Lab 17 on Thursday
Prokaryotes and Viruses
Chapter 25
Learning Objectives
• Describe prions and the diseases they cause
• Describe general characteristics of Prokaryotes
• Compare and contrast bacteria, archeae and
eukarya
• Compare and contrast gram positive and gram
negative bacteria
• Describe gram + and gram- bacteria
• Describe the three main bacterial shapes
Why It Matters
• Prokaryotes: The smallest organisms
Prions
• Infectious proteins with no associated
nucleic acids
• Misfolded versions of normal cellular
proteins that can induce other normal
proteins to misfold
Prion Diseases
Degenerate nervous system in mammals
– Scrapie: Brain disease in sheep
– Mad cow disease (Bovine spongiform
encephalopathy): Spongy holes and protein
deposits in brain tissue
– Creutzfeldt-Jakob disease: Rapid mental
deterioration, loss of vision and speech,
paralysis
– Kuru-cannibalistic tribe in New Guinea,
Brain Tissue Damaged by BSE
25.1 Prokaryotic Structure and
Function
• Prokaryotes are simple in structure
compared with eukaryotic cells
• Prokaryotes have the greatest metabolic
diversity of all living organisms
• Prokaryotes differ in whether oxygen can
be used in their metabolism
25.1 (cont.)
• Prokaryotes fix and metabolize nitrogen
• Prokaryotes reproduce asexually or, rarely,
by a form of sexual reproduction
• In nature, bacteria may live in communities
attached to a surface
Prokaryotes
• Colonize a great diversity of habitats
• Are small but complex cells
• Have great metabolic diversity
• Adapt rapidly to their environments
Three Common Shapes in
Prokaryotes
• Spherical: cocci
• Rodlike: bacilli
• Spiral: vibrios (curved) and spirilla (helix)
Prokaryotic Genomes
• Prokaryotic chromosome
– Single, circular DNA molecule
– Packaged into nucleoid
• No nucleolus
• No nuclear membranes
• Plasmids
– Small circles of DNA
– Genes supplement nucleoid genes
– Replicate independently (along with main
DNA)
Plasmid
Pili
Cytoplasm
Folded DNA
containing
molecule
(in the nucleoid) ribosomes
Flagellum
Capsule
Plasma
Peptidomembrane glycan layer
Outer
membrane
Cell wall
Fig. 25-3, p. 528
Prokaryotic Ribosomes
• Bacterial ribosomes
– Smaller than eukaryotic ribosomes
– Protein synthesis similar to eukaryotes
• Archaeal ribosomes
– Size similar to bacteria
– Different structure
– Protein synthesis is combination of bacterial
and eukaryotic processes
Prokaryotic Cell Wall
• Protects plasma membrane
– Helps withstand osmotic pressure
– Prevents action of detergent-like chemicals
• Made of peptidoglycans
– Polysaccharide polymers connected by short
polypeptides
Gram Stain
• Gram stain technique
– Stain with crystal violet, then with iodine
• Fixes dye to cell wall
– Wash with alcohol
– Stain again with fuchsin or safranin
• Gram-positive bacteria
– Appear purple because crystal violet retained
• Gram-negative bacteria
– Appear pink because crystal violet lost
Gram-Positive Bacteria
• Single, relatively thick peptidoglycan layer
Capsule
may be
present
a. Gram-positive bacterial cell wall
Peptidoglycan
layer
Plasma membrane
Cell
wall
Cytoplasm
Fig. 25-4, p. 529
Gram-Negative Bacteria
• Two-layered walls; relatively thin
peptidoglycan sheath surrounded by outer
membrane
b. Gram-negative bacterial cell wall
Capsule
Outer membrane
Peptidoglycan layer
Plasma membrane
Cell
wall
Cytoplasm
Fig. 25-4, p. 529
Slime Coat
• Capsule
– Slime attached to cells
• Slime layer
– Loosely associated with cells
• Protects bacteria from desiccation, antibiotics, viruses,
antibodies, and enzymes
•
Helps bacteria adhere to surfaces
Pili
• Rigid protein
shafts extend
from cell walls
• Mostly in Gramnegative bacteria
• Help bacteria
attach to each
other or to
surfaces
Obtaining Carbon and Energy
(1)
• Autotrophs (auto = self; troph =
nourishment)
– Use carbon dioxide as their carbon source
• Heterotrophs
– Obtain carbon from organic molecules
Obtaining Carbon and Energy
(2)
• Chemoautotrophs
– Obtain energy by oxidizing inorganic or
organic substances
• Phototrophs
– Use light as energy source
Organic molecules
Carbon source
CO2
Energy source
Oxidation of molecules
Light
CHEMOAUTOTROPH
PHOTOAUTOTROPH
Found in some bacteria Found in some photoand archaeans; not
synthetic bacteria, in
found in eukaryotes
some protists, and
in plants
CHEMOHETEROTROPH PHOTOHETEROTROPH
Include some bacteria
and archaeans, and
also in protists, fungi,
animals, and plants
Found in some
photosynthetic
bacteria
* Inorganic molecules for chemoautotrophs
and organic molecules for chemoheterotrophs.
Fig. 25-8, p. 531
Prokaryotes and Oxygen:
Aerobes
• Aerobes
– Require oxygen for cellular respiration
– Oxygen is the final electron acceptor
• Obligate aerobes
– Cannot grow without oxygen
Prokaryotes and Oxygen:
Anaerobes
• Anaerobes
– Do not require oxygen to live
• Obligate anaerobes (poisoned by oxygen)
– Use fermentation or type of respiration in which
inorganic molecules (NO3– or SO42–) are final electron
acceptors
• Facultative anaerobes
– Use O2 when present
– Use fermentation under anaerobic conditions
Prokaryotes and Nitrogen (1)
• Nitrogen fixation
– Conversion of atmospheric nitrogen (N3) to ammonia
(NH3)
– Ammonia ionized to ammonium (NH4+) for
biosynthesis
• Nitrogen-fixing bacteria include
– Some cyanobacteria
– Free-living Azotobacter
– Bacteria such as Rhizobium that are symbiotic with
plants
Prokaryotes and Nitrogen (2)
• Nitrification
– Conversion of ammonium (NH4+) to nitrate
(NO3–)
– Two-step conversion by nitrifying bacteria
• Some types of bacteria convert ammonia to nitrite
(NO2–)
• Other types convert nitrite to nitrate
Prokaryote Reproduction
• Binary fission
– Asexual reproduction
– Produces exact copies of parent
• Conjugation
– Two cells connected by pilus
– Part of DNA of one cell is transferred to
another cell (usually plasmids)
Endospore
• Develops inside some bacteria when
environmental conditions are unfavorable
• Metabolically inactive
• Highly resistant to heat, desiccation, attack
by enzymes or chemicals
Endospore: Clostridium tetani
25.2 Domain Bacteria
• Molecular studies reveal more than a
dozen evolutionary branches in the
Bacteria
• Bacteria cause diseases by several
mechanisms
• Pathogenic bacteria commonly develop
resistance to antibiotics
Classification of Prokaryotes
Bacteria
• 12 separate evolutionary branches
• Six most important groups:
– Proteobacteria
– Green bacteria
– Cyanobacteria
– Gram-positive
– Spirochetes
– Chlamydias
The Proteobacteria (1)
•
Gram-negative bacteria
– Purple sulfur (photoautotrophic)
– Purple nonsulfur (photoheterotrophic)
– Purple photosynthetic pigment
• Free-living proteobacteria (chemoheterotrophs)
– Some cause human diseases
• Bubonic plague, Legionnaire’s disease, gonorrhea,
gastroenteritis, dysentery
– Some plant pathogens
• Rot, scabs, wilts
The Cyanobacteria
• Gram-negative
photoautotrophs
• Blue-green color
• Photosynthesis
similar to plants
• Release oxygen as
byproduct of
photosynthesis
b.
c.
Heterocyst
Resting spore
Fig. 25-13, p. 535
The Gram-Positive Bacteria (1)
• Primarily chemoheterotrophs
• Many pathogenic species
– Anthrax
– Staphylococcus
• Food poisoning, skin
infections, toxic shock
syndrome, pneumonia,
meningitis
– Streptococcus
• Strep throat,
pneumonia,
scarlet fever,
kidney infections
The Gram-Positive Bacteria (2)
• Some beneficial species
– Lactobacillus
• Lactic acid fermentation used to produce pickles,
sauerkraut, yogurt
• Mycoplasmas
– Naked cells that have lost their cell walls
– Smallest known cells (0.1 to 0.2 µm in
diameter)
The Spirochetes
• Gram-negative spiral-shaped bacteria
– Propelled by rotation of flagella
– Enables movement in thick mud and sewage
• Beneficial or harmless species
– Spirochetes in termite intestine digest plant fiber
– Treponema in mouth
• Pathogenic species
– Syphilis, relapsing fever, Lyme disease
The Chlamydias
• Gram-negative bacteria
– Cell walls with membrane outside
– Lack peptidoglycans
• Intercellular
parasites that
cause diseases
in animals
Bacterial Disease Mechanisms
•
Exotoxins
– Toxic proteins leaked or secreted
• Clostridium botulinum (botulism exotoxin)
•
Endotoxins
– Toxins only released when bacteria die or lyse
• E. coli, Salmonella, Shigella
•
Exoenzymes
– Enzymes secreted that digest plasma membrane
• Streptococcus, Staphylococcus, Clostridium
Resistance to Antibiotics
• Pathogenic bacteria may develop resistance to
antibiotics
– Mutation of their own genes
– Acquiring resistance genes from other bacteria
• Resistant strains difficult to treat with conventional
antibiotics
• Resistance is a form of evolutionary adaptation
25.3 Domain Archaea
• Archaea have some unique characteristics
• Molecular studies reveal three
evolutionary branches in the Archaea
The Archaea
• Archaea are more closely related to
domain Eukarya than domain Bacteria
• Characteristics
– Some features like bacteria
– Some features like eukaryotes
– Some unique features
Characteristics of Bacteria,
Archaea,
and Eukarya