Lecture 3: Professional pathogens Flashcards
Describe the term infection
When an organism enters the body, increases in number and damages the host in the process (invasion, replication, damage).
Describe the term colonisation
Establishment of an organism within the host
Describe the term pathogen
An organism which can evade the immune defences of the normal human host to cause infection.
Describe the term commensal
An organism which lives on us / in our gut but doesn’t cause infection.
Describe the term symbiotic relationship
Mutual benefit
Describe the term parasite
Unequal benefit, used colloquially for eukaryotic infections (malaria, tape worms) but technically applies to infection caused by bacteria too.
What is a true or ”professional” pathogen?
An organism which can cause disease in any susceptible host
What is an “opportunistic” pathogen?
An organism which only causes disease in immunocompromised patients
Describe the term virulence
Fundamental properties of the organism which determine how it causes disease.
What determines whether a bacterial species can cause disease?
Dependant on the virulence of the organism. Low pathogenicity organisms may have the ability to cause disease if present in immunocompromised individuals as they have a weakened immune system.
What is the difference between gram positive and gram negative bacteria?
Gram positive bacteria only have a thick peptidoglycan cell membrane where as gram negative have a think peptidoglycan membrane and an LPS membrane with porins.
What are the cell wall components of gram positive bacteria?
Do NOT have LPS but do have lipoteichoic acid and peptidoglycans, which can activate the immune response in similar ways using different Toll-like receptor (TLRs).
Name a gram positive bacteria
Staphylococcus aureus
Streptococcus pneumoniae
Describe the structure S.aureus under a microscope
Gram positive (PURPLE) cocci in clusters which look like bunches of grapes down a microscope. Golden colonies when cultured on agar.
What are the main virulence mechanisms of S.aureus?
- Surface proteins which mainly function as adhesion molecules- tissue adhesion, colonisation and ‘clocking’
- A group of secreted proteins; cytotoxins, exfoliative toxins, enterotoxins (superantigens), and complement inhibitors.
- Capsule- masks cell surface features from recognition by immune system.
- Cell wall associated enzyme called Coagulase- stimulates clotting and plays a role in immune evasion.
- Qarum sensing regulatory system- controls gene expression
How can be Coagulase test be used to distinguish S.aureus from other Staph species?
Coagulase only found in S.aureus
What are enterotoxins (superantigens)?
Enterotoxins (superantigens) are secretory proteins that stimulate massive T cell activation
Describe Streptococcus pneumoniae
Gram positive (PURPLE) diplococci bacteria which is the cause of the most commonest form of pneumonia (~40%). Can disseminate to cause endocarditis (infection of heart valves), upper respiratory tract infections, and meningitis.
What are the main virulence mechanisms o Streptococcus pneumoniae?
- Specific adhesins for respiratory mucosa
- Pneumolysin- binds host cell wall cholesterolmand forms pores to trigger lysis of host phagocytic cells.
- Secretory IgA protease- breaks down secreted immunoglobulin IgA preventing mucosal clearance
- Capsule- masks cell surface features from recognition by immune system
- Toxins
Where is S.aureus commonly seen?
S. aureus is a common cause of infections in patients who need vascular access (line) devices for e.g. cancer treatment. S. aureus line infections commonly seed in the blood to distant body sites such as heart valves, bones and joints and this is because of their adhesions.
What happens following ingestion of S.aureus?
Ingestion of S.aureus enterotoxins causes a rapid brief illness that is characterised by vomiting and minimal diarrhoea.
Name a gram negative bacteria
Meningococcal (Neisseria meningitidis)
How does LPS in gram negative bacteria trigger sepsis?
LPS interacts with TLRs (esp. TLR4) on monocyte / macrophage cells and the vascular endothelium. Results in systemic activation of inflammatory pathways, coagulation and clotting pathways. Clinically this manifests as “SEPSIS”.
Describe the structure of Neisseria meningitidis under microscope
Gram negative (pink) cocci
What are the main virulence mechanisms of meningococcal (Neisseria meningitidis)?
- Adhesins- allow for adhesion to respiratory epithelium and meninges.
- LPS- interacts with TLRs to trigger systemic activation of inflammatory pathways.
- Capsule- masks cell surface features from recognition by immune system.
What test is used to identify the bacterial capsule?
Indian ink stain
