Streptococcus

• Name Group Haemolysis pattern

  Streptococcus pyogens A beta haemolysis

  Streptococcus agalactiae B beta haemolysis

  Streptococcus faecalis D non or alpha haemolysis

  Viridans streptococci || usually not typed or untyable || alpha haemolys

  Streptococcus pneumoniae none alpha haemolysis
• Classification

  (A)Depends on haemolysis pattern on blood agar

  • beta haemolysis - complete haemolysis
  • alpha hemolysis - incomplete haemolysis, green pigment
  • gamma haemolysis - non-haemolytic
• Classification

  (B)Depends on cell wall group specific substance(Lancefield classification)

  • contains carbohydrates
  • grouping into Lancefieldgroups A-H and K-U

  (C)Capsular polysaccharides

  • S.pneumoniae is more than 90 types

  (D)Biochemical reaction
• Streptococcus pyogens

  • local or systemic invasion, post streptococcal immunologic disorders
  • PYR - positive
  • susceptible to bacitracin ( agalactiae --> R)
• Aetiology

  (4)Antigenic structure

  (a)M protein

  • major virulence factor
  • resist phagocytosis by inhibiting activation of alternate complement pathway
  • type specific
  • determinant for rheumatic fever
• Aetiology

  (5)Enzymes and toxins

  (a)Streptokinase(fibrinolysin)

  (b)Deoxyribonucleases

  (c)Hyaluronidase(spreading factor)

  (d)Pyrogenic exotoxins(erythrogenic toxin)

  • associated with streptococcal toxic shock syndrome and scarlet fever
• Aetiology

  (5)Enzymes and toxins

  (e)Haemolysins

  • streptolysin O
  • antibody to streptolysin O, antistreptolysin O (ASO)
  • ASO excess of 160-200 units is considered abnormally high and suggests either recent infection or persistently high antibody levels caused by exaggerated immune response
  • streptolysin S is responsible for haemolytic zones around streptococcal colonies.
• Pathogenesis

  (A)Diseases attributable(due to) invasion

  • erysipelas
  • cellulitis
  • necrotizing fasciitis (streptococcal gangrene)
  • puerperal fever
  • bacteremia or sepsis
• Pathogenesis

  (B)Diseases attributable to local infection

  • sore throat or pharyngitis
  • pyoderma (pyo=pus, derma=skin)

  - called impetigo

  - more widespread infection occurs in eczematous or wounded skin or in burns and may progress to cellulitis

  - skin infections are attributable to glomerulonephritis but do not lead to rheumatic fever
• Pathogenesis

  (C)Invasive group A streptococcal infections

  1. Streptococcal toxic shock syndrome
  2. Scarlet fever

  • in association with pharyngitis or with skin or soft tissue infection
  • rash appears on the trunk after 24 hours of illness and spreads to involve extremities.
• Pathogenesis

  (C)Invasive group A streptococcal infection

  3. Post streptococcal diseases (rheumatic fever, acute glomerulonephritis)

  • 1 - 4 weeks after which nephritis or rheumatic fever may develops
  • these diseases are not attributable to direct effect of bacteria but represent hypersensitivity response
  • nephritis is preceded by infection of the skin, rheumatic fever is preceded by infection of the respiratory tract.
• Laboratory diagnosis

  (1)Microscopic examination of smear

  • gram stained, examined under ordinary light microscope showed gram positive cocci arranged in long chains of not more than 4 cocci

  (2)Culture

  • crystal violet blood agar
  • discoid colonies with beta haemolysis on blood agar plate
  • bacitracin sensitivity test is positive, group A
• Laboratory diagnosis

  (3)Serology

  (a)Anti-streptolysin O test (ASO test)

  (b)Other serologic tests

  • anti Dnase test, anti hyaluronidase test, anti streptokinase test, anti NADase test

  (4)Antibiotic sensitivity test
• Enterococci

  • enterococcus faecalis is the most common and others enterococcus faecium.
  • enterococci causes nosocomial infections particularly in intensive care units.
  • most common sites of infection are urinary tract, wounds, biliary tract and blood
  • cause meningitis and bacteremia in neonates.
  • In adults, enterococci can cause endocarditis.
• Antibiotic resistant of enterococci

  (1) Intrinsic resistance + Extrinsic resistance

  (2) Resistance to aminoglycosides

  (3) Vancomycin resistance

  • vancomycin resistance enterococci (VRE)

  (4) Beta lactamase production and resistance to beta lactams

  (5) Trimethoprim sulfamethoxazole (TMP-SMZ) resistance