2. Virulence and Virulence Factors Flashcards
How do we know a bacterium is a pathogen?
Koch’s Postulates
Briefly summarise Koch’s Postulates
- Bacterium should be found in all people with disease, & in correct location
- Bacterium should be isolated from infected site & maintained in pure culture
- Pure culture should be capable of causing disease
- The same bacterium should be isolated from the intentionally infected host
What are some problems associated with Koch’s postulates?
- Doesn’t always account for host factors
- Not always possible to isolate organisms in pure culture
- Some diseases involve polymicrobial infections
- Cultivation of bacteria can lead to loss of virulence factors
What is virulence?
The ability of a bacterium to cause disease
What is a virulence factor?
A bacterial product or structure or strategy which contributes to virulence
Define infection
An infection occurs when a bacterium (including those capable of causing disease) becomes established in the body
True or False: Not all bacteria that infect cause disease
True
Define disease
An infection producing symptoms that are detrimental to the host
How do we know that a specific attribute of a pathogen is a virulence factor?
Molecular Koch’s Postulates - attempts to define a virulence factor (gene product) rather than a pathogen
What is the most important thing to know re Molecular Koch’s Postulates?
Disrupting the gene in a virulent strain should diminish virulence, or, introducing the gene to an avirulent strain should make it virulent
What are examples of virulence factors that mediate colonisation & survival in infection?
Adherence, motility, invasion & intracellular residence, evasion of host defences
What virulence factors damage the host (determine disease)?
Toxins: endotoxins (lipid A), exotoxins
Enzymes: hyaluronidase, DNAse
What are some factors that have been implicated in order for bacteria to survive antibacterial features of the mucosal surface?
- Motility & chemotaxis
- sIgA proteases (sIgA binds Ag & mucin)
- Iron acquisition (produce siderophores to compete against transferrin)-
What genes are pili encoded by?
Pap genes
What genes are fimbriae encoded by?
Fim genes
What do fimbriae tightly bind to?
Mannose residues (found on most human cells)
What are pili?
Protein hair-like structures found on the outside of many bacterial cells - long hair-like structures that have a protein at their tip that binds to host sugars
What is an example of an infection that best represents the function of both pili and fimbriae?
- UTI caused by E.coli
- Fimbriae stick to bladder cells, pili stick to kidney cells
- Fimbriae and pili help the bacterial cell to attach to host cells (don’t get washed out in urination)
Specificity of binding to host cell carbohydrate depends on…
the pilus tip
What do afimbrial adhesins do?
Mediate tight binding between bacterium & host cell
What is an example of a bacterium that has an adhesin?
Streptococcus pyogenes has an adhesin, protein F, which binds to fibronectin → causes throat & wound infections
What are invasins?
A collective term for proteins that are important for bacterial invasion
How do bacteria like Shigella and Listeria enter host cells which are not naturally phagocytic?
Cause rearrangements in the host cytoskeleton (actin) - do this using bacterial surface proteins called invasins. Can also rearrange actin within the cell to promote movement between cells
What is an example of a bacteria that targets M cells for entry?
Salmonella
What are capsules and what do they do?
Capsules are loose unstructured polysaccharide polymers that can prevent uptake by macrophages
Some capsular material is non-immuogenic. Give examples
- Streptococcus pyogenes - produces a capsule called hyaluronic acid
- Neisseria meningitidis - sialic acid
How can complement be evaded?
Can evade complement by altering the major target, LPS, by attaching sialic acid to the LPS O antigen. Also produces enzymes which degrade complement proteins
What happens when sialic acid is attached to the LPS O antigen?
The LPS essentially becomes invisible to complement
What bacterial protein binds Fc portion of Ab and neutralises the Ab response to that bacteria?
Protein A of Staphylococcus aureus
What are endotoxins?
LPS (intergral part of Gram negative outer membrane)
Where are exotoxins mostly produced?
Inside Gram positive bacteria as part of their growth & metabolism. They are then secreted/released following lysis into the surrounding medium.
How is LPS chimeric?
LPS has a lipid component (Lipid A) and a sugar glycol component (O antigen)
What is the toxic component of LPS?
It is released by G-ve bacteria as they die & acts as a toxin in eukaryotic cells - an indication to the host that G-ve bacteria are in the vicinity & the host prepares the immune system
What happens if LPS is found in the wrong place at the wrong time?
Death by septic shock
What do LPS binding proteins bind to?
Lipid A part of LPS → delivers LPS to circulating immune cells e.g. macrophages
TLR4 surface protein on macrophages binds to?
CD14, which interacts with LPS binding protein → signalling cascade in macrophage → cytokine production
What is shock?
Shock describes a set of events which lead to the collapse of the circulatory system, primarily, and can progress to multiple organ system failure and death (80%)
What are the 3 main categories of exotoxin?
- A-B toxin (simple or complex)
- Membrane disrupting (pore-forming or membrane-damaging)
- Superantigen (responsible for toxic shock)
How are simple A-B toxins formed?
They are produced as a single protein from a single gene, cleaved by proteins - remain connected by a disulphide bridge (joins A & B fragments together)
What does the B subunit do?
B subunit binds to target cell and delivers A subunit to that cell
What are complex A-B toxins made up of?
One A protein (may contain 2 subunits linked by a disulphide bridge), multiple B proteins (usually 5). A & B produced independently from separate genes. A subunit proteolytically cleaved into A1 & A2. A2 part responsible for interacting with B subunits - together deliver A1 subunit to target cells. A1 subunit has the toxic activity.
What happens when a toxin binds to its receptor?
Triggers an endocytic response → toxin is taken up into endosome → active subunit is delivered across endosomal membrane into cytoplasm
What is an example of a membrane disrupting toxin?
Phospholipase A2 - cleaves phospholipid heads, destabilises the membrane, cell dies
True or False: Membrane-disrupting toxins are often indiscriminate and attack many cell types
True
What do superantigens do?
Superantigens bring a MHC and a TCR together in a non-specific way, without the presence of an antigen → much bigger immune response → toxic shock syndrome (inappropriate massive immune response)
How do superantigens trick T cells?
They trick T cells into thinking it has recognised a foreign antigen
What happens when a T cell interacts with a superantigen?
Cytokine storm → shock → death
What are hydrolytic enzymes?
Enzymes which can contribute to the progression of an infection, but are not normally categorised as toxins i.e. proteases, hyaluronidase (cause tissue damage), DNAse (reduce viscosity)
How is virulence measured?
2 strains of the same species (e.g. Salmonella typhi vs Salmonella enterica)
or
A parent and mutant (lacking a potential virulence factor)
Which strain of Salmonella only infect humans?
S. typhi
S. typhimurium causes similar effects in mice
What is a very important measure of virulence?
LD50 (number of cells to cause death in 50% of animals) - can be used to compare virulence of different strains or mutants
(the lower the number, the more virulent the organism)
True or False: ID50 will always be lower or equal to LD50
True
What other features can be used to measure virulence?
Numbers in a particular organ, symptomatology, luciferase (light)
Advantages and disadvantages of cultured cells
Advantages: human origin, cost efficient, defined, readily available, few ethical concerns
Disadvantages: incorrect gene expression, lack of polarisation, loss of traits over time, often transformed (cancer-derived, different metabolism)
What can you use cultured cells to measure?
Specific attributes of pathogens such as adherence, cytotoxicity and invasion (but obv can’t reproduce symptoms, organ specific traits, systemic spread, response to immune system etc.)
