bacterial pathogenesis Flashcards
Local infection
- Entry of pathogen + multiplication at site
Disseminated infection
- Entry of pathogen + multiplication + spread
NM as opportunistic pathogens
- 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
- Nosocomial infection
functions of virulence factors
- 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
Genes for VF’s means of transfer
- Mostly on plasmids and bacteriophages
- Horizontal gene transfer
○ Plasmids
○ Bacteriophages
○ Transposons
Vf genes can be expressed together called pathogenicity islands
- Horizontal gene transfer
HGT via plasmids
- Ds circular DNA
- Self replicating
- Encode for toxins, adhesins, invasion, drug resistance
- Transmitted via conjugation
If VFs absent on plasmids = no disease
HGT via bacteriophages
- Virus infects bacteria
- Encode for toxins
Lysogenic phage integrates into the bacterial genome
- Encode for toxins
HGT via Transposons
- 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
Pathogenicity islands
- Large areas of chromosomal DNA that contain clusters of genes encoding VFs
Permanently integrated and dedicated to virulence
how does virulence gene expression regulate
- 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
Updated Koch’s postulate
- The ‘virulence’ gene is always found in strains with a particular virulence phenotype
- The gene should be expressed in the host
- Mutation (inactivation) of that particular gene abolishes (reduces) the virulence phenotype
- Reintroduction of the gene reconstitutes the virulence phenotype
portals of entry
○ Epithelial and mucosal membranes ○ Skin ○ Respiratory tract ○ Gastrointestinal tract ○ Urogenital tract Eye
innate immunity
○ Physical barriers § shedding ○ Chemical and biochemical barriers § Secretions § Reflex response (sneeze, cough) pH
colonisation
- 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
adhesion
○ Contact with host cell - docking
○ Binding - specific - surface interactions
○ Mediated via adhesins - ligands that bind to complementary surfaces receptors on host cells
what are the 3 types of adhesions
pili/fimbriae
afimbrial adhesions
bacterial capsules
pili/fimbriae
□ 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
afimbrial adhesions
□ 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
what are bacterial capsules. what are there functions
□ Polymeric structure surrounds the bacterial cell membrane □ Protection □ Sticky adhesion □ To each other (biofilm) □ To surfaces (plaque on teeth
what is the function of invasins
○ Enter into and through epithelial/mucosal layer, through basement membrane through BV into blood
○ Invade through cells - transcytosis
○ Allows entry to non-phagocytic cells
Zipper mechanism
○ 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
Trigger mechanism
○ 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
mechanisms to avoid immune response
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
what is Phagocytosis
what is the process
- 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- Degradation of pathogen by microbiocidal agents
complement
Set of serum proteins important in innate immune system
avoiding phagocytosis prior and during
- 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
- During
how do capsules avoid immune response
- Hide PAMPS (LPS) - prevent phagocytosis
- Prevent complement binding
Avoid ab
- Prevent complement binding
Specific enzymes used to avoid an immune response
- C5a peptidase
○ Degrade C5a - reduce leucocyte recruitment
IgA proteases - cleaves IgA
Fc receptor capture of Ab
- Blocks complement binding
Avoid detection of antibodies
Host mimicry
- Copy structures to minimise eliciting Ab detection
Capsules with sugars that mimic host surface glycoproteins
Antigenic variation /phase variation
- Change surface components
Phase variation - switch on and off expression of genes
Fe acquisition
- 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
Damage to host
- Multiplication = damage
- Extracellular enzymes
Exaggerate inflammatory response
- Extracellular enzymes
Toxins
- Direct effect ○ Damage at site of infection ○ Secreted - Indirect effect Exaggerated immune response via toxin
Exotoxins
- 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
- Heat-liable toxins (proteins)
Type I Exotoxin - superantigens
- 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
Type II - membrane disrupting exotoxins
- 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
Type III - A-B toxins
- 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
Endotoxins
- 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
