differently
than others
Metabolic Diversity
• No characteristic of prokaryotes
illustrates their diversity better
than the way they obtain energy
Heterotrophs
• Must take inorganic molecules for
both energy and a supply of
carbon
Photoheterotrophs
• These organisms are
photosynthetic using sunlight for
energy but they also need to take
in organic compounds as a
carbon source
Photoautotrophs
• Use light energy to convert
carbon dioxide and water to
carbon compounds and oxygen in
a process similar to that used by
green plants
Ex.) cyanobacteria (blue-green
algae)
Chemoautotrophs
• Can perform chemosynthesis
• Make organic carbon molecules
from carbon dioxide
• Unlike photoautotrophs, however
they do not require light as a
source of energy. Instead they
use energy directly from chemical
reactions
Releasing Energy
• Like all organisms, bacteria need
a constant supply of energy
• This energy is released by the
process of cellular respiration or
fermentation or both
Obligate aerobes
• Require a constant supply of
oxygen in order to live
Obligate anaerobes
• Do not require oxygen and if fact
may be killed by it
Facultative anaerobes
• Can survive with or without
oxygen
Growth and Reproduction
• Bacteria can grow really fast
• If unlimited space and food were
available to a single bacterium and if
all of its offspring divided every 20
minutes in just 48 hours they would
reach a mass of approximately 4000
times the mass of the earth!
Binary Fission
• When a bacterium has grown so
that it has nearly doubled in size,
it replicates its DNA and divides in
half producing 2 identical
daughter cells
Binary Fission
• Bacterial
reproduction,
asexual
reproduction
Conjugation
• Many bacteria are also able to
exchange genetic information by
a process called conjugation
• This transfer of genetic
information increases genetic
diversity
Spore Formation
• When growth conditions become
unfavorable, many bacteria form
structures called spores
Endospore
• A type of spore formed when a
bacterium produces a thick
internal wall that encloses a thick
internal wall that encloses its DNA
and a portion of its cytoplasm
Endospore
Importance of Bacteria
• Bacteria are vital to maintaining the
living world
• Some are producers that capture
energy by photosynthesis
• Others are decomposers that break
down the nutrients in dead matter and
the atmosphere
• Still other bacteria have human uses
Decomposers
• As decomposers, bacteria help
the ecosystem recycle nutrients,
therefore maintaining equilibrium
in the environment
Nitrogen Fixers
• You may recall that plants need nitrogen to
make amino acids, the building blocks of
protiens
• Nitrogen gas (N2) makes up approximately 78
percent of Earth’s atmosphere
• However, plants can’t use nitrogen gas directly
• Nitrogen must first be changed chemically to
ammonia (NH3) or other nitrogen compounds
Nitrogen fixation
• Process which turns unusable
nitrogen gas into useful nitrogen
containing compounds
• Allows nitrogen atoms to continually
cycle through the biosphere
• Many plants have symbiotic
relationships with nitrogen fixing
bacteria
Human Uses of Bacteria
• Used in food and beverage
production
• Industries: petroleum, water,
mining, drugs
• Inside of us (symbiosis)
–E.coli
• Drug research
19 – 2 Viruses
Viruses
• Particles of nucleic acid, protein, and
sometimes lipids
• Viruses can reproduce only by
infecting living cells
• A typical virus is composed of a core
of DNA or RNA surrounded by a
protein coat
• Viruses are very small. They can only
be seen with an electron microscope
Capsid
• A viruses protein coat
• The capsid proteins of a typical virus bind to
receptors on the surface of a cell and “trick” the
cell into allowing it inside
• Once inside, the viral genes are expressed and
causes the host cell to make copies of the virus
and in the process the host cell is destroyed
• Because viruses must bind precisely to proteins
on the cell surface and then use a hosts genetic
system, most viruses are highly specific to the
cells they infect
Viral Infection
• Once the virus is inside the host
cell, two different processes may
occur
Lytic Infection
• In a lytic infection, a virus enters a
cell, makes copies of itself, and
causes the cell to burst
A literary approach to lytic
virus infections
In its own way, a lytic virus is similar to a
desperado in the Old West. First, the outlaw
eliminates the town’s existing authority (host cell
DNA). Then, the desperado demands to be
outfitted with new weapons, horses, and riding
equipment by terrorizing the local people (using
the host cell to make proteins). Finally, the
desperado forms a gang that leaves the town to
attack new communities (the host cell bursts,
releasing hundreds of virus particles).
Lysogenic Infection
• In a lysogenic infection, a virus integrates its
DNA into the DNA of the host cell, and the viral
genetic information replicates along with the
host cells DNA
• Unlike lytic viruses, lysogenic viruses do not lyse
the host cell right away. Instead, a lysogenic
virus remains inactive for a period of time
• Eventually, only one of a number of factors may
activate the DNA of a prophage which will then
remove itself from the host cell DNA and direct
the synthesis of new viruses particles
Retroviruses
• Viruses that contain RNA as their
genetic information
• When retroviruses infect a cell,
they produce a DNA copy of their
RNA
• Ex.) Human Immunodeficiency
Virus (HIV) and Feline leukemia
virus
Viruses and Living Cells
• Viruses must infect a living cell in
order to grow and reproduce
• They also take advantage of the
host’s respiration, nutrition and all
the other functions that occur in
living things
• Therefore, viruses are considered
to be parasites
Parasites
• organisms that live on or in a host
organism from which it obtains
nutrients, and it usually does
harm to the host
Are viruses alive?
Cells and Viruses
Characteristic
Cell
Structure
Cell membrane, cytoplasm;
eukaryotes also contain
nucleus and organelles
Reproduction
Independent cell division either
asexually or sexually
Genetic Code
DNA
Growth and Development
Yes; in multicellular organisms,
cells increase in number and
differentiate
Obtain and Use Energy
yes
Response to Environment
yes
Change Over Time
yes
Virus
19 – 3 Diseases Caused
by Bacteria and Viruses
• Bacteria and viruses are
everywhere in nature, but only a
few cause disease
Pathogens
• Disease causing agents
• All viruses reproduce by infecting living cells,
and disease results when the infection
causes harm to the host
• All bacteria require nutrients and energy:
however, disease results when bacteria
interfere with the host’s ability to obtain
enough of those elements to function
properly
Bacterial Disease in Humans
• Bacteria produce disease in one of
two general ways
1. Some bacteria damage the cells and
tissues of the host by breaking down
the cells for food
2. Other bacteria release toxins that
travel throughout the body
interfering with the normal activity of
the host
Preventing Bacterial Disease
• Many bacterial disease can be
prevented by stimulating the
bodies immune system with
vaccines
Vaccine
• A preparation of weakened or killed
pathogen
• When injected into the body, a
vaccine sometimes prompts the body
to produce immunity to the disease
• If a bacterial infection does occur, a
number of drugs can be used to
attack and destroy the invading
bacteria
Antibiotics
• Compounds that block the growth
and reproduction of bacteria NOT
virus
Controlling Bacteria
• There are various methods used
to control bacterial growth,
including sterilization,
disinfectants, and food processing
Sterilization by Heat
• Many bacteria cannot survive
high temperatures for a long time,
so most can be killed by exposure
to high heat
• Ex.) pasteurization
Disinfectants
• Chemical solutions that kill
pathogenic bacteria (chlorine)
Food Storage and Processing
• Food that is stored at a low
temperature will stay fresh longer
because bacteria cannot reproduce
fast at cold temperatures
• Also, a lot of the processing
procedures that are used in the food
industry raise the temperature of food
to a point where the bacteria are
killed
Viral Disease in Humans
• Like bacteria, viruses produce
disease by disrupting the body’s
normal equilibrium
• Unlike bacterial diseases, viruses
can’t be treated with antibiotics
• The best way to protect against
most viral diseases lies in
prevention by the use of vaccines
Viral Disease in Animals
• Viruses produce serious animal diseases
Ex.) Foot-and-mouth disease, Rous
sarcoma
Viral Disease in Plants
• Many viruses infect plants
• Ex.) Tobacco mosaic virus, potato
yellow dwarf virus
Viroids and Prions
• Scientists have discovered two
virus-like particles that also cause
disease
Viroids
• Single stranded RNA molecules
that have no surrounding capsid
• Cause disease in plants
Prions
• Proteins that cause disease in animals
Ex.) Mad cow disease