Lecture 7 Prokaryotes (Bacteria) Flashcards
Key Concepts
➢ Structural and functional adaptations contribute to prokaryotic success
➢ Genetic diversity:Rapid reproduction, mutation, and genetic recombination
➢ Diverse nutritional and metabolic adaptations
➢ Prokaryotes have both beneficial and harmful impacts on humans
What characteristics enable prokaryotes to reach huge population sizes and thrive in diverse environments
slide 3
THE PROKARYOTIC CELL
slide 4
A. STRUCTURE
Essential structure
- cell wall
- cell membrane
- cytoplasm
- nuclear material
Particilar structures
-capsule
-flagella
-pili
-spore
slide 5
Morphology of Prokaryotic Cells: Shapes
● Two types most common
○ Coccus: spherical
○ Rod: cylindrical
● Variety of other shapes
○ Vibrio, spirillum, spirochete
○ Pleomorphic (many shapes)
○ Great diversity often found in low
nutrient environments
slide 6
Prokaryotic Cell Reproduction
- Binary Fission (figure 12.12)
- Origin of replication
- High rate of replication
- Short generation time (20mn)
slide 7
Groupings - bacteria divide by binary fission
○ Cells often stick together following division
○ Form characteristic groupings
○ Examples:
■ Neisseria gonorrhoeae (diplococcus)
■ Streptococcus (long chains)
■ Sarcina (cubical packets)
■ Staphylococcus - grapelike clusters)
slide 8
External Organisation
▪ Cell Wall
▪ Cell Membrane
▪ Capsule
▪ Flagella
▪ Pilus
1.The Cell Membrane - boundary of the cell
○ Phospholipid bilayer embedded with proteins
■ Hydrophobic tails face in; hydrophilic tails face out
○ Serves as semipermeable membrane
○ Proteins serve numerous functions
■Selective gates
■Sensors of environmental conditions
slide 10
- Cell Wall - protection
Cell wall is strong, rigid structure that prevents cell lysis
○ Architecture distinguishes two main types of bacteria
■ Gram-positive
■ Gram-negative
○ Made from peptidoglycan (only in bacteria)
slide 11
The Gram stain
Hans Christian Joachim Gram (1853–1938)
● Danish physician working at morgue in Berlin
● Worked for Dr. Carl Friedlander
○ Attempting to identify cause of pneumonia
● Gram was developing methods to stain bacteria
● With one method, bacteria stained unequally
○ Some retained dye, others did not
○ Revealed two different kinds of bacteria
● Basis for modern Gram stain
○ Identifies two major groups of bacteria according to cell wall
structure and chemistry
○ Gram-positive and Gram-negative
The peptidoglycan
The strength of the bacterial cell walls is due to a layer of peptidoglycan, a material found only in bacteria
slide 13
The Gram-Positive and Gram negative
slide 14
Treatment implications
● Lipid portions of the lipopolysaccharides in the walls of many gram- are toxic,
causing fever or shock.
● Outer membrane of a gram- helps protect it from the body’s defenses.
● Gram- more resistant to antibiotics - outer membrane impedes the entry of
some drugs.
slide 15
Antibacterial Substances That Target Peptidoglycan
● Peptidoglycan makes good target since unique to bacteria
○ Can weaken to point where unable to prevent cell lysis
● Penicillin interferes with peptidoglycan synthesis
○ Prevents cross-linking of adjacent glycan chains
○ Usually more effective against Gram-positive bacteria than
Gram-negative bacteria
■ Outer membrane of Gram-negatives blocks access
■ Derivatives have been developed that can cross
● Lysozyme breaks bonds linking glycan chain
○ Enzyme found in tears, saliva, other bodily fluids
○ Destroys structural integrity of peptidoglycan molecule
Bacteria That Lack a Cell Wall
● Mycoplasma species have extremely variable shape
● Penicillin, lysozyme do not affect
● Cytoplasmic membrane contains sterols that increase
strength
- Capsules - adhere to surfaces
Gel-like layer outside cell wall that protects or allows attachment to
surface
* Capsule: distinct, gelatinous
* Slime layer: diffuse, irregular
* Most composed of glycocalyx (sugar shell) although some are polypeptides
* Allow bacteria to adhere to surfaces
* Once attached, cells can grow as biofilm (Polysaccharide encased community)
* Example: dental plaque
* Some capsules allow bacteria to evade host immune system
- Capsules - adhere to surfaces
-Capsule > condensed layers; closely surrounds the bacterium
-slime layer > loosely adherent: nonuniform in density and thickness
-Capsule/slime layer: also called glycoalyx
- Capsules - function to adhere to surfaces
● Prevents phagocytosis of bacteria
● Attached bacteria to surface
slide 20
- Flagella - involved in motility
- Spin like propellers to move cell
- Some important in disease
- Numbers and arrangements help with characterization
- Peritrichous - distributed over entire surface
- Polar flagellum: single flagellum at one end of cell
- Pili - involved in conjugation
● Pili (sing. pilus) are shorter than flagella
● Types that allow surface attachment termed fimbriae
● Twitching motility, gliding motility involve pili
● Sex pilus used to join bacteria for DNA transfer
Internal Organisation
▪ Genetic Material
▪ Ribosome
▪ Cytoplasm
▪ Endospore
- Genetic Material
▪ Chromosome forms gel-like region: the nucleoid
* Single circular double-stranded DNA
* Packed tightly via binding proteins and supercoiling
▪ Plasmids are circular, supercoiled, dsDNA
* Usually much smaller; few to several hundred genes
* May share with other bacteria; antibiotic resistance can
spread this way
Specialized membranes
slide 26
Bacterial chromosome
- Circular chromosome
- One double-stranded (ds) DNA molecules
– Vibrio cholera has 2 chromosomes
– Rhizobium melilotus 3 chromosomes - Size ~million base pairs (bp) in length (E.coli ~ 4.6M bp)
– Human genome ~6 billion bp - 90% DNA codes for proteins
– Human DNA~ only 1% codes for protein - Essential genes
Plasmids and transposons
- Some bacteria possess a plasmid
- Small circular DNA molecules
- One or 2 copies /cell (some have many copies)
- Non- essential genes
- Small size ~ thousands bp
- Function:
– gene transfer,
– antibiotic resistance ,
– killing other bacteria - Transposons –
- moving DNA sequences
Theta Replication
A plasmid replicates
independently of its
bacterial
chromosome:
Replication begins at the origin of replication (ori) and continues around the circle. In this
diagram, replication is taking place in both directions; in some plasmids, replication is in
one direction only.
slide 29
Rolling Circle Replication
slide 30
- Ribosomes - Involved in protein synthesis
- Facilitate joining of amino acids
- Relative size expressed as S (Svedberg)
- Reflects density: how fast they settle when centrifuged
- Prokaryotic ribosomes are 70S
- Made from 30S and 50S
- Eukaryotic ribosomes are 80S
- Important medically: antibiotics
impacting 70S ribosome do not
affect 80S ribosome
slide 31
- Cytoplasm
Cytoskeleton: internal protein framework
* Once thought missing in bacteria
* Bacterial proteins similar to eukaryotic cytoskeleton have
been characterized
* Likely involved in cell division and controlling cell shape
Storage granules: accumulations of polymers
* Synthesized from nutrients available in excess
* Carbon, energy storage:
– Glycogen
Gas vesicles: controlled to provide buoyancy
Higher concentration of salts than eukaryotes
- Endospores - Unique type of dormant cell
- Produced by members of Bacillus, Clostridium
- May remain dormant for 100 years or longer
- Extremely resistant to heat, desiccation, chemicals, ultraviolet
light, boiling water - Endospores that survive can germinate to become vegetative cell
- Found virtually everywhere
- Endospores - Example of Infections
● Anthrax – Bacillus anthracis
● Tetanus – Clostridium tetani
● Botulism – C. botulinum
● Gas gangrene – C. perfringens
Sporulation vs Germination
▪ Sporulation triggered by carbon, nitrogen limitation
* Starvation conditions begin 8-hour process
* Endospore layers prevent damage
* Exclude molecules (e.g., lysozyme)
* Cortex maintains core in dehydrated state, protects
from heat
* Core has small proteins that bind and protect DNA
* Calcium dipicolinate - protective role
▪ Germination triggered by
heat, chemical exposure
- Endospores - Medical implications
slide 36
B. Genetic Change in Bacteria
Organisms adapt to changing environments
● Natural selection favors those with greater fitness
● Bacteria adjust to new circumstances
o Regulation of gene expression
o Genetic change
● Change in organism’s DNA alters genotype
o Sequence of nucleotides in DNA
o Bacteria are haploid, so only one copy, no backup
● May change observable characteristics, or phenotype
o Also influenced by environmental conditions
Genetic change in bacteria occurs by two mechanisms:
1. Mutations
2. Horizontal Gene Transfer
- Mutation
Changes the existing nucleotide sequence
* mutation passed on to the progeny (daughter cells)
through vertical gene transfer.
* modified organism and daughter cells = mutants
slide 39
- Mutation - Antibiotic Resistance
slide 40
- Mutation - MEGA-plate experiment
slide 41
- Genetic Recombination
● Horizontal gene transfer
● Movement of DNA from one organism to another.
● changes are passed on to the progeny by vertical transfer.
slide 42
Importance
Microorganisms commonly acquire genes from other cells,
the process of horizontal gene transfer
Movement of DNA from one cell (the donor) to another (the
recipient) - rapid spread of antibiotic resistance, such as
for Staphylococcus aureus
3 main mechanisms
* Conjugation: direct transfer of DNA from one
bacterium to another
* Transformation: bacterium takes up free DNA
* Transduction: bacterial viruses take DNA from one
bacterium to another
Lederberg and Tatum’s
experiment demonstrated
that bacteria undergo
genetic exchange
slide 44
Davis’s U-tube
experiment demonstrated
that bacterial genetic
exchange requires
direct contact
● U-shaped tube that was divided into two
compartments by a filter with fine pores.
● Filter allowed a liquid medium to pass
from one side of the tube to the other,
but the pores of the filter were too small
to allow the passage of bacteria.
● Two auxotrophic strains of bacteria
were placed on opposite sides of the
filter, and suction was applied alternately
to the ends of the U-tube, causing the
medium to flow back and forth between
the two compartments.
slide 45
Conjugation
- Direct transfer via connection tube is one-way traffic from
donor cells to recipient cells. - It is not a reciprocal exchange of genetic information.
3 Requirements - F+
cells: donor cells containing F factor - F–
cells: recipient cells lacking F factor - Sex pilus: connection tube
- Conjugative plasmids direct their own transfer - Replicons
- F plasmid (fertility) of E. coli – only for transfer
- Other plasmids encode resistance to some antibiotics
- Spread resistance easily
F plasmid
● F plasmid of E. coli - F+
cells have, F–
do not
● Encodes proteins required for conjugation including:
○ Sex pilus (F pilus)
○ Brings cells into contact
○ Enzyme cuts plasmid
○ Single strand transferred
○ Complementary strands synthesized
○ Both cells are now F+
slide 47
R plasmids
Natural gene transfer and antibiotic resistance
● R plasmids – contain antibiotic resistance genes
● genes located on R plasmids that can be transferred naturally to other bacteri● R plasmids have evolved in the past 60 years since the beginning of
widespread use of antibiotics.
● The transfer of R plasmids is not restricted to bacteria of the same or even
related species.
slide 48
The F factor is transferred during conjugation between F+ and F− cells
slide 49
How do you get chromosomal genes transferred?
Hfr cells (high-frequency strains): donor cells with F factor integrated into the donor bacterial chromosome
The F factor is integrated into the bacterial chromosome in an Hfr cell.
At a frequency of only
about 1 in 10,000.
(low frequency)
Bacterial genes may be transferred from an Hfr cell to an F− cell in conjugation.
slide 51
Characteristics of E.Coli cells with different types of F factor
slide 52-53
Transformation
● A bacterium takes up DNA from the medium.
● Recombination takes place between introduced genes
and the bacterial chromosome.
● Competent cells: cells that take up DNA
● Transformants: cells that receive genetic material
Slide 54
Transformation can be used to map bacterial genes
slide 55
Transduction
slide 56
AB Resistance:
Mutation and horizontal gene transfer
- Rapid reproduction enables bacterial cells
carrying resistance genes to quickly
produce large numbers of resistant offspring - Resistance genes spread rapidly within
and among bacterial species by horizontal
gene transfer
Diverse nutritional and metabolic adaptations
Prokaryotes can be categorized by how they obtain
energy and carbon:
● Phototrophs obtain energy from light
● Chemotrophs obtain energy from chemicals
● Autotrophs require CO2
or related compounds as a
carbon source
● Heterotrophs require an organic nutrient to make other
organic compounds
Diverse nutritional and metabolic adaptations
Energy and carbon sources are combined to give 4
major modes of nutrition:
● Photoautotroph
● Chemoautotroph
● Photoheterotroph
● Chemoheterotroph
slide 59
The need for oxygen
Prokaryotic metabolism varies with respect to O2
● Obligate aerobes require O2
for cellular respiration
● Obligate anaerobes are poisoned by O2
and live by
fermentation or use substances other than O2
for
anaerobic respiration
● Facultative anaerobes can use O2
if it is present or
carry out fermentation or anaerobic respiration if not
Impacts on humans and animals
● Negative impact - Pathogens (half of human diseases)
● Positive impact
○ Mutualistic Bacteria - intestines (digestion, absorption)
○ Food production (Cheese,yogurt, Beer and wine, sauerkraut)
○ Research and technology (Cloning, recombinant DNA)
○ Medicine (CRISPR-Cas9 system, insulin production)
○ Production - produce natural plastics PHA
○ Bioremediation - remove pollutants from soil, air, or water
