•Halobacterium
- Extremely salt-tolerant
- Compensating water loss
•Why are prokaryotes the most
abundant on Earth?
•Description of prokaryotes
•Shapes of prokaryotes
• cell wall
- behavior in hypertonic
aaa environment
- salt used to preserve aaaa
food
- composition:
aaaaapeptidoglycan
• gram stain
- procedure
- analysis of results
Chapter 27 Bacteria & Archaea
Overview: Masters of Adaptation
• Archae genus in which the archaea have red membrane pigments, some of which
capture light energy that drives ATP synthesis
• one of the most salt-tolerant species on Earth; thrive in salinities that dehydrate &
kill other cells
• compensates for water lost through osmosis by pumping potassium ions into the
cell until the ionic concentration inside matches outside
• prokaryotes are able to adapt to a broad range of habitats
27.1 Structural & Functional Adaptations Contribute to Prokaryotic Success
• most prokaryotes are unicellular, have diameters of 0.5 – 5 µm, have a variety of
shapes, & achieve all of an organism’s life function within a single cell
1. cocci are spherical
o pairs = diplococci ; chains of many cells = streptococci ; clusters =
staphylococci
2. bacilli are rod-shaped
o chains = streptobacilli
3. spiral include spirilla & spirochetes
Cell-Surface Structures
• a key feature of prokaryotes is the cell wall, which maintains cell shape, protects
the cell, and prevents it from bursting in a hypotonic environment
• in a hypertonic environment, most prokaryotes lose water & plasmolyze (shrink
away from their wall).
o So, salt can be used to preserve foods because it causes prokaryotes to lose
water, preventing them from multiplying rapidly
• Cell walls contain peptidoglycan, a polymer composed of modified sugars crosslinked by short poplypeptides. This molecular fabric encloses the entire bacterium
& anchors other molecules that extend from its surface
• Archael cell walls contain a variety of polysaccharides & proteins but lack
peptidoglycan
• Gram stain- technique used to classify many bacterial speciies into 2 groups based
on differences in cell wall composition
• Stain samples with crystal violet dye & iodine, then rinse in alcohol, & stain with
red dye (i.e. safranin)
• The structure of a bacterium’s cell wall determines the staining response
o Gram-positive bacteria have simpler walls with large amount of
peptidoglycan, which traps the crystal violet in the cytoplasm. The alcohol
rinse doesn’t remove the crystal violet, which masks the red safranin dye
o Gram-negative bacteria have less peptidoglycan & are structurally more
complex, with an outer membrane that contains lipopolysaccharides; their
thinner layer of peptidoglycan is located in a layer between the plasma &
outer membrane. The crystal violet’s easily rinsed from the cytoplasm, &
the cell appears red from the dye
- medical applications
- treatment implications
- antibiotics
• capsule
- function
• fimbriae
• pilli
• taxis
• flagella
• convergent evolution
Gram staining is valuable in medicine for quickly determining if a patient’s infection is
due to gram-negative or to gram-positive bacteria.
• The lipid portions of the lipopolysaccharides in the walls of gram-negative bacteria are
toxic, causing fever or shock. Their outer membrane helps protect it from the body’s
defenses. They’re more resistant than gram-positive to antibiotics because the outer
membrane impedes the entry of drugs
• But, there are some gram-positive species that have virulent strains that are resistant to
1(+) antibiotics
o Ex: multi-drug resistant staphylococcus aureus
• The effectiveness of antibiotics (i.e. penicillin) derives from their inhibition of
peptidoglycan cross-linking. The resulting cell wall will not be functional, especially in
gram-positive bacteria.
o Such drugs destroy many species of pathogenic bacteria without adversely
human cells, which don’t have peptidoglycan
• Capsule- sticky layer of well-defined & dense polysaccharide or protein that surrounds
cell wall of many prokaryotes
o If less well organized, it is called a slime layer
• They enable prokaryotes to adhere to their substrate or to their individuals in a colony,
may protect against dehydration, & may shield pathogenic prokaryotes from attacks by
their host’s immune system
• Fimbriae- hairlinke appendages that some prokaryotes have that help them stick to their
substrate or another
o Ex: bacteria that causes gonorrhea uses fimbriae to fasten itself to the
mucuous membranes of its hosts
• Pili- appendages that put two cells together prior to DNA transfer from one cell to
another; “sex pilli”
Mobility
• ~50% of all prokaryotes are capable of taxis, a directed movement toward or away
from a stimulus
o Ex: prokaryotes that exhibit chemotaxis change their movement pattern in response
to chemicals They move toward nutrients or oxygen (positive chemiostaxis) or away
from a toxic substance (negative chemostasis)
• Prokaryotic flagella differ from eukaryotic.
o They’re 1/10 the width & aren’t covered by extension of the plasma membrane.
o molecular composition
o Mechanism of propulsion: among bacterial & archael flagella, they’re similar in size
& rotation mechanism, but are composed of different proteins
• The structural & molecular comparisons suggest that flagella of bacteria, archaea, &
eukaryotes arose independently. They all perform similar functions but are not related
by common descent, so they are analogous, not homologous structures
Evolutionary Origins of Bacterial Flagella
• Bacterial flagellum has 3 main parts: motor, hook, & filament that are composed of 42
different kinds of proteins
• There’s a lot of evidence that indicates that bacterial flagella originated as simpler
structures that were stepwisely modified over time
•
Half of the proteins are necessary for its function; the others are inessential or
not coded in the genomes of some species
• Ex: a set of 10 proteins in the motor are homologous to 10 similar proteins in a
secretory system found in bacteria
o Secretory system is a protein complex that enables a cell to secrete
certain macromolecules
• Ex: 2 other proteins in the motor are homologous to proteins that function in
ion transport
• The proteins that comprise the rod, hook, & filament are all related to each
other & are descended from an ancestral protein. This suggests that the
bacterial flagellum evolved as other proteins were added to an ancestral
secretory system
• The process in which existing structures take on new functions through descent
with modification
Internal Organization & DNA
• Prokaryotes’ cells are simpler in their internal structure & physical arrangement
of their DNA and lack complex compartmentalization
• Some prokaryote cells have specialized membranes, the infoldings of the
plasma membrane, that perform metabolic functions
• It is structurally different & has less DNA. The genome consists of a circular
chromosome with many fewer proteins
• They lack a membrane-bounded nucleus; their chromosome is located in the
nucleoid, a region of cytoplasm. May have plasmids, smaller rings of
independently replicating DNA molecules that carry a few genes
• Fundamentally similar in both eukaryotes & prokaryotes, but there are
differences
o Ex: prokaryotic ribosomes are smaller & differ in their protein & RNA
content
§ These differences allow certain antibiotics (i.e. erythromycin &
tetracycline) to bind to ribosomes & block protein synthesis in
prokaryotes but not in eukaryotes, so people can use these to
kill bacteria growth without harming themselves
Reproduction & Adaptation
•
• exaptation
• Prokaryotes’ cells are much
simpler than eukaryotes’
• specialized membranes
• different genome
• transcription & translation