Session 6-Streptococci Flashcards
Describe the gram stain of streptococci
Gram positive cocci chains
What are the classifications of streptococci by haemolysis?
1) alpha haemolysis
2) beta haemolysis
3) gamma haemolysis (non-haemolytic)
Give an example of an alpha haemolytic streptococci
Viridans streptococci eg. Streptococcus pneumoniae
Give an example of a beta haemolytic streptococci
Streptococcus pyogenes
Give an example of a non-haemolytic streptococcus
Enterococcus faecalis
Why do alpha haemolytic streptococci look wet?
Due to capsule - makes bacteria more virulent because protects bacteria against antibody attack
What is the difference between alpha and beta haemolytic streptococci?
Alpha - partial breakdown of RBCs
Beta - complete breakdown of RBCs so clear zone around colony
What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci pyogenes?
1) A (beta)
2) pyogenic
What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci agalactiae?
1) B (beta, gamma)
2) pyogenic
What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci dysgalactiae?
1) C, G (beta)
2) pyogenic
What is the:
1) Lancefield antigen
2) Sherman group
Of Streptococci pneumoniae?
1) NA (alpha)
2) viridans but can be pyogenic
What are the virulence factors of streptococcus pyogenes? (8)
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
What are the actions of the hyaluronic acid capsule of streptococcus pyogenes?
- Inhibits phagocytosis by neutrophils and macrophages
- Poor immunogen because of similarity to human connective tissue hyaluronate
What is the action of the M protein of streptococcus pyogenes?
-resistance to phagocytosis by inhibiting activation of alternative complement pathway on bacterial cell surface
What is the action of streptolysins O and S?
Lysis of erythrocytes, neutrophils and platelets
What is the action of DNAses?
Degradation of DNA, inactivating phagocytosis
What is the action of hyaluronidase?
Degradation of hyaluronic acid in CT
What is the action of streptokinase?
Dissolution of clots through conversion of plasminogen to plasmin
What is the action of streptococcal pyrogenic exotoxins?
Cleaves IgG bound to group A strep
How is streptococcal pharyngitis spread?
Thorough droplets - coughing in close range
What do untreated patients with streptococcal pharyngitis develop?
M protein specific antibody
What are the clinical features of streptococcal pharyngitis?
- Abrupt onset sore throat
- Fever
- Headache
- Lymphoid hyperplasia
- Tonsillopharyngeal exudates
How can streptococcal pharyngitis lead to scarlet fever?
Infection with streptococcal pyrogenic exotoxins strain of streptococcus pyogenes which is very virulent
What are the signs of scarlet fever?
- High fever
- Sepsis
- Arthritis
- Jaundice
What are the suppurative complications of streptococcal pharyngitis?
1) peritonsillar cellulitis/abscess
2) retropharyngeal abscess
3) mastoiditis, sinusitis
4) meningitis, brain abscess
True or false: acute rheumatic fever can follow on from any streptococcal infection
FALSE - only from pharyngitis, acute post-streptococcal glomerulonephritis can follow on from any strep infection
What is acute rheumatic fever?
Inflammation of heart, joints and CNS
How can acute rheumatic fever lead to an increased risk of endocarditis?
Inflammation of heart valves -> rheumatic heart disease/rheumatic valvular damage -> increased risk of endocarditis
What are the possible mechanisms of acute rheumatic fever?
- 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
What is acute post-streptococcal glomerulonephritis?
Acute inflammation of renal glomerulus
Which skin infections can streptococcus pyogenes cause? (4)
1) impetigo
2) erysipelas
3) cellulitis
4) necrotising fasciitis
What is the most common cause of glomerulonephritis?
Impetigo
What is erysipelas?
Dermis infection with lymphatic involvement on face and lower limbs
What is cellulitis?
Skin and subcutaneous tissue infection
What are the risk factors of cellulitis?
- Impaired lymphatic drainage
- Illicit injecting drug use
What is necrotising fasciitis?
Infection of deeper subcutaneous tissues and fascia
How can necrotising fasciitis be treated?
- surgery to cut away the necrotic tissue
- high dose of IV immunoglobulin, which is full of antibodies which neutralise the strep toxins
What is streptococcal toxic shock syndrome?
- deep tissue infection with strep pyogenes
- bacteraemia
- vascular collapse (septic shock)
- organ failure
What causes streptococcal toxic shock syndrome?
Entry of group A strep into deeper tissues and bloodstream
How do streptococcal pyogenic exotoxins cause toxic shock syndrome?
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
