Session 7 - Allergies and Infections on surfaces

Question 1

Define microbiota

Answer 1

Ecological community of commensal, symbiotic and pathogenic MOs that share our body space

Question 2

How are surfaces within the body infected?

Answer 2

Surface infections occur by:

• Invasion into host
• Migration – e.g. E. coli UTI
• Inoculation – Coagulase negative staphylococcus prosthetic join infection
• Haematogenous – e.g. viridans Strep endocarditis from the mouth

Question 3

What is an infection of the skin called?

Answer 3

cellulitis

Question 4

infection of pharynx called?

Answer 4

pharyngitis

Question 5

infection of eye called?

Answer 5

conjunctivitis

Question 6

Infection of GI called?

Answer 6

gastroenteritis

Question 7

Infection of Urinary tract called?

Answer 7

UTi

Question 8

Lung infection called?

Answer 8

pneumonia

Question 9

Bone infection called?

Answer 9

Osteomyelitis

Question 10

Infection of plural cavity called?

Answer 10

Empyema

Question 11

Infection of large blood vessels called?

Answer 11

Vasculitis

Question 12

Infection of heart valves called?

Answer 12

Endocarditis

Question 13

Infection of joints?

Answer 13

Septic arthrits

Question 14

What bacterias causes endocarditis in prosthetic valves <1 post-op?

Answer 14

Coagulase negative staph aureus

Question 15

What bacterias cause endocarditis in native and prosthetic valves (>1 yr post op)?

Answer 15

Viridans streptococci, staph aureus, HACEK group

Question 16

Describe the pathogenesis of infections at a surface.

Answer 16

1) First step is adherence to host cells or the prosthetic surface – Via use of pili or fimbriae
2) Formation of the biofilm:

• A mixture of polysaccs and short chain nucleotides produced by bacteria
• Allows bacteria to survive and multiply on surfaces

3) Invasion and multiplication of bacteria
4) Host response

• Pyogenic (neutrophils -> pus)
• Granulomatous – Produces nodular inflammatory lesions.

Some bacteria are better at sticking to certain surfaces than others.

Question 17

How do biofilms form?

Answer 17

Formation occurs via quorum sensing:

• Quorum sensing is communication between bacteria, signalling molecules produced into the bacterial environment with the intention of binding to receptors on other bacteria to signal there are other bacteria of the same type around.
• If there are a lot of similar types of bacteria around it signals to the bacteria that this is a good place to settle down and reproduce.
• Positive feedback process.

Question 18

How can you diagnose an infected surface?

Answer 18

Difficult but blood cultures and tissue material sonication

Question 19

How can you treat an infected surface?

Answer 19

Antibiotics (dont penetrate well into biofilm) or removal of infected tissue

Question 20

How do you prevent infection on a surface?

Answer 20

• Maintain surface integrity
• Prevent bacterial surface colonisation
• Remove colonised bacteria

Question 21

Define hypersensitivity

Answer 21

Antigen specific immune responses that are either inappropriate or excessive and result in harm to host.

Question 22

How does hypersensitivity develop? What are the names of the 2 phases?

Answer 22

1) Sensitisation phase – First encounter with antigen, IgE specific to antigen forms. IgE binds to surface of mast cells
2) Effector phase – Allergen binds to 2 different IgE forming a cross link which activates the mast cell to degranulate. Depending on where the mast cell is different clinical symptoms are experienced.

Question 23

Why do people have allergies? What phenotypes are involved?

Answer 23

Hygiene hypothesis:

• TH1 phenotype – less likely to produce IgE antibodies in response to allergens if raised with low antibiotic use, poor sanitation, with large family sizes and with livestock
• TH2 phenotype – More likely to produce IgE in response to allergens if raised with high antibiotic use, good sanitation, small family sizes and away from livestock.

Question 24

What is clinical cross-reactivity?

Answer 24

• Allergy to one type of allergen can result in an allergy to another type of structurally similar allergen
• Depends on structural homology
• E.g. allergy to cow’s milk carries a 92% risk of being allergic to goat’s milk.

Question 25

What happens in an allergic reaction?

Answer 25

• Mast cells are cells containing a large number of granules which contain chemical mediators
• When activated degranulation results in the release of mediators • Mediators include histamine, tryptase, TNF-alpha, CCL3 etc etc.

Anaphylaxis results

Question 26

What happens during anaphylaxis?

Answer 26

Anaphylaxis results in:

• Serious hypotension – Lots of blood vessels vasodilate, dropping BP
• Angioedema
• Wheezing – Bronchi constrict

Question 27

How do you treat anaphylaxis?

Answer 27

Treatment – Adrenaline intramuscular injection:

• Reverses peripheral vasodilation and reduces oedema
• Reverses airway obstruction
• Increases force of myocardial contraction
• Inhibits mast cell activation

Question 28

What 3 methods can be employed to diagnose an allergy?

Answer 28

1) Clinical history – allergens, atopy (predisposition towards hypersensitivity)
2) Blood tests – Serum allergen-specific IgE, serum mast cell histamine
3) Skin prick tests using a range of allergens – Wheel and flare reactions

Question 29

How can you manage an allergy?

Answer 29

• Allergy avoidance
• Education – how to recognise symptoms, use epipen etc.
• Medic alert identification – wear a band identifying allergies
• Drugs – anti histamines, anti-IgE IgG, injectable adrenaline
• Emerging evidence that slow exposure to very small amounts of allergen increasing over time can desensitise patients