Streptococcus Flashcards
streptococi characteristics
(especially pyogenes)
- Gram positive
how do you classify streptocci
haemolysis
explain haemolysis
Use agar with blood (horse or sheep)- with RBC.
- Iron is a requirement for many bacteria, therefore haemolysis to get iron
partial haemolysis
alpha haemolysis
e.g. viridans (green) streptococcus
(live in the mouth)
complete haemolysis
beta haemolysis
- streptocccus pyogenes
no haemolysis
gamma non-hamolysis
e.g. enterococcus faecalis
streptococcus classification schemes
streptococcus pyogenes virulence factors
- hyaluornic acid capsule
- M protein
- Adhesins
- Streptolysins O and S
- DNAases A, B, C and D
- Hyaluronidase
- streptokinase
- streptooccal pyrogenic exotoxins
Hyaluronic acid capsule
- Inhibits phagocytosis by neutrophils and macrophages
- Poor immunogen because of similarity to human connective tissue hyaluronate
M protein
- Resistance to phagocytosis by inhibiting activation of alternative complement pathway on bacterial cell surface
- >150 antigenically different serotypes as a consequence of nucleotide variants of the M gene
- Resistance to phagocytosis by inhibiting activation of alternative complement pathway on bacterial cell surface
Adhesins (Lipoteichoic acid, M protein, fibronectin binding proteins)
Adherence is first step in colonisation/ infection
Streptolysins O and S
Lysis of erythrocytes, neutrophils, platelets
DNAases A, B, C and D
Degradation of DNA
Hyaluronidase
Degradation of hyaluronic acid in connective tissue
Streptokinase
Dissolution of clots through conversion of plasminogen to plasmin
Streptococcal pyrogenic exotoxins
Cleaves Ig G bound to group A steep
Streptococcus pyogenes M protein
If you stain bacteria correctly you can see fuzzy layerà M proteins
Streptococcal pharyngitis (tonsillitis)
- Causative organism: streptococcus pyogenes
- Most common at 5-15 years
- Droplet spread
- Association with overcrowding
- Untreated patients develop M protein specific antibodies
clinical features of streptococcal pharyngitis
- Abrupt onset of sore threat
- Malaise, fever, headache
- Lymphoid hyperplasia
- Tonsillopharyngeal exudates
- Throat swab –> Group A strep
Complications of streptococcal pharyngitis
scarlet fever
suppurative complications
acute rheumatic fever
acute post-streptococcal glomerulonephritis
Scarlet fever
- Due to infection with streptococcal pyrogenic exotoxin strain of S. Pyogenes
- Local or haematogenous spread
- High fever, sepsis, arthritis, jaundice
- 1800s epidemic with 20% mortality
Suppurative complications
- Peritonsillar cellulitis/abscess (contiguous)
- Retropharyngeal abscess
- Mastoiditis, sinusitis, otitis media
- Meningitis, brain abscess (haematogenous spread)
Acute rheumatic fever
- Inflammation of the heart, joints, CNS
- Follow on from pharyngitis
- No bacteria anymore (no point using antibiotics)
- Rheumatogenic M types
- Mechanisms (possible)
- Auto-immune
- Serum sickness
- Binding of M protein to collagen
- ASO, ASS induced tissue injury
Acute post-streptococcal glomerulonephritis
- Acute inflammation of renal glomerulus
- M type specific but not same as ARF M types
- Antigen-antibody complexes in glomerulus
Streptococcus pyogenes skin infections
impetigo
erysipelas
cellulitis
necrotising fasciitis
impetigo
- Childhood infection (2-5 years)
- Initial skin colonisation followed by intradermal inoculation
- No ARF but impetigo is most common cause of glomerulonephritis
- Flucloxacillin
Erysipelas
- Dermis infection with lymphatic involvement
- Face, lower limbs
- Facial lesions frequently preceded by pharyngitis
- Lower limb infection usually secondary to invasion of skin via trauma, skin disease or local fungal infection
Cellulitis
- Skin and subcutaneous tissue infection
- Impaired lymphatic drainage and illicit injecting drug use important risk factor
Necrotising fasciitis (may require surgery)
- Infection of deeper subcutaneous tissue and fascia
- Rapid, extensive necrosis
- Usually secondary to skin break
- Severe pain, even before gross clinical changes
- High fever, fulminant course, high mortality (20-70%)
normal skin vs necrotising fascitis
Streptococcal toxic shock syndrome
*
- Deep tissue infection with strep pyogenes
- And
- Bacteraemia
- And
- Vascular collapse
- And
- Organ failure
Deep tissue infection with strep pyogenes- from health to death in hours
- Entry of group A strep into deeper tissue sand bloodstream
- Streptococcal pyrogenic exotoxins stimulate T cells through binding to MHC class II APC and V-B region of T cell receptor inducing:
- Monocyte cytokines (TNF-alpha, IL-1B, IL-6) and lymphokines (TNF-B, IL-2, IFN-Y)
- M-protein fibrinogen complex formation
