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Lecture 7 Prokaryotes (Bacteria) Flashcards

1
Q

Key Concepts

A

➢ 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

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2
Q

What characteristics enable prokaryotes to reach huge population sizes and thrive in diverse environments

A

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3
Q

THE PROKARYOTIC CELL

A

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4
Q

A. STRUCTURE

A

Essential structure
- cell wall
- cell membrane
- cytoplasm
- nuclear material
Particilar structures
-capsule
-flagella
-pili
-spore

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5
Q

Morphology of Prokaryotic Cells: Shapes

A

● 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

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6
Q

Prokaryotic Cell Reproduction

A
  • Binary Fission (figure 12.12)
  • Origin of replication
  • High rate of replication
  • Short generation time (20mn)

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7
Q

Groupings - bacteria divide by binary fission

A

○ Cells often stick together following division
○ Form characteristic groupings
○ Examples:
■ Neisseria gonorrhoeae (diplococcus)
■ Streptococcus (long chains)
■ Sarcina (cubical packets)
■ Staphylococcus - grapelike clusters)

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8
Q

External Organisation

A

▪ Cell Wall
▪ Cell Membrane
▪ Capsule
▪ Flagella
▪ Pilus

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9
Q

1.The Cell Membrane - boundary of the cell

A

○ 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

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10
Q
  1. Cell Wall - protection
A

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)

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11
Q

The Gram stain

A

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

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12
Q

The peptidoglycan

A

The strength of the bacterial cell walls is due to a layer of peptidoglycan, a material found only in bacteria

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13
Q

The Gram-Positive and Gram negative

A

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14
Q

Treatment implications

A

● 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.

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15
Q

Antibacterial Substances That Target Peptidoglycan

A

● 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

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16
Q

Bacteria That Lack a Cell Wall

A

● Mycoplasma species have extremely variable shape
● Penicillin, lysozyme do not affect
● Cytoplasmic membrane contains sterols that increase
strength

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17
Q
  1. Capsules - adhere to surfaces
A

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

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18
Q
  1. Capsules - adhere to surfaces
A

-Capsule > condensed layers; closely surrounds the bacterium
-slime layer > loosely adherent: nonuniform in density and thickness
-Capsule/slime layer: also called glycoalyx

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19
Q
  1. Capsules - function to adhere to surfaces
A

● Prevents phagocytosis of bacteria
● Attached bacteria to surface

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20
Q
  1. Flagella - involved in motility
A
  • 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
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21
Q
  1. Pili - involved in conjugation
A

● 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

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22
Q

Internal Organisation

A

▪ Genetic Material
▪ Ribosome
▪ Cytoplasm
▪ Endospore

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23
Q
  1. Genetic Material
A

▪ 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

24
Q

Specialized membranes

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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
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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
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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
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Rolling Circle Replication
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2. 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
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3. 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
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4. 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
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4. Endospores - Example of Infections
● Anthrax – Bacillus anthracis ● Tetanus – Clostridium tetani ● Botulism – C. botulinum ● Gas gangrene – C. perfringens
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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
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4. Endospores - Medical implications
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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
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1. 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
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1. Mutation - Antibiotic Resistance
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1. Mutation - MEGA-plate experiment
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2. 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
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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
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Lederberg and Tatum’s experiment demonstrated that bacteria undergo genetic exchange
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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
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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
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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
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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
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The F factor is transferred during conjugation between F+ and F− cells
slide 49
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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)
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Bacterial genes may be transferred from an Hfr cell to an F− cell in conjugation.
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Characteristics of E.Coli cells with different types of F factor
slide 52-53
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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
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Transformation can be used to map bacterial genes
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Transduction
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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
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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
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Diverse nutritional and metabolic adaptations
Energy and carbon sources are combined to give 4 major modes of nutrition: ● Photoautotroph ● Chemoautotroph ● Photoheterotroph ● Chemoheterotroph slide 59
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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
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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

POB 2 (25 decks)

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