Infection 20 - Streptococci

Question 1

Describe the structure of streptococci

How are streptococci classified?

Answer 1

• Streptococci are all gram-positive cocci - arranged in chains
• Classified on the type of haemolysis they posses

1) Alpha haemolysis (viridans) - e.g.: streptococcus pneumoniae, goes green on blood agar (partial breakdown of RBC’s)
2) Beta haemolysis - e.g.: streptococcus pyogenes, complete haemolysis (most virulent)
3) Gamma/Non-haemolytic - e.g.: Enterococcus faecialis, no haemolytic capacity.

• Lancefield serological classification via cell wall antigens
• Sherman classification into 4 groups based on physical properties (pyogenic = pus forming)

Question 2

What is the classification of streptococcus pyogenes?

Answer 2

• Lancefield group A beta-haemolytic streptococcus

- Gram positive cocci (in chains)

Question 3

Name a couple of virulence factors that streptococcus pyogenes has + what they do

Answer 3

1) M protein - provides resistance to phagocytosis by inhibiting activation of alternative complement pathway on bacterial cell surface.
2) Hyaluronic acid capsule - inhibits phagocytosis by neutrophils + macrophages
3) Adhesins - helps bacteria to bind and is important first step in colonisation.

Question 4

What is the most common infection streptococcus pyogenes causes?
How is it spread?
What are the clinical features?

Answer 4

• Streptococcal pharyngitis (strep throat)
• Spread by droplets in crowded environments
• Sore throat, malaise, fever, headache, tonsillophayngeal exudate (pus on tonsils)

Question 5

What are the potential complications of streptococcal pharyngitis?

Answer 5

1) Scarlet fever - due to infection with pyrogenic exotoxin strains which leaves IgG). Causes high fever, sepsis, arthritis, jaundice + typical rash.
2) Suppurative complications - basically pus forming
3) Rheumatic fever - inflammation of heart, joints + CNS. Follows a month or two after pharyngitis, due to M-protein antibodies reacting with host antigens.
4) Glomerulonephritis - Acute inflammation of renal glomerulus, due to antigen-antibody in glomerulus sticking in glomerulus stimulating immune response

Question 6

What 4 skin infections can streptococcus pyogenes cause?

Answer 6

1) Impetigo - childhood infection, causing honeycomb crusting on face
2) Erysipelas - dermis infection with lymphatic involvement, facial lesions preceded by pharyngitis
3) Cellulitis - skin + subcutaneous infection
4) Necrotising fasciitis - Infection of deeper subcutaneous tissue, rapid + extensive necrosis.

Question 7

What is streptococcal toxic shock syndrome?

Answer 7

• Deep tissue infection w/strep pyogenes AND bacteraemia AND vascular collapse AND organ failure
• From health to death within hours
• Entry of group A strep into deeper tissues + bloodstream
• Streptococcal pyrogenic exotoxins stimulate T-cells through binding to MHC class ll molecules