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Streptococci

Categorization
Cell Wall: Gram Positive Shape: Cocci
Biochemistry: Catalase Negative Metabolism: Microaerophilic
Classification
  • Overview
    • Both staphylococci and streptococci are gram positive cocci; therefore, it is important to understand how these organisms are differentiated. Additionally, it is important to appreciate how different strains of Streptococci are differentiated from one another.
  • Differentiation From Staphylococci
    • Gram Stain: Streptococci appear as strings of beads on gram stain whereas staphylococci appear as grape-like clusters
    • Catalase: All streptococci are catalase negative, and thus do not bubble when exposed to hydrogen peroxide (H2O2).
  • Differentiation Between Strains
    • Hemolytic ability (Ability to lyse erythrocytes)
      • Streptococci are plated on blood agar plate and area surrounding colonies are observed
      • Beta-hemolytic: Completely hemolyze RBCs, white area surrounds colonies
      • Alpha-hemolytic: Partially hemolyze RBCs, green area surrounds colonies
      • Gamma-hemolytic: Cannot hemolyze RBCs, red area surrounds colonies.
    • Antigenic characteristics
      • Streptococci can be differentiated based on the antigenic characteristics of a carbohydrate in streptococcal cell wall. These Lancefield antigens are given letter names A, B, C, D, and so on. However, note that some strains do not have Lancefield antigens.
Member Organisms