Streptococci

Question 1

What is the morphology of streptococci pyogenes?

Answer 1

• individual cocci arranged in chain form
• spherical/oval in shape
• diplococcal appearance usually seen
• gram +ve (stain purple) but staining can be lost eventually in culture
• group A strains produce capsules of hyaluronic acis (prevents phagocytosis)
• hyaluronic acid binds to CD44 binding protein on epithelial cells
• hairlike pilli (partly made of M protein and lipoteichoic acids)

Question 2

Is streptococcus pyogenes an anaerobe or aerobe? What conditions does it grow best in?

Answer 2

• facultative anaerobe

- grows best at 37 degrees celsius

Question 3

What appearance can the colonies of Strep. pyogenes take?

Answer 3

• glossy or matte
• matte colonies produce M protein (virulent)
• glossy not very harmful

Question 4

What is the structure of the M protein? What is its function?

Answer 4

M protein is the major virulence factor of S. pyogenes

• M protein looks like hair-like projections of the cell wall
• resist phagocytosis by WBC due to inhibition of the complement pathway

Question 5

What are the virulence factors for Streptococci pyogenes?

Answer 5

• streptokinase (fibrolysin)
  this encourages conversion of plasminogen (in blood plasma) to the enzyme plasmin. This digest fibrina and other proteins, facilitating the release of bacteria from blood clots
• deoxyribonucleases
  This breaks down DNAses and
• hyaluronidases
  breaks down hyaluronic acid in connective tissue
• exotoxins
  these exotoxins have been shown to cause scarlet fever and STSS (streptococcal toxic shock syndrome)
• hemolysins

Question 6

Streptococci pyogenes can manifest itself in different forms clinically. What is this?

Answer 6

Erysipelas

Fiery red skin rash with brawny edema and margin that demarcates rash from healthy skin advances quickly

Question 7

Streptococci pyogenes can manifest itself in different forms clinically. What is this?

Answer 7

Cellulitis - another skin infection caused by streptococci pyogenes, however, it is not raised and does not clearly demarcate healthy skin from infected skin (this is the difference between erysipelas and cellulitis)

Question 8

Streptococci pyogenes can manifest itself in different forms clinically. What is this?

Answer 8

Necrotising fascilitis (gangrene) - necrosis of the skin

Question 9

What is puerperal fever?

Answer 9

Infection caused by streptococci pyogenes - infection of the uterus after delivery (endometritis)

Question 10

What is strep throat? What are the clinical findings?

Answer 10

infection of the pharynx caused by streptococci

- thin serous discharge and little fever, infection tends to spread to the mastoid and middle ear

Question 11

What is streptococcal pyoderma?

Answer 11

impetigo (local infection of superficial layers of skin)
- eroded vesicles covered with crusted pus
Those with ezcema or skin burns/wounds are more prone to this kind of infection
- clinically identical picture observed in staphylococcus aureus

Question 12

Explain how streptococcal pyogenes infection can lead to rheumatic fever. What are the clinical findings?

Answer 12

Rheumatic fever typically develops 1-4 weeks after strep. throat (it is NOT a consequence of skin infections)

Clinical findings

• inflammation of all parts of the heart (myocardium, pericardium, endocardium)
• inflammation of the heart typically leads to deformed heart valves and small perivascular granulomas (Aschoff bodies)
• recurrent strep throat infections can lead to recurrent episodes of rheumatic fever