33.4 Bacterial Pathogenesis Flashcards
What is virulence?
The degree of damage inflicted by a pathogen.
What are Koch’s postulates, as a concept?
Four criteria that were designed to establish a causative relationship between a microbe and a disease.
State all of Koch’s postulates.
In order for a microbe to be confirmed as the cause of a disease:
- The microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms.
- The microorganism must be isolated from a diseased organism and grown in pure culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be reisolated from the diseased experimental host and identified as being identical to the original specific causative agent.
In other words, it’s checking at lots of different stages that a pathogen is the one causing the disease.
What are molecular Koch’s postulates?
A set of criteria that must be satisfied to show that a gene found in a pathogenic microorganism codes for a product that is the cause of a given disease.
(It differs from Koch’s postulates in that Koch’s postulates seek to find the microbe causing disease, while molecular Koch’s postulates seek to find the exact genes causing disease)
Who established molecular Koch’s postulates?
Stanley Falkow
Genes that satisfy molecular Koch’s postulates are often refer to as…
Virulence factors
State all of molecular Koch’s postulates.
In order to prove that a gene is the virulence gene that causes a disease, it must be shown that:
- The virulence factor is expressed by the pathogen
- Deletion of the gene causes attenuation of the virulence
- Complementation with the gene should restore virulence
What are septicaemia and bacteraemia?
Septicemia is a clinically significant form of bacteremia complicated by toxemia, fever, malaise, and often shock.
What is the definition of bacteraemia?
The presence of bacteria in the blood
Which bacteria is the most common cause of bacteraemia and sepsis?
Staphylococcus aureus
What is the definition of septicaemia?
The presence and multiplication of large amounts of bacteria in the blood
It is potentially life-threatening
Which bacteria are the most common cause of septicaemia?
S.aureus, S.pneumoniae, E.coli
What is the definition of sepsis?
dysregulated immune response to infection –> organ dysfunction
What is the definition of septic shock?
Sepsis associated with hypotension that does not improve with fluid treatment.
Happens when bacteria damage small blood vessels - leak fluid to surrounding tissues
Give some other examples of bacteria that can cause bacteraemia/sepsis/septic shock.
E. coli
Pneumococcus
Group A streptococcus
Salmonella
Pseudomonas aeruginosa
Name the key attributes that are necessary for a bacterium to be virulent. (5)
- Has specific invasion methods (tropism)
- Can damage host cells
- Can evade host immune system
- Can acquire nutrients e.g. iron
- Can do host to host transmission
What is host tropism and what enables it?
*The infection specificity of certain pathogens to particular hosts and host tissues.
* Tropism occurs do to:
* Invasion
* Motility
* Attachment/Adhesions
What are two examples of pili involved in UPEC (uropathogenic E. coli) infections? What is the role of each?
- Pap pilus -> Upper UTI (kidneys)
- Type I pilus -> Lower UTI (urethra, prostate, bladder)
Describe the role of the type I pilus of UPEC in UTIs.
Type I pilus causes lower UTIs:
- The lectin domain of the FimH tip adhesin of the type I pilus of UPEC (uropathogenic E. coli) binds to mannosylated uroplakins (found on plaques) in the bladder.
- This allows the UPEC to bind to the bladder, causing the UTI.
- Understanding this structure allows the design of anti-infectives.
What are bacterial toxins?
Factors which poison or intoxicate host cells, killing them or altering their function.
What are the two types of toxin?
- Exotoxins -> Secreted protein toxins
- Endotoxins -> Lipopolysaccharides (LPS) from the outer membrane of Gram-negative bacteria (cell-associated)
Can bacterial toxins be entirely responsible for pathogenesis?
- In some cases
*disease can be replicated by simply injecting the toxin, and the bacteria causes disease only by release of the toxin.
*E.g. diphtheria toxin can cause the same changes to the tonsils as diphtheria bacteria. - In other cases, there are many factors involved in the disease.
What are the different methods of immune evasion?
Host mimicry, antigenic variation, recruitment of immune modulators, subversion of immune responses.
How does N. meningitidis use host mimicry to evade the host immune response?
The capsule contains molecules that mimic human NCAM-I in serogroup B
So not recognised as foreign
What is the role of lipopolysaccharides (LPS) in an infection?
- Elicits inflammatory responses in humans
- Elicit complement activation via the alternative pathway
- Are involved in virulence and pathogenesis, by for example:
- Allowing attachment and invasion
- Acting via molecular mimicry to appear like host cells
Give some examples of molecular mimicry enabled by the O-antigens of the LPS of Gram-negative bacteria.
- H. pylori -> Mimic Lewis blood group
- Campylobacter -> Mimic GM1 ganglioside
What are bacterial capsules and what is their role?
- High molecular weight polymers on the surface of bacteria
- They are structurally diverse and can be structural mimics of host molecules
- They are important in resistance to the host’s immune response
What is antigenic variation (in bacteria)?
The mechanism by which a bacterium alters the proteins or carbohydrates on its surface and thus avoids a host immune response.
How does antigenic variation occur?
The sequence of a gene coding for a protein can change by:
- Point mutation
- Uptake of DNA by horizontal gene transfer
What are immune modulators?
Regulate immune response to help maintain balance.
*E.g. CKs (signalling)/ Hormones (such as cortisol)/ GFs (cells growth/ differentiation/ repair).
They can stimulate or suppress the immune response to help fight off infections and prevent excessive inflammation/ autoimmune responses.
What is the mechanism of S. aureus using protein A?
Uses protein A to bind to the Fc domain of IgG to prevent its action
Directly opposes host defence
Avoids bacterium being opsonised/attenuated/destroyed
Why is an IC life cycle beneficial for bacteria?
It allows them to avoid immune responses.
Give some examples of intracellular pathogens (which are adapted to survive within host cells).
- Listeria
- Shigella
- Salmonella
- Mycobacteria
- Coxiella
- Legionella
- Mycobacterium
What are the different strategies for IC pathogens to combat the endosome/ phagosome?
What type of bacterium are Shigella and Salmonella?
O-antigen-possessing Gram-negative intracellular bacteria
Where do Shigella and Salmonella infect?
- Shigella -> Colon
- Salmonella -> Small intestine
How do Shigella and Salmonella enter cells (from the GI tract)?
- Cross the epithelial barrier via M-cells (sentinel antigen-presenting cell that present antigens to macrophages underlying them)
- These M-cells are phagocytic by nature
- After this, the bacteria are then taken up by macrophages, in which pyroptotic cell death happens (in the case of Shigella)
- Sa**lmonella tend to prefer to stay in the macrophages and replicate there, while Shigella prefer to escape
- After this, the bacteria invade epithelial cells through the basolateral membrane using the type 3 secretion system
- Finally, the cell undergo cell-to-cell spread to adjacent cells
What are the symptoms of Shigella and Salmonella? Why?
- They can lead to inflammatory diarrhoea
- This is because they cause pyroptotic cell death (of phagocytes), which leads to release of cytokines that attract neutrophils
- This leads to the breakdown of the integrity of the epithelial barrier
For which bacteria do secretion systems have a role in cell entry?
- Shigella
- Salmonella
- Pseudomonas
What are secretion systems in bacteria?
Multi-protein systems that allow a wide range of substrates to be secreted from the cytosol into the extracellular space.
Describe the type 3 secretion systems in bacteria.
- Found in many different Gram-negative bacteria
- Type 3 secretion systems transport substrates across 3 cell membranes -> The inner cell membrane, outer cell membrane and the cell membrane of the host
- They have a basal body (like flagella do), needle and translocon
- Essentially they can be viewed as syringes that pump toxins directly into the host cell
- The exact function of the secretion system varies between bacterial species, depending on what substrates are secreted
What is the role of T3SS in these bacteria?
Describe the type 3 secretion systems in Salmonella.
There are two type 3 secretion systems:
- For entry -> SPI-1
- For intracellular replication -> SPI-2
Give an example of a Gram-positive intracellular pathogen.
Listeria -> Causes food poisoning and more serious diseases in the immunocompromised, elderly and pregnant women
What is the importance of secretion systems during bacteria entry into cell?
Many pathogens use dedicated protein secretion systems to secrete virulence factors from the cytosol of the bacteria into host cells or the host environment.
How can Mycobacterium tuberculosis survive inside host cells?
- Has a cell wall containing mycolic acids
- This prevents fusion of the lysosome with the phagosome that the bacterium is in, so it can survive
What are the main ways in which intracellular pathogens can survive within host cells? Which bacteria use each strategy?
Once taken up by a phagocyte, the bacterium is within an endosome, which the phagocyte plans to fuse a lysosome with. The bacteria can avoid this by:
- Escaping the endosome -> Shigella, Listeria
- Prevent fusion with the lysosome -> Salmonella, Mycobacteria
- Being able to survive in the phagolysosome -> Coxiella, Legionella, Mycobacterium
Give three examples of extracellular pathogens
- Neisseria meningitidis
- Staphylococcus aureus
- Streptococcus pneumoniae
What are superantigens?
- A type of bacterial toxin that act by inappropriately activating T-cells.
- They do this by forming cross-bridges between MHC molecules on antigen-presenting cells and the T-cell receptor (between conserved regions).
- This leads to non-specific activation, leading to mass release of cytokines and upregulation of T-cell activity
How do clostridial toxins work?
- The toxin acts by modifying host cell GTPase proteins by glucosylation, leading to changes in cellular activities.
- This involves actin condensation and cell rounding, which is followed by death of the cell
What type of bacterium is Neisseria meningitidis and where does it infect?
Non-O-antigen possessing Gram negative coccus, typically residing in the upper airway
Does Neisseria meningitidis cause disease?
- It is found in about 10% of the population and it does not cause disease in most
- In some individuals, it causes severe systemic disease that may involve bacterial meningitis.
What causes Neisseria meningitidis to be pathogenic in some individuals?
There are two features:
- The bacterium reaches very high concentrations in the systemic circulation (up to 1013 bacteria/individual)
- The bacterium blebs (sheds membrane vesicles) as it grows within the body -> This frees up endotoxin lipid A (part of LPS) from the Gram-negative outer membrane
What is the immune response to Neisseria meningitidis? What is the evidence for this?
- The immune response is mainly via the complement pathway
- We know this because of high rates of meninogcoccal disease in individuals with complement pathway defects
How are pathogenic strains of Neisseria meningitidis able to reach such high concentrations in the blood and therefore cause disease?
- Due to the capsule, of which there are 13 serogroups:
- In serogroup B, the bacterial capsule has alpha 2‐8 linked polysialic acid -> This is the same molecule that is on neural cell adhesion molecule 1 in infancy, meaning that the immune system does not respond to it -> Molecular mimicry
- This also means that vaccines against serogroup B strains are not feasible
- The bacteria can also bind complement factor H (via factor H binding protein) -> This factor is responsible for downregulating complement, so the bacteria are less susceptible to complement.
- The bacteria is regulated by temperature via RNA thermocensors -> These are sequences in the untranslated region of mRNA that fold into a hairpin loop if the temperature is low, so that no translation can happen. If the temperature rises, the RNA is translated.
What is Endotoxic shock?
Septic shock due to the release of endotoxins by gram-negative bacteria.
- endotoxins and lipopolysaccharides in the cell walls released during reproduction and destruction of the bacteria.
Characterised by pattern of organ failure.
What causes endotoxic shock?
Lipid A potently stimulating innate leukocyte PRRs –> powerful host response
What are the two ways in which host cells can recognise endotoxins? How are endotoxins targeted?
Endotoxins can bind to:
- TLR4 on the surface of host cells -> Leads to inflammatory cytokine production
- Caspase-4 receptors on the inside of cells -> Leads to cell apoptosis
Endotoxins are targeted by antimicrobial peptides (AMPs).
What are some different subtypes of extracellular toxins in bacterial infections?
What are AB toxins?
- Toxins that are secreted by certain bacteria, including:
- Cholera
- Diphtheria
- Pertussis toxin (associated with whooping cough)
- E. coli
- They consist of B subunits, for attachment to the host cell, and A subunits for toxicity within the cell
Why do bacteria need to regulate their virulence gene expression?
Expression of virulence factors is costly to maintain and reduces bacterial fitness
Also need to express diff genes for certain environments/competition
How does quorum sensing work?
quorum sensing → regulation of gene expression in response to fluctuations in cell-pop density
*Quorum sensing bacteria produce + release autoinducers. As cell density ↑, conc ↑.
*Detection of minimal threshold stimulatory concentration leads to alterations in gene expression.
How does thermosensing work to regulate virulence gene expression?
Diff temp causes diff effects - e.g. heat shock leads to relaxation of plasmids, cold shock leads to negative supercoiling
So certain genes are only accessible for transcription in certain temps
What are two-component regulatory systems?
Simple stimulus-response coupling mechanisms that allow bacteria to sense and respond to changes in the environment
What are the two typical components of two-component regulatory systems and what do they do?
- A membrane-bound histidine kinase that senses the environmental stimulus
- A corresponding response regulator that activates virulence genes
