Professional Pathogens Flashcards
Infection
When an organism enters the body, increases in number and damages the host in the process
A Pathogen
An organism which can evade the immune defences of the normal human host to cause infection. Pathogens almost always colonise before causing infection
A Commensal
An organism which lives on us or in our gut but doesn’t cause infection
Colonisation
When an organism lives on us but is not causing infection
Symbiosis
Mutual benefit
Parasite
Unequal benefit
Professional pathogens (give example too)
Almost always cause disease. e.g. malaria, HIV, Neisseria meningitidis, Staphylococcus aureus, Streptococcus pneumoniae
Opportunistic pathogens (give example too)
Only cause disease in immunocompromised patients. e.g. Staphylococcus epidermidis, candida albicans
Why is there a spectrum between professional and opportunistic pathogens (a spectrum of pathogenicity)?
Because organisms vary in virulence
How can low virulence organisms become pathogens?
If a patient is immunocompromised.
Pathogenicity
Probability that an organism is causing disease in patient
What does pathogenicity depend on?
Virulence, where sample is from (sterile or not), immune state of patient (steroids, diabetes, alcohol, T-cell deficiencies, neutropenia, burns, trauma, venous catheters), how sample was taken
Brief profile of Staphylococcus aureus
Commensal of anterior nares, Gram positive cocci in clusters
Name the virulence factors of S. aureus that allow it to be a good commensal and also a pathogen
Surface proteins (adhesins and protein A and coagulase), secreted proteins (toxins), capsule
Staphylococcal capsule
Polysaccharide capsule, immunologically inert surface that allows S. aureus to evade immune response
Protein A
Found on surface of S. aureus. Binds antibodies away from the antigen-complementary receptor, allowing immune evasion.
Adhesins
Proteins on surface of S. aureus. Allow bacteria to stick to different tissues (via proteins) and also bind host proteins so that immune system cannot recognise bacteria
Coagulase
Stimulates clotting, thus stopping white blood cells reaching site.
Name the staphylococcal toxins secreted by S. aureus and describe their functions
Cytotoxins (forms pores in host cells, lysing them, including polymorphs), exofoliative toxins (cause skin to fall off, degrade connective tissue). Enterotoxins (In gut. Stimulate massive T cell activation and thus allows immune evasion. Ingestion of this toxin leads to vomiting).
Why are there such massive immune responses to Gram negative bacteria?
Because of the lipopolysaccharide (endotoxins) on cell surface. Innate immune system very sensitive to these so strong immune responses.
Which bacterium causes meningococcal septicaemia?
Neisseria meningitidis
Which bacterium is responsible for the commonest form of pneumonia?
Streptococcus pneumoniae
Sputum
A mixture of saliva and mucus coughed up from the respiratory tract, typically as a result of infection
Mucosa
mucous membrane
Spore forms of bacteria are resistant to:….. which makes them ideal for……
Drying out, alcohol, antibiotics (so can relapse after treatment). Ideal for survival in hospitals, allowing transfer between patients
When do bacteria form spores?
When they are not dividing
Give an example of a spore forming bacteria
Clostridium difficile
Give two examples of Gram negative bacteria?
Neisseria meningitidis, Streptococcus pneumoniae
Describe the effects on Clostridium difficile of taking antibiotics
C. difficile is resistant to many antibiotics. It will overgrow, producing toxins and diarrhoeal disease
Virulence determinants and their importance, giving example
Secreted proteins, surface proteins, capsules etc. Important for identification of different species of bacteria and distinguishing between less and more virulent bacteria e.g distinguish virulent S. aureus from S. epidermis or distinguish S. aureus from less virulent strains. Less virulent will lack some of the virulence factors.
