Bacterial Pathogens and Diseases 1 Flashcards
What is a pathogen ?
A micro-organism capable of causing disease
What is pathogenicity ?
The ability of an infectious agent to cause disease
What is Virulence ?
The quantitive ability of an agent to cause disease. To what extent does it cause disease
Whta is Toxigenicity ?
The ability of a micro-organism to produce a toxin that contributes to the development of disease
What are the virulence mechanisms ?
The mechanisms through which a pathogen can cause disease : Adherence factors (molecules and proteins that allow the attachment of bacteria to find a niche and start colonisation )
Biofilms
(Molecules that allow bacteria to form complex structures for macromolecules)
Invasion of Host cells and tissues
To evade phagocytosis , colonise tissues )
Toxins-Endotoxins and exotoxins
What are exotoxins ?
These are a diverse heterogeneous group of proteins produces and secreted by living bacterial cells (not byproducts or waste )
Produced by gram negative and positive bacteria
Cause disease symptoms in host during disease
Act via a variety of diverse mechanisms
What are gram negative bacteria ?
Gram negative bacteria have a thin peptidoglycan layer and have no outer lipid membrane
They do not retain the violet stain
What are gram positive bacteria ?
Gram positive bacteria have a thick peptidoglycan layer and no outer lipid membrane
These stain violet
What selective advantages do exotoxins give bacteria ?
They help transmission of disease They evade immune response They enable biofilm formation They enable attachment to host cells They escape from phagosomes
They allow for colonisation ,niche establishment and carriage -Evolutionary advantage
What is a example of bacteria found on mucosal surfaces ?
Staphylococcus aureus
What exotoxins does Staphylococcus auerus contain ?
Haemolytic toxins
- Cause cells to lyse by forming pores
- Important cause of features of S.aerus disease
- alpha , beta and gamma toxins , Panton Valentine Leukocidin (PVL) , LukAB, LukED, LukMF
Phenol soluble modulins PSM
-Aggregate the lipid bilayer of host cells -lysis
(Majority of S.aureus in humans is asymptomatic carriage in the nose )
What are some functions of the toxins found in S.aureus ?
A. When the S.auerus is phagocytosed , the alpha toxins can block the fusion of lysosomes to phagolysosome so the organism can stay.
Modulins can aid the escape of the S.auereus from the phagolysosome
B. Some molecules e.g. phenol soluble modulins can be harmful to other micro-organisms ( competition).This allows it to find a niche within the nasal cavity. It also allows S.aeurus which is a cocci (no flagella) to slide through surfaces to colonise the nasal mucosa.
C.
What are Cocci bacteria ?
A coccus (plural cocci) is any bacterium or archaeon that has a spherical, ovoid, or generally round shape.
Outline the genetics of exotoxins
Can be encoded by chromosomal genes Shiga toxin in Shigella dysenteriae, TcdA and TcdB in C.difficile
Many toxins coded by extrachromosomal genes
(these can be transferred /exchanged )
-Plasmids -Bacillus anthracis toxin , tetanus toxin
-Lysogenic bacteriophage
(a virus which infects and replicated within bacteria /archaea)
e.g. streptococcal pyrogenic exotoxins in Scarlet Fever , Diphtheria toxins
Describe the classification of Exotoxins
1.Membrane Acting toxins -Type 1
(get in contact with cell surface receptors and interfere with processes)
2.Membrane Damaging Toxins -Type 2
(Physically damage )
3.Intracellular Toxins - Type 3
(Interfere with cellular processes)
Issues with this classification =Many toxins have more than one type activity
As mechanisms better understood this classification tend to break down
Outline the main characteristics of the membrane acting toxins
Act :
Act from without the cell
(outside the cell)
Interfere:
Interfere with the host cell signalling by inappropriate activation of host cell receptors
Target :
Target receptors include:
Guanylyl cyclase -increases intracellular cGMP
Adenylyl cyclase - Increases intracellular cAMP
Rho proteins
Ras proteins
Give an example of a type 1 exotoxin
E.coli Stable heat toxin
It will affect the downstream signalling .
The toxin affects the cystic fibrosis transmembrane receptor causing changes in important ion transporters within tissues.
E.g. Chlorine , bicarbonate ,sodium transporters are affected
Effect is an increase in sodium chloride ions and this can cause diarrhoea.
Outline the Type 2 exotoxins and how they work
These cause damage to host cell membrane
They can interact with the receptor a channel
polymerising a channel causing disruption of ions.
(Action potentials )
1.Insert channels to host cell membrane
-Beta sheet toxins S.aureus –Alpha toxin , gamma toxin , PVL
alpha helix toxins -dipheria toxin
-Can produce pores in membrane by polymerising.
2.Enzymatical damage e.g. S.aureus beta -haemolysin, PSM
Not receptor function but receptor itself
Receptor mediated
Receptor independent
How do Membrane damaging toxins function ?
Receptor Mediated interaction
Polymerise a pore , forms a hole in the membrane .
Doesn’t affect function of receptor.
Receptor independent may attach to membrane and alter the phospholipid bilayer structure .
Outline the type 3- Intracellular toxins function
These are active within the cell which means they need to gain access.
Usually two components -AB toxins
Receptor binding and translocation function -B
(can bind to host cell receptor) -Internalised through receptor mediated endocytosis
Toxigenic (enzymatic )- A
May be single or multiple B units e.g. Cholera toxin AB(5)- has electrostatic interactions and this makes it heat sensitive.
AB are covalently linked and heat resistance
What are some example of the enyzmatic A componenet of intracellular toxins ?
Wide variety of activities
These can modify enzymatic activity
ADP-ribosyl transferases -Exotoxin A of Pseudomonas aeruginosa
pertussis toxin
Glucosyltransferases e.g. TcdA and TcdB of Closetridium difficile.
can affect ribosomal Rna and inhibit protein synthesis
Deamidase - dermo necrotic toxin of Bordetella pertussis
Protease -Clostridial neurotoxins :botulism and tetanus
These can destroy proteins
and affect presynaptic structure
Adenylcyclase -e.g. EF toxin of Bacillus anthracis
This can affect production of cAMP which is a key messenger for many processes
Give some further examples of intracellular toxins -type 3 other
Type 3 secretion and toxin injections
e.g. YopE in Yersinia species
This bacteria has managed to inject its toxins into host cells.
Produce multi-protein complexes which anchor to the membrane. Inner membrane and outer membrane + protein complex which forms a needle and injects the toxins.
Type IV secretion and toxin injection
e. g. CagA in Helicobacter pylori
- This is a different secretion process
Outline super antigens and inflammatory cytokines
Exotoxins are able to induce inflammatory cytokine response.
Antigen presenting cell
Cytokines which are released:
IL1, IL1 beta , TNF, IL6 , interferon gamma , IL18
Due to hyperactivation of T cells which is not antigen specific
Mechanisms : Super antigen : non specific bridging of the MHC class 2 and T-cell receptor leading to cytokine production e.g. Staphylococcal exfoliative toxin A , Toxic shock syndrome Toxin 1 (TSST1)
Via activation of the different inflammasome leading to release of IL1 beta and IL18 e.g. S.auereus toxin A, PVL
Activate cell upon detection of pathogen patterns, death by paraptosis
What are toxoids?
Toxins can be inactivated by using formaldehyde/glutaraldehyde which creates toxoids
These are inactive proteins but still highly immunogenic - form the basis of vaccines.
E.g.
Tetanus Vaccine
Diphtheria
Pertussis (Acellular)
Treatment of toxin mediated disease can be affected by administering preformed antibodies to the toxin
Diphtheria antitoxin-horse antibodies
Tetanus-Pooled human immunoglobulin (specific /normal)
Botulism-horse antibodies
Experimental and research-Monoclonal antibodies
Give an example of a toxin mediated disease
Toxins in Clostridium dificile disease:
This is a gram positive bacillus (rod shaped0
Anaerobic
Spore forming -highly resistant
Toxin-producing
Can be carried asymptomatically in the gut -if controlled by microbiota
3 toxins
What is the epidemiology of C.dificile ?
Common hospital acquired infection worldwide
Spread through ingestion of spores-dormant in environment
Coloniser of the human gut up to 5% in adults
Risk factors-Antibiotic use, age ,antacids and prolonged hospital stay
How can C.difice be caused by antibiotics?
They act to destroy the microbial ecosystem within the gut
Antibiotics provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes.
Allows C.dificile colonisation +growth
All antibiotics have potential for causing disease.
Examples:
Some antibiotics are worse: 2nd and 3rd generation cephalosporins Quinolones Clindamycin Aminoglycosides Trimethoprim Vancomycin
Outline the pathogenesis of C.dificle (Toxin-mediated disease)
Mediated by the toxin it contains:
Cytotoxin A- TcdA coded by the tcdA gene
Cytotoxin B-TcdB coded by tcdb gene
Binary toxin-C.diff transferase (CDT)-minor role in disease
Tcd A and Tcd B -Type 3 AB toxins
The A component of toxins are glycosylating enzymes
There is a receptor binding domain which binding domain which interacts with the cell for internalisation.
Hydrophobic domain which allows the escape of the active A component into the intracellular space. The GTD and CPD domain (A part of the toxin which mediate disease)
What are the components of the TcdA and TcdB ?
GTD_glucosyltransferase domain
CPD-Cycteine protease domain
DD-delivery hydrophobic domain
RBD-receptor binding domain
There is a receptor binding domain which binding domain which interacts with the cell for internalisation.
Hydrophobic domain which allows the escape of the active A component into the intracellular space. The GTD and CPD domain (A part of the toxin which mediate disease)
How does the toxin interact with the membrane and get internalised ?
- The toxin will bind to specific host cell receptors
- Receptor mediated endocytosis =internalised and cause damage
- Endosome will acidify which leads to creation of a pore-mediated by hydrophobic domain
- This allows release of active component A of toxin
- GTD will interact with Rho GTPases
- Deactivation effects within the host cell
Consequences:
Cytopathic effects changing structure of cell
cytoskeleton breakdown, epithelial cell permeability increase ,epithelial to epithelial cell contact loss
Cytotoxic
Activation of production of reactive oxygen species which are very toxic and damaging -can induce apoptosis
Outline cytopathic and cytotoxic
Patchy necrosis with neutrophil infiltration
Epithelial ulcers
Pseudomembranes,leuokocyes
fibrin mucous cell debris
May be asymptotic
Watery diarrhoea
dysentery
Pseudomembranous colitis
How can C.dificiel be diagnosed ?c
Clinical signs and symptoms
Raised white cell count in the blood
Detection of organisms and toxins in stool
2 phase test :
Glutamamte dehydrogenase -detects if C.dificile organism present
2.Toxin enzyme linked immunosorbent assay (ELISA) for TcdA and TcdB toxins)
Detection of tcdA/tcdB gene -PCR
Colonoscopy-pseudomembranous colitis
How can c.dificiel be treated ?
Treatment is 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-Faecal transplant (recover microbiota from a patient )
What is the Veroocytotoxin Escheria coli (VTEC) disease ?
VTEC or Shiga-toxin (Stx) producing E.coli (STEC) can cause disease mild to life threatening disease
Stx carried by some E.coli -most commonly 0157:H7
- Identified usually by growth on sorbitol MacConkey agar (SMac) -does not feremtn sorbitol and hence is clear
- other less common types n
What is the epidemiology of VTEC disease
E.coli 0157:H7 naturally colonises the gastrointestinal tracts of cattle who are generally asymptomatic
Transmission :
Predominantly via consumption of contaminated food/water
Person to person
Animal to person
Low infectious dose
What is the pathogenesis of VTEC diseases?
Toxin -Shiga like toxin (SLT)= shigatoxin (STx) = Verocytotroxin
Stx,Stx1,Stx1a,1c,1d Stx2a,2c,2d -variations in a.a sequence
gene carried on lysogenic bacteria
Type 3 exotoxin -AB 5
Enzymatic component A= N-Glycosidase
Bound to 5 B subunits
What is the Mechanism of VTEC disease?
Bind to receptor globotriaosylceramide Gb3 or glovbotetraosylceramide (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 dissociate
A1 binds to 28S RNA subunit -blocks protein synthesis
This can also cause a haemolytic uermic syndrome ( condition that affects blood /blood vessels )
Outline STEC pathogenesis
STEC closely adheres to the epithelial cells of the gut mucosa.
Stx is transported from the intestine to the kidney and other tissues is possibly through polymorphonuclear neutrophils (PMNs).
Bind to glomerular endothelial cells of kidney,cardiovascular and central nervous system.
very high levels of Gb3 in kidey so kidneys most affected.
Thought that Stx favours inflammation resulting in microvascular thrombosis and inhibition of fibrinolysis.
What can STEC disease lead to ?Symptoms
Abdominal cramps Watery diarrhoea Haemolytic uraemic syndrome Anaemia Renal failure Thrombocytopenia Lethargy Severe headache Convulsions Encephalopathy
How can STEC be diagnosed and treated ?
Clinical signs and symptoms
Haemotological and biochemical evidence
Stool culture -Grow on SMAc
PCR for Stx genes
Treatment :
Supportive including renal dialysis and blood product transfusion
Antibiotics have little to no role
