Difficulty: Introductory | Prerequisites: Streptococcus classification and pathogenesis (Part 1 of these notes)
This second set of notes covers the clinical side: what streptococcal infections look like, how they are diagnosed, and how they are treated. If Part 1 was about knowing which organism you are dealing with and why it is dangerous, this part is about recognising the disease and managing it. You should already be comfortable with the Lancefield groups and haemolysis types before working through this material.
Streptococcal infections range from localised problems (strep throat, impetigo, cellulitis) to serious systemic diseases (pneumonia, neonatal sepsis) and immune-mediated complications (rheumatic fever, glomerulonephritis). Diagnosis relies on throat swabs, rapid antigen tests, blood cultures, and imaging. Penicillin remains the first-line treatment for most streptococcal infections, and prevention centres on prompt antibiotic therapy, vaccination (for S. pneumoniae), and basic hygiene.
Pharyngitis (strep throat)
Inflammation of the pharynx caused by Group A Streptococcus pyogenes, presenting with sore throat, fever, and swollen lymph nodes. In simple terms, this is the classic "strep throat" that school-age children frequently get.
Impetigo
A superficial skin infection, typically caused by Group A strep, characterised by honey-coloured crusts on the skin. Think of it as a crusty, weeping skin rash that spreads by contact, common in young children.
Cellulitis
A deeper skin and soft-tissue infection presenting as a painful, red, swollen area, often with fever. In simple terms, it is a spreading bacterial infection in the skin layers below the surface.
Rheumatic fever
A post-infectious, immune-mediated inflammatory condition that can follow untreated Group A strep pharyngitis, affecting the heart, joints, skin, and nervous system. Think of it as the body's immune system accidentally attacking its own tissues (especially the heart valves) after fighting off strep throat.
Glomerulonephritis (post-streptococcal)
Inflammation of the kidney glomeruli occurring after a streptococcal infection, presenting with haematuria, proteinuria, and oedema. In simple terms, immune complexes from the strep infection lodge in the kidneys and cause damage.
Rapid antigen detection test (RADT)
A point-of-care test that detects Group A streptococcal carbohydrate antigen from a throat swab within minutes. Think of it as the quick in-clinic strep test that gives a result while you wait.
Antibiotic stewardship
The coordinated effort to use antibiotics appropriately: right drug, right dose, right duration, only when needed. In simple terms, it is the practice of not over-prescribing antibiotics so that bacteria do not develop resistance.
Pharyngitis (strep throat)
Caused by Group A strep (S. pyogenes)
Symptoms: sore throat, fever, headache, swollen cervical lymph nodes, sometimes tonsillar exudate
Diagnosis: rapid antigen detection test (RADT) for a quick result; throat culture is the gold standard if RADT is negative
Treatment: oral penicillin or amoxicillin; macrolides (e.g. azithromycin) for penicillin-allergic patients
Why it matters: untreated strep throat is the gateway to rheumatic fever
Impetigo
Superficial skin infection, most common in children
Symptoms: honey-coloured crusts, usually on the face or extremities
Treatment: topical antibiotics (e.g. mupirocin) for mild cases; oral antibiotics for widespread infection
Cellulitis
Deeper infection of skin and subcutaneous tissue
Symptoms: painful, red, warm, swollen area of skin, often with fever
Treatment: oral antibiotics for mild cases; intravenous antibiotics for severe or rapidly spreading infection
Pneumonia
Typically caused by Streptococcus pneumoniae (alpha-haemolytic, not Group A)
Symptoms: fever, productive cough, difficulty breathing, pleuritic chest pain
Diagnosis: chest X-ray (consolidation pattern), sputum culture, blood culture
Treatment: antibiotics, commonly penicillin or a respiratory fluoroquinolone
Neonatal infections (Group B strep)
Streptococcus agalactiae colonises the vaginal tract and can be transmitted during delivery
Causes early-onset neonatal sepsis and meningitis
Prevention: intrapartum antibiotic prophylaxis for colonised mothers
Rheumatic fever
Follows untreated Group A strep pharyngitis (not skin infections)
Symptoms: fever, migratory polyarthritis (joint pain moving between joints), carditis (heart inflammation), subcutaneous nodules, erythema marginatum (skin rash), Sydenham chorea (involuntary movements)
The heart damage (rheumatic heart disease) can be permanent, affecting the mitral and aortic valves
Prevention: prompt antibiotic treatment of strep throat
Post-streptococcal glomerulonephritis
Can follow either pharyngitis or skin infections caused by Group A strep
Symptoms: haematuria (blood in urine, often described as "cola-coloured"), proteinuria, oedema, hypertension
Diagnosis: urinalysis, serological tests (elevated ASO titre, low complement levels)
Treatment: largely supportive; most cases in children resolve on their own
Throat swabs: used for rapid antigen detection and culture in suspected pharyngitis
Blood cultures: essential for systemic infections (sepsis, endocarditis)
Serological tests: detect antibodies against streptococcal antigens (e.g. anti-streptolysin O titre); useful for confirming prior infection in rheumatic fever or glomerulonephritis
Imaging: chest X-ray for pneumonia; echocardiography for suspected rheumatic heart disease
First-line antibiotic: penicillin for most streptococcal infections. Group A strep has remained reliably susceptible to penicillin.
Alternatives: macrolides or cephalosporins for penicillin-allergic patients
Vaccination: pneumococcal vaccines (PCV13, PPSV23) protect against S. pneumoniae infections. No licensed vaccine yet exists for Group A strep.
Hygiene: handwashing, wound care, and respiratory hygiene reduce transmission
Antibiotic stewardship: appropriate prescribing prevents the emergence of resistance, which is an increasing concern among streptococcal species
The rapid strep test is one of the most common point-of-care diagnostics in general practice. The reason doctors insist on treating confirmed strep throat with a full course of antibiotics, even when symptoms resolve quickly, is to prevent rheumatic fever. In low-resource settings where access to antibiotics is limited, rheumatic heart disease remains a leading cause of cardiovascular morbidity in young people.
Students often think rheumatic fever can follow any streptococcal infection. It follows untreated Group A strep pharyngitis specifically. Skin infections (impetigo) do not lead to rheumatic fever, though they can lead to glomerulonephritis.
A common mistake is assuming that a negative rapid antigen test rules out strep throat. The RADT can produce false negatives, which is why a throat culture is recommended as a follow-up in children and adolescents when the rapid test is negative.
Students sometimes confuse pneumococcal pneumonia with Group A strep infections. Streptococcus pneumoniae is a different species from S. pyogenes: pneumoniae is alpha-haemolytic and has no Lancefield group, while pyogenes is Group A and beta-haemolytic.
There is a misconception that antibiotics treat rheumatic fever itself. They do not reverse the immune-mediated damage. Antibiotics prevent rheumatic fever by treating the initial strep throat before the immune response causes harm.
Know which diseases are direct infections (pharyngitis, impetigo, cellulitis, pneumonia) versus post-infectious sequelae (rheumatic fever, glomerulonephritis). This distinction is heavily tested.
Be able to list the Jones criteria for rheumatic fever: carditis, polyarthritis, chorea, erythema marginatum, subcutaneous nodules. Exams often give a clinical scenario and ask you to identify the condition.
Penicillin as the first-line treatment for Group A strep is a commonly tested fact. Know the alternatives for penicillin-allergic patients.
Understand why intrapartum prophylaxis is given for Group B strep: the bacteria colonise the birth canal and can cause neonatal sepsis and meningitis during delivery.
The "cola-coloured urine" description in a clinical vignette is a strong hint at post-streptococcal glomerulonephritis.
True or False: Rheumatic fever can follow untreated streptococcal impetigo.
Fill in the blank: The gold-standard diagnostic test for strep throat is a ____.
True or False: Penicillin is the first-line treatment for most streptococcal infections.
Fill in the blank: Post-streptococcal glomerulonephritis presents with haematuria, proteinuria, and ____.
True or False: Pneumococcal pneumonia is caused by a beta-haemolytic Streptococcus.
Answers: 1. False (rheumatic fever follows untreated pharyngitis, not skin infections). 2. Throat culture. 3. True. 4. Oedema (and often hypertension). 5. False (S. pneumoniae is alpha-haemolytic).
Q: A 7-year-old presents with a sore throat, fever, and tonsillar exudate. The rapid antigen test is negative. What should the next step be, and why?
A: A throat culture should be sent. Rapid antigen tests can produce false negatives, and a missed Group A strep pharyngitis in a child risks the development of rheumatic fever if left untreated.
Q: Explain why treating strep throat with antibiotics prevents rheumatic fever but cannot reverse rheumatic heart disease once it has developed.
A: Antibiotics eliminate the Group A strep infection before the immune system mounts the cross-reactive response that damages cardiac tissue (molecular mimicry). Once rheumatic heart disease has set in, the valve damage is structural and immune-mediated, not caused by ongoing bacterial presence, so antibiotics cannot undo it.
Q: A newborn develops sepsis within 24 hours of birth. Blood cultures grow Group B Streptococcus. What preventive measure could have reduced this risk?
A: Intrapartum antibiotic prophylaxis for the mother. Pregnant women are screened for Group B strep colonisation at 35 to 37 weeks, and those who test positive receive intravenous antibiotics (usually penicillin) during labour to reduce vertical transmission.
Q: A patient presents two weeks after a skin infection with cola-coloured urine and facial oedema. What condition is most likely, and what tests would you order?
A: Post-streptococcal glomerulonephritis. Order urinalysis (looking for haematuria and proteinuria), serum complement levels (typically low C3), and anti-streptolysin O (ASO) titre (elevated, confirming recent streptococcal infection).
Q: Why is there a pneumococcal vaccine but no Group A strep vaccine?
A: S. pneumoniae has a polysaccharide capsule with a limited number of serotypes that can be targeted by conjugate or polysaccharide vaccines. Group A strep has over 200 M protein serotypes, and the M protein shares structural similarity with human cardiac proteins, raising the risk that a vaccine could inadvertently trigger an autoimmune response similar to rheumatic fever. Developing a safe, broadly protective Group A vaccine remains an active area of research.
Rheumatic fever and glomerulonephritis connect directly to immunology: molecular mimicry, immune complex deposition, and complement activation are all concepts that appear in later units. Antibiotic stewardship ties into the broader microbiology theme of antimicrobial resistance. Neonatal Group B strep screening is a topic that bridges microbiology with obstetrics and public health. The diagnostic methods here (culture, serology, rapid antigen testing) apply across all of bacteriology, not just streptococcal infections.
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