Streptococci

Question 1

How does streptococci gram stain?

Answer 1

Gram positive (purple) cocci. Make chains!

String of Pearls = Strep Pyrogenes.

Question 2

How do you classify haemolysis?

Answer 2

A haemolysis - Viridans (green forming - group of organisms not a single organism) streptococci e.g. Streptococcus pneumoniae. Don’t frequently cause invasive disease (but cause endocarditis)

B haemolysis - Streptococcus pyogenes - very virulent as ability to haemolysis

Non-haemolytic gammas Enterococcus faecalis e.g. if capsule.

Alpha-hemolytic species cause oxidization of iron in hemoglobin molecules within red blood cells, giving it a greenish color on blood agar. Beta-hemolytic species cause complete rupture of red blood cells. On blood agar, this appears as wide areas clear of blood cells surrounding bacterial colonies. Gamma-hemolytic species cause no hemolysis.

Question 3

How do you classify streptococci?

Answer 3

According to ability to haemolyse.

But can subdivide B haemolytic strain further as different strains of antibiotics are present on surface. They are classified by the Lancefield antigen.

E.g. Group A B haemolytic strep = Strep pyogenes

Sherman also classified the species according physiological properties (but this is less commonly used).

Now, also classified according to 16S ribosomal RNA sequences now.

Question 4

Why is Strep Pyogenes virulent?

Answer 4

Lots of virulence factors.

Question 5

What are Strep Pyogenes virulence factors?

Answer 5

• Hyaluronic acid capsule
• M protein
• Adhesions, including lipoteichoic acid, M protein, Fibronectin binding proteins.
• Streptolysins O and S
• DNAses A,B,C and D
• Hyaluronidase
• Strepkinase
• Streptococcal pyrogens exotoxins.

Question 6

What is streptococcal pharyngitis?

Answer 6

Streptococcus pyogenes

Painful sore throat

Peak incidence at 5-15 years

Droplet spread

Association with over crowding (Schools, households, student accommodation..)

Untreated patients develop M protein specific antibody so, not routinely treated with antibiotics. Also less likely to develop resistance.

Question 7

What are the clinical features of Streptococcal Pharyngitis?

Answer 7

• Abrupt onset sore throat Malaise,
• Fever,
• Headache,
• Lymphoid hyperplasia,
• Tonsillopharyngeal exudates.
• A throat swap would show group A strep.

Question 8

What are the complications of streptococcal pharyngitis?

Answer 8

Scarlet fever

• Due to infection with streptococcal pyreogenic exotoxins strain of S. pyogenes.
• Local or haematogenous spread.
• High fever, sepsis, arthritis. Jaundice.
• 1800’s epidemic wit 20% mortality.
• Isolate and treat with antibiotics.

Suppurative complications

• Peritonsillar cellulitis / abscess.
• Retropharyngeal abscess.
• Mastoiditis, sinusitis, Otis media - getting into bone.
• Meningitis, brain abscess - from getting into carotid artery.

Acute Rheumatic fever (immunological)

• Inflammation of heart, joints, CNS.
• Follow on from pharyngitis,
• Only rheumatogenic M types cause this.
• Possible mechanisms :
  
  • Autoimmune,
  • Serum sickness,
  • binding of M protein to collagen,
  • ASO, ASS induced tissue injury.
• Endocarditis could be a late complication of R fever.

Acute post-strep glomerulonephritis (inflammation of glomerulus)

• Acute Inflammation of renal glomerulus,
• M type specific but NOT same as ARF M types.
• Antigen-antibody complexes in glomerulus.

Question 9

What skin infections can Strep pyogenes cause?

Answer 9

• Impetigo
• Erysipelas
• Cellulitis
• Necrotising fasciitis

Question 10

What is impetigo?

Answer 10

• Childhood infection 2-5 years
• Initial skin colonisation, followed by intradermal innoculation
• No ARF but impetigo is most common cause of glomeruonephritis

Question 11

What is Erysipelas?

Answer 11

• Dermis infection with lymphatic involvement.
• Face and lower limb
• Facial lesions, frequently preceded by pharyngitis
• Lower limb infection usually secondary to invasion of skin via trauma, skin disease or local fungal infection.

Question 12

What is cellulitis?

Answer 12

• Skin and subcutaneous tissue infection - usually only on one of the lower limbs.
• Impaired lymphatic drainage and illicit injecting drug use important risk factors.

Question 13

What is necrotising fasciitis?

Answer 13

• This is an infection of the deeper subcutaneous tissues and fascia.
• This can spread to the muscles.
• Rapid, extensive necrosis
• Usually secondary to skin break (group A strep in skin)
• Severe pain, even before gross clinical changes
• High fever, fulminant course, high mortality (20-70%).
• Patients are SEVERELY unwell.
• Probably will need to amputate.
• The ‘sweep test’ to see if you can move hand freely.
• Also need antibiotic treatment - if group A strep then penicillin.

Question 14

What is streptococcal toxic shock syndrome?

Answer 14

• Deep tissue infection with Strep pyogenes AND bacteraemia AND vascular collapse AND organ failure.
• They can go from health to death in hours.
• Entry of group A strep into deeper tissues and bloodstream.
• Streptococcal pyrogenic exotoxins stimulate T-cells through binding to MHC class II antigen presenting cells and V-B region of T cell receptor inducing monocytes, cytokines (TNF-a, IL-1B, IL6) and lymphokines
• M protein fibrinogen complex and formation.