# Streptococcus > Streptococcus for FMGE Microbiology: Lancefield groups, bacitracin and optochin tests, rheumatic fever, pneumococcus capsules and enterococcus traits. - Canonical URL: https://prepelephant.com/topics/fmge/microbiology/streptococcus-fmge - Exam / course: FMGE · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Streptococcus", PrepElephant, https://prepelephant.com/topics/fmge/microbiology/streptococcus-fmge ## Direct answer Catalase-negative gram-positive cocci in chains organise themselves by two parallel systems the exam expects you to carry: Lancefield cell-wall carbohydrate groups (A for Streptococcus pyogenes, B for S. agalactiae, D for the enterococci) and haemolysis on blood agar — beta (complete), alpha (partial, green) or gamma (none). Group A beta-haemolytic streptococci, bacitracin-sensitive, cause pharyngitis, impetigo, scarlet fever and then the nonsuppurative sequelae rheumatic fever and post-streptococcal glomerulonephritis; group B (CAMP test positive, hippurate hydrolysed) colonises the vagina and causes neonatal sepsis and meningitis. Streptococcus pneumoniae, an alpha-haemolytic, optochin-sensitive, bile-soluble, capsule-carrying diplococcus, causes lobar pneumonia and meningitis and is covered by both the 23-valent polysaccharide and conjugate vaccines; enterococci, bile-esculin-positive and growing in 6.5 per cent salt, bring vancomycin resistance to the ward. ## What you must remember - First divide by catalase (staphylococcus positive, streptococcus negative); then by haemolysis; then by bench tests — bacitracin for group A, optochin and bile solubility for pneumococcus, CAMP and hippurate for group B, bile esculin and salt tolerance for enterococci. - Group A virulence: M protein (types rheumatogenic — pharyngitis strains drive rheumatic fever; nephritogenic skin strains drive glomerulonephritis), hyaluronic acid capsule, streptolysin O (immunogenic, basis of the ASO titre) and streptolysin S (oxygen-stable, surface haemolysis). - Scarlet fever: erythrogenic (SpeA) toxin phage-encoded — rough "sandpaper" rash, circumoral pallor, Pastia lines, strawberry tongue, desquamation later. - Rheumatic fever follows pharyngitis (not skin infection) after two to three weeks; glomerulonephritis follows either throat or skin infection and does not recur the way rheumatic fever does. - Group B streptococcus screening at 35–37 weeks of pregnancy, with intrapartum penicillin prophylaxis, prevents early-onset neonatal sepsis. - Pneumococcus: capsule is the key virulence factor (quellung swelling with type-specific antiserum), IgA protease aids colonisation; resistance is by penicillin-binding protein changes, treat meningitis by MIC. - Viridans streptococci: Streptococcus mutans causes dental caries; S. sanguinis groups are the classic subacute bacterial endocarditis organisms after dental procedures. - Enterococcus faecalis and E. faecium: treat endocarditis with synergistic penicillin or vancomycin plus gentamicin; linezolid and daptomycin cover vancomycin-resistant strains. ## A sore throat walked through properly A twelve-year-old presents with fever, tender anterior cervical nodes and tonsillar exudate without cough — a Centor-score picture favouring streptococcal pharyngitis over viral. A rapid antigen test or throat swab confirms group A. Penicillin V for ten days remains first-line everywhere including Indian practice; the duration matters, because a full course prevents rheumatic fever even though it barely shortens the sore throat. Two weeks later a migratory polyarthritis with a new murmur and a raised erythrocyte sedimentation rate would fulfil revised Jones criteria — secondary prophylaxis with monthly benzathine penicillin then begins. Had the presentation instead been impetigo with a three-week gap to periorbital oedema, tea-coloured urine and a low C3, that would be post-streptococcal glomerulonephritis: supportive care, no antibiotic prophylaxis, and essentially no recurrence risk. The pathway teaches the central doctrine — which site of infection leads to which sequela, and what prophylaxis each demands. ## How the exam frames it Bench-test matching is the commonest frame: bacitracin-sensitive beta-haemolytic colonies equal group A; optochin-sensitive alpha-haemolytic diplococci equal pneumococcus; bacitracin-resistant equal group B; catalase is the gateway separating the genera — and candidates who jump to coagulase lose the mark, because coagulase belongs to staphylococci. The second frame is sequela directionality: rheumatic fever follows pharyngitis only, glomerulonephritis follows both pharyngitis and skin infection, and only rheumatic fever warrants long-term secondary prophylaxis. The third is the pneumococcus capsule — asking which virulence factor the polysaccharide vaccine targets, or why the conjugate vaccine (part of India's universal immunisation programme) works in infants while the 23-valent does not. ASO titre questions ask what a rise means (recent infection) rather than a diagnosis of rheumatic fever by itself. ## Frequently asked questions ### Which bedside tests differentiate group A, group B and pneumococcus? Group A streptococci are bacitracin-sensitive beta-haemolytic; group B are bacitracin-resistant, CAMP-positive; Streptococcus pneumoniae is alpha-haemolytic, optochin-sensitive and bile-soluble. ### Why does rheumatic fever follow pharyngitis but not skin infection? Pharyngitis strains carry rheumatogenic M proteins and the throat's immune context produces cross-reactive antibodies against cardiac tissue; skin strains with nephritogenic proteins cause glomerulonephritis instead, so the site and M type decide the sequela. ### What is the role of the ASO titre? Streptolysin O is strongly immunogenic, so a rising or significantly elevated anti-streptolysin O titre documents recent group A infection — supporting evidence in Jones criteria, never a standalone diagnosis. ### How is neonatal group B streptococcal disease prevented? Vaginorectal screening at 35–37 weeks and intrapartum intravenous penicillin for colonised women; the CAMP test or hippurate hydrolysis identifies the organism in the laboratory. ### Which streptococcus causes dental caries and which causes subacute endocarditis? Streptococcus mutans causes caries through glucan production on enamel; viridans group organisms such as S. sanguinis cause subacute bacterial endocarditis after dental procedures — the reason for prophylaxis in high-risk cardiac patients. ### How is enterococcus identified and its endocarditis treated? Bile-esculin positivity and growth in 6.5 per cent sodium chloride; endocarditis needs synergistic cell-wall agent plus aminoglycoside, with linezolid or daptomycin reserved for vancomycin-resistant strains.