bacterial pathogenesis

Question 1

Local infection

Answer 1

• Entry of pathogen + multiplication at site

Question 2

Disseminated infection

Answer 2

• Entry of pathogen + multiplication + spread

Question 3

NM as opportunistic pathogens

Answer 3

• Change in circumstance / niche
  ○ Breach/injury at body surface, increase in numbers, altered immune status of host
  
  • Nosocomial infection
    ○ NM transferred to or between compromised individuals

Question 4

functions of virulence factors

Answer 4

• Enter host and colonise
  
  • Adhesion via adhesins that are ligands for host cell receptor
  • Invade and multiply host niches
  • Cause cell damage
  • Evade immune system - avoid phagocytosis (e.g. capsules), inflammatory response
  • Cause damage via toxins and cytotoxic proteins, degradative enzymes
    Resist antibiotics

Question 5

Genes for VF’s means of transfer

Answer 5

• Mostly on plasmids and bacteriophages
  
  • Horizontal gene transfer
    ○ Plasmids
    ○ Bacteriophages
    ○ Transposons
    Vf genes can be expressed together called pathogenicity islands

Question 6

HGT via plasmids

Answer 6

• Ds circular DNA
  
  • Self replicating
  • Encode for toxins, adhesins, invasion, drug resistance
  • Transmitted via conjugation
    If VFs absent on plasmids = no disease

Question 7

HGT via bacteriophages

Answer 7

• Virus infects bacteria
  
  • Encode for toxins
    Lysogenic phage integrates into the bacterial genome

Question 8

HGT via Transposons

Answer 8

• Jumping genes
  
  • Sections of DNA that can mover around in and out of the genome and between plasmids and bacteriophages
  • Well known for transferring
    ○ Antibiotic resistance
    ○ Toxins
    Pili

Question 9

Pathogenicity islands

Answer 9

• Large areas of chromosomal DNA that contain clusters of genes encoding VFs
  Permanently integrated and dedicated to virulence

Question 10

how does virulence gene expression regulate

Answer 10

• To conserve energy
  
  • Lack of nutrients, oxygen concentration, osmolarity, bile salts, temperature, antimicrobial compounds, oxidative stress
  • Switch on and off
    Regulated in response to population density = quorum sensing

Question 11

Updated Koch’s postulate

Answer 11

1. The ‘virulence’ gene is always found in strains with a particular virulence phenotype
   
   2. The gene should be expressed in the host
   3. Mutation (inactivation) of that particular gene abolishes (reduces) the virulence phenotype
2. Reintroduction of the gene reconstitutes the virulence phenotype

Question 12

portals of entry

Answer 12

○ Epithelial and mucosal membranes
		○ Skin
		○ Respiratory tract
		○ Gastrointestinal tract
		○ Urogenital tract
Eye

Question 13

innate immunity

Answer 13

○ Physical barriers
			§ shedding
		○ Chemical and biochemical barriers
			§ Secretions
			§ Reflex response (sneeze, cough)
pH

Question 14

colonisation

Answer 14

• Outcompete NM - biofilms
  
  • Multiply
  • Remain at site = localised
  • May invade further
    H. Pylori
  • Urease
    ○ Cleaves urea in stomach to produce ammonia which neutralise acid
  • Flagella
    Creates motility driven by chemotaxis according to pH gradient

Question 15

adhesion

Answer 15

○ Contact with host cell - docking
○ Binding - specific - surface interactions
○ Mediated via adhesins - ligands that bind to complementary surfaces receptors on host cells

Question 16

what are the 3 types of adhesions

Answer 16

pili/fimbriae
afimbrial adhesions
bacterial capsules

Question 17

pili/fimbriae

Answer 17

□ Attachment to surface, motility DNA transfer
□ Long filamentous hair like
□ Pili - thick tubular structure
□ Fimbriae - thin and shorter than pili
□ Adhesion via tip of pilus
□ Composed of protein subunits (pilin)
□ Host receptors - glycoprotein

Question 18

afimbrial adhesions

Answer 18

□ Short monomeric or trimeric structures - not ordered structure like pili/fimbriae
□ Widely spread
□ Establish close and strong adhesion
□ Cell attachment to host cells via glycoprotein, integrin

Question 19

what are bacterial capsules. what are there functions

Answer 19

□ Polymeric structure surrounds the bacterial cell membrane
			□ Protection
			□ Sticky adhesion
				□ To each other (biofilm)
				□ To surfaces (plaque on teeth

Question 20

what is the function of invasins

Answer 20

○ Enter into and through epithelial/mucosal layer, through basement membrane through BV into blood
○ Invade through cells - transcytosis
○ Allows entry to non-phagocytic cells

Question 21

Zipper mechanism

Answer 21

○ Cell surface receptor/ligand interactions
○ Binding of invasins to integrins activates receptor which signal to initiate polymerisation to form pseudopods and surround pathogen
Mediated from outside cell

Question 22

Trigger mechanism

Answer 22

○ Syringe-like structure that injects bacterial proteins into host cell
○ Initiation of actin polymerisation to form pseudopods and surround pathogen
Interaction from inside host cell

Question 23

mechanisms to avoid immune response

Answer 23

Avoid recognition by complement proteins, Ab and phagocytes
	- Hide intracellularly
	- Mask surface antigens - capsules
	- Antigen variation/phase variation
	- Use mimicry
	- Inhibit/destroy Ab - bind Fc of antibody, IgA protease
Avoid phagocytosis
Modify inflammation e.g. toxins

Question 24

what is Phagocytosis

what is the process

Answer 24

• Microbe destruction by phagocytes of innate immune system
  
  • Pathogen has PAMPS e.g. LPS - if detected, may be coated with C3b and or Ab
  • Phagocyte recognises
    ○ Pattern recognition receptors
    ○ C3b receptor
    ○ Fc receptor
    1. Engulf pathogen in a phagosome
    2. Phagosome fuses with lysosome to form phagolysosome
    
    3. Degradation of pathogen by microbiocidal agents

Question 25

complement

Answer 25

Set of serum proteins important in innate immune system

Question 26

avoiding phagocytosis prior and during

Answer 26

- Prior ○ Form biofilms - difficult for phagocytes to recognise individual cells and induce apoptosis - During ○ Inhibit internal phagocytic pathway to survive ○ Block assembly of NADPH oxidase Block fusion of lysosome

Question 27

how do capsules avoid immune response

Answer 27

- Hide PAMPS (LPS) - prevent phagocytosis - Prevent complement binding Avoid ab

Question 28

Specific enzymes used to avoid an immune response

Answer 28

- C5a peptidase ○ Degrade C5a - reduce leucocyte recruitment IgA proteases - cleaves IgA

Question 29

Fc receptor capture of Ab

Answer 29

- Blocks complement binding | Avoid detection of antibodies

Question 30

Host mimicry

Answer 30

- Copy structures to minimise eliciting Ab detection | Capsules with sugars that mimic host surface glycoproteins

Question 31

Antigenic variation /phase variation

Answer 31

- Change surface components | Phase variation - switch on and off expression of genes

Question 32

Fe acquisition

Answer 32

- Secretion of Siderophores - high affinity iron-chelators - Express receptors for iron capture - Express high-affinity iron-binding receptor for haemoglobin Express toxins to release host Fe

Question 33

Damage to host

Answer 33

- Multiplication = damage - Extracellular enzymes Exaggerate inflammatory response

Question 34

Toxins

Answer 34

``` - Direct effect ○ Damage at site of infection ○ Secreted - Indirect effect Exaggerated immune response via toxin ```

Question 35

Exotoxins

Answer 35

- Secreted out of bacteria or released by bacterial lysis - Heat-liable toxins (proteins) ○ Type I - active immune response ○ Type II membrane disrupting toxins ○ Type III A-B toxins - active inside cells - Heat-stable toxins (non-protein) E.g. glycopeptides, polyketides

Question 36

Type I Exotoxin - superantigens

Answer 36

- Very potent - Form superantigen - MHC class II complex in APC's, activates CD4 T cells without antigen - T-cell activation without antigen, triggers intense immune response and toxicity Polyclonal activation of pro-inflammatory cytokines leading to cytokine storm

Question 37

Type II - membrane disrupting exotoxins

Answer 37

- Damage by cell membrane lysis = haemolysin ○ Phospholipases § Hydrolyses phosphatidylcholine in host cell membrane causing lysis ○ Pore-forming toxins Insertion of pores in cell membrane causing cell rupture via osmosis

Question 38

Type III - A-B toxins

Answer 38

- Binary toxin = A subunit + B subunit - A = catalytic activity - B = binds to host cell receptor 1. B binds to host cell receptor - 2. toxin endocytosed 3. Inside vacuole A separates from B 4. A component gains access to cytoplasm, exerts effect - Diphtheria toxin (DT) - B targets respiratory cells - A blocks protein synthesis = cell death - Cholera toxin (CT) - B targets intestinal cell - A causes ion efflux = fluid loss - diarrhoea - Botulinum toxin (BoNT) - A endopeptidase, blocks acetylcholine release - inhibits muscle contraction

Question 39

Endotoxins

Answer 39

- Within bacteria, usually cell wall components, release when cell wall breaks or cell death - Gram -ve - LPS, LOS - Gram +ve - LTA, - LPS induces exaggerated immune response - increased inflammatory cytokines - variable O-antigen LOS indices sepsis and inflammation - no O-antigen