Strep

• Streptococci


  • Gram positive cocci in chains, spherical to ovoid
  • Non-motile
  • Capnophilic – increased CO2
  • Catalase (-)
• Medium of choice

  Sheep's blood agar
• First step in identification of Streptococci

  1. Inoculate in Sheep's blood agar

  2. Determine the hemolysis pattern
• Selective medium

  Phenyl ethyl alcohol agar
• Smith & Brown classification of Streptococci

  • Alpha hemolytic
  • Beta hemolytic
  • Gamma hemolytic
  • Alpha prime
• Alpha hemolytic Streptococci

  • Can cause incomplete/partial hemolysis on BAP
  • Can cause greening of agar
  • Includes Streptococcus pneumoniae, Viridans Streptococci: S. mitis & S. mutans
• Beta hemolytic Streptococci

  • Can cause complete hemolysis on BAP
  • Can produce clear zones around the colonies
  • Includes S. pyogenes and S. agalactiae
• Gamma hemolytic Streptococci


  • Do not cause hemolysis on BAP
  • Includes E. faecalis, E. faecium, & S. bovis
• Alpha prime Streptococci


  • Has 2 patterns of hemolysis
  • Colony is surrounded by inner alpha and outer beta
  • No single organism is reported as alpha prime
  • Usually happens because of prolonged refrigeration
• Bergey's classification of Streptococci


  • Pyogenic
  • Viridans
  • Enterococci
  • Lactic
• Pyogenic Streptococci


  • Unable to grow at 10oC and 45oC
  • Includes S. pyogenes
• Viridans Streptococci


  • Able to grow at 45oC but not at 10oC
  • Includes S. mitis, S. mutans, S. sanguis, S. salivarius, S. constellatus, and S. intermedius
• Enterococci


  • Able to grow at 10 and 45oC
  • Includes E. faecalis
• Lactic Streptococci


  • Able to grow at 10oC but not at 45oC
  • Includes S. lactis and S. cremoris
• Lancefield classification


  • Most significant in identifying and classifying beta hemolytic Streptococci
  • Based on the extraction of C carbohydrate from the Streptococcal cell wall
  • All Streptococci except Viridans and S. pneumoniae are placed under Lancefield
• Lancefield classification of beta hemolytic Streptococci

  • Group A
  • Group B
  • Group C, F, G
• Group A Streptococci - Streptococcus pyogenes


  • Flesh eating bacteria
  • Major throat pathogen
  • Causes diseases like pharyngitis /Strep throat; Necrotizing fasciitis, Erysipelas (a skin infection), Scarlet fever
• Necrotizing fasciitis

  a.k.a galloping gangrene; rapidly progressing skin infection
• Erysipelas


  Different from Erysipeloid (caused by Erysipelothrix rhusiopathiae)
• Scarlet fever


  a form of pharyngitis with rashes; manifest with "strawberry tongue"
• Complications of repeated episodes of pharyngitis caused by Group A Streptococci

  • Acute glomerulonephritis or Bright's disease
  • Rheumatic fever
• Growth/Colony morphology of Group A Streptococci

  Best isolated on BAP, addition of SXT can enhance its recovery from heavily contaminated specimens
• Media for Initial isolation of Group A Streptococci

  Sheep's blood agar
• Tests for identification/Susceptibility of Group A Streptococci

  • Bacitracin (0.02-0.04 units) – Sensitive/Susceptible
  • PYR (+)/Pyrrolidonyl Arylamidase (PYR) Test
• PYR Test

  • Used for the presumptive identification of group A beta-hemolytic Streptococci and Enterococci
  • Reagent: p-dimethylaminocinnamaldehyde
  • Procedure: Get filter paper, place inoculum in the filter paper, add reagent to the colonies
  • Positive: development of red color
• Virulence factors of Streptococcus pyogenes

  • Streptolysin
  • Streptolysin O
  • Streptolysin S
  • Erythrogenic Toxin or Pyrogenic Toxin
  • M protein
  • Protein F
  • Hyaluronidase
  • Streptokinase
• Streptolysin O

  • Antigenic/Immunogenic (stimulates the body to produce anti-streptolysin O or ASO)
  • Oxygen labile
  • Can cause sub-surface hemolysis on BAP (hemolysis is produced when incubated anaerobically)
• Streptolysin S

  • Non-antigenic
  • Oxygen stable
  • Can cause surface hemolysis (hemolysis is produced only when incubated aerobically)
• M protein

  • Major virulence factor
  • Prevents phagocytosis
  • Found in the cell wall of S. pyogenes
• Protein F

  Promotes/enhances attachment to host/epithelial cells
• Hyaluronidase

  Spreading factor; enhances ability of the organism to invade tissues
• Streptokinase

  Promotes fibrinolysis; responsible for the dissolution of clots
• Diagnostic tests for Scarlet fever

  • Dick's test
  • Schultz Charlton Test Or Blanche Phenomenon
• Dick's test

  • Susceptibility test for Scarlet fever
  • To determine if the patient is at risk for developing scarlet fever
  • Positive: erythema (redness) at the site of injection
• Schultz Charlton Test Or Blanche Phenomenon

  • Test to determine if rashes are due to scarlet fever or not
  • (+) result: gradual disappearance or fading of rashes following the injection of anti-erythrogenic toxin
  • This means that the erythrogenic toxin was neutralized by anti-erythrogenic toxin
• Group B Streptococci - Streptococcus agalactiae

  • A.k.a. Group B-Beta Hemolytic Streptococci
  • Normal flora of the GIT, pharynx, and vaginal tract
  • Causes septicemia
  • Regarded as the # 1 cause of Neonatal meningitis
  • In adults: postpartum endometriosis
• Identification tests for Group B Streptococci

  • CAMP test (+)
  • Hippurate Hydrolysis (+)
  • Bacitracin & SXT RESISTANT
  • LIM broth, Granada – may be used for Group B Streptococci
• Major virulence factor of Group B Streptococci

  Capsule (prevents phagocytosis)
• CAMP test

  • Media: BAP
  • Known organism: S. aureus (beta hemolytic)
  • Unknown organism: Suspected S. agalactiae (beta)
  • (+) result: enhanced hemolysis as shown by arrowhead zone of beta hemolysis
  • (-) result: no enhanced hemolysis; but there is still hemolysis
• Hippurate test

  • Detects hydrolysis of sodium hippurate to benzoic acid and glycine
  • Positive result is due to the enzyme: hippuricase/hippurate hydrolase
  • Traditionally, ferric chloride is used to detect benzoic acid
  • Nowadays, (rapid test) ninhydrin reagent is used to detect glycine
  • Positive result: development of purple color