Overview
Streptococcus pneumoniae, formerly called Diplococcus pneumoniae because it occurs in pairs, was renamed in 1974 due to its close relationship with the Streptococcus genus. It normally colonises the upper respiratory tract mucosa, and 40–70% of healthy adults carry it asymptomatically.
Most pneumococcal disease is endogenous — the organism invades from its own carrier site rather than being newly acquired. This lesson covers its morphology, cultural and biochemical identification, virulence factors, the diseases it causes, and the polysaccharide and conjugate vaccines used for prevention.
Learning Objectives
After this lesson you will be able to…- Describe the classification and morphological characteristics of Pneumococcus.
- Discuss the cultural and biochemical characteristics used for identification.
- Explain the mechanisms of virulence and pathogenicity.
- Enumerate the diseases caused by Pneumococcus.
- Describe how to culture and identify Pneumococcus from clinical specimens.
- Discuss the pneumococcal vaccines, their schedule and doses.
Clinical Story
Why This MattersA 4-year-old boy is brought to the clinic with high fever, rapid breathing and chest pain following a bout of influenza. The physician suspects bacterial pneumonia and sends a sputum sample to the lab. The technologist must recognise the lancet-shaped, alpha-haemolytic diplococci on culture and confirm the identity using the bile solubility and optochin sensitivity tests before the child can be started on the correct antibiotic.
Core Concepts
Pneumococci are Gram-positive, slightly elongated, oval to lanceolate (lancet)-shaped diplococci, 0.5–1.25 µm in diameter, usually occurring in pairs or short chains. One end of each coccus is broad and the other pointed, giving the typical lancet shape, with the broad ends of the pair in apposition. A thick capsule surrounds the pair — most apparent in smears from clinical exudate, but usually lost on culture.
Pneumococci are fastidious and require enriched media (blood agar) to grow, with an optimum temperature of 37°C (range 25–42°C) and pH 7.8. Growth is enhanced by 5–10% CO₂. On blood agar, colonies are alpha-haemolytic, dome-shaped and mucoid after 18 hours. Autolysin (Lyt A) causes characteristic autolysis of the culture between 18–24 hours, changing plateau-shaped colonies to ones with a lysed, depressed centre. Bile salts enhance this autolysis.
Pneumococcus is an aerotolerant anaerobe that ferments many sugars and hydrolyses inulin (differentiating it from other streptococci); it is oxidase and catalase negative. Three specific tests confirm identity: the bile solubility test (10% sodium deoxycholate lyses pneumococcal colonies/broth within minutes), the optochin sensitivity test (a zone of inhibition ≥15 mm around an optochin disc), and the Quellung reaction (capsule swelling in the presence of type-specific antiserum, used for serotyping).
The outermost capsule is made of polysaccharide, which diffuses into host tissue during infection and is called the 'specific soluble substance' (SSS). Based on capsular polysaccharide type, pneumococci are classified into Type I, II, III, and a heterogeneous Group IV containing more than 90 serotypes. Other antigens include the somatic 'C' carbohydrate (used to detect C-reactive protein) and nucleoprotein.
The capsule is the single most important determinant of virulence, protecting the organism from phagocytosis — unencapsulated variants cannot cause disease. Pneumococci also produce pneumolysin, a virulent toxin that damages cell membranes, is cytotoxic, and activates complement. Autolysin-mediated lysis of bacteria in tissue also releases products that contribute to tissue damage.
Predisposing factors include cardiopulmonary disease, prior viral respiratory infection (e.g. influenza), splenectomy, and complement defects. Pneumococci cause sinusitis, otitis media, mastoiditis, lobar pneumonia, bronchopneumonia, acute exacerbation of chronic bronchitis, joint infections, endocarditis, meningitis, bacteraemia, septicaemia, metastatic abscesses and conjunctivitis.
Two vaccines are used: the 23-valent purified polysaccharide vaccine (PPV23), covering serotypes 1, 2, 3, 4, 5, 6B, 7F, 8, 9N, 9V, 10A, 11A, 12F, 14, 15B, 17F, 18C, 19F, 19A, 20, 22F, 23F, 33F — about 70% efficacious in high-risk adults but poorly immunogenic under age 2, with low immune memory and no herd immunity; and the pneumococcal conjugate vaccine (PCV), created by conjugating polysaccharide with carrier proteins to improve immunogenicity even in infants.
Laboratory Principle
Pneumococcal identification relies on three complementary principles: the fragile cell wall is rapidly lysed by bile salts (bile solubility test) because deoxycholate activates the organism's own autolysin; optochin (ethyl hydrocuprein hydrochloride) selectively inhibits pneumococcal ATPase at low concentration, unlike other alpha-haemolytic streptococci; and the polysaccharide capsule becomes optically visible and swollen when it binds specific antibody, refracting light differently under the microscope (Quellung/capsule-swelling reaction) — a direct antigen-antibody visualisation.
Equipment Required
Reagents & Materials
| Reagent / Material | Concentration / Grade | Purpose | Storage |
|---|---|---|---|
| Blood Agar / Chocolate Agar | 5–10% sheep blood | Primary culture of Pneumococcus | 2–8°C, protect from light |
| Optochin Discs | 5 mg ethyl hydrocuprein hydrochloride | Optochin sensitivity test | 2–8°C, desiccated |
| Sodium Deoxycholate | 10% solution | Bile solubility test | Room temperature, tightly capped |
| Polyvalent Pneumococcal Antiserum | Commercial preparation | Quellung / capsule swelling reaction and slide agglutination | 2–8°C |
| Methylene Blue Stain | 1% aqueous | Counterstain for Quellung test | Room temperature |
| Gram Stain Kit | Crystal violet, iodine, alcohol, safranin | Microscopic morphology | Room temperature |
Step-by-Step Procedure
Collect sputum for respiratory infection, aspirate/pus for otitis media, blood for septicaemia, or CSF for suspected meningitis under aseptic conditions.
Make a smear from the specimen, Gram stain it, and look microscopically for typical lancet-shaped, Gram-positive diplococci surrounded by a thick capsule.
Mix a drop of specimen with a drop of polyvalent/type-specific antiserum to detect specific soluble substance directly and identify the serotype early.
Inoculate blood agar and chocolate agar; incubate at 37°C in 5–10% CO₂ overnight (18 hrs) and examine for alpha-haemolytic, dome-shaped, mucoid colonies.
Perform the bile solubility test, optochin sensitivity test and inulin fermentation test on isolated colonies to confirm the identity of S. pneumoniae.
Mix culture suspension with polyvalent or type-specific antiserum plus methylene blue and examine for capsular swelling, or perform latex agglutination to confirm serotype.
Flow Diagram
Quality Control
Run a known optochin-sensitive S. pneumoniae strain and an optochin-resistant viridans streptococcus alongside every batch of optochin discs to confirm the zone of inhibition criteria (≥14–16 mm sensitive). Include positive and negative bile solubility controls each time a new batch of 10% sodium deoxycholate is prepared.
Participate in a recognised External Quality Assessment Scheme (EQAS) for bacterial identification and antimicrobial susceptibility testing; submit blinded pneumococcal isolates periodically and compare results against the reference laboratory to verify ongoing accuracy of the bile solubility and optochin methods.
Reference Values
Key Diagnostic Parameters⚠️ Reference ranges may vary between laboratories. Always apply your laboratory's established reference intervals.
Clinical Interpretation
| Finding | Possible Significance | Action / Follow-up |
|---|---|---|
| Alpha-haemolytic, optochin-sensitive, bile-soluble diplococci | Confirms Streptococcus pneumoniae | Report as S. pneumoniae; correlate with clinical picture |
| Alpha-haemolytic, optochin-resistant, bile-insoluble | Viridans streptococci (commensal, not pneumococcus) | Report as viridans streptococci; consider contamination/normal flora |
| Positive Quellung reaction with type-specific antiserum | Confirms capsular serotype for epidemiological/vaccine correlation | Forward isolate for serotyping/surveillance if indicated |
| Pneumococci isolated from normally sterile site (blood/CSF) | Invasive pneumococcal disease — septicaemia or meningitis | Urgent antibiotic therapy and clinical correlation |
Common Errors & How to Avoid Them
Cause: Overnight cultures older than 24 hours may show self-lysed, depressed colonies that are mistaken for absent growth.
Prevention: Read plates promptly at 18–24 hours and recognise the lysed, plateau-with-depressed-centre appearance as a positive identification clue, not a negative result.
Cause: Optochin zones vary with medium and CO₂ conditions; testing without blood agar and CO₂ incubation gives unreliable zones.
Prevention: Always perform optochin testing on blood agar incubated in a CO₂-enriched atmosphere and use the correct 15 mm cut-off.
Cause: Repeated subculturing converts smooth (S) encapsulated colonies to rough (R) unencapsulated mutants.
Prevention: Use primary isolates or early subcultures for capsule-dependent tests such as Quellung, and store stock cultures by lyophilisation.
Laboratory Tips from the Bench
If a colony looks alpha-haemolytic but you're unsure whether it's pneumococcus or viridans streptococcus, do the bile solubility test first — it's faster and cheaper than ordering an optochin disc, and a positive clearing within minutes is highly specific.
Always examine blood agar plates within 18–24 hours; pneumococcal colonies self-destruct due to autolysin, so a plate read a day late may look like 'no growth' when it actually grew and lysed.
Remember 'BOQ' for pneumococcal ID: Bile-soluble, Optochin-sensitive, Quellung-positive — the three tests that separate S. pneumoniae from look-alike viridans streptococci.
Important Notes
Unencapsulated pneumococci are avirulent. Any test relying on capsule visualisation (Quellung) should ideally be performed on fresh clinical material or early subcultures, since serial subculture promotes loss of capsule (S→R variation).
Griffith's 1928 transformation experiment, which used virulent and non-virulent pneumococci, laid the foundation for the discovery that DNA is the genetic material — a landmark moment in molecular biology history.
Interactive Quiz
Test Your KnowledgeFlashcards
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Clinical Case Study
Apply Your KnowledgePresents with 2-day history of high fever (39.5°C), rapid breathing, cough with rust-coloured sputum, and right-sided chest pain aggravated by breathing. Recently recovered from influenza-like illness.
The Gram stain morphology, alpha-haemolysis, optochin sensitivity and bile solubility together confirm Streptococcus pneumoniae as the causative organism of community-acquired lobar pneumonia, likely secondary to preceding influenza infection which predisposed the child to bacterial superinfection.
- →Post-viral bacterial pneumonia is a classic pneumococcal presentation.
- →Bile solubility and optochin sensitivity together are near-diagnostic for S. pneumoniae.
- →Pneumococcal conjugate vaccination reduces this risk in children under 2 years.
Frequently Asked Questions
Autolysin (Lyt A) progressively lyses the bacterial cells from the centre outward as the culture reaches stationary phase, especially after 18–24 hours, giving the colony a collapsed, umbilicated appearance.
Growth without added CO₂ is poor because pneumococci are capnophilic; a 5–10% CO₂ atmosphere (candle jar or incubator) markedly improves colony size and yield.
Capsule synthesis is energy-costly and repeated in-vitro subculture selects for unencapsulated (rough) variants; in the host, immune pressure favours retention of the capsule because it is essential for survival against phagocytes.
Quick Revision
10-Minute ReviewKey Takeaways
- Pneumococcus is a leading cause of community-acquired pneumonia, otitis media, and bacterial meningitis worldwide.
- Correct laboratory identification relies on the triad of alpha-haemolysis, optochin sensitivity, and bile solubility.
- The polysaccharide capsule is both the key virulence factor and the basis of vaccine design.
- Griffith's transformation experiments with pneumococci founded the field of molecular genetics.
- Vaccination strategy differs by age group — conjugate vaccines are preferred in infants and young children.
Competency Checklist
Track Your MasteryReferences
- National Institute of Open Schooling. Microbiology — Lesson 16: Pneumococcus.
- Ananthanarayan R, Paniker CKJ. Textbook of Microbiology. Universities Press.
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology.