Streptococci

Question 1

How can streptococci be classified?

Answer 1

gram positive cocci in chains

alpha-haemolytic: ‘viridians’ streptococci e.g streptococcus pneumoniae
beta-haemolytic: streptococcus pyogenes
gamma (non-haemolytic): enterococcus faecalis

can be further classified into lancefield groups based on antigens

e.g Lancefield group A beta-haemolytic streptococcus

Question 2

What are the virulence factors for streptococcus pyogenes?

Answer 2

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 emm gene.

Adhesins, including lipoteichoic acid, M protein, fibronectin binding proteins: Adherence is first step in colonisation/infection

Streptolysins O and S: Lysis of erythrocytes, neutrophils, platelets

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 strep. Member of superantigenic Spe family (clonal T-cell proliferation)

DNAases A, B, C, D: Degrades DNA

Question 3

What are the clinical features of streptococcal pharyngitis?

Answer 3

• streptococcus pyogenes
• droplet spread
• untreated patients develop m protein specific antibody (preferably dont give antibiotics)
• abrupt onset sore throat
• malaise, fever, headache, lymphoid hyperplasia
• throat swabs show group A strep

complications:

• scarlet fever- due to exotoxin-high fever, sepsis, jaundice, arthritis
• suppurative complications e.g. abscess
• acute rheumatic fever
• acute post-streptococcal glomerulonephritis

Question 4

What are some skin infections caused by streptococcus pyogenes?

Answer 4

• impetigo
• cellulitis
• necrotising fascitis

Question 5

What is streptococcal toxic shock syndrome?

Answer 5

• deep tissue infection with strep pyogenes
• bacteraemia
• vascular collapse
• organ failure

health to death in hours potentially

entry of group A strep into deeper tissues and bloodstream. streptococcal pyrogenic exotoxins stimulate t cells through binding mhc 2 apc’s inducing monocytic cytokines and lymphokines (damage). there is m protein fibrinogen complex formation which bind to polymorphonuclear leukocytes which causes further endothelial damage as they bind endothelial cells and release hydrolytic substances.