Overview
Streptococci are Gram positive cocci arranged in chains or pairs, forming part of the normal flora of humans and animals. The most important pathogen in the genus, Streptococcus pyogenes, causes pyogenic infections with a characteristic tendency to spread β unlike the localized lesions of staphylococci β and is also responsible for important non-suppurative sequelae such as acute rheumatic fever and glomerulonephritis.
This lesson covers the classification of streptococci by hemolysis and Lancefield grouping, the morphology and virulence factors of S. pyogenes, the spectrum of suppurative and non-suppurative disease, and the laboratory diagnostic approach including culture, ASO titer, and other serological tests.
Learning Objectives
After this lesson you will be able toβ¦- Describe the morphological and physiological characteristics of bacteria in the genus Streptococcus.
- List the features by which Streptococcus pyogenes, Streptococcus agalactiae, Streptococcus mutans and Enterococcus faecalis can be identified.
- Describe the spectrum of diseases caused by the various streptococci.
- Describe and compare suppurative and non-suppurative infections of streptococci.
- Describe the laboratory diagnosis of streptococcal infections.
Clinical Story
Why This MattersA 9-year-old child recovers from a sore throat two weeks ago, and now presents with painful, swollen joints and a new heart murmur. The throat culture at the time was never sent β but a raised ASO (antistreptolysin O) titer confirms recent Streptococcus pyogenes infection, linking the earlier "simple" pharyngitis to a potentially life-altering case of acute rheumatic fever.
Core Concepts
Streptococci are first divided into obligate anaerobes (peptostreptococci) and aerobic/facultative anaerobic streptococci, which are classified by their hemolytic pattern on 5% horse blood agar:
Alpha (Ξ±) hemolytic β partial hemolysis with greenish discolouration ("viridans streptococci"); includes Streptococcus pneumoniae.
Beta (Ξ²) hemolytic β sharply defined, complete clear zone of hemolysis; most pathogenic streptococci belong here.
Gamma (Ξ³) or non-hemolytic β no change in the medium; includes the enterococcus group.
Hemolytic streptococci are further classified serologically by Lancefield (1933) into 20 groups (AβV, excluding I and J) based on the group-specific C carbohydrate antigen. The great majority of human pathogenic hemolytic streptococci belong to Group A (Streptococcus pyogenes), which can be further subdivided by M, T, and R surface protein antigens (Griffith typing).
Individual cocci are 0.5β1.0 ΞΌm, spherical/oval, arranged in chains (chain formation results from division in one plane with incomplete daughter cell separation). They are Gram positive, non-motile, non-sporing, and capsulated.
They are aerobes and facultative anaerobes growing best at 37Β°C, requiring media enriched with blood/serum. On blood agar, colonies are circular, semitransparent, with a clear zone of hemolysis. Virulent fresh isolates produce "matt" (finely granular) colonies; avirulent strains produce "glossy" colonies; well-capsulated strains produce "mucoid" colonies.
Biochemically, they ferment sugars (sorbitol, maltose, lactose, mannitol) producing acid but no gas, and are catalase negative β the key test differentiating them from Staphylococcus.
Streptolysin O β oxygen labile, antigenic (antibody = ASO, used diagnostically), cardiotoxic, leucotoxic. Streptolysin S β oxygen stable, serum soluble, responsible for surface hemolysis on blood agar plates.
Pyrogenic exotoxin (erythrogenic/scarletinal toxin) β causes scarlet fever, positive Dick test, acts as a superantigen.
Streptokinase β activates plasminogen to plasmin, aiding spread of infection; therapeutically important in thromboembolic disease. DNAase (streptodornase) β liquefies pus. Hyaluronidase β breaks down connective tissue, aiding spread.
M protein is the most important virulence factor, inhibiting phagocytosis; antibody to M protein is protective. The cell wall's group-specific C carbohydrate is the basis of Lancefield grouping.
Suppurative infections β respiratory (tonsillitis/pharyngitis), skin and soft tissue (erysipelas, impetigo, cellulitis), genital (puerperal sepsis), abscesses of liver/lung/kidney/brain.
Non-suppurative complications β acute rheumatic fever (throat infection, longer latent period >2β5 weeks, essential prior sensitization, markedly raised ASO titer, Aschoff's nodules) and acute glomerulonephritis (throat or skin infection, shorter latent period 1β3 weeks, specific nephritogenic serotypes, moderately raised or normal ASO, generally good prognosis with spontaneous resolution).
Group B (S. agalactiae) β leading cause of neonatal meningitis; hippurate hydrolysis positive.
Group D (Enterococci, e.g. E. faecalis) β grows in 40% bile, 6.5% NaCl, pH 9.6, 45Β°C; causes UTI, wound infection, endocarditis.
Viridans group (alpha hemolytic, non-groupable) β normal oral/URT flora; S. sanguis causes bacterial endocarditis after dental procedures; S. mutans causes dental caries by producing acid and dextran from sucrose.
Laboratory Principle
In acute suppurative infections, diagnosis relies on culture and direct demonstration of the organism, because streptococci grow readily on blood agar and produce a characteristic hemolytic pattern. In non-suppurative complications (rheumatic fever, glomerulonephritis), the causative organism is often no longer present, so diagnosis instead relies on demonstrating a raised antibody titer β most importantly Antistreptolysin O (ASO) β against a streptococcal exotoxin. Streptolysin O is antigenic and its corresponding antibody rises in serum following infection, providing indirect but reliable retrospective evidence of a recent streptococcal infection.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Lancefield group antisera | Group-specific | Serological grouping via precipitation test | Refrigerated (2β8Β°C) |
| ASO reagent kit | Standardized titration reagents | Antistreptolysin O titer estimation | Refrigerated, per manufacturer instructions |
| Bacitracin disc | 0.04 units | Maxted's rapid presumptive identification of S. pyogenes | Refrigerated, protect from moisture |
| Rapid antigen test kits | Group A streptococcal antigen | Rapid diagnostic screening for S. pyogenes | Room temperature, per kit instructions |
Step-by-Step Procedure
Collect throat swab, pus, or blood for isolation of S. pyogenes; vaginal swab, blood, CSF, or ear swab for S. agalactiae; urine and blood for enterococci. Collect under aseptic precautions and transport in Pike's medium if delay is expected.
Perform Gram staining; chain-forming, non-motile Gram positive cocci suggest Streptococci. Note: microscopy has no diagnostic value in throat or genital specimens since streptococci are part of the resident flora there.
Plate on sheep blood agar and incubate at 37Β°C anaerobically with COβ. Observe colonies and hemolysis pattern. Use Maxted's bacitracin disc method for rapid presumptive identification of S. pyogenes (wide zone of inhibition = sensitive).
Confirm Lancefield group and Griffith (M protein) type using group-specific antisera and precipitation/agglutination methods as needed for epidemiological purposes.
For suspected rheumatic fever or glomerulonephritis, estimate the ASO titer (raised in most cases), and consider Anti-DNAase B titer (significant if >300β350) or Anti-hyaluronidase test, which is especially useful in pyoderma-associated disease where ASO may not rise.
Flow Diagram
Quality Control
Run known Group A Streptococcus pyogenes as a bacitracin-sensitive control and a Group B or D control strain as bacitracin-resistant with every batch, to confirm disc potency and correct interpretation.
Participate in an external proficiency scheme for both culture-based identification and ASO titer estimation, since titer interpretation can vary between laboratories and reagent lots.
Reference Values
Typical Findingsβ οΈ ASO titer thresholds vary by age, population, and laboratory reference range. Always apply your local laboratory's established cut-offs.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Beta hemolytic, bacitracin sensitive GAS | Streptococcus pyogenes β classic pharyngitis/skin pathogen | Treat with penicillin; monitor for rheumatic fever risk if untreated |
| Markedly raised ASO titer, joint pain, carditis | Suggestive of acute rheumatic fever | Initiate penicillin prophylaxis; cardiology referral |
| Hematuria, edema after skin/throat infection | Suggestive of acute post-streptococcal glomerulonephritis | Supportive management; usually resolves spontaneously with good prognosis |
| Alpha hemolytic streptococcus from blood after dental work | Viridans group endocarditis, especially in patients with pre-existing valve disease | Blood cultures, echocardiography, prolonged antibiotic therapy |
| Group B streptococcus in vaginal/neonatal specimen | Risk of neonatal sepsis/meningitis | Intrapartum antibiotic prophylaxis per protocol |
Common Errors & How to Avoid Them
Cause: Horse blood agar can support growth of Haemophilus hemolyticus, whose colonies can be confused with streptococci.
Prevention: Use sheep blood agar, which inhibits H. hemolyticus and gives cleaner, more interpretable hemolysis patterns.
Cause: Streptococci are part of the normal resident flora of the throat and genital tract, so seeing them on a Gram stain does not confirm infection.
Prevention: Always rely on culture (and hemolysis/serotyping) rather than direct microscopy for these specimen types.
Cause: ASO titers often fail to rise significantly after streptococcal skin infections (pyoderma), leading to false reassurance.
Prevention: Order anti-hyaluronidase or anti-DNAase B testing in addition to (or instead of) ASO when pyoderma is the likely antecedent infection.
Laboratory Tips from the Bench
Streptolysin O is oxygen labile β its hemolytic activity is best demonstrated in pour plates or broth with reducing agents, not on the surface of standard plates, where only Streptolysin S activity is visible.
Store fresh S. pyogenes isolates in Robertson's cooked meat medium at 4Β°C if immediate testing is not possible β the organism dies quickly at room temperature.
Remember "CAMP down the fever" β Catalase-negative (unlike Staph), Alpha/Beta/Gamma hemolysis classification, M protein virulence, Pyrogenic exotoxin causes fever (scarlet fever) β the four pillars of Streptococcus pyogenes identity.
Important Notes
Unlike acute glomerulonephritis, acute rheumatic fever essentially requires prior sensitization to Group A streptococcal antigens and has a much longer latent period (2β5 weeks) β repeated attacks are common, making long-term penicillin prophylaxis essential for affected patients.
The Streptozyme test (passive slide hemagglutination against multiple extracellular streptococcal antigens) is positive across all types of streptococcal infection, making it a useful general screening tool when a specific antibody test is unavailable.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgeTwo weeks after an untreated sore throat, presents with migratory joint pain affecting the knees and ankles, low-grade fever, and a new systolic murmur on cardiac auscultation.
A markedly raised ASO titer with normal complement, migratory arthritis, and new cardiac murmur two to five weeks after an untreated streptococcal sore throat is classic for acute rheumatic fever β a non-suppurative complication rather than active infection (hence the negative current throat culture).
- βNon-suppurative complications are diagnosed by antibody demonstration, not by culture, since the organism is usually gone.
- βComplement levels remain normal in rheumatic fever but are typically low in glomerulonephritis β helpful for distinguishing the two.
- βLong-term penicillin prophylaxis is essential after rheumatic fever to prevent repeated attacks and cumulative cardiac damage.
Frequently Asked Questions
The catalase test is the key differentiator β Staphylococcus is catalase positive (produces bubbles with HβOβ) while Streptococcus is catalase negative. Morphologically, Staphylococcus forms grape-like clusters while Streptococcus forms chains.
Rheumatic fever requires prior immune sensitization to streptococcal antigens that cross-react with heart tissue β this immune-mediated cross-reactivity takes longer (2β5 weeks) to manifest than the immune-complex deposition mechanism of glomerulonephritis (1β3 weeks).
Yes. ASO titers can be raised in several unrelated conditions including pneumococcal pneumonia, tuberculosis, gonorrhea, hepatitis, and rheumatoid arthritis, so results must always be interpreted alongside the full clinical picture.
Quick Revision
10-Minute ReviewKey Takeaways
- Streptococci are Gram positive, catalase-negative cocci classified by hemolysis and Lancefield grouping.
- Streptococcus pyogenes (Group A) is the most important human pathogen in the genus.
- M protein, streptolysins, and various enzymes drive the virulence of S. pyogenes.
- Suppurative infections spread locally; non-suppurative sequelae (rheumatic fever, glomerulonephritis) are immune-mediated.
- Culture diagnoses acute infection; antibody titers (ASO, Anti-DNAase B) diagnose non-suppurative complications.
- Other clinically important streptococci include Group B (neonatal disease), Enterococci, and the viridans group.
Competency Checklist
Track Your MasteryReferences
- Ananthanarayan R, Paniker CKJ. Textbook of Microbiology. 10th ed. Universities Press.
- Collee JG, Fraser AG, Marmion BP, Simmons A. Mackie & McCartney Practical Medical Microbiology. 14th ed. Churchill Livingstone.
- National Institute of Open Schooling (NIOS). Microbiology β Streptococcus, Module Notes.