Session 10

Question 1

What does streptococcus pyogenes look like?*

Answer 1

String of pearls (SP)

- Cocci chain

Question 2

What kind of bacteria is strep pyogenes?

Answer 2

• Gram positive cocci

- Chain

Question 3

What does it mean to classify streptococci by haemolysis?

Answer 3

Classify by how good they are at breaking up red blood cells in blood agar - bacteria need access to iron and break down blood cells, meaning that they can grow vigorously

Question 4

What are the types of haemolysis classifications?

Answer 4

• Alpha haemolysis (viridans), partial haemolysis
• Beta haemolysis (complete haemolysis)
• Gamma haemolysis (non-haemolytic) -

Question 5

How does haemolysis appear?

Answer 5

Alpha: green
Beta: translucent patch on plate
Gamma: no change as no haemolysis occurs

Question 6

What is streptococcus pyogenes?*

Answer 6

Lancefield Group A, Beta haemolytic streptococcus

Question 7

What is the Lancefield classification?*

Answer 7

Classification of beta-haemolytic streptococci based on their cell wall antigens (eg. A, B, C, G)

Question 8

What is the Sherman classification?*

Answer 8

Classification of streptococci based on:

• Viridans (partial haemolysis)
• Pyogenes (pus forming)
• Enterococci (gut)
• Lactic streptococci (produce lactic acid, vaginal flora)

Question 9

How else can bacteria be classified?

Answer 9

Sequencing DNA

Question 10

What are the virulence factors for S. pyrogenes?

Answer 10

• Hyaluronic acid capsule
• M protein
• Adhesins (lipoteichoic acid, fibronectin binding protein)
• Strepsolysins O and S
• DNAses A B C D
• Hyaluronidase
• Streptokinase
• Streptococcal pyrogenic exotoxins

Question 11

What is the action of the hyaluronic acid capsule?

Answer 11

Inhibits phagocytosis by neutrophils and macrophages (similar to human hyaluronate)

Question 12

What is the action of M protein?*

Answer 12

Inhibits activation of a complement pathway on bacterial cell surface and so is resistant to phagocytosis

Question 13

What is the action of adhesins?

Answer 13

Allow infection by allowing adherence

Question 14

What is the action of streptolysins (O + S)?

Answer 14

Neutrophil, platelet and erythrocyte lysis

Question 15

What is the action of DNAses?

Answer 15

DNA breakdown

Question 16

What is the action of hyaluronidase?

Answer 16

Degrading hyaluronic acid in connective tissue

Question 17

What is the action of streptokinase?

Answer 17

Plasminogen converted to plasmin, so clot dissolution

Question 18

What is the action of the exotoxins?

Answer 18

Cleaves IgG bound to group A strep

Question 19

What is streptococcal pharyngitis?*

Answer 19

An infection caused by S. pyrogenes that spreads through droplets and is associated with overcrowding

Question 20

What happens if streptococcal pharyngitis is left untreated?

Answer 20

Patients will develop M protein resistance (antibody) and is less susceptible to reinfection Q

Question 21

What are the clinical features of streptococcal pharyngitis?

Answer 21

• Abrupt onset of sore throat
• Malaise
• Headache
• Fever
• Lymphoid hyperplasia
• Tonsillopharyngeal exudate

Question 22

How do you diagnose pharyngitis?

Answer 22

Throat swab that will show group A strep

Question 23

What are the complications of strep pharyngitis?

Answer 23

• Scarlet fever
• Suppurative complications
• Acute rheumatic fever
• Acue post-streptococcal glomerulonephritis

Question 24

What is scarlet fever and how is it caused?

Answer 24

• Caused by infection with pyrogenic exotoxin
• An illness characterised by high fever, red rash, sepsis, arthritis and jaundice
• Can have local or haematogenous spread

Question 25

What are suppurative complications?*

Answer 25

Complications resulting in pus formation: - Retropharyngeal abscesses - Peritonsillar cellulitis/abscesses - Sinusitis - Mastoiditis (local) - Meningitis and brain abscesses (haematogenous)

Question 26

What is acute rheumatic fever and what causes it?

Answer 26

- Inflammation of the heart joints that can follow on from pharyngitis - Caused by rheumatogenic M types

Question 27

What are the possible mechanisms of rheumatic fever?

Answer 27

- Autoimmune - Serum sickness (reaction to injection of serum) - M protein binding to collagen

Question 28

What is acute post-streptococcal glomerulonephritis?

Answer 28

- Acute inflammation of glomerulus

Question 29

What causes acute glomerulonephritis?

Answer 29

- M types (not same as acute rheumatic fever)

Question 30

How does rheumatic fever cause illness?*

Answer 30

- Haemolytic streptococci causing throat infection | - Immunological response leading to rheumatic fever affecting joints, skin, heart and brain (chorea)

Question 31

What skin infections can strep pyrogenes cause?

Answer 31

- Impetigo - Erysipelas - Cellulitis - Necrotising fascitis

Question 32

What is impetigo?*

Answer 32

A childhood skin infection occurring under age of 5, caused by colonisation and then intradermal inoculation - Can cause glomerulonephritis

Question 33

What is erysipelas?*

Answer 33

An infection of the dermis causing red, swollen and sore patches. Often preceded by pharyngitis if facial lesions - Can spread to lower limbs too via trauma or skin disease

Question 34

What is cellulitis?*

Answer 34

An infection of the skin and subcutaneous tissues | - Associated with impaired lymphatic drainage and IV drug use

Question 35

What is necrotising fascitis?*

Answer 35

Infection of fascia and deeper subcutaneous tissues that causes rapid and extensive necrosis and is usually secondary to a skin break - Severe pain - High mortality (20-70%)

Question 36

What is streptococcal toxic shock syndrome?

Answer 36

A deep tissue infection with strep pyrogenes - causes bacteraemia, vascular collapse and organ failure

Question 37

What is the pathogenesis of streptococcal toxic shock syndrome?*

Answer 37

- Group A streptococcus invades blood - Sheds M protein - M protein forms a complex with fibrinogen - M protein-fibrinogen binds to neutrophils and activates them - Neutrophils produce respiratory burst which damages endothelium - Causes vascular leakage, DIC, hypotension - Streptococcal pyrogenic exotoxins stimulate T cells through binding to MHC II APCs