Streptococcus Classification
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Direct answer
Streptococci are gram-positive cocci in chains, catalase negative, classified by haemolysis on blood agar into beta-haemolytic, alpha-haemolytic and non-haemolytic types, and by Lancefield grouping of the cell-wall carbohydrate into groups A to V, of which A, B, C, D and G matter clinically. Group A Streptococcus pyogenes causes pharyngitis, skin infections and their post-infectious complications, Group B Streptococcus agalactiae causes neonatal sepsis, and the alpha-haemolytic pneumococcus causes lobar pneumonia and meningitis. Penicillin remains the drug of choice, and no penicillin resistance has been documented in Streptococcus pyogenes.
What you must remember
- Haemolytic classification: beta-haemolytic streptococci give complete clear haemolysis; alpha-haemolytic viridans streptococci and pneumococcus give partial green haemolysis; non-haemolytic types leave red cells intact.
- Lancefield groups: based on the cell-wall C carbohydrate; group A is Streptococcus pyogenes and group B is Streptococcus agalactiae; enterococci, once group D streptococci, are now a separate genus.
- Group A identifiers and factors: beta haemolysis with bacitracin sensitivity; virulence rests on M protein, capsule, streptolysin O and S, streptokinase, streptodornase, hyaluronidase and erythrogenic toxin.
- Post-streptococcal complications: rheumatic fever follows pharyngitis with rising anti-streptolysin O titres, while glomerulonephritis follows throat or skin infection with nephritogenic M types.
- Group B identifiers: CAMP test positive and bacitracin resistant; it causes neonatal meningitis and sepsis from maternal genital colonisation.
- Pneumococcus: alpha-haemolytic, lancet-shaped capsulated diplococci, optochin sensitive and bile soluble, showing the quellung reaction and draughtsman colonies.
- Viridans and enterococci: viridans streptococci cause dental caries and subacute endocarditis; enterococci grow in bile-aesculin and 6.5 per cent salt and may be vancomycin resistant.
Common confusion
Every green haemolysis is called viridans streptococcus, forgetting that pneumococcus is also alpha-haemolytic — optochin sensitivity and bile solubility separate them. Bacitracin and optochin discs are interchanged: bacitracin identifies group A, optochin identifies pneumococcus. Keep the complications apart — rheumatic fever follows throat infection alone and recurs, whereas glomerulonephritis follows throat or skin infection and does not recur.
Exam-focused takeaway
In theory, write both classifications in parallel, then describe Streptococcus pyogenes fully with enzymes, toxins, diseases and complications, closing with penicillin as the drug of choice. In viva, expect bacitracin versus optochin, the CAMP test and anti-streptolysin O interpretation. At the practical, beta-haemolytic colonies with a negative catalase test identify a streptococcus, and capsulated lancet diplococci point to pneumococcus, for which vaccines exist.
Frequently asked questions
What are the two main classifications of streptococci?
By haemolysis on blood agar — beta, alpha and non-haemolytic — and by Lancefield grouping of the cell-wall carbohydrate, clinically groups A, B, C, D and G.
How is Streptococcus pyogenes identified?
Beta haemolysis, catalase negativity, bacitracin sensitivity and pyrrolidonyl arylamidase positivity, with typing by M protein.
How do rheumatic fever and glomerulonephritis differ?
Rheumatic fever follows group A pharyngitis, may recur and involves the heart with a rising anti-streptolysin O titre; glomerulonephritis follows throat or skin infection, causes haematuria and oedema, and does not recur.
Which tests separate pneumococcus from viridans streptococci?
Optochin sensitivity and bile solubility, both positive in pneumococcus, along with the capsulated diplococci and draughtsman colonies.
What is the CAMP test?
A plate test in which group B streptococcal haemolysin enhances Staphylococcus aureus haemolysis, forming an arrowhead zone — it identifies Streptococcus agalactiae.
Which streptococci cause endocarditis?
Viridans streptococci after dental procedures and enterococci, both classically subacute; group A streptococcus rarely, in acute endocarditis.