Bacterial pathogens and disease Flashcards
Define pathogen, pathogenicity, virulence, toxigenicity
Pathogen: A microorganism capable of causing disease
Pathogenicity: The ability of an infectious agent to cause disease
Virulence: the quantitative ability of an agent to cause disease
Toxigenicity: the ability of a microorganism to produce a toxin that contributes to the development of disease
What mechanisms give virulence to bacteria?
Adherence factors, proteins and molecules that allow bacteria to attach to surface
Biofilm, allows bacteria to produce 3D communities and structures
Invasion of host cells and tissues, allowed by mechanisms – allows colonisation
Toxins – endotoxins and exotoxins
What are exotoxins?
Heterogenous group of proteins produced and secreted by living bacterial cells
Produced by both gram negative and gram-positive bacteria
NOT the result of breakdown products, produced by living bacteria.
Cause disease symptoms in host during disease
Act via a variety of diverse mechanisms
What selective advantage does having exotoxins give to the bacteria?
Cause disease? May help transmission of disease by promoting carriage, however in severe disease host may be a literal and evolutionary dead end.
Evade immune response
Enable biofilm formation
Enable attachment to host cells
Escape from phagosomes
allow for colonisation, niche estabilishment and carriage - evolutionary advantage
What do exotoxins produce in the case of staphylococcus aureus?
Haemolytic toxins
- Cause cells to lyse by forming pores on the membrane
- Important cause of features of S.Aureus disease
- a, B, Y toxins, Panton Valentine leukocidin (PVL), LukAB, LukEF, LukMF
Phenol soluble modulins PSM
- Aggregate the lipid bilayer of host cells – causes cell lysis, damages lipid bilayer
Majority of S.aureus in humans is asymptomatic carriage in the nose and respiratory tract, and skin.
How does PSM help bacteria, starting cell cell phagocytososing bacteria
A. Cell, phagocytosing bacteria. PSMs allow bacteria to escape from phagosome. Alpha toxins block the binding of lysosomes to the phagosome, avoiding the destruction by acidification and enzymes released by lysosomes.
B. PSM useful at helping S.aureus in displacing other bacteria in the environment. By killing all the bacteria, it can find a niche and be dominant.
C. S.Aureus releases PSM as it allows it to slide through certain media like throats and mucosal sites.
D. Alpha toxins, beta toxins and PSM help initiate formation of biofilms, a community of bacteria that first need to find a niche and attach, accumulate and then interact with other molecules and different organisms to form a secondary structure.
Alpha and beta toxins allow this process. PSM helps chunks of that biofilm move and aids carriage.
What can exotoxins be encoded by?
Encoded by chromosomal genes Shiga toxin in Shigella dysenteriae, TcdA and TcdB in C.difficle.
Many toxins coded by extrachromosomal genes :
- Plasmids – Bacillus anthracis toxin, tetanus toxin
- Lysogenic bacteriophage (viruses of bacteria) – e.g streptococcal pyrogenic exotoxins in Scarlet Fever, Diphtheria toxin
toxins can be transferred, species that could not cause disease now cause disease
Toxins can be classified by the toxins activity
What are the issues with this classification?
- Membrane acting toxins – type 1
- Membrane damaging toxins – type 2
- Intracellular toxins – type 3
2.
Many toxins may have more than one type of activity
As mechanisms better understood by this classification tends to break down
Where do type 1, mechanism activating toxins:
Act
Interfere
Target
Act:
- Act from without the cell
Interfere:
- Interfere with the host cell signalling by inappropriate activation of host cell receptors
Target:
Target receptors include
- Guanylyl cyclase – increased intracellular cGMP
- Adenyl cyclase – increased intracellular cAMP
- Rho proteins
- Ras proteins
acting on cGMP and cAMP messes up central signalling pathways of host cells + conserved pathways, potential to cause severe diseases as most cells rely on production of cGMP and cAMP.
How do type 2, membrane damaging toxins cause damage to the host cell membrane?
1.
- Insert channels into the host cell membrane, disrupting the potential and transport of ions
- Beta sheet toxins e.g S.aureus a-toxin, y toxin, PVL
- Alpha helix toxins e.g diphtheria toxin
- Enzymatical damage that damages the cell e.g S.aureus Beta – haemolysin, PSM
Or
- Receptor mediated
- Receptor independent
Where are intracellular toxins, type 3 active?
What two components is it made of?
- Active within the cell – must gain access to the cell
Usually 2 components – AB toxins
- Receptor binding and translocation function – B
- Toxigenic (enzymatic) - A
- May be single or multiple B units e.g Cholera toxin AB5 – 5 B subunits.
Component B is the one that binds to the cell surface receptor to allow for internalisation of the toxin.
A is toxic and has enzymatic activity or tampers with metabolism of the cell – acts within cell.
Enzymatic component A of type 3 toxins has a wide variety of activities
ADP – ribsoyl transferases e.g Exotoxin A of Pseudomonas aeruginosa, pertussis toxin
Glucosyltransferases e.g TcdA and TcdB of Clostridium difficile
Deamidase e.g dermonecrotic toxin of Bordetella pertussis
Protease e.g Clostridial neurotoxins: botulism and tetanus
Adenylcyclase e.g EF (edema factor) toxin Bacillus antrhacis
Bacteria have different types of secretory mechanisms
Give examples
Secretions
And toxin injections
Multi-molecule complexes that act like needles.
Can have multi-meric secretory system, a pump of toxins like toxin CagA in Heliobacter pylori
On the left is an inner and outer membrane of a cell, a subunit protrudes and interacts with the host membrane to inject toxin.
e.g YopE in Yersinia species
What do exotoxins induce the release of?
Induce release of inflammatory cytokines
IL1, IL1B(beta), TNF, IL6, interferon y, IL18
How do supernatigens interact with the immune system and cause disease?
Superantigen: non-specific bridging of the MHC class II and T-cell receptor leading to cytokine production
e.g Staphyloccal Exfoliative Toxin A, toxic Shock Syndrome Toxin I (TSST1)
Via activation of the different inflammasome leading to release of IL1 Beta and IL18 e.g S.aureus toxin A, PVL
What happens when exotoxins bypass antigen presentation and cause clonal activation?
Bypass and crosslink the MHC to the T-cell receptor, do this regardless of the specificity of the MHC peptide complex and T cell receptor.
Consequence of this that it produces activation of many different clones.
Leads to a huge inflammatory response, a great amount of cytokine production, and then induce toxic shock.
example of this is Staphlyococcal toxins that cause this, transmitted through tampons – difficult to treat and can lead to fatality so not always beneficial to toxin.
Toxins can be supernatigens or inflammasome, what is an inflammasome?
Inflammasome is complex set of pathways, able to detect infection or damage, leading onto maturation and production of cytokines – leading to an immune response.
How can toxins be inactivated?
What are toxoids?
- Toxins can be inactivated using formaldehyde or glutaraldehyde – toxoids
- Toxoids are inactive proteins but still highly immunogenic – form the basis of vaccines
- Tetanus vaccine
- Diphtheria
- Pertussis (acellular)
How is toxin mediated disease treated?
By adminstering performed antibodies to the toxin
- Diptheria antitoxin – horse antibodies
- Tetanus – pooled human immunoglobulin. Specific or normal
- Botulism – horse antibodies
- Experimental and research – monoclonal antibodies
antibodies don’t have function in immune system but block toxin.
Describe the microbiology of clostridium difficile disease
Gram-positive bacillus
Anaerobic
Spore-forming – produces spores that are resistant to specific environments etc so can transmit easily
Toxin-producing – like TcdA and TcdB
Can be carried asymptomatically in gut
3 toxins
What is the epidemiology of c.difficle?
Common hospital acquired infection worldwide
Spread by ingestion of spores – remain dormant in environment
Coloniser of the human gut up to 5% in adults – some of those patients need risk factors to develop the disease
Risk factors – antibiotic use, age, antacids and prolonged hospital stay
How do antibiotics treat c.difficle?
- Act by disrupting the microbial ecosystem within gut
- Antibiotics provide a competitive advantage to spore forming anaerobes over non-spore forming anaerobes
- Allows C.difficile colonisation and growth
- All antibiotics have potential for causing disease
Some antibiotics worse than others
- 2nd and 3rd generation cephalosporins
- Quinolones
- Clindamycin ? - not clear
Others less likely
- Aminoglycosides
- Trimethoprim
- Vancomycin
What is the pathogenesis of C.difficile?
What two effects does GTD binding to Rho GTPases lead to?
Receptor binding domain binds to receptor, toxin internalised
Endosome acidifies, allows for increase in activity of CPD (cysteine protease domain)
and interaction of DD (delivery hydrophobic domain) allows for the release of the toxin from the endosome
GTD (glucotransferase domain) released from the endosome to the host cell cytoplasm thanks to protease activity of CPD
Binds to Rho GTPases, leading onto two effects
- Cytopathic effects – cytoskeleton breakdown, loss of cell-cell contacts, increased epithelial permeability - and other functions that depend on cytoskeleton like renewal of cells, division of stem cells etc. Prolonged damage.
- Cytotoxic effects – activation of the inflammasome, increase in ROS levels (causes bystander damage) , induction of programmed cell death
What is seen in the gut of someone with c.difficile disease?
- Patchy necrosis with neutrophil infiltration
- Epithelial ulcers
- Pseduomembranes – leucocytes, fibrin, mucous, cell debris
Starts asymptomatic, many of us can carry C.difficile without having a problem if it’s not predominant
When it starts producing symptoms it produces watery diarrhoea
Induces Pseudomembranous Colitis
Some circumstances it can lead onto toxic megacolon and peritonitis – fecal matter enters peritoneal cavity and we end up with peritonitis
How is C.difficle diagnosed?
What does the 2 phase test include?
Clinical signs and symptoms – watery diarrhoea
Raised white cell count in blood
Detection of organisms and toxins in stool
2 phase test
- Glutamate dehydrogenase – detects if C.difficile organism is present
- Toxin enzyme linked immunosorbent assay (ELISA) for TcdA and TcdB toxins
Detection of tcdA and tcdB genes – PCR
Colonoscopy – psuedomembranous colitis
How is C.diffcile disease treated?
Dependent on severity and presence of surgical complications
Ideally removal of offending antibiotic – not always possible
Antibiotics fidaxomicin or metronidazole or vancomycin
Surgery – partial, total colectomy
Recurrent – faceal transplant
What disease can verocytotoxin Edcherichia coli cause?
VTEC or shiga-toxin (stx) producing E.coli (STEC) can cause disease, mild to life threatening
Stx carried by some E.coli - most commonly 0.157:H7
- Identified usually by growth on sorbitol MacConkey agar (SMac) - does not ferment sorbitol and hence is clear
- Other less common types not identified using Smac
Where does E.coli naturally colonise?
How is it transmitted?
E.coli O157:H7 naturally colonises the gastrointestinal tracts of cattle who are generally asymptomatic
Transmission
- Predominantly via consumption of contaminated food and water
- Person to person, particularly in child day-care facilities
- Animal to person e.g in petting zoos, farms or camp groups
Very low infectious dose
What is the pathogenesis of the Shigatoxin and verocytotoxin?
Gene carried on
Type of exotoxin
Enzymatic components A and B
Toxin – Shiga like toxin (SLT) = shigatoxin (Stx) = verocytotoxin (VTEC)
Stx, Stx1, Stx1a, 1c, 1d, Stx2a, 2c 2d – variations in a.a sequence
Gene carried on lysogenic virus
Type III exotoxin – AB5
Enzymatic component A = N-Glycosidase
Bound to 5 B subunits
What is the mechanism of Stx and VTEC?
Binds to receptor globo-tri-aosyl-ceramide Gb3 or globo-tetra-osyl-ceramide (Gb4) on host cell membrane
Bound toxin internalised by receptor mediated endocytosis
Carried by retrograde trafficking via the golgi apparatus to the endoplasmic reticulum
The A subunit is cleaved off by membrane bound proteases
Once in the cytoplasm A1 and A2 disassociated
A1 binds to 28S RNA subunit – blocks protein synthesis
STEC pathogenesis
What cells does it to adhere to?
Where are there high levels of Gb3
What does Stx favour
STEC closely adheres to the epithelial cells of the gut mucosa
The route by which Stx transported from the intestine to the kidney and other tissues is debated, possible polymorphonuclear neutrophils (PMNs)
Bind to the glomerular endothelial cells of kidney, cardiovascular and central nervous system - high levels of Gb3 in kidney so kidneys most affected
Stx favours inflammation resulting in microvascular thrombosis and inhibition of fibrinolysis
What are the symptoms of STEC disease?
Who is at greatest risk?
Can be severe and life threatening
Children <5 years at greatest risk
Abdominal cramps, watery or bloody diarrhoea – may not be present
Haemolytic uraemic syndrome
- Anaemia
- Renal failure
- Thrombocytopaenia
Less common are neurological symptoms
- Lethargy
- Severe headache
- Convulsions
- Encephalopathy
How is STEC diagnosed?
Clinical signs and symptoms
Haematological and biochemical evidence
Stool culture – growth on SMac
PCR for stx genes
How is STEC treated?
Supportive including renal dialysis and blood product transfusion
Antibiotics have little to no role
Difficult to treat
