Fagan L13

Question 1

What are the classes of pathogens?

Answer 1

Pathogen – an organism that, by its actions, causes harm to its host
Commensal – a commensal organism benefits from the interaction with the host and the host neither benefits nor is harmed. Can have subtypes including symbionts and those that give organism benefits in exchange for substances such as carbs for vitamin B12.
Opportunistic pathogen – an organism that can cause disease in its host given the right set of circumstances but is otherwise a commensal
Zoonotic pathogen – an organism with an animal reservoir (either as a pathogen or commensal) that can be transmitted to and cause disease in humans

Question 2

What were the 4 of Koch’s postulates?

Answer 2

1. The microorganism must be found in abundance in all organisms suffering from the disease but not in healthy organisms.
2. The microorganism must be isolated from a diseased organism and grown in pure culture.
3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be re-isolated from the diseased experimental host and identified as being identical to the original causative agent.

Question 3

What are the circumstances that make Koch’s postulates untrue?

Answer 3

• 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.
• When the organism cannot be grown in pure culture – usually because of complex or completely unknown nutritional requirements. E.g. Chlamydia, leprosy, syphilis.
• 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.
• When 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.

Question 4

What are strict or professional pathogens?

Answer 4

• highly adapted organisms for which the pathogenic lifestyle is their main or only option – often can’t survive outside of the host called obligate pathogens
• often have nutritional requirements that can only be satisfied through pathogenesis
• often access a unique niche in the host with little competition from other microorganisms
• 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!)

Question 5

What are some examples of strict pathogens?

Answer 5

Helicobacter pylori – live in the stomach, cause stomach ulcers and cancer, no competitors
Neisseria gonorrhoeae – sexually transmitted infection of the genitourinary tract
Shigella dysenteriae – dysentery, gut infection
Mycobacterium tuberculosis – TB, lung (mostly) and some systemic infections causing several million deaths per year.
Chlamydia trachomatis – intracellular sexually transmitted infection, genitourinary tract, eyes, lungs, etc. Obligate pathogen so can’t survive outside of human cells

Question 6

What are opportunistic pathogens?

Answer 6

• only cause disease when host defences are compromised in some way
• when not being a pathogen, they live in the environment such as in water or exist as commensals
• often not efficiently spread between hosts but there are exceptions as they can live in other areas rather than human cells unlike professional pathogens so not had the same evolutionary specificity!

Question 7

What are examples of opportunistic pathogens?

Answer 7

Pseudomonas aeruginosa – wound/burn infections, lung infections in cystic fibrosis
Clostridioides difficile – inflammatory infections of the large intestine but only after antibiotic treatment
Staphylococcus epidermidis – skin and wound infections
Staphylococcus aureus – skin and wound infections, endocarditis, osteomyelitis, etc.
All of these require some additional factor that compromises the normal host defence, e.g. a barrier breech (burn, cut or puncture wound), inhibition of clearance (mucus in CF lung), loss of colonisation resistance (antibiotic induced dysbiosis), immunocompromisation (HIV, anti-rejection meds, etc.).

Question 8

What are facultative pathogens?

Answer 8

• probably the majority
• well-adapted for multiple lifestyles, equally at home in environmental niches and when causing disease
• flexibility usually means a larger genome

Question 9

What is an example of a facultative pathogen?

Answer 9

Escherichia coli – gut and UTI infections, sometimes leading to sepsis or meningitis. M ost E. coli are harmless commensals and environmental organisms, only a few have the requisite virulence factors (more on these later!) to cause human disease, often encoded on plasmids or mobile genetic elements so non-pathogenic species can easily pick up the required virulence factors to make it pathogenic. This can be seen in most opportunistic and facultative pathogens.