Streptococci

• Streptococcaceae


  • Family
  • Catalase negative (-) - streptococci and enterococci
  • Catalase positive - staphylococci and micrococci (bubble formation)

  
• Gram positive (+) cocci


  • In pairs and in chains
  • Appear more elongated than staphylococci

  
• Streptococci and enterococci


  • No bubble formation
  • In chains when growth in broth cultures
• Streptococci and enterococci


  • Facultative anaerobe, aerotolerant anaerobes, capnophilic (requires increasing amount of carbon dioxide (more than 5-10%))
  • Can ferment glucose but no gas
  • Colonies: small [pinpoint] and transparent
• Staphylococci


  • Colonies: medium-sized/pinhead sized, opaque
• Classification of streptococci


  1. Colonial morphology and hemolytic reactions on blood agar

  2. Serologic specificity of the cell wall group-specific substance

  
• Beta-hemolytic pattern


  Clear surrounding, complete lysis of RBCs

  
• Alpha-hemolytic pattern


  Greenish discoloration, partial hemolysis of RBCs
• Non-hemolytic pattern


  No change, no RBCs lysed
• Lancefield Classification


  Based on the C carbohydrate (polysaccharide) in the cell wall
• Lancefield Groups


  • Group A: Streptococcus pyogenes
  • Group B: Streptococcus agalactiae
  • Group C: Streptococcus dysgalactiae, Streptococcus equi
  • Group D: Enterococcus spp.
  • Group F: Streptococcus anginosus group

  
• Beta hemolytic groups


  • Groups A, B, and C
• Alpha hemolytic


  Streptococcus pneumoniae (Viridans streptococci)
• Non-hemolytic


  Group D streptococci, Enterococcus spp.

  
• Group antigens are classified into certain hemolysis types, but they do not demonstrate such types all the time
• Antigens are acid labile except for antigen B (acid stable)
• Virulence factors of Streptococcus pyogenes

  • M protein
  • Protein F (fibronectin-binding protein) and lipoteichoic acid
  • Hyaluronic acid capsule
  • Streptolysin O (SLO) and Streptolysin S
  • Deoxyribonuclease (DNase)
  • Streptokinase or fibrinolysin
  • Hyaluronidase
  • Streptococcal pyrogenic exotoxins

  
• M protein


  Encoded by the genes emm, resist phagocytosis and adhere the bacterial cell to mucosal cells, over 150 serotypes
• Streptolysin O (SLO)


  Can initiate a highly immune response, detected by Anti-streptolysin O (ASO) test
• Deoxyribonuclease (DNase)


  Potent protease, interferes with phagocytosis, immunogenic
• Streptokinase or fibrinolysin


  Allows the bacteria to escape from blood clots
• Hyaluronidase


  Spreading factor, solubilizes ground substance of mammalian connective tissues
• Streptococcal pyrogenic exotoxins


  Erythrogenic toxin, cause scarlet fever
• Streptococcus pyogenes infections


  • Bacterial pharyngitis (strep throat)
  • Pyodermal infections (impetigo, cellulitis, erysipelas, wound infection, arthritis, scarlet fever)
  • Necrotizing fasciitis (type 2)
  • Streptococcal toxic shock syndrome
  • Poststreptococcal sequelae (rheumatic fever, acute glomerulonephritis)
• Streptococcus agalactiae


  • Capsule resists phagocytosis, component is sialic acid
  • Produces hemolysin, CAMP factor, neuraminidase, DNase, hyaluronidase, and protease (not involved in virulence)

  
• Streptococcus agalactiae infections


  Invasive disease of newborns (vertical transmission, colonization in vagina and rectal area)
• Specimen collection for Streptococcus pyogenes

  1. Throat swab, pus, cerebrospinal fluid or other sterile body fluid, or blood

  2. Swab: tongue should be depressed and swab rubbed over posterior pharynx and tonsillar areas

  3. Exudate: touched with swab

  4. Avoid tongue and uvula

  
• Specimen collection for Streptococcus agalactiae

  1. Vaginal and rectal material with swabs between 35 and 37 weeks of gestation

  2. Transport medium: Todd-Hewitt broth (10 microgram/mL colistin) and Lim broth (15 microgram/mL nalidixic acid)

  
• Direct microscopic examination


  • Gram positive (+) cocci round or oval-shaped, occasionally forming elongated cells

  
• Culture


  1. BAP, BAP containing sulfamethoxazole (SMZ) or colistin and polymixin B

  2. 18 to 24 hours of incubation at 35° C to 37° C aerobically or anaerobically

  
• Macroscopic examination of colonies

  • Streptococcus pyogenes: small, transparent, and smooth with a well-defined area of β-hemolysis

  Streptococcus agalactiae: grayish white mucoid colonies surrounded by a small zone of β-hemolysis, StrepB Carrot Broth: orange or red pigment after 6 hours incubation

  
• Microscopic examination


  • Gram positive (+) cocci with some short chains

  
• Biochemical tests


  Catalase test

  Bacitracin test (Taxo A)

  Pyrrolidonyl-a-nathylamide hydrolysis test (PYR test)

  Hippurate hydrolysis test (for S. agalactiae)

  CAMP test

  
• Catalase test


  Principle: catalase mediates breakdown of hydrogen peroxide (30% H2O2) into oxygen and water

  Result: (+) bubble formation for staphylococci and micrococci, (-) no or few bubble formation for streptococci and enterococci
• Bacitracin test (Taxo A)

  Principle: determine effect of bacitracin (0.04 Unit) on organism

  Result: (+) zone of inhibition = sensitive (S. pyogenes), (-) no zone = resistant (other beta-hemolytic streptococci)

  
• PYR test


  Principle: differentiate S. pyogenes from nonhemolytic Group D strep by presence of L-pyrroglutamylaminopeptidase enzyme

  Result: (+) bright red color = S. pyogenes, (-) no color change or orange = other beta-hemolytic streptococci

  
• Hippurate hydrolysis test


  Principle: determine capability to hydrolyze hippurate

  Result: (+) deep purple color = S. agalactiae, (-) colorless or slightly yellow pink = other beta-hemolytic streptococci
• CAMP test


  Principle: detection of CAMP factor that acts synergistically with beta-lysin of S. aureus

  Result: (+) enhanced hemolysis at juncture = S. agalactiae, (-) no enhancement = other beta-hemolytic streptococci

  
• Will not proceed immediately to bacitracin, check for the hemolytic pattern first (Beta: Bacitracin; Alpha: Optochin; Non: Bile esculin)

  
• Streptococcus pneumoniae


  • Member of S. mitis group, does not have C carbohydrate (no Lancefield classification), cell wall has C substance that reacts with CRP