The Bacterial Cell - Lecture 1 Flashcards
(95 cards)
1
Q
What are the main structural differences between bacteria and eukaryotes?
A
- Cell envelope
- Ribosomes
- Genetics - chromosomes + plasmids
- Metabolism and protein secretion
- Pathogenicity
2
Q
How is the cell wall structured in bacteria?
A
- Outer membrane
- Peptidoglycan cell wall
- Periplasm
- Inner membrane
3
Q
Why are antibiotics not harmful to humans?
A
When new antibiotics are found, it is because the toxicity is targeting something in the bacteria that the host (e.g. humans) don’t have.
4
Q
How does the cell envelope differ in Gram-negative and Gram-positive bacteria?
A
Gram-negative peptidoglycan membrane is much thinner than it is in Gram-positive
5
Q
What are the components of the inner plasma membrane?
A
- Phospholipid bilayer (hydrophobic tails, hydrophilic heads)
- Membrane proteins
6
Q
What are the main functions of the inner plasma membrane?
A
- Permeability barrier
- Transport of solutes
- Energy generation
- Location of enzyme systems
7
Q
How can the inner membrane of bacteria relate to eukaryotes?
A
All membrane-associated functions of eukaryotic organelles are performed in the bacterial inner membrane
8
Q
What are the key features of the periplasmic space?
A
- Aqueous component
- Densely packed with proteins
9
Q
What are the functions of the periplasmic space?
A
- Potentially harmful enzymes are sequestered away from the cytoplasm here (such as RNAse, proteases etc.)
- Transportation of proteins that are involved in functions such as secretion/uptake
10
Q
What is RNAse?
A
- RNAse is an enzyme that ingests RNA
- It is dangerous for bacteria as they need RNA for transcription and translation of the DNA
11
Q
What are they key components of the bacterial cell wall?
A
- Peptidoglycan (murein)
- Techoic acids in Gram-positive bacteria
12
Q
What are the functions of the bacterial cell wall?
A
- Rigid exoskeleton
- Prevents osmotic lysis
- Confers shape
13
Q
What are the functions of a eukaryotic cell wall?
A
- Acts as a protective armour to prevent damage and penetration of cells by bacterial infections and viruses
- Prevents osmotic lysis - ensures that the cell cannot swell and burst or shrink and die
14
Q
How thick is the cell wall in Gram-negative and Gram positive bacteria?
A
Gram-negative = few nm so quite thin
Gram-positive = 30-100nm so quite thick
15
Q
What anchors surface proteins in Gram-negatives?
A
The surface proteins are attached from cell walls on Gram-positive bacteria and attach to the outer membrane of the Gram-negative bacteria
16
Q
What does the Gram-negative outer membrane consist of?
A
- Lipid bilayer NOT a phospholipid bilayer
- Glycolipids, mainly lipopolysaccharide
17
Q
What are the functions of Gram-negative outer membranes?
A
- Anchor for adhesin proteins to allow interaction with host cells
- Effective but selective permeability barrier
- Prevents entry of many molecules
18
Q
What does the outer membrane prevent the entry of?
A
- Bile salts
- Antibiotics
- Lysozymes (it attacks peptidoglycan)
19
Q
What is an adhesive protein
A
A protein that allows it to stick to a host
20
Q
Why is it harder to find antibiotics for Gram-negative bacteria?
A
The outer membrane prevents against antibiotics from entering
21
Q
What are the two parts of a lipopolysaccharide (LPS)?
A
- Highly variable chain of sugars - the O antigen is outside the cell
- Lipid A - embedded in the outer membrane
22
Q
What would lipopolysaccharides look like on agar?
A
- The O+ colonies would be smooth and hydrophilic
- The O- colonies would be rough and hydrophobic
23
Q
How are lipopolysaccharides involved in immunology?
A
- They play a role in immune evasion
- They are highly stimulatory to our innate immune system
- Lipid A = pathogen-associated molecular pattern (PAMP)
24
Q
Why is it beneficial for the LPS to have varying chains of sugars?
A
The highly variable outer antigen allows the bacteria to evade the hosts immune system
25
What happens to the LPS if the outer membrane breaks open?
- Lipid A is released
- The immune system goes into overdrive
26
Give an example of an endotoxin
LPS (lipid A)
27
When are endotoxins released?
When a bacterial cell breaks (lyses); they activate the immune system and the hosts inflammatory responses
28
How are endotoxins released?
- Via outer membrane vesicles (AKA blebbing)
- Cell lysis or disintegration
29
What can endotoxins potentially activate?
1. Macrophages
2. Cytokines
3. Inflammatory response
30
What is a porin?
Transmembrane proteins forming water filled pores across the outer membrane of Gram-negative bacteria
31
What do porins do?
They are large channels which allow passive diffusion of small hydrophilic molecules such as sugars and amino acids across the membrane
32
What do most porins form?
General, non-specific channel
HOWEVER some bacteria form more specific channels
33
Why is it good that the PG cell wall is not found in human cells?
1. Antibacterial drugs are designed to attack this target
2. It is a target for attack by lysozymes
34
Why is penicillin not toxic to humans?
The target for penicillin is missing from our cell biology, thus leading to its extremely low toxicity
35
What is peptidoglycan made from?
Peptide and a carbohydrate
36
What is the carbohydrate in peptidoglycan made from?
N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM)
37
What is a common sequence of a bacterial pentapeptide?
1. L-Ala
2. D-Glu
3. A2pm (or L-Lys)
4. D-Ala
5. D-Ala
1 attaches to the carbohydrate
5 is lost during cross-linking
38
How is peptidoglycan formed?
1. Assembled in the cytoplasm
2. NAG formed from fructose-6-phosphate
3. Mur enzymes (MurA to MurF) catalyse the formation of NAM from NAG and adds 5 amino acids onto NAM
4. NAM-5AA is coupled to lipid carrier bactoprenol (lipid I) in the inner membrane
5. NAG coupled to lipid I by MurG to make lipid II
6. It is then flipped across the IM into periplasmic space by flippase enzymes
7. In the periplasm, NAM and NAG subunits joined and cross-linked to form peptidoglycan matrix
39
What are flippases?
They flip the peptidoglycan to the other side of the inner membrane to the periplasm
40
What is a transglycosylase?
They join carbohydrates together
41
What are the construction steps of peptidoglycan?
1. Chains of repeating NAM-NAG subunits are assembled by transglycosylase enzymes
2. Peptide sidechains are cross-linked by transpeptidase enzymes
3. Cross-linking leads to a mesh like layer, so therefore a strong peptidoglycan molecule
42
What do transglycosylases do?
They join NAM-NAG subunits via glycosidic bonds
43
What do transpeptidases do?
They cross-link pentapeptide sidechains
44
What are transpeptidases also known as?
Penicillin Binding Proteins (PBPs)
45
Which molecules does cross-linking occur between?
D-Ala at position 4 and AA at position 3
- can either occur directly or via a short peptide bridge
46
What are the two main classes of penicillin binding proteins?
- High molecular mass (HMM)
- Low molecular mass (LMM)
47
What can High molecular mass PBPs do that low molecular mass PBPs cant?
Transglycosylation
48
What do a lot of Gram-positive cell walls contain?
Techoic acid
49
What is lipotechoic acid?
- Linked to lipids
- Membrane bound
50
What is a capsule?
The polysaccharide layer outside of the cell wall
51
What does the capsule do?
1. Immune evasion - helps bacteria evade macrophage digestion
2. Adhesion - helps bacteria stick to the host
52
Do all bacteria have a capsule?
No, however pathogens often do.
53
How big are ribosomes in bacteria?
70S
54
How big are ribosomes in eukaryotes?
80S
55
Where is the nucleus located in bacteria?
It floats in the cytoplasm - it is not membrane bound
56
What does the bacterial ribonucleoprotein consist of?
- A 50S subunit (active site) with 5S rRNA + 23S rRNA + 31 proteins
= A 30S subunit (decoding) with 16S rRNA (1540 nucleotides) + 21 proteins
57
What does the 30S ribosome subunit do?
It ensures fidelity of decoding by establishing accurate codon-anticodon interactions
58
What does the 50S subunit do?
It catalyses peptide bond formation and elongation of the nascent protein
59
What are bacterial ribosomes sensitive to?
1. Chloramphenicol
2. Tetracycline
3. Gentamicin
4. Erythromycin
5. Streptomycin
60
What do the antibiotics interact with?
Specific regions of rRNA or the ribosomal protein
61
What are some examples of antibiotics interacting with ribosomal proteins?
1. Tetracycline - binds to A-site in the 30S subunit sterically hindering fit of amino-acyl-tRNA
2. Erythromycin - binds to entrance of ribosomal exit tunnel, blocking the path of the nascent peptide chain
62
What are some important mobile genetic elements?
- Plasmids
- Transposons e.g. jumping genes
- Bacteriophage
63
What is a jumping gene?
A gene that can hop from chromosomes to plasmids
64
What is the medical significance of mobile genetic elements of bacteria?
- We can make antibiotic susceptible bacteria resistant
- We can make harmless/commensal bacteria into pathogens
65
What are some common facts about Bacillus cereus?
- Common environmental bacterium
- Forms spores
- Causes mild diarrhoea
- Plasmids pXO1 and pXO2 are absent
66
What are some common facts about Bacillus anthracis?
- Causes anthrax
- Deadly pathogen
- Plasmids pXO1 and pXO2 encode anthrax virulence factors - the toxin and capsule
67
What are some common characteristics of plasmids?
- Smaller than chromosomes
- Circular and supercoiled
- Have an extra-chromosomal accessory genetic element
- Found in all 3 domains of life
68
What are the core components of a plasmid?
- DNA sire - origin of replication
- Replicon - a replication protein
69
What is mpf?
Mating pair formation protein
70
What is PenR?
An antibiotic resistance gene
71
What does a mating pair formation consist of?
- Conjugative pilus
- Cell-envelope spanning channel
72
What does DNA processing and transfer (Tra) consist of?
1. Relaxase enzymes cut one strand at origin of transfer oriT
2. The cut strand is unwound and transferred as the new strand is synthesised
3. ssDNA copies of plasmid DNA is in the recipient cell
4. Complementary strand is synthesised to create DNA
73
What is bacterial conjugation?
The passing or sharing of a plasmid
74
Define mobilisation in terms of bacteria
If there are two plasmids in a cell and one is conjugative, the other plasmids can move with the conjugative one
75
What is a host range?
Plasmids cant move to and replicate in ALL strains, they only work with certain groups of bacteria
76
What defines narrow host range plasmids?
Can only be maintained in a single bacterial host or a closely related group
77
What defines a broad host range plasmid?
They can replicate in multiple unrelated bacteria
78
What is an example of a broad host range plasmid?
RK2
- it can stably replicate in almost all gram-negative bacteria
- it transfers by conjugation to almost any bacterium, even in eukaryotes
79
What is more problematic for humans: narrow host ranges or broad host ranges?
Broad host ranges because it allows for more bacteria to spread antibiotic resistance
80
What are three types of bacterial plasmids?
1. Synthetic
2. Resistance
3. Virulence
81
Who were jumping genes discovered by?
Barbara McClintock
82
What are some common facts about transposons?
- Common in bacterial genomes
- They are defined segments of DNA capable of moving from one site to another
- Can cause mutation and recombination
- Can carry antibiotic resistance genes and can move from chromosomes to plasmids.
83
Define "independent of homology" in terms of transposons
Can move itself randomly, they don't have to be in pairs
84
What are the two types of movement from a jumping gene?
1. From one site to another on a chromosome
2. From a chromosomes to a plasmid, or a plasmid to a chromosome
85
What are some features of transposition?
- It is an enzyme catalysed process - transposases are involved
- It is often a replicative process, resulting in duplication of the transposon sequence
- Can also be non-replicative
86
What are the two types of metabolism that bacteria have?
1. Catabolic metabolism - breaks down things for food
2. Anabolic metabolism - can make their own food
87
Why during metabolism do bacteria secrete enzymes?
To suit their environment. However, these enzymes can be dangerous for human hosts if they start destroying internal environments
88
What does MacConkey Agar contain?
- Neutral red (pH indicator)
- Bile salts and crystal violet (inhibits growth of most gram positive)
- Sugar (e.g. lactose of sorbitol)
89
What do proteases do for the bacteria?
- Degrade tissue barriers to pathogen invasion
- Proteolytic digestion of host tissue components
- May target specific host molecules involved
90
What is secreted during protein secretion?
- Surface appendages
- Nutrient scavengers
- Toxins
- Host manipulation factors
- Antibacterials
- Biofilm formation factors
91
What are the three types of exportation across the inner membrane?
1. Sec (general secretion) pathway
2. Signal Recognition particle (SRP) pathway
3. Twin-Arginine Transport (Tat) pathway
92
What are two differences between Sec and Tat pathways?
1. Tat can export a whole folded protein, Sec cant do that as its too narrow
2. Sec unravels the protein before transportation
93
What are some key characteristics of signal peptides?
- Targeting mechanism for specific export/secretion
- Specific interactions occur with secretion machinery or chaperone
94
What are the Sec precursor proteins?
- N(amino)-terminal signal sequence: 10-30aa
- Cleavage 9removed) after secretion
95
What happens to proteins after they cross the Gram-positive inner membrane?
- They are released into the extracellular environment
- Or they are incorporated into the cell wall by peptide-anchoring mechanisms
