6. Streptococci Flashcards
What do streptococci look like on a gram stain?
Gram positive - purple
Chains of cocci - string of pearls
How are streptococci classified by haemolysis?
alpha haemolysis - partial breakdown
beta haemolysis - complete breakdown
non-haemolytic (gamma)
What does alpha hemolytic strep look like on blood agar?
Green appearance
What does beta haemolytic strep look like on blood agar?
Clear halo visible
What is an example of an alpha haemolytic strep?
Strep. pneumoniae - wet appearance due to capsule
What is an example of beta haemolytic strep?
Strep pyogenes
What are the virulence factors of streptococci pygogenes?
Hyaluronic acid capsule M Protein Streptolysins DNAases Hyaluronidase Streptokinase Exotoxins
What are M proteins, how do they act as virulence factors?
Proteins on bacterial surface that inhibit complement activation and phagocytosis.
Also help with adherence.
What does hyaluronidase do?
Disrupts ground substance of connective tissue, aiding spread of infection.
What does streptokinase do?
Converts plasminogen to plasmin, lysing clots.
How does the bacterial capsule act as a virulence factor?
Made of hyaluronic acid, non-immunogenic as same as found in CT. Prevent phagocytosis,
What do streptolysins O and S do?
Cell lysis of erythrocytes, neutrophils and platelets, release of lysosomal enzymes.
What is the significance of the exotoxin released by strep pyogenes?
Streptococcal pyrogenic toxin
“super antigen family”
Cleaves IgG bound to group A strep
What infection does strep pyogenes commonly cause in people aged 5-15?
Streptococcus pharyngitis
How it Strep pharyngitis spread?
Droplet spread
What are complications that can arise from strep pharyngitis?
Scarlet fever
Acute rheumatic fever
Acute post-streptococcal glomerulonephritis
What suppurative (pus-forming) complications can occur following strep pharyngitis?
Peritonsillar abscess
Retropharyngeal abscess
Meningitis
Brain abscess
How does scarlet fever arise from strep pyogenes infection?
Strep. pyogenes exotoxin strain
What are the symptoms of scarlet fever?
High fever, sepsis, arthritis, jaundice
What is acute rheumatic fever?
Inflammatory disease affecting the heart, joints and CNS
What is acute post-streptococcal glomerulonephritis and how is it caused by strep pyogenes?
Acute inflammation of the renal glomerulus as due to antigen-antibody complexes in the glomerulus
What skin infections can strep pyogenes cause?
Impetigo
Erysipelas
Cellulitis
Necrotising fasciitis
Which age group is impetigo most common?
2-5 year olds
What complication is most common following strep progenies impetigo?
Most common cause of glomerunephritis
No rheumatic fever
What is necrotising fasciitis?
Infection of deeper subcutaneous tissues and fascia leading to rapid, extensive necrosis.
How can streptococci cause toxic shock syndrome?
Can happen when group A strep enters deeper tissues and blood stream.
Release of super antigen exotoxins that stimulate T cells through MHCII, inducing mass cytokine release.
