Bacteria Flashcards
Describe size and shape of Helicobacter pylori
- Stomach ulcers
- Helical shape
- penetrate viscous mucus of the stomach
- Mutants that form rod attenuated in ability to cause disease- less effective less able to penetrate viscous mucus
Describe size and shape of Proteus mirabilis
- Urinary tract infections
- Long cell length
- Enables cells to align and use a group swarming motility
- Smaller cells can’t move in a group
Describe structure of typical prokaryotic cell
- Internal structure is loose and messy
- Some specialisation but in general far less organised than eukaryotic
- Eukaryotes have Membrane bound organelles, Nucleus and nuclear membrane and Have distinct functions
Describe bacterial cell wall
- The cell envelope provides an interface between bacteria and their environment.
- Maintains cell integrity
- Protein or sugar capsule (protective)
- External structures (pili, flagella) for adhesion or mot
- Range of osmotics and stresses
- When placed in distilled water red blood cells will burst (haemolysis)
- Bacteria retain their shape and integrity even when placed in distilled water.
Describe Gram +ve cell wall
- Thick cell wall
- Multiple layers of peptidoglycan,
- interspersed with teichoic acids.
- E.g Staphylococcus, Streptococcus, Clostridia.
Describe Gram –ve cell wall
- Thin peptidoglycan cell wall,
- Enclosed by outer membrane of lipopolysaccharides (LPS).
- No teichoic acids.
- Cavity between inner membrane and outer LPS membrane – periplasmic space – where peptidoglycan is located.
- E.g. Escherichia, Proteus, Pseudomonas
What is gram staining
- Fix bacterial smear onto glass slide using heat.
- Stain with crystal violet solution – cells appear blue/violet colour.- binds to peptidoglycan component
- Fix with Gram’s iodine (I/KI solution) – cells same colour as above.
- Decolourise with alcohol or acetone – some cells appear blue/violet, others colourless.
- Stain with safranin solution (red colour)
- Wash with water – Gram positive cells appear purple/violet colour- thick peptidoglycan layer, Gram negative cells appear pink/red colour.
Describe peptidoglycan
- Made up of linear polysaccharide chains (glycan strands) up to 200 disaccharide units in length, cross-linked by short peptide chains.
- Glycan strands composed of alternating units of:
- N-acetylglucosamine
- N-acetylmuramic acid
- Joined by Beta 1-4 glycosidic links.
- One large continuous molecule surrounding the cell rather like chain mail- provides strength and mechanical integrity
How do many antibiotics work
- Antimicrobial agents acting on peptidoglycan- not contained in human cells
- Many antibiotics act to inhibit production of peptidoglycan
- penicillins, cephalosporins, bacitracin, vancomycin, teicoplanin
- Can survive without peptidoglycan synthesis if isotonic environment But if not:
- Add b-lactam antibiotic
- Cell wall weakens and cytoplasmic membrane forces its way out
- Cell wall breaks down leaving the cytoplasmic membrane as the outermost structure.
- The resulting spheroplast is osmotically fragile and so will very easily take up water, swell and BURST.
Describe Staphylococcus aureus cell wall and how it can be broken down
- contains a pentaglycine cross-link
- B-Lactams antibiotics prevent this cross linking
- Has to be actively growing peptidoglycan- pentaglycine cross-link blocked
- Lysozyme is an antimicrobial enzyme present in many body fluids, e.g tears, saliva
- Actively degrades and breaks up the glycan strands- cleaving bond between sugar units
- Can degrade already synthesised peptidoglycan
Describe structure of Cytoplasmic membrane
- A highly selective permeability barrier constructed of phospholipids and proteins
- Larger molecules can only enter if their transport is mediated
- Prokaryotic membranes do not contain cholesterol- eukaryotic membranes do
- Prokaryotic membranes are very fluid
How does the Cytoplasmic membrane act as a key site or energy generation
- Charged ions such as H+ cannot move freely across membrane, so gradients can form
- 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
- Electron transport system- ATP synthesis
- Chemiosmotic theory
What is LPS- endotoxin and what consequences does it have
- This feature of Gram negative cells can continue to cause damage and injury even after cells have died. For example:
- Solutions sterilized by autoclaving will be free from live cells, but may still be lethal if injected into a patient.
- Patients with Gram-negative septicaemia are liable to suffer toxic shock and other morbidities even if given antibiotics that kill bacteria causing the infection
- This is due to the toxic effect of lipopolysaccharide (LPS) outer membrane. Also commonly referred to as endotoxin for this reason.
Describe the LPS subunit
- Repeating chain of sugars (polysaccharide).
- As many as 200 sugar residues
- Core also consists of polysaccharide, but generally ~5 sugar residues with side chains such as phosphoethanolamine
- Lipid A region- conserved
- Lipid A- A dimer of phosphoglucosamine residues with 2 fatty acid chains linked to each sugar residue.
- Fatty acids may be branched or unbranched
What are the General effects of LPS
- Fever
- Intravascular coagulation (due to activation of blood clotting factors)
- Vasodilatation leading to hypotension (low blood pressure)
- Shock (due to hypotension)
- If above symptoms are severe, effects of endotoxin can be fatal.
Describe Neisseria meningitidis
- Gram –Ve diplococcus.
- A major cause of bacterial meningitis, which has a septicaemic phase as part of its pathogenesis.
- Can replicate to high numbers in blood stream.
- Sheds LPS into blood during septicaemia which can cause severe complications.
- Treatment with antibiotics (and disruption of cell wall) also results in enormous LPS release which may be lethal to patient.
- But Gram –Ve sepsis also usually fatal
What are capsules
- Many Gram negative bacteria and some Gram positive bacteria produce extracellular polysaccharides (EPS).
- May be discrete capsule or a more generalised layer of slime.
3.
What can EPS can function in:
- Adherence (e.g. Streptococcus mutans)
- Resistance (to antimicrobials, desiccation and macrophages)
- Immune evasion and virulence (e.g. Klebsiella pneumoniae, Streptococcus pneumoniae)
How does streptococcus pneumoniae depend upon its polysaccharide capsule for pathogenicity
- Streptococcus pneumoniae depends upon its polysaccharide capsule for pathogenicity. -virulence
- In absence of capsule infecting dose increased 10,000 times.
- Human host responds very well immunologically to protein antigens but less well to polysaccharides.
- Hence, capsule shields cell from detection by immune system. – shields from antigens
Describe bacterial chromosomes
- Initially internal structure of bacteria thought to be an unorganised “soup” of proteins, nucleic acids and other metabolites
- We now know that there is a degree of organisation within the bacterial cell.
- The bacterial nucleoid is an example –chromosomal DNA is localised to an area of the cytoplasm, and anchored to the cytoplasmic membrane
- Consists of a series of highly folded DNA loops but NOT bound by a membrane as observed for nucleus of eukaryotic cells
Describe plasmids
- Cells can contain other genetic elements such as plasmids.
- These are autonomously replicating, extrachromosomal.
- Usually circular dsDNA.- ds=double stranded
- Can transfer between species (Horizontal gene transfer).
- Encode for accessory functions conferring advantage to cell: e.g. production of toxins or antibiotic resistance.