Lecture 3: Professional pathogens

Question 1

Describe the term infection

Answer 1

When an organism enters the body, increases in number and damages the host in the process (invasion, replication, damage).

Question 2

Describe the term colonisation

Answer 2

Establishment of an organism within the host

Question 3

Describe the term pathogen

Answer 3

An organism which can evade the immune defences of the normal human host to cause infection.

Question 4

Describe the term commensal

Answer 4

An organism which lives on us / in our gut but doesn’t cause infection.

Question 5

Describe the term symbiotic relationship

Answer 5

Mutual benefit

Question 6

Describe the term parasite

Answer 6

Unequal benefit, used colloquially for eukaryotic infections (malaria, tape worms) but technically applies to infection caused by bacteria too.

Question 7

What is a true or ”professional” pathogen?

Answer 7

An organism which can cause disease in any susceptible host

Question 8

What is an “opportunistic” pathogen?

Answer 8

An organism which only causes disease in immunocompromised patients

Question 9

Describe the term virulence

Answer 9

Fundamental properties of the organism which determine how it causes disease.

Question 10

What determines whether a bacterial species can cause disease?

Answer 10

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.

Question 11

What is the difference between gram positive and gram negative bacteria?

Answer 11

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.

Question 12

What are the cell wall components of gram positive bacteria?

Answer 12

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).

Question 13

Name a gram positive bacteria

Answer 13

Staphylococcus aureus

Streptococcus pneumoniae

Question 14

Describe the structure S.aureus under a microscope

Answer 14

Gram positive (PURPLE) cocci in clusters which look like bunches of grapes down a microscope. Golden colonies when cultured on agar.

Question 15

What are the main virulence mechanisms of S.aureus?

Answer 15

1. Surface proteins which mainly function as adhesion molecules- tissue adhesion, colonisation and ‘clocking’
2. A group of secreted proteins; cytotoxins, exfoliative toxins, enterotoxins (superantigens), and complement inhibitors.
3. Capsule- masks cell surface features from recognition by immune system.
4. Cell wall associated enzyme called Coagulase- stimulates clotting and plays a role in immune evasion.
5. Qarum sensing regulatory system- controls gene expression

Question 16

How can be Coagulase test be used to distinguish S.aureus from other Staph species?

Answer 16

Coagulase only found in S.aureus

Question 17

What are enterotoxins (superantigens)?

Answer 17

Enterotoxins (superantigens) are secretory proteins that stimulate massive T cell activation

Question 18

Describe Streptococcus pneumoniae

Answer 18

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.

Question 19

What are the main virulence mechanisms o Streptococcus pneumoniae?

Answer 19

1. Specific adhesins for respiratory mucosa
2. Pneumolysin- binds host cell wall cholesterolmand forms pores to trigger lysis of host phagocytic cells.
3. Secretory IgA protease- breaks down secreted immunoglobulin IgA preventing mucosal clearance
4. Capsule- masks cell surface features from recognition by immune system
5. Toxins

Question 20

Where is S.aureus commonly seen?

Answer 20

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.

Question 21

What happens following ingestion of S.aureus?

Answer 21

Ingestion of S.aureus enterotoxins causes a rapid brief illness that is characterised by vomiting and minimal diarrhoea.

Question 22

Name a gram negative bacteria

Answer 22

Meningococcal (Neisseria meningitidis)

Question 23

How does LPS in gram negative bacteria trigger sepsis?

Answer 23

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”.

Question 24

Describe the structure of Neisseria meningitidis under microscope

Answer 24

Gram negative (pink) cocci

Question 25

What are the main virulence mechanisms of meningococcal (Neisseria meningitidis)?

Answer 25

1. Adhesins- allow for adhesion to respiratory epithelium and meninges.
2. LPS- interacts with TLRs to trigger systemic activation of inflammatory pathways.
3. Capsule- masks cell surface features from recognition by immune system.

Question 26

What test is used to identify the bacterial capsule?

Answer 26

Indian ink stain