Classification of Streptococci: Hemolysis, Group-Specific Substance (Lancefield Classification), Capsular Polysaccharides, Biochemical Reactions.

Question 1

Hemolysis

Answer 1

Streptococci are classified by their hemolytic patterns on blood agar: 1) Alpha-hemolytic: partial (greenish) hemolysis due to hydrogen peroxide (e.g., Streptococcus pneumoniae, viridans streptococci). 2) Beta-hemolytic: complete clear zone of hemolysis (e.g., S. pyogenes [Group A], S. agalactiae [Group B]). 3) Gamma-hemolytic: no hemolysis (e.g., Enterococcus faecalis). This classification helps narrow down species based on colony appearance.

Question 2

Group-specific substance (Lancefield classification)

Answer 2

The Lancefield classification groups beta-hemolytic streptococci (mainly) based on cell wall carbohydrate antigens (C substance). Rebecca Lancefield identified groups A to V (excluding I and J). Group A: S. pyogenes; Group B: S. agalactiae; Group D: Enterococcus and non-enterococcal group D streptococci. The Lancefield grouping is determined using latex agglutination tests or serologic methods. Alpha-hemolytic streptococci (e.g., S. pneumoniae, viridans group) lack Lancefield antigens.

Question 3

Capsular polysaccharides

Answer 3

Certain streptococci produce a capsule that contributes to virulence by resisting phagocytosis. 1) Streptococcus pneumoniae has a prominent polysaccharide capsule, with over 90 serotypes; capsule is the major virulence factor and basis for vaccine production. 2) S. pyogenes has a hyaluronic acid capsule, which is non-immunogenic because it mimics human connective tissue. 3) S. agalactiae (Group B) also possesses a polysaccharide capsule important for neonatal virulence. Capsules can be visualized by quellung reaction or immunofluorescence.

Question 4

Biochemical reactions

Answer 4

Biochemical tests help differentiate streptococci: 1) Catalase: all streptococci are catalase-negative (unlike staphylococci). 2) Bacitracin sensitivity: S. pyogenes (Group A) is sensitive; S. agalactiae (Group B) is resistant. 3) CAMP test: S. agalactiae is CAMP-positive (arrowhead hemolysis with S. aureus). 4) Hippurate hydrolysis: positive in S. agalactiae. 5) Optochin sensitivity and bile solubility: S. pneumoniae is sensitive and bile soluble; viridans group is resistant. 6) Growth in 6.5% NaCl: Enterococcus grows; non-enterococcal Group D does not. These reactions help distinguish species for diagnosis and treatment.