Streptococcus pneumoniae

Ms. Neha Sharma

Introduction

  • Streptococcus pneumoniae is a significant human pathogen responsible for various serious infections, including pneumonia, meningitis, and otitis media.
  • Understanding its characteristics, pathogenic mechanisms, and effective laboratory diagnosis is crucial for managing infections caused by pneumococci.
  • Continuous surveillance and research are essential to address the challenges of antibiotic resistance and improve prevention strategies through vaccination.

 

General Character

  • Genus: Streptococcus
  • Species: Streptococcus pneumoniae
  • Family: Streptococcaceae
  • Gram Staining: Pneumococci are Gram-positive cocci, appearing purple due to a thick peptidoglycan layer.
  • Shape and Arrangement:
    • Shape: They are lancet-shaped cocci, often described as elliptical.
    • Arrangement: Typically found in pairs (diplococci) or short chains.
  • Oxygen Requirements: S. pneumoniae is a facultative anaerobe, meaning it can grow in aerobic and anaerobic conditions but prefers aerobic environments.

 

Morphology

  • Cell Wall Structure:
    • It comprises a thick peptidoglycan layer, which is crucial for structural integrity and provides resistance to lysis.
    • Contains teichoic and lipoteichoic acids, contributing to the cell wall’s properties and pathogenicity.
  • Capsule:
    • S. pneumoniae possesses a polysaccharide capsule that is a major virulence factor. The capsule protects against phagocytosis and is the basis for its classification into different serotypes.
  • Surface Structures:
    • The capsule is critical for virulence, preventing opsonization and phagocytosis by immune cells.

 

Cultural Characteristics

  • Growth Media:
    • Blood Agar: Grows well on blood agar, where it typically exhibits α-hemolysis (partial hemolysis) due to hydrogen peroxide production.
    • Chocolate Agar: Often used for enhanced growth, especially in clinical specimens.
    • Selective Media: It can be grown on bile-esculin agar but is not strictly necessary.
  • Colony Appearance:
    • Colonies are small, mucoid, and glistening, often with a greenish hue around them due to partial hemolysis.
  • Temperature and pH Range:
    • The optimal growth temperature is around 37°C, with a preferred pH range of 6.5 to 7.5.

 

Biochemical Reactions

  • Catalase Test: S. pneumoniae is catalase-negative, which helps distinguish it from staphylococci.
  • Bile Solubility Test: S. pneumoniae is bile-soluble, which means it can be lysed by bile salts, differentiating it from other alpha-hemolytic streptococci (like S. mitis).
  • Optochin Sensitivity Test: S. pneumoniae is sensitive to optochin (disk diffusion test), another distinguishing feature.
  • Fermentation: It ferments carbohydrates, producing lactic acid, but does not produce gas during fermentation.

 

Pathogenicity

  • Virulence Factors:
    • Capsule: Prevents phagocytosis and is a key virulence factor.
    • Pneumolysin: A toxin that damages host tissues, activates the immune response, and contributes to inflammation and tissue injury.
    • Autolysins: Enzymes that can contribute to releasing pneumolysin and other virulence factors during bacterial lysis.
    • Surface adhesins: Facilitate adherence to respiratory epithelium, aiding colonization and infection.
  • Clinical Infections:
    • Pneumonia: The most common disease caused by S. pneumoniae, often called pneumococcal pneumonia. Symptoms include cough, fever, chest pain, and difficulty breathing.
    • Meningitis: S. pneumoniae is one of the leading causes of bacterial meningitis in adults and children.
    • Otitis Media: A common cause of middle ear infections, especially in children.
    • Sinusitis: This can also lead to sinus infections.
    • Bacteraemia: This can result in bloodstream infections, leading to sepsis.

 

Laboratory Diagnosis

  • Specimen Collection: Clinical specimens can include sputum, blood, cerebrospinal fluid (CSF), or middle ear fluid.
  • Microscopic Examination:
    • A Gram stain reveals Gram-positive diplococci, often within white blood cells, indicating an active infection.
  • Culture Techniques:
    • Specimens are inoculated onto blood agar and incubated in a CO₂-enriched atmosphere to enhance growth.
    • Identification involves observing colony morphology and performing biochemical tests (bile solubility, optochin sensitivity).
  • Antigen Detection: Rapid antigen tests can detect pneumococcal polysaccharides in urine or CSF, providing quick results.
  • Molecular Methods: PCR techniques can be used for rapid identification and serotyping of pneumococci, especially in severe infections or when cultures are negative.

 

Antibiotic Resistance

  • Resistance Patterns: S. pneumoniae has shown increasing resistance to penicillin and other antibiotics, complicating treatment options.
  • Multidrug-Resistant Strains: The emergence of strains resistant to macrolides, tetracyclines, and fluoroquinolones has been reported.
  • Treatment Options: Empirical treatment often includes ceftriaxone or vancomycin, particularly in severe cases. Antibiotic susceptibility testing is essential for guiding treatment.

 

Prevention

  • Vaccination is the most important measure.

    • PCV13/PCV15/PCV20 for infants, elderly, and immunocompromised.

    • PPSV23 for adults ≥65 years and high-risk groups.

  • Maintain respiratory hygiene:

    • Cover mouth/nose while coughing or sneezing

    • Use masks during infections

    • Frequent handwashing

  • High-risk groups require special care:

    • Asplenic, HIV-positive, elderly, chronic diseases

    • Combined PCV + PPSV23 schedule

  • Avoid smoking, as it increases colonization and pneumonia risk.

  • Prevent viral infections (especially influenza) through vaccination and hygiene practices.

  • Improve immunity through good nutrition and healthy lifestyle.

  • Reduce overcrowding and ensure good ventilation in closed spaces.

 

MCQs

1. Streptococcus pneumoniae is:

A. Gram-negative coccus
B. Gram-positive diplococcus
C. Acid-fast bacillus
D. Spirochete

2. The characteristic shape of S. pneumoniae is:

A. Kidney-shaped diplococci
B. Comma-shaped
C. Lancet-shaped diplococci
D. Coccobacilli

3. S. pneumoniae is commonly found as part of normal flora in:

A. Skin
B. Nasopharynx
C. Small intestine
D. Stomach

4. Major virulence factor of S. pneumoniae:

A. Lipid A
B. Capsule
C. Pili
D. Flagella

5. Colony appearance on blood agar is:

A. Large mucoid colonies
B. Metallic sheen colonies
C. “Draughtsman” or sunken colonies
D. Swarming growth

6. Type of hemolysis S. pneumoniae produces:

A. Beta hemolysis
B. Gamma hemolysis
C. Alpha hemolysis
D. Variable hemolysis

7. S. pneumoniae is bile:

A. Resistant
B. Sensitive
C. Partially resistant
D. Unaffected

8. S. pneumoniae is optochin:

A. Resistant
B. Sensitive
C. Negative
D. Indeterminate

9. Quellung reaction is used to detect:

A. Flagella
B. Capsule swelling
C. Cell wall teichoic acid
D. M protein

10. Pneumococcal pneumonia classically presents with:

A. Dry cough
B. Rust-colored sputum
C. Bloody diarrhea
D. Painless ulcers

11. Most common cause of bacterial pneumonia in adults:

A. K. pneumoniae
B. S. aureus
C. S. pneumoniae
D. H. influenzae

12. “Lancet-shaped diplococci” are seen in:

A. Gonorrhea
B. Pneumococcal infections
C. Meningitis from meningococcus
D. Diphtheria

13. In sputum smear, S. pneumoniae appears as:

A. Chains
B. Clusters
C. Pairs (diplococci)
D. Single bacilli

14. S. pneumoniae is a leading cause of:

A. Endocarditis
B. Otitis media
C. Gastroenteritis
D. UTI

15. Population at highest risk for pneumococcal infection:

A. Young healthy adults
B. Alcoholics and elderly
C. Teenagers
D. Athletes

16. Capsule of S. pneumoniae is composed of:

A. Peptidoglycan
B. Teichoic acid
C. Polysaccharide
D. Lipoprotein

17. Lobar pneumonia caused by S. pneumoniae typically affects:

A. One lobe of lung
B. Multiple lobes
C. Both lungs uniformly
D. Bronchi only

18. Pneumococcal meningitis is most common in:

A. Adults
B. Neonates
C. Elderly
D. Immunocompromised

19. Most important test to differentiate S. pneumoniae from viridans streptococci:

A. Gram stain
B. Optochin sensitivity
C. Catalase test
D. Coagulase test

20. S. pneumoniae is catalase:

A. Positive
B. Negative
C. Variable
D. Weak

21. Common complication of pneumococcal pneumonia:

A. Cavitation
B. Pleural effusion
C. Abscess
D. Pneumothorax

22. Pneumolysin is a:

A. Exotoxin
B. Cytotoxin
C. Neurotoxin
D. Enterotoxin

23. The capsule protects S. pneumoniae against:

A. Complement
B. Neutrophil phagocytosis
C. Antibodies
D. All of the above

24. Pneumococcal vaccine available as 13-valent is:

A. PPSV23
B. BCG
C. PCV13
D. DPT

25. PPSV23 vaccine is:

A. Polysaccharide vaccine
B. Conjugate vaccine
C. Live vaccine
D. Toxoid vaccine

26. PCV vaccines induce:

A. T-cell independent immunity
B. T-cell dependent immunity
C. No immunological memory
D. Immediate hypersensitivity

27. The drug of choice for pneumococcal infection is historically:

A. Penicillin
B. Vancomycin
C. Chloramphenicol
D. Tetracycline

28. Increasing resistance to penicillin is due to:

A. Beta-lactamase
B. Altered penicillin-binding proteins (PBPs)
C. Decreased capsule
D. Loss of pili

29. Pneumococcus is identified in CSF by:

A. India ink
B. Latex agglutination
C. Acid-fast staining
D. Albert stain

30. S. pneumoniae fermentation pattern:

A. Ferments glucose only
B. Ferments several sugars producing lactic acid
C. No fermentation
D. Ferments lactose only

31. Pneumococcal infection commonly follows:

A. Typhoid fever
B. Viral URTI or influenza
C. Dengue fever
D. Malaria

32. In blood agar, S. pneumoniae colonies become “draughtsman” due to:

A. Hemolysis
B. Autolysis
C. Toxin secretion
D. Capsule breakdown

33. Nasopharyngeal colonization is mediated by:

A. Pili
B. Capsule
C. Teichoic acid
D. Surface adhesins

34. Pneumococcal meningitis CSF findings include:

A. High protein, low glucose
B. Low protein, high glucose
C. High RBCs
D. Low WBCs

35. Pneumococcal bacteremia is common in:

A. Asplenic patients
B. Diabetics
C. Pregnant women
D. Athletes

36. Pneumococcal vaccine is contraindicated in:

A. Pregnancy
B. Immunocompromised states
C. Anaphylaxis to vaccine components
D. HIV

37. Most common cause of community-acquired meningitis in adults:

A. Listeria
B. Neisseria
C. Streptococcus pneumoniae
D. Haemophilus influenzae

38. Pneumococcus is bile soluble because of:

A. Capsule
B. Pneumolysin
C. Autolysin enzyme (amidase)
D. Bile salt permeability

39. The gram stain of S. pneumoniae shows:

A. Pink bacilli
B. Purple diplococci
C. Clustered cocci
D. Spiral rods

40. Pneumococcal pneumonia classically shows which sign on chest X-ray?

A. Ground-glass opacity
B. Lobar consolidation
C. Miliary pattern
D. Reticulonodular pattern

✅ Answer Key (Separate Section)

1-B
2-C
3-B
4-B
5-C
6-C
7-B
8-B
9-B
10-B
11-C
12-B
13-C
14-B
15-B
16-C
17-A
18-D
19-B
20-B
21-B
22-B
23-D
24-C
25-A
26-B
27-A
28-B
29-B
30-B
31-B
32-B
33-D
34-A
35-A
36-C
37-C
38-C
39-B
40-B