Bacterial pathogenesis and immune evasion Flashcards
commensalism
a relationship that is beneficial to the bacteria but does not help or harm the host.
opportunistic pathogen
an infectious microorganism that is normally a commensal or does not harm its host but can cause disease when the host’s resistance is low
balanced pathogenicity - what is it meant by this term?
balanced pathogenicity is having a balance between microbe and host - based on the properties of both
e. g. microbe - adhesions, toxins, capsule, etc.
e. g. host - natural barriers, defensive cells, complement
Genetics can also tip the balance either way
name some general concepts of Bacterial Pathogenicity
Host defences and susceptibilities
- innate/adaptive immunity (also antimicrobial peptides, turnover, iron binding proteins)
- age
- genetics
Genetic and Molecular Basis for Virulence
- genes (chromosomes, plasmids)
- bacteriophages, mobile genetic elements
Host-mediated Pathogenesis
-immunopathology – e.g. TB
Intracellular Growth
rickettsia, mycoplasm, chlamydia, TB, salmonella
e.g. if one is under the age of 1 then you have a very immature immune system that doesn’t respond to bacterial virulence factors very well
What’s unique that about that bacterial pathogen that causes them to cause that disease, what genes in that organism?
define immunopathology
E.g. in TB, granuloma formation
The host damage that occurs with TB infection is purely driven by our own immune response or (immunopathology) the organism cannot be killed by our innate or adaptive immune system so get granulomatous inflammation chronically
Virulence factors - 3 main roles
- Promote colonisation and adhesion
- Evade host defences
- Promote tissue damage
Why do we need virulence factors?
Bacteria need to be able to attach to the host and grow divide to cause disease
Examples of virulence factors
- Adherence factors, which colonise mucosal sites via pili
- Invasion factors, surface components that damage the tissue structure they adhere to
- Capsules, which prevent: antibodies binding, complement, phagocytosis and opsonisation
- Polysaccharides - protect from opsonisation and phagocytosis.
- Endotoxins - proteins that have an inhibitory effect on metabolic activity within host, damaging nerves/enterocytes/ciliated lung epithelial cells. Also lipopolysaccharides (g -ve) and lipteichoic acids (g +ve), cause fever, inflammation, lethal shock
- Exotoxins - produce toxins/enzymes, e.g. cytotoxins, neurotoxins
- Siderophores - proteins released by bacteria that bind to iron and take it back to bacteria, they need it for growth. iron-binding factors to compete with the host for iron
Types of Infection
- Local
surface infection; wound
skin or URT, but haven’t penetrated any deep tissue or gut infection such as cholera release toxins that cause disease without penetration - Invasive
penetrate barriers spread
drive a specific response normally innate inflammatory – this means it’s made it into tissue and circulation - Systemic
via blood to other sites
the damage that the immune response does to the host to contain the infection
Host defences - name some
Innate and adaptive immunity
Natural barriers - skin, gut, lungs, eyes, GU tract
Non-specific:
physical conditions (dry, acidic), sloughing, microflora, lysozyme,
toxic lipids, lactoferrin, lactoperoxidases, tight junctions, bile, mucin,
ciliated epithelia, bile, cryptdins, phagocytes, intraepithelial lymphocytes
Adaptive:
MALT, SALT, GALT, secretory IgA
Defences of tissue and blood
Usually involves tissue damage and controlled by feedback mechanisms
non-specific:
transferrin, complement, acute phase proteins (released by liver)
phagocytes- monocytes and macrophages, PMN’s -neutrophils
adaptive:
antibodies
macrophage activation
T cells
Stages of Infection
- Acquisition
- Colonisation – adherence
- Penetration – break down the barrier
- Multiplication and Spread –create a niche where it can get nutrients
- Immune avoidance
- Damage
- Transmission
- Resolution
NB - Not all microbes need all stages.
e.g. cholera does not need to worry about immune evasion, as it sits on the surface of the gut, only thing that is going to clear it is secretory IgA, but by the time there is enough IgA then it has already produced its toxin, caused watery diarrhoea, multiplied to infect a new individual
Mechanisms of microbial responses to immunity
Extracellular (eg pneumolysin, superantigens)
Capsule (inhibition C3 and Ig deposition, phagocytosis)
Surface structures (eg protein A, M protein, LPS)
Direct secretion into cells (eg type 3 secretion systems)
PAMPs and PRRs - example
LPS and TLR4 Cells
Links innate immunity recognition to adaptive immunity via signalling
Importantly – cells respond to signals, TLR4 respnds to lipopolysaccharide
There are intracellular receptors NOD4 drive signalling cascade that lead to changes in transcription that will induce a procytokine response
Immune response to some important bacteria
look at table
‘discuss virulence factors in relation to bacterial infections’
Neisseria meningitides – its nasopharyngeal carriage rather than infection, infection occurs when natural barrier has been compromised or maybe they had a cold before damage mucousal surface, or some phase variations may switch on genes that allow penetration to randomly become a more virulent strain than before
TB is an intracellular organism , antibodies don’t really have much affect on it, need the right cytokine profile from the right T cell population to engage with macrophages activating them to kill the intracellular bacteria – this does not work on TB, once infected you cant really get rid of it
gram +ve envelope
Large peptidoglycan layer on the outside on which teichoic and lipoteichoic acid sits in it – potent immune-stimulatory molecules
gram -ve envelope
Outermembrane is complex full of these LPS chains
Also proteins on the surface that interact with host immunity, to evade immunity
When it lives in the host for a long time then it can change those outer membrane proteins
name some bacterial structures involved in mobility and attachment
Flagella, fimbriae and pili on the outside of some bacteria (vibrio cholera, salmonella, e.coli)
They may be associated with movement such as in the gut with flagella
what are Bacterial Adhesins
Cell-surface components that facilitate adhesion to other cells or surfaces (Skin, Urogenital, GI & respiratory tracts)
Bacterial Adhesin receptors
glycolipids, glycoproteins, transmembrane proteins, mucins, CD’s
extracellular matrix – elastin, laminin, fibronectin, vitronectin, hyaluronan, heparin, collagen
examples of bacterial adhesins
Fimbriae or pili Capsular polysaccharides LPS Lipoteichoic acid Outer membrane proteins Flagella Curli
- E coli “P” fimbriae (pili) bind P blood group on uroepithelial cells
- Neisseria gonorrhoeae pili attach to mucosal cells – non-piliated mutants are less pathogenic.
- Vibrio cholera fimbriae bind intestinal epithelial cell receptors.
- Strep pyogenes lipoteichoic acid binds to epithelial cell.
Host-pathogen interactions - Strep pyogenes
Strep pyogenes group A strep – engulfed by polymorphonucleosome
it is not being engulfed such that the host is in control, rather some bacteria influence the way a phagocyte will phagocytose to the advantage of the bacteria
they can express proteins or carbohydrate meaning they will be engulfed through a different receptor mechanism pathway that doesn’t stimulate the respiratory burst
M protein of Streptococcus pyogenes acts as adhesin and structural component of cell wall
Host-pathogen interactions - E coli
E.Coli in the gut
-causes the cell to exude out a pedestal upon which the bacteria sits, so has a close interaction with this cell type to the extent that it can live on the surface without damage to the bacteria
Host-pathogen interactions - N gonorrhoeae
Fimbriae of Neisseria gonorrhoeae allow the bacterium to adhere to tissues
Fimbrae about 2/3 times the size of the bacteria so can easily attach to target
Once it has penetrated:
it will switch of some of the genes that it needed to adhere to evade the immunity
small number of phase variants are the ones that will survive and take over, then will enter cell and express tight adhesins allowing it to survive inside the cell
what is e coli
Commensal gut organism
but can become a pathogen, causing: diarrhoea, dysentery, Haemolytic uraemic syndrome (HUS), Urinary tract and kidney infections, Septicaemia, Pneumoniae and meningitis
Different strains of E. coli – different repertoire of genes
e.g. entero-haemorrhagic E. coli – cause haemolytic uraemic syndrome -> life threatening – they probably have 300 genes different from simple enterotoxigenic E. coli
Enteropathogenic Escherichia coli (EPEC) cause the pedestal – it damages the villi, so it can no longer regulate fluid, so get diarrhoea – repair -> acute secretory
Enteroinvasive Escherichia coli (EIEC) have mechanisms that allow them to enter by damaging surface – then will express proteins that allows them to survive inside the cell and spread through epithelial layer, dysentery -> blood and puss in faeces due to tissue destruction + shigella like toxin leads to systemic effects on endothelial cells on kidney, haemolytic uraemic syndrome
Bacterial host cell attachment – Mechanism summary
Pili or fibrillae protruding from the bacterial surface.
A - Pili or fibrillar protruding from bacterial surface. These proteinaceous appendages bind to host cell surface molecules, usually carbohydrates, by adhesin proteins located at the distal tip of the pilus/fibrillar organelle.
B - Bacterial/epithelial cell interactions mediated by afimbrial adhesin proteins.
C - Some bacteria establish intimate associations with eukaryotic cells by intimin proteins, resulting in cytoskeletal rearrangements, host cell signalling, possible internalisation of the bacteria, and in many cases systemic disease.
D - Some bacteria secrete their own receptor protein, which is internalised by the target host cell, phosphorylated, and embedded in the eukaryotic cell as a new receptor for tight binding by the bacterium.
Bacteria interfere with cellular functions
- examples
- adhesion to receptors (adhesins/integrins)
- block phagocytosis (yersinia)
- membrane disruptions (pore forming toxins)
- -ve protein synthesis
- manipulation of lysosomal function (e.g. S typhi)
- endosomal trafficking
- manipulation of cytoskeleton
how do bacteria interfere with innate immunity?
- influence phagocytosis! (macrophages and PMN’s)
- complement (diff flashcard)
- kill cell via leucocidins - staphs
- prevent opsonisation via protein A, which binds to the Fc portion of IgG - staphs
- block contact via capsules - meningococcus, HibB
intracellular pathogens:
- promote uptake via C3R, Fc receptors (no ROI)
- prepares cell for invasion- class III secretion, Shigella
- down regulates P-L fusion - M. tuberculosis
- escape P-L to cytoplasm - Listeria
- resist oxidative killing via catalase/peroxidase production - blocks MHC, IFN receptors, TNF release
how do bacteria survive inside the cell?
- Directs phagocytosis via CR3 – no ROI
- Actin rearrangement - +ve engulfmant
- Type 3 secretion systems – prepares cell
- Resists digestion and ROIs, in PLs - SOD, catalase
- Escape into cytoplasm, e.g. Listeria; Legionella
- Inhibits PL fusion, maintains early endosome and blocks acidification
e. g. mycobacteria - Controls antigen presentation - stops CTLs or Pφ activation
what is listeria?
Listeria monocytogenes (listeria) is a bacterium that causes an illness called listeriosis.
how does listeria invade?
cell-cell invasion, involves a pore forming toxin called listeriolysin
Listeria reorganises actin filaments within the cell – allows it to travel through the cell and from cell to cell
Stages – attachment and effacing lesions by EPEC
bfp – bundle forming pili – mediate initial attachment
Activation of Type 111 secretion systems –
secretion of bacterial translocated intimin receptor Tir. Receptor for the adhesion protein, intimin.
Cytoskeletal rearrangements – pedestal formation
This strain can secrete its own receptor when it comes into contact with the cell.
It activates a secretion process, the protein secreted is a receptor which goes into the target cell then becomes expressed on the surface of the target cell, to allow the bacteria to adhere, forms a tight intimin interaction between receptor and the ligand
explain how class III secretory systems work further
There are about 8 different ways that bacteria can secrete proteins and into the target cell.
This image shows bacteria with loose attachment to cell, it makes a hypodermic needle called a secretory apparatus, that responds to the environment, allows upregulation of some genes to make this bridging structure
It then injects proteins or Yops into the cell some which are receptors to be presented, other effector proteins affect the cell in different ways
Yop E - actin rearrangement via GTPases, cell rounding, anti-phagocytic
YopH - Tyrosine phosphatase, -ve phagocytosis
YopT - Actin rearrangement
Gram negative bacteria secretion systems
• ‘injects’ effectors into host cell: MOLECULAR SYRINGES
• examples:
Salmonella gut epithelium invasion
modification macrophage phagosome Yersinia inhibition neutrophil and macrophage function Shigella macrophage invasion and escape from phagosome cell-to-cell spread E. coli EHEC and EPEC adhesion to gut epithelium Legionella inhibition phagolysosomal fusion
how do bacteria interfere with innate immunity?
- complement
- failure to trigger - LPS/capsules
- downregulation of binding - coating with non-fixing with IgA
- block MAC - C5a proteases
roles of complement?
induces inflammatory response promotes chemotaxis ↑ phagocytosis by opsonisation ↑vascular permeability mast cell degranualtion lysis of cell membranes
complement pathways
look at table
what is MBL?
part of complement pathway
Mannose-binding lectin - an acute phase protein
Binds MBL-associated serine protease (MASP)
avoidance of complement via polysaccharide capsules?
Bacterial polysaccharide capsules:
-poor activators
-long side chains (O antigens) fix C3b - prevent access to membrane
-capsules rich in sialic acid promote interaction
with H and I factors and thus promotes MAC dissociation - eg Group B Streps, Neisseria meningitidis, E.coli K1
- outer layer of G-ve bacteria can rapidly shed C5b-C9 MAC
- C3a and C5a proteases
- coat self with non-complement fixing IgA
e.g. The M-protein of S. pyogenes binds factor H of the complement pathway
and this leads to the degradation of the opsonin C3b by factor I and the formation of C3 convertase
Streptococcus pneumoniae
inhalation into the lung
colonises by the nasopharynx due to adhesion molecules, produces secretory IgA proteases during colonisation so they can invade further after post-viral damage - viral infection will damage structure of epithelial cells.
switches on genes for pneumolysin, a toxin that damages pneumocyte membranes - destroys defensive barriers allowing bacteria to replicate there
escapes phagocytosis via capsules
Inflammatory process driven by teichoic acids
pneumonia - bacteraemia - meningitis and septicaemia
immune response to imp bacteria?
look at table
Antigenic Variation - Definitions
successive expression of alternative forms of an antigen - in a specific clone or its progeny
Phase variation - ON/OFF of an antigen at low frequency
- occurs: during course of infection in an individual host or during spread of microbe through a community
what is antigenic variation a strategy for
immune evasion and pathogenesis
Antigenic Diversity/ polymorphisms (slow, rare)
- genetically stable and alternative forms of antigens
in a population of microbes
e.g. serotypes of Strep.pneumoniae
Neisseria gonorrhoeae - Paradigm of immune evasion as a pathogenic mechanism
- surface components interact with host cell
- components vary at high frequency in a population of bacteria
- variation to avoid immune response
Phase and antigenic variation in Neisseria affects what?
affects cell surface components: capsule, pilus, opa, outer and inner membrane
All of these structures can undergo either:
- Phase variation i.e. an ON-OFF switch (capsule, Opa’s)
- Antigenic variation e.g. pilins (or both phase and antigenic)
Mechanisms of phase variation in bacteria?
Intra-genomic recombination e.g. N.gonorrhoeae Pillins
Host derived Environmental triggers of toxin Synthesis and Release
Bacterial toxin production - tightly controlled
Regulatory elements - sensitive to environmental signals found in host
1. Diphtheria toxin - totally repressed by adequate Iron. Limiting iron (i.e. in blood) - toxin de-repressed = expressed
- Cholera toxin and related virulence factors (adhesins) - controlled by environmental osmolarity and temperature
- B. pertussis, induction of different virulence components – staggered attachment factors are produced initially to establish the infection, toxins are synthesized and released later to counter the host defences and promote bacterial survival.
How bacteria avoid the immune response – innate and adaptive summary
INNATE:
- C3a and C5A proteases
- opsonisation (binding) inhibition
- complement activation inhibition
- survival inside macrophages
ADAPTIVE
- antigen presentation inhibition
- blockage of cell cycle progression
- apoptosis inhibition
- superantigens and appropriate immune activation
- Ig binding proteins (protein A)
- sIgA proteases
host mimicry - M protein
Phase and antigenic variation
Virulence Factors that promote Bacterial colonisation and survival?
pili - adherence to mucosal surfaces
non-fimbrial adhesins - tight binding to cells
sIgA proteases - prevent trapping in mucin
siderophores - iron acquisition
capsules - prevent phagocytosis and block complement
altered LPS - serum resistance, no MAC
Ca5 peptidases - downregulation of complement signalling
surface protein variation - evade immunity
toxic proteins/enzymes - kill PMN’s, damage tissues
communications in Infections - Host-pathogen interactions
Signalling – TLRs, PRRs and PAMPs
- Inter- and intra – cellular communications in eukaryotic cells
- signalling
e.g. G-proteins
ion channels
second messengers e.g. cAMP
actin, microtubules - Bacterial signalling - 2 component signal transduction systems
Quorum sensing
Cytokine mimics - Bacterial reception of host signals
adaptation through gene expression
