Paper II
Question
Classification of Streptococci with examples. Write about the diseases caused by Streptococcus pyogenes. Add a note on laboratory diagnosis. (2+3+3)
Answer
Classification of Streptococci
1. By haemolysis on blood agar:
- Beta-haemolytic — complete haemolysis (e.g., Streptococcus pyogenes, Streptococcus agalactiae).
- Alpha-haemolytic — partial (greenish) haemolysis (e.g., Streptococcus pneumoniae, viridans streptococci).
- Gamma (non-haemolytic) — no haemolysis (e.g., Enterococcus species, some group D streptococci).
2. Lancefield grouping (based on cell wall carbohydrate (C) antigen): Group A (S. pyogenes), Group B (S. agalactiae), Group D (Enterococcus and non-enterococcal group D streptococci), and other groups (C, G, etc.).
Diseases caused by Streptococcus pyogenes (Group A Streptococcus):
- Suppurative infections: pharyngitis/tonsillitis, impetigo/pyoderma, cellulitis, erysipelas, necrotizing fasciitis, streptococcal toxic shock syndrome, scarlet fever (mediated by pyrogenic exotoxins).
- Non-suppurative sequelae: Acute rheumatic fever (following pharyngitis, via molecular mimicry between streptococcal M protein and cardiac tissue) and Acute post-streptococcal glomerulonephritis (following pharyngitis or skin infection, immune-complex mediated).
Laboratory diagnosis:
- Direct microscopy — Gram stain showing Gram-positive cocci in chains.
- Culture — on blood agar, showing beta-haemolytic colonies; Bacitracin sensitivity used as a presumptive test to distinguish Group A (S. pyogenes, bacitracin-sensitive) from other beta-haemolytic streptococci.
- Lancefield grouping — by latex agglutination, confirming Group A antigen.
- Rapid antigen detection test — for streptococcal pharyngitis, direct antigen detection from throat swab, providing rapid point-of-care results.
- Serology — Antistreptolysin O (ASO) titre, used to demonstrate a recent preceding streptococcal infection, particularly important in the workup of suspected rheumatic fever or post-streptococcal glomerulonephritis.
- Throat swab culture — remains the gold standard for confirming active streptococcal pharyngitis.