Bacterial Pathogens and Diseases I (Exotoxins) Flashcards
what is a pathogen?
a microorganism capable of causing disease
what is meant by pathogenicity?
the ability of an infectious agent to cause disease
what is meant by virulence?
the quantitative ability of an agent to cause disease
what is meant by toxigenicity?
the ability of a microorganism to produce a toxin that contributes to the development of disease
what are virulence factors?
Factors that bacteria have genetically acquired and encoded in their genome. They are necessary for the bacteria to undergo the process that leads to disease
examples:
adherence factors (to get into tissues)
biofilms
invasion of host cells and tissues
toxins – endotoxins and exotoxins
what are exotoxins?
Heterogeneous group of proteins produced and secreted by living bacterial cells into the host tissues, causing disease symptoms
Produced by both gram negative and gram positive bacteria
What selective advantages do exotoxins give to the bacteria?
help cause disease
Evade immune response Enable biofilm formation Enable attachment to host cells. Escape from phagosomes Allowing carriage without disease
example of toxins
Haemolytic toxins and Phenol soluble modulins
where is s. aureus in our body?
commensal in our nasal cavity – it isn’t doing any damage or releasing toxins
how does s. aureus colonise?
in a bio film
PSMs give it gliding properties – useful for when it lives in the nose. Also contains toxins that prevent that phagolyosome fusion and therefore prevent bacteria death. PSM allows bacteria to escape from the early phagosome.
Haemolytic toxins to damage other organisms and prevent them from growing – bacteria are in competition, need to kill off other bacteria so they can grow.
Haemolytic toxins
cause cells to lyse by forming pores
Important cause of features of S. aureus disease.
Phenol soluble modulins
interfere with and aggregate the lipid bilayer of host cells, leading to lysis
bacterial chromosome?
single circular piece of double stranded DNA
where are toxin genes encoded?
most toxin genes are encoded on the chromosome, but not all of them - many toxins coded by extrachromosomal genes (plasmids)
how do bacteria exchange genetic information?
conjugation, transduction and transfection
plasmids play a role
what are plasmids?
plasmids are independently replicating small pieces of circular DNA that exist in the bacteria separate from the chromosome, can be passed onto other bacteria via conjugation, transduction and transfection – this is partly how bacteria exchange genetic information
explain transfection
toxins can be transferred from 1 bacteria to another via bacteriophages
a bacteriophage is a virus that infects a bacteria, and as a result it picks up some of the genes in the bacteria, and transfers it to the next bacteria it infects
Classification of Exotoxins
- Membrane Acting Toxins – Type I
- Membrane Damaging Toxins – Type II
- Intracellular Toxins – Type III
Membrane Acting toxins – Type I
act - on the outside of cells or on the membrane
interfere - host cell signaling by inappropriate activation of host cell receptor
target receptors:
- Guanylyl cyclase, increase intracellular cGMP
- Adenyl cyclase, increase intracellular cAMP
- Rho and Ras proteins (enzymes involved in generating GTP)
an example of a type 1 membrane acting toxin
ecoli, produces an STa toxin
once STa engages with the receptor, it causes an increase in cGMP
Membrane Damaging toxins – Type II
cause damage to the host cell membrane
- insert channels into host cell membrane (beta sheet or alpha helix toxins)
- enzymatical damage
Receptor mediated or Receptor Independent
explain the difference between receptor mediated and receptor independent (type II membrane damaging toxins)
receptor mediated is where a toxin binds to the toxic specific receptor, and defined (hexametric or octametric) pores forms (alpha toxins)
receptor independent is where the toxin attaches itself to the membrane and causes it to disintegrate, and there are formation of short lived pores (many alpha type PSM’s)
Intracellular toxins – type III
these toxins go inside cells
2 components: A and B
A part- activity, toxigenic and enzymatic
B part- receptor binding and translocation function
May be single or multiple B units e.g. Cholera toxin AB5. 5 binding subunits: Active domain sitting on the top.
Once the toxin is inside the cell, it releases the A part which has biological activity, usually involves some sort of enzymatic degradation.
The protein is held together by electrostatic attractions, meaning its usually susceptible to heat. On the other hand if it’s a single polypeptide chain, heat will not affect or inactivate it.
Another type of type III toxin squirts effector proteins to disrupt the cell
the enzymatic component A of Type III toxins has a wide variety of activities - give some examples:
ADP – ribosyl transferases - e.g. Exotoxin A of Pseudomonas aeruginosa, pertussis toxin.
Glucosyltransferases – e.g. TcdA from Clostridium difficile, modifies ribosomal RNA so protein synthesis doesn’t happen efficiently
Deamidase – e.g. dermonecrotic toxin of Bordetella pertussis.
Protease – e.g. Clostridial neurotoxins: botulism & tetanus, damage to presynaptic vesicles on neuromuscular junction or in the brain
Adenylcyclase - e.g. EF toxin of Bacillus anthracis
Toxins can have an effect via dysregulation of immunity- what does this mean?
- they can induce inflammatory cytokine release
- they can act as super antigens, covalently cross links MHC class II to the t-cell receptor. Activates huge numbers of T-cell populations, which will all produce inflammatory cytokines. The super-antigens causes clonal activation of T-cells, leading to sepsis and septic shock.
what are toxoids?
Toxins, which are inactivated using formaldehyde or glutaraldehyde
they are inactive proteins but still highly immunogenic – form the basis for vaccines
examples: Tetanus Vaccine, Diphtheria or Pertussi
(also, we can use antibodies against toxins as part of the treatment, so tetanus is treated by cooled human Ig that contains antibodies against the toxin)
microbiology of Clostridium difficile
gram-positive bacillus.
anaerobic.
toxin-producing.
can be carried asymptomatically in the gut.
3 toxins.
makes spores that can survive in the environment for months, difficult to kill
is clostridium difficile common?
yes it is a common hospital acquired infection worldwide
how does Clostridium difficile act and spread?
disruption of normal gut flora meaning c diff can grow and start to produce toxins that damage the epithelial surface of the gut
spread by ingestion of spores – remain dormant in environment
risk factors of c. diff?
age
prolonged hospital stay
antacids – for people with acid reflux problems. If there is a spore, usually the stomach acid can kill it off, but if you’re on antacids the spore can get into your cut and cause acute watery diarrhoea
antibiotics - provide a competitive advantage to spore forming anaerobes over non spore forming anaerobes, allowing c diff colonisation and growth
pathogenesis of c. diff?
Receptor binding domain
Hydrophobic domain that allows it to get across the membrane
Glucosyltransferase domain and Protease domain
- toxins get into the cell via receptor mediated endocytosis
- acidification of the endosome
- pore formation, releasing the toxic component
- disruption of cAMP and gCMP and disruption to ros and ras proteins.
what effects does c. diff have on the body?
watery diarrhoea
dysentery
pseudomembranous colitis (necrotic damage to the mucous membrane inside the colon, look via a colonoscopy)
toxic megacolon
raised white cell count in blood
detection of toxins in stool (Toxin ELISA)
C. diff treatment
depends on severity and presence of surgical complications
removal of offending antibiotic
surgery – partial, total colectomy
Recurrent – faecal transplant
what is verocytotoxcin?
some e.coli that has acquired a gene that encodes verocytotoxcin – a very damaging toxin, causes haemorrhagic acute watery diarrhoea
it is also called STEC
transmission of e coli?
consuming contaminated food and water
person to person, particularly in child day-care facilities
animal to person (petting zoos, dairy farms, camp grounds)
very low infectious dose
what type of toxin is verocytotoxcin/shiga toxin?
Stx toxin is a type II exotoxin with an AB5 structure
A is the enzymatic component, which is N-Glycosidase. This damages ribosomal RNA, inhibiting protein synthesis and causing cell damage. The A is bound to 5 B subunits
The gene is carried on lysogenic bacteria.
mechanism of action of the Stx (shiga) toxin?
Binds to Gb3 receptor on host cell membrane, and internalised by endocytosis
transported through the golgi apparatus into the ER where the ribosomes are sitting
Active subunit cleaved off, it will N-glycosylate the 28s ribosomal RNA in eukaryotic ribosomes and prevents protein synthesis
Pathogenesis of STEC?
b-binding domain of the toxins allows it to bind to endothelial cells as well as epithelial cells in the mucosa
Stx binds to glomerular endothelial cells of kidney, cardiovascular and CNS - v high levels of Gb3 in kidney so kidneys most affected
inflammatory response, causes thrombotic clotting of the bacteria, blocking the glomeruli – gives you haemolytic urine syndrome
STEC Disease
Shiga toxin-producing Escherichia coli, or Verotoxigenic E. coli/VTEC
severe and life threatening
children < 5 years are at the greatest risk
abdominal cramps, watery or bloody diarrhoea
haemolytic uraemic syndrome (anaemia, renal failure and thrombocytopaenia)
neurological symptoms are less common (lethargy, severe headache, convulsions)
STEC Diagnosis
Clinical signs and symptoms
Haematological and biochemical evidence.
Stool culture – Growth on SMac
PCR for Stx genes
STEC Treatment
Supportive including renal dialysis and blood product transfusion
Antibiotics have little to no role
