6. Streptococci Flashcards
Describe the gram stain appearance of streptococci.
gram positive (purple) cocci in chains
Which type of agar plates can be used to classify streptococcal species?
blood agar - classify based of haemolytic ability
What protein must a bacterium produce to perform haemolysis?
haemolysin
What are the 3 types of streptococci based on haemolytic ability?
- alpha haemolysis (incomplete erythrocyte lysis)
- causes wet green appearance of agar
- displayed by S. pneumoniae and viridians Streptococci (group of oral bacteria)
- beta haemolysis (complete erythrocyte lysis)
- causes white area around colonies
- displayed by e.g. S. pyogenes
- gamma haemolysis (non-haemolytic)
- e.g. Enterococcus faecalis
Name 2 systems by which beta-haemolytic streptococcal species can be further classified.
- Lancefield serological classification: based on cell wall antigen (reaction with antibodies, groups A-F affect humans)
- Sherman group: pyogenic, viridans, enterococcal or lactic
What is the haemolytic, Lancefield and Sherman classification of S. pyogenes?
- beta-haemolytic
- Lancefield antigen A
- pyogenic
What is the haemolytic, Lancefield and Sherman classification of S. pneumoniae?
- alpha-haemolytic
- no Lancefield antigen
- viridans (but can be pyogenic)
How is S. pyogenes transmitted?
- inhalation of droplets
2. skin-to-skin
What is the main disease caused by inhalation of droplets containing S. pyogenes?
Streptococcal pharyngitis
What are the clinical features of Streptococcal pharyngitis?
- abrupt onset sore throat
- fever
- malaise and headache
- lymphoid hyperplasia
- tonsillopharyngeal exudates
Name 4 possible complications of Streptococcal pharyngitis.
- acute rheumatic fever
- acute post-streptococcal glomerulonephritis
- scarlet fever
- suppurative complications
What is acute rheumatic fever?
Inflammation of heart, joints and CNS caused by rheumatogenic M-type streptococcal strains.
Suggest possible explanations for the occurrence of rheumatic fever.
i) autoimmune (Ab cross-react with own antigen)
ii) serum sickness (reaction to Ab-Ag complex)
iii) binding of M protein to collagen (around heart/joints), streptolysin ASO or ASS induced injury
What is acute post-streptococcal glomerulonephritis?
acute inflammation of renal glomerulus due to Ag-Ab complexes in glomerulus
caused by M-type specific strains (different to those in ARF)
What is scarlet fever and what are the symptoms?
Local or haematogenous spread of streptococcal pyrogenic exotoxin strain.
Symptoms:
- rash
- high fever
- sepsis
- arthritis
- jaundice
Name possible suppurative complications of pharyngitis and explain their occurence.
- peritonsillar cellulitis/abscess, retropharyngeal abscess, mastoiditis, sinusitis, otitis media, meningitis/brain abscess
- due to spread of pus from tonsil into lateral pharyngeal space and then to system
Name 4 possible S. pyogenes skin infections.
- impetigo
- erysipelas
- cellulitis
- necrotising fasciitis
What is impetigo?
Childhood infection (2-5yrs) involving initial S. pyogenes skin colonisation followed by intradermal inoculation.
Most common cause of glomerular nephritis.
What is erysipelas and what is it often preceded by?
S. pyogenes dermis infection with lymphatic involvement affecting face and lower limbs.
- facial lesions often preceded by pharyngitis
- lower limb infection usually secondary to skin invasion via trauma, skin disease or local fungal infection
What is necrotising fasciitis?
- infection of deeper subcutaneous tissues and fascia, usually secondary to skin break
- rapid, extensive necrosis – severe pain, even before gross clinical changes, high fever, fulminant course, high mortality (20-70%)
What is streptococcal toxic shock syndrome?
- deep tissue infection with S. pyogenes + bacteraemia + vascular collapse + organ failure – from health to death in hrs
- involves entry of group A Strep. into deeper tissues and bloodstream
How do S. pyogenes virulence factors cause streptococcal toxic shock syndrome?
- streptococcal pyrogenic exotoxins (superantigenic) overstimulate T cells through binding to MHC II APCs and V-β region of TCR (lower than normal), inducing monocyte cytokines (TNFα, IL-1 and IL-6) and lymphokines (TNFβ, IL-2, IFNγ)
- M protein-fibrinogen complexes bind to neutrophils promoting degranulation, respiratory burst and endothelial damage
How does S. pyogenes escape killing by the immune system?
- hyaluronic acid capsule - inhibits phagocytosis by macrophages and neutrophils. Poor immunogen due to similarity to human connective tissue hyaluronate.
- M protein (on surface) - inhibits activation of complement alternative pathway
- Streptolysins O and S - lysis of erythrocytes, neutrophils and platelets
Which product produced by S. pyogenes and other beta-haemolytic streptococci can be used as a medication?
streptokinase - dissolution of clots through conversion of plasminogen to plasmin
