Classification of Streptococcus

June 1, 2021 Sushil Humagain Bacteriology, Microbiology 0

  • The genus Streptococcus includes a large number of species of Gram-positive, catalase negative cocci.
  • They are non-motile and non-spore forming.
  • They are arranged in chains of varying length and also in pairs.
  • Some of them form the normal flora of man and animals while some like Streptococcus pyogenes are human pathogens.
  • There are different bases of classification of Streptococci:

1. On the basis of Oxygen requirement:

  • Aerobes and facultative anaerobes: E. g. Streptococcus spp
  • Strict or obligate anaerobes: E. g. Peptostreptococcus

2. Brown’s classification (On the basis of haemolytic pattern on blood agar):

  • Aerobic and facultative anaerobic Streptococci are further classified on the basis of haemolytic property or pattern on blood agar.
  • It was described by J.H. Brown in 1919.

  • On this basis, there are three groups of Streptococci.
    • Alpha-hemolytic Streptococci:
      • Exhibit incomplete hemolysis (1-2mm wide)
      • Impart a greenish discoloration around the colony with few persistent unhemolyzed RBCs
      • e.g. Viridians streptococci, Streptococcus pneumoniae
    • Beta-hemolytic Streptococci:
      • Exhibit a wide zone of complete hemolysis (2-4 mm wide)
      • Account for majority of Streptococcal diseases
      • e.g. Streptococcus pyogenes
    • Non-hemolytic Streptococci:
      • Also called gamma-hemolytic Streptococci
      • Don’t cause hemolysis at all
      • Usually non pathogenic
      • e.g. Streptococcus faecalis which are now grouped under a new genus Enterococcus.

3. Lancifield classification or serological classification:

  • Also known as serological grouping, it was put forward by Rebecca Lancefield in 1933.
  • Most strains of beta-hemolytic and some of alpha-hemolytic and non-hemolytic Streptococci are classified on this basis.
  • It is based on the difference of group specific polysaccharide antigen on the cell wall of Streptococci.
  • Streptococci are classified into 20 Lancifield groups from A to V (except I and J).
    • Group A: Streptococcus pyogenes
      • Group A streptococci i.e. pyogenes are further sub divided into approximately 80 serotypes by Griffith according to their specific surface proteins (M, T and R). M-protein is the most important one.
      • The pathogenicity of Streptococci depends on the presence of hyaluronic acid capsule and surface M-protein.
    • Group B: Streptococcus agalactiae
    • Group C: Streptoccus equi
    • Group D: Enterococcus
    • Others (group E to V)

4. On the basis of 16s rRNA sequence:

  • The genus Streptococcus is further subdivided into six clusters on the basis of comparative analysis of 16s rRNA sequences.
    • Pyogenic group:
      • Consists of predominantly beta-hemolytic species that are pathogenic in humans or animal species.
      • e.g. Streptococcus pyogenes, S. agalactiae
    • Anginosus group:
      • Found in the human oral cavity, gastrointestinal tract and genital tracts as normal flora
      • Sometimes isolated from infections
      • e.g. Streptococcus anginosus, S. intermedius
    • Mitis group:
      • Includes several oral streptococci
      • e.g. Streptococcus mitis, S. pneumoniae
    • Salivarius group:
      • Consists of both dairy streptococci and species found in the human oral cavity
      • e.g. Streptococcus salivarious, S. thermophiles
    • Bovis group:
      • Inhabits the intestinal tract of several animals species and sometimes humans
      • e.g. Streptococcus bovis, S. equines
    • Mutans group:
      • Mainly colonize tooth surfaces of humans and a number of animal species.
      • e.g. Streptococcus mutans, S. ferus

5. Shermann’s Divisions (On the basis of physiological characteristics):

  • It was proposed by Sherman in 1937.
  • On the basis of hemolytic reaction, group carbohydrate antigens, and phenotypic tests (primarily fermentation and tolerance tests), he organized streptococci into the following four divisions:
    • Pyogenic division:
      • Included beta-hemolytic strains with defined group antigens (A, B, C, E, F, and G).
      • This division of the streptococci is similar to that of today’s identification systems based on serological classification.
      • e.g. Streptococcus pyogenes
    • Viridans division:
      • Included streptococcal species that were neither beta-hemolytic nor were tolerant to high-pH growth conditions.
      • They couldn’t tolerate salt, and did not grow at 10°C.
      • Most of them are normal flora of upper respiratory tract of humans.
    • Lactic division:
      • Included strains used in manufacture of dairy products.
      • Not associated with human infections
      • Reclassified as the genus, Lactococcus in the mid-1980s.
      • They are non-hemolytic groups.
      • e.g. Lactococcus
    • Enterococci division:
      • Can grow in broths at high pH and can tolerate high salt concentrations
      • Has a wide temperature range (10 to 45°C) for growth
      • They are the normal flora of human intestine
      • e.g. Enterococcus

Classification of Streptococcus