Virulence factors Flashcards
what is a pathogen?
any organism that, by its actions, causes harm to its host
what is a commensal?
a commensal organism benefits from the interaction with the host, and the host neither benefits nor is harmed
- commensalism covers a range of ecosystem interactions between bacteria and host e.g. symbiotes
- commensal is a simplified term for harmless microorganisms
what is an opportunistic pathogen?
an organism that can cause disease in its host only if it is given the right set of circumstances, otherwise it is a commensal
- they don’t always cause disease
- only cause disease when host defences are compromised in some way
- when not being a pathogen, they live in the environment (soils, water) or exist as commensals of the host
- often not efficiently spread between hosts as they do not need to, but there are exceptions
what is a facultative pathogen?
Facultative pathogens are organisms for which the host is only one of the niches they can exploit to reproduce
- an organism that may resort to parasitic activity, but does not absolutely rely on any host for completion of its life cycle
- well-adapted for multiple lifestyles, equally at home in environmental niches and when causing disease
- flexibility in different environments usually means a larger genome
what is a zoonotic pathogen?
an organism with an animal reservoir (either as a pathogen or commensal) that can be transmitted to and cause disease in humans
how do infectious diseases progress?
- Pathogen infects the host - can lead to asymptomatic colonisation, or disease
- Disease can lead to resolution of infection and recovery, or death
what do pathogens use to colonise and harm the host?
virulence factors
what are virulence factors which enable pathogens to colonise the host?
adhesins, invasins, nutrient acquisition (pathogen scavenges limited nutrients such as iron), motility, chemotaxis to detect chemical gradients to swim towards nutrients or away from danger
- This is what causes infection
what are the virulence factors which enable pathogens to harm the host?
Endotoxins, exotoxins, proteases, DNase, lipase, haemolysins
- DNase and proteases allow bacteria to utilise host tissues for essential molecules to survive – makes a nutrient broth of the host
- Depletes the host
- This is what causes disease
how can hosts defend against colonisation and damage?
Host defences:
- physical barriers (skin, gut epithelium prevent pathogen access to deep tissues)
- complement cascade,
- macrophages,
- antimicrobial peptides,
- adaptive immunity (T cells and B cells)
Push interaction with pathogen to the less harmful side of the infection
- Pathogen is suppressed and cleared by the immune system
what are the 4 criteria of Koch’s Postulates?
- The microorganism must be found in abundance in all organisms suffering from the disease but not in healthy organisms.
- The microorganism must be isolated from a diseased host and grown in pure culture.
- The cultured microorganism should cause disease when introduced into a healthy organism.
- The microorganism must be re-isolated from the diseased experimental host and identified as being identical to the original causative agent.
how are Koch’s Postulates used in practice?
- There are many different ways of satisfying the four postulates
- But the basic idea remains the same – the organism must be detected in association with disease, cultured in the lab, reproduce the disease in some kind of model organism and then be reisolated from that experimental infection
- Incredibly powerful definition and common practice in infectious disease research
- but not always possible to tick all boxes
what are examples were Koch’s Postulates aren’t enough to define a pathogen?
- When the pathogenic organism secretes something that causes disease symptoms, so the organism itself doesn’t need to be present to see disease, e.g. botulism by Clostridium botulinum – lethal botulinum toxin - cant address first criteria
- When the organism cannot be grown in pure culture – usually because of complex or completely unknown nutritional requirements. E.g. Chlamydia, leprosy, syphilis - Cannot address second criteria
- When growth in the lab leads to loss of virulence – so disease cannot be recapitulated in a model organism. E.g. Puumala virus cultured in vero cells can no longer infect bank voles, the virus’ natural host - Cannot address third criteria – reinfection into healthy host does not lead to disease
- no animal model is available so in the absence of human experimentation, disease cannot be reproduced in the lab. E.g. too many to list – this is the most common problem
how is disease caused?
- Disease can be caused by infection of pretty much any body-site
- Where the infection occurs in the body results in different diseases
- Disease symptoms will reflect the damage caused by the infecting organism and the response by the immune system
what are strict/professional pathogens?
bacteria that definitely cause disease
- highly adapted organisms for which the pathogenic lifestyle is their main or only option – often can’t survive outside of the host – obligate pathogens e.g. chlamydia
- often have nutritional requirements that can only be satisfied through pathogenesis = feed themselves through causing disease to host
- often access a unique niche in the host with little competition from other microorganisms - evolved to exploit areas with minimal competition
- usually transmit very efficiently between hosts
- some are so adapted to the in-host lifestyle that their genome has reduced in size to the point that they can’t live independently (remember efficiency!)
