Pathogenicity Flashcards
What is a pathogen?
An organism which causes harm to it’s host through it’s own actions
What is a commensal?
An organism which benefits from it’s host but bring neither benefit or harm to it’s host.
What is an opportunistic pathogen?
An organism which can cause disease in it’s host but only when given the right set of conditions
What is a zoonotic pathogen?
An organism which has an animal reservoir (either as a pathogen or commensal) and can cause disease in humans
Describe disease progression
Pathogens infect host either causing disease or leading to an asymptomatic carrier. Disease can kill the host, be neutralized (asymptomatic carrier) or resolved entirely and the host recovers.
Pathogens use colonisation virulence factors to infect hosts such as Adhesins, Invasins, nutrient acquisition, motility, chemotaxis.
Pathogens use damage virulence factors to cause disease, such as exo/endotoxins, proteases, DNases, Lipases, Haemolysins.
Host defences include physical barriers, complement, macrophages, antimicrobial peptides and the adaptive immune response.
What are kochs postulates?
- Microorganism must be found in abundance in all individuals suffering from the disease but not in healthy individuals
- Microorganism must be able to be isolated and grown in a pure culture
- Cultured microorganism must cause disease when reintroduced into a healthy experimental host.
- Microorganism must be reisolated from the diseased experimental host and identified as being identical to the original causative microorganism.
Describe challenges to Koch’s postulates
- Some microorganisms can cause disease without being present: Clostridium botulinum secretes toxins which can remain present after the bacteria has died.
- <1% of bacteria can be cultured in vitro due to complex or unknown nutritional requirements.
- Cultured pathogens can lose their virulence, there isn’t selective pressure against mutations in virulence factors. Puumala virus loses it’s virulence when cultured in vero cells.
- There’s isn’t always an animal model for reinfection. Syphilis only survives in humans or the 9-banded armadillo. Smallpox is exclusive to humans.
Tell me about strict/professional pathogens
They are highly adapted to their niches, unable to survive outside of their host due to nutritional requirements only satisfied through pathogenesis. They can access unique niches that yields very little competition with other microbes. They transmit efficiently between hosts and have a reduced genome.
Give some examples of strict/professional pathogens
- Helicobacter pylori - only microbe that survives in the stomach, causes stomach cancer and gastric ulcers but can protect against oesophageal adenocarcinoma and GORD.
- Neisseria gonorrhoeae, an STI in the genitourinary tract.
- Shigella dysenteriae, a gut infection that leads to dysentery.
- Mycobacterium tuberculosis causes TB, infects the lungs and can lead to systemic infections.
- Chlamydia trachomatis, intracellular STI in the genitourinary tract, can infect lungs and eyes too.
Tell me about opportunistic pathogens
Will only cause disease when the conditions are right, typically when the host immune system is compromised or after antibiotics when the microbiota has been disrupted. Lives in the environment or host as a commensal. Not very efficient at transmitting between hosts
Give some examples of opportunistic pathogens
- Pseudomonas aeruginosa, causes wound/burn infection, infects lungs in cystic fibrosis patients.
- Clostridioides difficile, inflammatory infection of large intestines. Typically occurs post-antibiotic treatment, very common in hospitals.
- Staphylococcus epidermis, causes skin and wound infections.
- Staphylococcus aureus, causes skin and wound infections, can lead to endocarditis, osteomyelitis.
Tell me about facultative pathogens
Well-adapted to multiple lifestyles, equally at home in a niche or as a pathogen. This flexibility is usually conferred in a large genome.
E. coli can act as a facultative pathogen, gut and UTI infections can lead to sepsis and meningitis.
Tell me about the boundary between opportunistic and facultative pathogens
This often depends on the strain of the pathogen as some strains can be more pathogenic than others. Virulence factors are typically encoded in mobile elements or plasmids allowing them to be easily transferred via horizontal transfer. STEC, Shiga toxin-producing E. coli picked up a shigella toxin and causes bloody diarrhoea.
What’s the difference between Antiseptics, Disinfectants and Antibiotics?
Antiseptics and Disinfectants are too toxic for internal use, antiseptics are used on the skin while disinfectants are used on inanimate surfaces. Antibiotics are low molecular mass compounds injected or ingested into the human body with minimal side effects.
Tell me about ideal properties of antibiotics
Either kills or inhibits bacterial growth (bactericidal for immunocompromised individuals)
Selective toxicity; exploits differences between the pathogen and the host.
Pharmacokinetics; drug is distributed around the body.
