Antibacterial Responses

Question 1

Key steps in infection

Answer 1

entry, invasion and colonisation of host tissue, evasion of immunity, tissue damage

Question 2

Why is the microbiota important?

Answer 2

It is a mechanism of protection to infection both ecological and immunological

This shows that not all bacteria cause disease

Question 3

How do bacteria survive in the body?

Answer 3

by the ability to evade the effector mechanism of immunity

Some bacteria establish latent or persistent infection and the immune system does not clear the microbe

Question 4

Give examples of the mechanical defence of different organs

Answer 4

(flow of fluid)
(Epithelial cells joined by tight junctions)

Skin - perspiration, sloughing off of skin

GI tract - mucus, food and saliva

Respiratory tract - mucus and by cilia air flow

Urogenital tract - urine, mucus, sperm

Eyes - flow of fluid tears

Question 5

Give examples of the chemical defence of different organs

Answer 5

Skin - sebum (fatty acids, lactic acid, lysozyme)

GI tract - acidity, enzymes (proteases)

Respiratory tract - Lysozyme in nasal secretions

Urogenital tract - Acidity in vaginal secretions, spermine and zinc in semen

Eyes - lysozymes in tears

Question 6

Give examples of the microbiological defence of different organs

Answer 6

Normal flora of the named organ

Question 7

Anti-microbial peptides

Answer 7

capable of killing by penetrating microbial membranes thus disrupting their integrity.

They are active against bacteria, fungi and many enveloped and non-enveloped viruses.

Question 8

Types of Anti-microbial peptides

Answer 8

𝛂-defensins and β-defensins.

𝛂-defensins are secreted mainly by neutrophils and by Paneth cells

β-defensins are secreted by a broad range of epithelial cells, in particular, those in the respiratory tract, the skin and the urogenital tract.

Question 9

How do toxins work?

Answer 9

Binds to cellular receptors
Endocytosis of toxin/receptor complexes
Dissociation of toxin to release active chain, poisoning cell

Question 10

The complement system

Answer 10

Key effector function of the humoral response.

Serum and cell surface proteins interact with one another to generate products that eliminate extracellular bacteria

Question 11

What are the complement pathways?

What activates the pathways?

Answer 11

Classical - Antibodies
Lectin - Carbohydrate
Alternative - Specific PAMPs

Question 12

the function of the antibodies

Answer 12

• Neutralise bacterial toxins
• Trigger classical complement pathway by
binding of IgM to the bacterial cell surface
• Opsonisation; coating of bacteria with
antibody thereby aiding phagocytosis

Question 13

What does complement cause?

Answer 13

Inflammation, lysis and opsonisation

Question 14

Toxin neutralisation

Answer 14

• Many bacteria secrete protein toxins that cause disease by
disrupting the normal function of host cells
• In order to do this a bacterial toxin must bind to a receptor
• Thus, antibodies that bind to that receptor block the function of the toxin

Question 15

Triggering of the complement cascade

Answer 15

• IgM bound to the surface of a bacterium binds to a complement component that initiates the classical pathway of complement activation.
• As a consequence of this, the bacterial cell surface is coated in C3b facilitating its phagocytosis.

Question 16

What do neutrophils do?

Answer 16

Neutrophils perform phagocytosis and degranulation of granules- the intracellular killing of bacteria
Oxidative burst kills the bacteria
Neutrophils Extracellular Traps (NETS)

Question 17

What do pattern recognition receptors (PRRs) and PAMPs lead to?

Answer 17

Phagocytosis and the production of cytokines:

PAMP binding by PRRs activates the phagocyte
Phagocyte ingests microbes, enlarges and increases metabolic activity

Activated phagocytes elevate antimicrobial activity

If infection is detected through TLRs, pro-inflammatory cytokines(eg IL1 Il6 and Tnf-alpha) and chemokines are produced, leading to the initiation of an inflammatory response.

Question 18

How do helper T cells enhance the phagocytosis of extracellular pathogens?

Answer 18

Helper T cells enhance phagocytosis of extracellular pathogens by activating macrophages

The MHC class II pathway presents bacterial antigens derived from extracellular infections to helper CD4 T cells

1. Macrophage engulfs and degrades bacterium, producing peptides.
2. Bacterial peptides are bound by MHC Class II in vesicles.
3. Bound peptides are transported by MHC Class II to the cell surface.
4. Helper T cell recognises complex of peptide antigen with MHC Class II and activates macrophage.

Question 19

Overall, what is involved in innate immunity and adaptive immunity?

Answer 19

The innate immune response to bacterial infection:
Complement

Pattern recognition of bacteria
Phagocytosis
Opsonisation
Cytokine secretion
Inflammation

The adaptive immune response to bacterial infection
CD4+ and CD8+ T cells
Antibodies