The Bacterial Cell

Question 1

What are the differences and similarities between Gram negative and Gram positive bacteria?

Answer 1

G+ has no outer membrane and a thick cell wall
G- has an outer membrane and a thin cell wall
They both contain an inner membrane

Question 2

Inner (plasma) Membrane components and function

Answer 2

Contains phospholipid bi-layer and membrane proteins
Acts as a permeability barrier and transports solutes
It’s where the enzyme systems are for energy generation e.g. ATP

Question 3

What are the major bacterial lipids?

Answer 3

Phosphatidyglycerol (PG), diphosphatidyglyceroL (cardiolipin, CL) and phosphatidylethanolamine (PE)

Question 4

Phospholipid structure

Answer 4

Hydrophilic head and hydrophobic tail
Contains a phosphate group

Question 5

Periplasmic space

Answer 5

Aqueous compartment between cell membrane and cell wall/cell membrane and outer membrane in G- and G+ bacteria. It isolates potentially harmful enzymes away from the cytoplasm.
Contains many proteins and transports them.

Question 6

Cell wall components and function

Answer 6

It keeps the shape of the cell and prevents osmotic lysis.
Contains peptidoglycan (murein)
Has a rigid exoskeleton
Anchors surface proteins in Gram positive bacteria

Question 7

What are techoic acids?

Answer 7

Cell surface glycopolymers found in Gram positive cell walls.
They provide flexibility and rigidity to the bacterial cell wall and controls ion movement in and out the cell. They contribute to the bacterial cell surface overall negative charge.

Question 8

Cell wall thickness

Answer 8

Gram positive - thick (30-100nm) layer
Gram negative - Thin (few nm) layer

Question 9

Gram-negative Outer Membrane

Answer 9

Has a lipid bilayer NOT a phospholipid bi-layer. It has phospholipids but in the inner layer.
Outer layer of outer membrane made of glycolipids with Lipopolysaccharide (LPS) as the main one.
Acts as an anchor for adhesin proteins and a selective permeability barrier.

Question 10

How does the Gram negative Outer Membrane protect the cell?

Answer 10

It prevents/slows the entry of Bile salts, Antibiotics and Lysozymes (attack the PG)

Question 11

What is the Lipopolysaccharide (LPS)?

Answer 11

Major component of outer leaflet of Gram-negative bacteria.
Made of 3 parts.
Contributes to structural integrity of the bacteria cell.

Question 12

Where is LPS found?

Answer 12

Only in bacteria.

Question 13

What is Lipid A?

Answer 13

Lipid A - allows LPS to imbed itself into the outer leaflet
Lipid A is also referred to as endotoxin and can cause fever and shock (e.g. septic shock)

Question 14

What is the Core oligosaccharide?

Answer 14

Short chain of sugar molecules that links lipid A to the O- antigen in outer membrane of G- bacteria.
Maintains structural integrity of lipopolysaccharide.
Interacts with host cells and modulates immune responses (pathogenicity)

Question 15

What is the O antigen?

Answer 15

Hydrophilic carbohydrate chains, component of LPS, highly variable in structure among bacteria species.
E.g. O157:H7 causes severe infections (eating contaminated beef)
O antigens are toxic, account for some of virulence of certain G- bacteria.

Question 16

What are Porins?

Answer 16

Transmembrane proteins forming water-filled pores across G- cell outer membrane.
Allow passive diffusion of small hydrophilic molecules.

Question 17

Specific Porins

Answer 17

In some bacteria there are more specific channels. e.g. Pseudomonas aeruginosa.

Specificity is determined by physical/chemical properties of the channel e.g. lumen diameter, amino acids lining.

Some porins restrict flow of certain antibiotics to keep the bacterial cell alive (intrinsic antibiotic resistance porins).

Question 18

Peptidoglycan Cell Wall

Answer 18

Type of cell wall in most bacteria
Protects cell from osmotic lysis, gives it shape
PG not found in human cells
Lysozymes attack the PG cell wall
PG aka murein

Question 19

Peptidoglycan structure

Answer 19

Peptido+glycan = Protein+Carb
The carbohydrates are called
N-acetylglucosamine (NAG) and N-acetyl muramic acid (NAM).

The protein is a tetrapeptide sidechain which is crosslinked.

Question 20

What is the Peptidoglycan Sacculus?

Answer 20

Net-like layer made of glycan chains connected by short peptides.
Has a molecular mass of > 3 billion Da

Question 21

What is the difference between a protein and peptide?

Answer 21

Peptides can have from 2 to 50 amino acids, proteins made up of 50+ amino acids.
Proteins are larger and more complex (1,2,3 and 4 structure), peptides are shorter chains of amino acids.

Question 22

What is the common pentapeptide sequence?

Answer 22

1) L-Ala
2) D-Glu
3) A₂pm (L-Lys)
4) D-Ala
5) D-Ala#

Question 23

Peptidoglycan Biosynthesis

Question 29

What is the capsule and its functions?

Answer 29

A polysaccharide layer outside the cell wall.
-Its very slippery so macrophages can’t catch on
-Adhesion

Question 30

2 types of capsule?

Answer 30

True capsule (capsular polysaccharide) and exopolysaccharide.

Question 31

What is the true capsule?

Answer 31

Discrete polysaccharide layer non-covalently bound to outer membrane proteins (OMP’s)

Question 32

What is the exopolysaccharide?

Answer 32

Matrix which embeds cells called slime layer/biofilm

Question 33

What are mobile genetic elements (MGE’s) and importance?

Answer 33

Type of moving genetic materials which can move within the genome or move between species.
They can carry AMR genes and make other bacteria’s resistant to certain antimicrobials.
They can turn commensal bacteria into harmful bacteria.

Question 34

Main MGE’s.

Answer 34

Plasmids, transposons and bacteriophages

Question 35

Plasmids

Answer 35

Circular, supercoiled extrachromosomal DNA found in all 3 domains of life. Name normally starts with lower case.

Question 36

Plasmid host range.

Answer 36

narrow host range - plasmid can move to closely related bacterial species

broad host range - plasmid is promiscuous - can replicate in multiple unrelated bacteria