Session 6-Streptococci

Question 1

Describe the gram stain of streptococci

Answer 1

Gram positive cocci chains

Question 2

What are the classifications of streptococci by haemolysis?

Answer 2

1) alpha haemolysis
2) beta haemolysis
3) gamma haemolysis (non-haemolytic)

Question 3

Give an example of an alpha haemolytic streptococci

Answer 3

Viridans streptococci eg. Streptococcus pneumoniae

Question 4

Give an example of a beta haemolytic streptococci

Answer 4

Streptococcus pyogenes

Question 5

Give an example of a non-haemolytic streptococcus

Answer 5

Enterococcus faecalis

Question 6

Why do alpha haemolytic streptococci look wet?

Answer 6

Due to capsule - makes bacteria more virulent because protects bacteria against antibody attack

Question 7

What is the difference between alpha and beta haemolytic streptococci?

Answer 7

Alpha - partial breakdown of RBCs

Beta - complete breakdown of RBCs so clear zone around colony

Question 8

What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci pyogenes?

Answer 8

1) A (beta)

2) pyogenic

Question 9

What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci agalactiae?

Answer 9

1) B (beta, gamma)

2) pyogenic

Question 10

What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci dysgalactiae?

Answer 10

1) C, G (beta)

2) pyogenic

Question 11

What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci pneumoniae?

Answer 11

1) NA (alpha)

2) viridans but can be pyogenic

Question 12

What are the virulence factors of streptococcus pyogenes? (8)

Answer 12

1) hyaluronic acid capsule
2) M protein
3) adhesins
4) streptolysins O and S
5) DNAses A, B, C and D
6) hyaluronidase
7) streptokinase
8) streptococcal pyrogenic exotoxins

Question 13

What are the actions of the hyaluronic acid capsule of streptococcus pyogenes?

Answer 13

• Inhibits phagocytosis by neutrophils and macrophages

- Poor immunogen because of similarity to human connective tissue hyaluronate

Question 14

What is the action of the M protein of streptococcus pyogenes?

Answer 14

-resistance to phagocytosis by inhibiting activation of alternative complement pathway on bacterial cell surface

Question 15

What is the action of streptolysins O and S?

Answer 15

Lysis of erythrocytes, neutrophils and platelets

Question 16

What is the action of DNAses?

Answer 16

Degradation of DNA, inactivating phagocytosis

Question 17

What is the action of hyaluronidase?

Answer 17

Degradation of hyaluronic acid in CT

Question 18

What is the action of streptokinase?

Answer 18

Dissolution of clots through conversion of plasminogen to plasmin

Question 19

What is the action of streptococcal pyrogenic exotoxins?

Answer 19

Cleaves IgG bound to group A strep

Question 20

How is streptococcal pharyngitis spread?

Answer 20

Thorough droplets - coughing in close range

Question 21

What do untreated patients with streptococcal pharyngitis develop?

Answer 21

M protein specific antibody

Question 22

What are the clinical features of streptococcal pharyngitis?

Answer 22

• Abrupt onset sore throat
• Fever
• Headache
• Lymphoid hyperplasia
• Tonsillopharyngeal exudates

Question 23

How can streptococcal pharyngitis lead to scarlet fever?

Answer 23

Infection with streptococcal pyrogenic exotoxins strain of streptococcus pyogenes which is very virulent

Question 24

What are the signs of scarlet fever?

Answer 24

• High fever
• Sepsis
• Arthritis
• Jaundice

Question 25

What are the suppurative complications of streptococcal pharyngitis?

Answer 25

1) peritonsillar cellulitis/abscess 2) retropharyngeal abscess 3) mastoiditis, sinusitis 4) meningitis, brain abscess

Question 26

True or false: acute rheumatic fever can follow on from any streptococcal infection

Answer 26

FALSE - only from pharyngitis, acute post-streptococcal glomerulonephritis can follow on from any strep infection

Question 27

What is acute rheumatic fever?

Answer 27

Inflammation of heart, joints and CNS

Question 28

How can acute rheumatic fever lead to an increased risk of endocarditis?

Answer 28

Inflammation of heart valves -> rheumatic heart disease/rheumatic valvular damage -> increased risk of endocarditis

Question 29

What are the possible mechanisms of acute rheumatic fever?

Answer 29

- auto-immune - serum sickness (reaction to antigen-antibody complexes) - binding of M protein to collagen (provokes immune response because foreign protein) - streptolycins O and S induced tissue injury

Question 30

What is acute post-streptococcal glomerulonephritis?

Answer 30

Acute inflammation of renal glomerulus

Question 31

Which skin infections can streptococcus pyogenes cause? (4)

Answer 31

1) impetigo 2) erysipelas 3) cellulitis 4) necrotising fasciitis

Question 32

What is the most common cause of glomerulonephritis?

Answer 32

Impetigo

Question 33

What is erysipelas?

Answer 33

Dermis infection with lymphatic involvement on face and lower limbs

Question 34

What is cellulitis?

Answer 34

Skin and subcutaneous tissue infection

Question 35

What are the risk factors of cellulitis?

Answer 35

- Impaired lymphatic drainage | - Illicit injecting drug use

Question 36

What is necrotising fasciitis?

Answer 36

Infection of deeper subcutaneous tissues and fascia

Question 37

How can necrotising fasciitis be treated?

Answer 37

- surgery to cut away the necrotic tissue | - high dose of IV immunoglobulin, which is full of antibodies which neutralise the strep toxins

Question 38

What is streptococcal toxic shock syndrome?

Answer 38

- deep tissue infection with strep pyogenes - bacteraemia - vascular collapse (septic shock) - organ failure

Question 39

What causes streptococcal toxic shock syndrome?

Answer 39

Entry of group A strep into deeper tissues and bloodstream

Question 40

How do streptococcal pyogenic exotoxins cause toxic shock syndrome?

Answer 40

Stimulate T cells through binding to MHC class II antigen-presenting cells and V-beta region of T cell receptor, inducing monocyte cytokines and lymphokines