5. Diphtheria Flashcards
What bacterium causes Diphtheria?
Corynebacterium diphtheriae - colonisation, A-B toxin
What does the 6 in 1 vaccine prevent against?
Diphtheria, tetanus, whooping cough (pertussis), Hib (Haemophilus influenzae B), polio, Hepatitis B
Features of Corynebacterium diphtheriae?
- G+ve
- Non spore-forming
- Non-motile
- Aerobic rod
Diphtheria is primarily a disease of…
Children
True or False: Diphtheria has a high fatality rate if left untreated
True
What does the 4 in 1 vaccine prevent against?
Diphtheria, tetanus, whooping cough, polio
Symptoms of diphtheria
Initially - malaise, sore throat, fever
Progresses to - irregular heartbeat, difficulty swallowing, coma, death
Bull neck is common
What happens when Corynebacterium diphtheria colonises the throat?
A pseudomembrane forms - can progress to lungs
Attachment aided by the pili encoded by Spa genes
What type of toxin is diphtheria toxin?
Simple A-B toxin - enters bloodstream, no invasion (bacteria doesn’t enter bloodstream the toxin is delivered)
Highly potent
What does the transmembrane domain do?
Delivers A subunit across membrane of endosome into cytoplasm
What is the natural response of the intoxicated cell?
To undergo endocytosis
What are the 3 functions in a simple A-B toxin?
A domain (light chain, catalytic domain) has the toxic activity (in this case, an ADP ribosylase). B domain (heavy chain) has 2 distinct structural domains that correlate with 2 distinct functions - receptor binding domain & transmembrane domain
What is the A subunit in the diphtheria toxin?
An ADP-ribosylase
Attaches to an ADP-ribose on elongation factor-2 at a modified histidine residue → protein synthesis ceases
What is a source of ADP-ribose?
NAD
How does the diphtheria toxin target cardiac cells & cause heart problems?
Receptor for B (heparin binding epidermal growth factor [HB-EGF]-like receptor) found mainly on cardiac cells
(serious symptom)
Process of diphtheria toxin:
- Toxin circulates in blood
- Cleavage of A-B toxin by surface associated host protease e.g. furin
- A and B domains separate but A domain still connected to transmembrane subdomain by disulphide bridge
- Endocytosis occurs & toxin taken up into endosome
- Aqueous lumen of endosome contains toxin now
- Endosome undergoes maturation process - acidification
- Decrease in pH (critical) - T domain conformational change
- T domain inserts into membrane & delivers A subunit across membrane into cytoplasm
- Cytoplasm’s reducing environment breaks disulphide bridge - releases A subunit into cytoplasm
- Toxin activated
- Disrupts cellular biochemistry
What is an important step in effective toxin delivery?
Endosome undergoing maturation process → acidification (transmembrane domain can now undergo a conformational change and insert into endosomal membrane)
Where is the toxin gene for Corynebacterium diphtheriae located?
On a temperate (lysogenic) bacteriophage
The bacterial regulator of diphtheria toxins (DtxR) is sensitive to what?
Iron levels
When there is high iron, DtxR-Fe binds to what?
DNA, and then no toxin is produced
What happens to DtxR when there is low iron (as in body)?
DtxR does not bind to DNA, and then the toxin is produced
Are you vaccinated against intoxication by Diphtheria toxin, or infection by Corynebacterium diphtheriae?
Intoxication
How is Diphtheria treated?
Toxin accounts for all symptoms so patient is dosed with equine antitoxin to bind free toxin (speed essential). Followed by a course of powerful antibiotics
What is used as the vaccine (toxoid)?
Formalin treated toxin
What other form of diphtheria can you get?
Cutaneous diphtheria - skin infection (ulceration & necrosis)
Therapeutic applications of diphtheria toxin?
- Replace binding domain with another binding domain which recognises a new target cell (e.g. one infected with AIDS or cancer cell). Will not direct the toxic A fragment to the target e.g. Tf-CRM107, fusion between transferrin & derivative of diphtheria toxin with increased toxicity
or - Use non-toxic derivative (inactive) of diphtheria toxin (CRM197) to increase immunogenicity of pneumococcal vaccine (PCV)
