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18.4 Bacteria and Archaea
KEY CONCEPT
Bacteria and archaea are both single-celled
prokaryotes.
18.4 Bacteria and Archaea
Prokaryotes are widespread on Earth.
• 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
Evaluate: Bacteria are often associated with illness.
Why is this a misconception?
18.4 Bacteria and Archaea
Bacteria and archaea are structurally similar but have
different molecular characteristics.
• Bacteria commonly come in three forms.
– rod-shaped, called bacilli
– spiral, called spirilla or spirochetes
– spherical, called cocci
Spirochaeta: spiral
Lactobacilli: rod-shaped
Enterococci: spherical
• Archaea have many shapes.
Spirochaeta: spiral
18.4 Bacteria and Archaea
• Bacteria and archaea have similar structures.
– plasmid
– flagellum
pili
plasma
– pili
membrance
chromosome
cell wall
plasmid
This diagram shows the typical structure
of a prokaryote. Archaea and bacteria
look very similar, although they have
important molecular differences.
flagellum
18.4 Bacteria and Archaea
• Bacteria and archaea have molecular differences.
– The amount of peptidoglycan within the cell wall can
differ between bacteria
GRAM NEGATIVE
GRAM POSITIVE
– Archaea have different lipids entirely
18.4 Bacteria and Archaea
• Gram staining identifies bacteria.
– stains polymer peptidoglycan
– gram-positive stains purple, more peptidoglycan
– 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.
18.4 Bacteria and Archaea
Bacteria have various strategies for survival.
• Prokaryotes exchange
genes during
conjugation. In
conjugation genetic
material transfers
between prokaryotes,
producing genetic
variation.
conjugation bridge
• Bacteria may survive by
forming endospores.
Endospores are a
specialized cell with a
TEM; magnification 6000x
thick, protective cell wall.
Endospores can last for
centuries.
Connect: Why are disinfectants alone not
enough to kill all types of bacteria?
18.4 Bacteria and Archaea
Prokaryotes provide nutrients to humans and other
animals.
• Prokaryotes live in digestive systems of animals.
– make vitamins
– break down food
– fill niches
Figure: These bacteria,
found in human
intestines, are beneficial
to our health. They
produce B vitamins and
keep out harmful
microbes. (colored SEM;
magnification 6300X)
18.4 Bacteria and Archaea
• Bacteria help ferment many foods.
– yogurt, cheese
– pickles, sauerkraut
– soy sauce, vinegar
18.4 Bacteria and Archaea
Prokaryotes play important roles in ecosystems.
• Prokaryotes have many functions in ecosystems.
– photosynthesize
– recycle carbon, nitrogen,
hydrogen, sulfur
– fix nitrogen
Figure: Root nodules of this white clover
contain nitrogen-fixing bacteria. Symbiotic
bacteria convert nitrogen from the
atmosphere (N2) into a form usable by the
clover. In return, the plant produces
carbohydrates that the bacteria can
consume. This SEM (90X) shows root
nodules, containing nitrogen-fixing
bacteria, of a white clover.
18.4 Bacteria and Archaea
• Bioremediation uses prokaryotes to break down
pollutants.
– oil spills
– biodegradable materials
Figure: Oil spill clean-up, men,
cleaning. Sometimes workers will
spray bacteria over the contaminated
are so the bacteria can digest the
chemicals, converting them into
harmless or less-harmless
compounds.
18.4 Bacteria and Archaea
Some bacteria cause disease.
• Bacteria cause disease by invading tissues or making
toxins.
• A toxin is a poison released by an organism.
Figure: Clostridium
botulinum causes a
serious illness called
botulism. Food
contamination by this
bacterium often
comes from improper
home canning.
18.4 Bacteria and Archaea
• Normally harmless bacteria can become destructive.
– may colonize new tissues
Figure:
Streptococcus
bacteria are
commonly found
on skin. They are
fairly harmless
unless they
come in contact
with tissues they
do not normally
colonize, such as
muscle or fat.
This can occur
through open
wounds.
18.4 Bacteria and Archaea
• Normally harmless bacteria can become destructive.
– immune system may be lowered
Apply: Potato salad left out at a picnic is sometimes a
source of food poisoning. Which bacterium mentioned
above is the most likely culprit? Explain.
18.4 Bacteria and Archaea
Antibiotics are used to fight bacterial disease.
• Antibiotics may stop bacterial cell wall formation.
• Antibiotics do not work on viruses.
• Prevention is best method to fight bacterial disease.
18.4 Bacteria and Archaea
Bacteria can evolve resistance to antibiotics.
• Bacteria are gaining resistance to antibiotics.
– overuse
A bacterium carries
– underuse
genes for antibiotic
resistance on a plasmid.
– misuse
• Antibiotics must be
A copy of the plasmid is
transferred through
used properly.
conjugation.
Infer: Why don’t antibiotics
affect our bodies’ cells?
Connect: How can you use
“superbugs” as an example
of natural selection?
Resistance is quickly
spread through many
bacteria.
18.4 Bacteria and Archaea
Archaea are extremophiles
• Archaea exist or “love” extreme conditions
– Extremely hot (thermophile
– High salt (halophile)
– High acidity (acidophile)
pH: Some bacteria can
live at a pH of 0
High temp: The highest
temperature archaea
have been known to live
at is 230 degrees
Fahrenheit (110 Celsius)