6. Streptococci

Question 1

Describe the gram stain appearance of streptococci.

Answer 1

gram positive (purple) cocci in chains

Question 2

Which type of agar plates can be used to classify streptococcal species?

Answer 2

blood agar - classify based of haemolytic ability

Question 3

What protein must a bacterium produce to perform haemolysis?

Answer 3

haemolysin

Question 4

What are the 3 types of streptococci based on haemolytic ability?

Answer 4

1. alpha haemolysis (incomplete erythrocyte lysis)
   
   • causes wet green appearance of agar
   • displayed by S. pneumoniae and viridians Streptococci (group of oral bacteria)
2. beta haemolysis (complete erythrocyte lysis)
   
   • causes white area around colonies
   • displayed by e.g. S. pyogenes
3. gamma haemolysis (non-haemolytic)
   
   • e.g. Enterococcus faecalis

Question 5

Name 2 systems by which beta-haemolytic streptococcal species can be further classified.

Answer 5

1. Lancefield serological classification: based on cell wall antigen (reaction with antibodies, groups A-F affect humans)
2. Sherman group: pyogenic, viridans, enterococcal or lactic

Question 6

What is the haemolytic, Lancefield and Sherman classification of S. pyogenes?

Answer 6

• beta-haemolytic
• Lancefield antigen A
• pyogenic

Question 7

What is the haemolytic, Lancefield and Sherman classification of S. pneumoniae?

Answer 7

• alpha-haemolytic
• no Lancefield antigen
• viridans (but can be pyogenic)

Question 8

How is S. pyogenes transmitted?

Answer 8

1. inhalation of droplets

2. skin-to-skin

Question 9

What is the main disease caused by inhalation of droplets containing S. pyogenes?

Answer 9

Streptococcal pharyngitis

Question 10

What are the clinical features of Streptococcal pharyngitis?

Answer 10

1. abrupt onset sore throat
2. fever
3. malaise and headache
4. lymphoid hyperplasia
5. tonsillopharyngeal exudates

Question 11

Name 4 possible complications of Streptococcal pharyngitis.

Answer 11

1. acute rheumatic fever
2. acute post-streptococcal glomerulonephritis
3. scarlet fever
4. suppurative complications

Question 12

What is acute rheumatic fever?

Answer 12

Inflammation of heart, joints and CNS caused by rheumatogenic M-type streptococcal strains.

Question 13

Suggest possible explanations for the occurrence of rheumatic fever.

Answer 13

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

Question 14

What is acute post-streptococcal glomerulonephritis?

Answer 14

acute inflammation of renal glomerulus due to Ag-Ab complexes in glomerulus

caused by M-type specific strains (different to those in ARF)

Question 15

What is scarlet fever and what are the symptoms?

Answer 15

Local or haematogenous spread of streptococcal pyrogenic exotoxin strain.

Symptoms:

• rash
• high fever
• sepsis
• arthritis
• jaundice

Question 16

Name possible suppurative complications of pharyngitis and explain their occurence.

Answer 16

• 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

Question 17

Name 4 possible S. pyogenes skin infections.

Answer 17

1. impetigo
2. erysipelas
3. cellulitis
4. necrotising fasciitis

Question 18

What is impetigo?

Answer 18

Childhood infection (2-5yrs) involving initial S. pyogenes skin colonisation followed by intradermal inoculation.

Most common cause of glomerular nephritis.

Question 19

What is erysipelas and what is it often preceded by?

Answer 19

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

Question 20

What is necrotising fasciitis?

Answer 20

• 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%)

Question 21

What is streptococcal toxic shock syndrome?

Answer 21

• 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

Question 22

How do S. pyogenes virulence factors cause streptococcal toxic shock syndrome?

Answer 22

• 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

Question 23

How does S. pyogenes escape killing by the immune system?

Answer 23

1. hyaluronic acid capsule - inhibits phagocytosis by macrophages and neutrophils. Poor immunogen due to similarity to human connective tissue hyaluronate.
2. M protein (on surface) - inhibits activation of complement alternative pathway
3. Streptolysins O and S - lysis of erythrocytes, neutrophils and platelets

Question 24

Which product produced by S. pyogenes and other beta-haemolytic streptococci can be used as a medication?

Answer 24

streptokinase - dissolution of clots through conversion of plasminogen to plasmin