Bacteria

Question 1

Describe size and shape of Helicobacter pylori

Answer 1

1. Stomach ulcers
2. Helical shape
3. penetrate viscous mucus of the stomach
4. Mutants that form rod attenuated in ability to cause disease- less effective less able to penetrate viscous mucus

Question 2

Describe size and shape of Proteus mirabilis

Answer 2

1. Urinary tract infections
2. Long cell length
3. Enables cells to align and use a group swarming motility
4. Smaller cells can’t move in a group

Question 3

Describe structure of typical prokaryotic cell

Answer 3

1. Internal structure is loose and messy
2. Some specialisation but in general far less organised than eukaryotic
3. Eukaryotes have Membrane bound organelles, Nucleus and nuclear membrane and Have distinct functions

Question 4

Describe bacterial cell wall

Answer 4

1. The cell envelope provides an interface between bacteria and their environment.
2. Maintains cell integrity
3. Protein or sugar capsule (protective)
4. External structures (pili, flagella) for adhesion or mot
5. Range of osmotics and stresses
6. When placed in distilled water red blood cells will burst (haemolysis)
7. Bacteria retain their shape and integrity even when placed in distilled water.

Question 5

Describe Gram +ve cell wall

Answer 5

1. Thick cell wall
2. Multiple layers of peptidoglycan,
3. interspersed with teichoic acids.
4. E.g Staphylococcus, Streptococcus, Clostridia.

Question 6

Describe Gram –ve cell wall

Answer 6

1. Thin peptidoglycan cell wall,
2. Enclosed by outer membrane of lipopolysaccharides (LPS).
3. No teichoic acids.
4. Cavity between inner membrane and outer LPS membrane – periplasmic space – where peptidoglycan is located.
5. E.g. Escherichia, Proteus, Pseudomonas

Question 7

What is gram staining

Answer 7

1. Fix bacterial smear onto glass slide using heat.
2. Stain with crystal violet solution – cells appear blue/violet colour.- binds to peptidoglycan component
3. Fix with Gram’s iodine (I/KI solution) – cells same colour as above.
4. Decolourise with alcohol or acetone – some cells appear blue/violet, others colourless.
5. Stain with safranin solution (red colour)
6. Wash with water – Gram positive cells appear purple/violet colour- thick peptidoglycan layer, Gram negative cells appear pink/red colour.

Question 8

Describe peptidoglycan

Answer 8

1. Made up of linear polysaccharide chains (glycan strands) up to 200 disaccharide units in length, cross-linked by short peptide chains.
2. Glycan strands composed of alternating units of:
3. N-acetylglucosamine
4. N-acetylmuramic acid
5. Joined by Beta 1-4 glycosidic links.
6. One large continuous molecule surrounding the cell rather like chain mail- provides strength and mechanical integrity

Question 9

How do many antibiotics work

Answer 9

1. Antimicrobial agents acting on peptidoglycan- not contained in human cells
2. Many antibiotics act to inhibit production of peptidoglycan
3. penicillins, cephalosporins, bacitracin, vancomycin, teicoplanin
4. Can survive without peptidoglycan synthesis if isotonic environment But if not:
5. Add b-lactam antibiotic
6. Cell wall weakens and cytoplasmic membrane forces its way out
7. Cell wall breaks down leaving the cytoplasmic membrane as the outermost structure.
8. The resulting spheroplast is osmotically fragile and so will very easily take up water, swell and BURST.

Question 10

Describe Staphylococcus aureus cell wall and how it can be broken down

Answer 10

1. contains a pentaglycine cross-link
2. B-Lactams antibiotics prevent this cross linking
3. Has to be actively growing peptidoglycan- pentaglycine cross-link blocked
4. Lysozyme is an antimicrobial enzyme present in many body fluids, e.g tears, saliva
5. Actively degrades and breaks up the glycan strands- cleaving bond between sugar units
6. Can degrade already synthesised peptidoglycan

Question 11

Describe structure of Cytoplasmic membrane

Answer 11

1. A highly selective permeability barrier constructed of phospholipids and proteins
2. Larger molecules can only enter if their transport is mediated
3. Prokaryotic membranes do not contain cholesterol- eukaryotic membranes do
4. Prokaryotic membranes are very fluid

Question 12

How does the Cytoplasmic membrane act as a key site or energy generation

Answer 12

1. Charged ions such as H+ cannot move freely across membrane, so gradients can form
2. Efflux of H+ out of the cell results in an electrochemical gradient that can be used to power a range of processes in the cell. -proton motive force
3. Electron transport system- ATP synthesis
4. Chemiosmotic theory

Question 13

What is LPS- endotoxin and what consequences does it have

Answer 13

1. This feature of Gram negative cells can continue to cause damage and injury even after cells have died. For example:
2. Solutions sterilized by autoclaving will be free from live cells, but may still be lethal if injected into a patient.
3. Patients with Gram-negative septicaemia are liable to suffer toxic shock and other morbidities even if given antibiotics that kill bacteria causing the infection
4. This is due to the toxic effect of lipopolysaccharide (LPS) outer membrane. Also commonly referred to as endotoxin for this reason.

Question 14

Describe the LPS subunit

Answer 14

1. Repeating chain of sugars (polysaccharide).
2. As many as 200 sugar residues
3. Core also consists of polysaccharide, but generally ~5 sugar residues with side chains such as phosphoethanolamine
4. Lipid A region- conserved
5. Lipid A- A dimer of phosphoglucosamine residues with 2 fatty acid chains linked to each sugar residue.
6. Fatty acids may be branched or unbranched

Question 15

What are the General effects of LPS

Answer 15

1. Fever
2. Intravascular coagulation (due to activation of blood clotting factors)
3. Vasodilatation leading to hypotension (low blood pressure)
4. Shock (due to hypotension)
5. If above symptoms are severe, effects of endotoxin can be fatal.

Question 16

Describe Neisseria meningitidis

Answer 16

1. Gram –Ve diplococcus.
2. A major cause of bacterial meningitis, which has a septicaemic phase as part of its pathogenesis.
3. Can replicate to high numbers in blood stream.
4. Sheds LPS into blood during septicaemia which can cause severe complications.
5. Treatment with antibiotics (and disruption of cell wall) also results in enormous LPS release which may be lethal to patient.
6. But Gram –Ve sepsis also usually fatal

Question 17

What are capsules

Answer 17

1. Many Gram negative bacteria and some Gram positive bacteria produce extracellular polysaccharides (EPS).
2. May be discrete capsule or a more generalised layer of slime.
   3.

Question 18

What can EPS can function in:

Answer 18

1. Adherence (e.g. Streptococcus mutans)
2. Resistance (to antimicrobials, desiccation and macrophages)
3. Immune evasion and virulence (e.g. Klebsiella pneumoniae, Streptococcus pneumoniae)

Question 19

How does streptococcus pneumoniae depend upon its polysaccharide capsule for pathogenicity

Answer 19

1. Streptococcus pneumoniae depends upon its polysaccharide capsule for pathogenicity. -virulence
2. In absence of capsule infecting dose increased 10,000 times.
3. Human host responds very well immunologically to protein antigens but less well to polysaccharides.
4. Hence, capsule shields cell from detection by immune system. – shields from antigens

Question 20

Describe bacterial chromosomes

Answer 20

1. Initially internal structure of bacteria thought to be an unorganised “soup” of proteins, nucleic acids and other metabolites
2. We now know that there is a degree of organisation within the bacterial cell.
3. The bacterial nucleoid is an example –chromosomal DNA is localised to an area of the cytoplasm, and anchored to the cytoplasmic membrane
4. Consists of a series of highly folded DNA loops but NOT bound by a membrane as observed for nucleus of eukaryotic cells

Question 21

Describe plasmids

Answer 21

1. Cells can contain other genetic elements such as plasmids.
2. These are autonomously replicating, extrachromosomal.
3. Usually circular dsDNA.- ds=double stranded
4. Can transfer between species (Horizontal gene transfer).
5. Encode for accessory functions conferring advantage to cell: e.g. production of toxins or antibiotic resistance.