Streptococci

Ms. Neha Sharma

Introduction

  • Streptococci are Gram-positive, spherical (cocci) bacteria that typically arrange in chains or pairs.

  • They belong to the family Streptococcaceae and are medically important pathogens.

  • Streptococci are non-motile, non-spore-forming, and usually non-capsulated (except some species).

  • They are facultative anaerobes, growing best in environments with reduced oxygen tension.

  • Streptococci are catalase-negative, which helps differentiate them from Staphylococci.

  • Many species are normal commensals of the upper respiratory tract, oral cavity, gastrointestinal tract, and skin.

  • Some species are pathogenic and cause diseases such as pharyngitis, pneumonia, rheumatic fever, endocarditis, and septicemia.

  • Classification is commonly based on hemolytic pattern on blood agar (alpha, beta, gamma hemolysis).

  • Further classification is done using Lancefield grouping based on cell wall carbohydrate antigens.

  • Streptococci are of major importance in clinical microbiology, laboratory diagnosis, and public health.

General Character

  • Genus: Streptococcus
  • Family: Streptococcaceae
  • Gram Staining: Streptococci are Gram-positive bacteria, appearing purple due to their thick peptidoglycan layer.
  • Shape and Arrangement:
    • Shape: They are spherical (cocci).
    • Arrangement: Streptococci typically occur in chains or pairs, resulting from division in one plane.
  • Oxygen Requirements: Streptococci can be classified based on their oxygen requirements:
    • Facultative anaerobes: Can grow in aerobic and anaerobic conditions (e.g., Streptococcus pneumoniae).
    • Obligate anaerobes: Require an oxygen-free environment for growth (e.g., Streptococcus pyogenes).

 

Morphology

  • Cell Wall Structure:
    • It comprises a thick peptidoglycan layer, crucial for maintaining shape and protecting against osmotic lysis.
    • The cell wall contains various polysaccharides, contributing to serological classification (Lancefield classification).
  • Capsule: Some species (e.g., S. pneumoniae) produce a polysaccharide capsule that enhances virulence by preventing phagocytosis.
  • Surface Structures:
    • Teichoic Acids: Present in the cell wall, involved in cell wall maintenance and regulation of cell growth.
    • M Protein: Found in the cell wall of certain species (e.g., S. pyogenes), it plays a key role in virulence by inhibiting phagocytosis and promoting adherence.

Cultural Characteristics

  • Growth Media:
    • Blood Agar: A differential medium that supports the growth of streptococci and allows for observing hemolytic patterns.
      • α-Hemolysis: Partial hemolysis (e.g., S. pneumoniae).
      • β-Hemolysis: Complete hemolysis (e.g., S. pyogenes).
      • γ-Hemolysis: No hemolysis (e.g., S. epidermidis).
    • Selective Media: Some species can be grown on selective media like bile esculin agar for certain enterococci.
  • Colony Appearance:
    • Colonies vary in size and color; β-hemolytic streptococci generally form clear zones around colonies on blood agar.
  • Temperature and pH Range:
    • Optimal growth occurs at 35-37°C. Some species can grow at temperatures as low as 10°C or as high as 45°C.
    • They prefer a neutral pH for optimal growth.

 

Biochemical Reactions

  • Catalase Test: Streptococci are catalase-negative, which distinguishes them from staphylococci.
  • Hemolysis Patterns:
    • Observed on blood agar as α, β, or γ hemolysis, used for preliminary classification.
  • Lancefield Classification: Based on the carbohydrate composition of antigens found on the bacteria’s cell wall:
    • Group A: Streptococcus pyogenes
    • Group B: Streptococcus agalactiae
    • Other groups include C, D (Enterococcus), F, and G.
  • Additional Biochemical Tests:
    • Bacitracin Sensitivity: S. pyogenes is sensitive, while S. agalactiae is resistant.
    • Camp Test: S. agalactiae produces a zone of enhanced hemolysis when combined with S. aureus.
    • Hippurate Hydrolysis: S. agalactiae is positive; S. pyogenes is negative.

Pathogenicity

  • Virulence Factors:
    • Toxins:
      • Streptolysins (O and S): Lyse red and white blood cells, contributing to tissue damage and inflammation.
      • Erythrogenic Toxin: Associated with scarlet fever.
    • Enzymes:
      • Hyaluronidase: Breaks down hyaluronic acid in connective tissues, aiding infection spread.
      • Streptokinase: Converts plasminogen to plasmin, promoting the breakdown of blood clots.
    • Adhesins: Promote attachment to host tissues, facilitating colonization.
  • Clinical Infections:
    • Streptococcus pyogenes: Causes pharyngitis (strep throat), impetigo, cellulitis, and severe invasive infections (necrotizing fasciitis, toxic shock syndrome).
    • Streptococcus agalactiae: Major cause of neonatal infections, including pneumonia and meningitis; also associated with infections in pregnant women.
    • Streptococcus pneumoniae: Causes pneumonia, meningitis, and otitis media. It is known for its polysaccharide capsule, a major virulence factor.
    • Enterococci (e.g., Enterococcus faecalis): Opportunistic pathogens that can cause urinary tract infections and endocarditis and are associated with antibiotic resistance.

 

Laboratory Diagnosis

1. Specimen Collection

Depends on the clinical condition:

  • Throat swab – Pharyngitis, tonsillitis

  • Sputum – Pneumonia

  • Blood – Septicemia, endocarditis

  • CSF – Meningitis

  • Pus / wound swab – Skin and soft tissue infections

Specimen should be collected aseptically and transported promptly.

2. Direct Microscopic Examination

  • Gram staining:

    • Gram-positive cocci

    • Arranged in chains or pairs

  • Presence of pus cells supports infection

Helps in presumptive diagnosis

3. Culture

  • Primary culture medium:

    • Blood agar (5% sheep blood)

  • Incubation:

    • 37°C for 18–24 hours

    • Facultative anaerobic conditions

4. Hemolysis on Blood Agar

Important for preliminary identification:

Type of Hemolysis Appearance Examples
Alpha (α) Greenish partial hemolysis S. pneumoniae, Viridans streptococci
Beta (β) Clear complete hemolysis S. pyogenes, S. agalactiae
Gamma (γ) No hemolysis Enterococci

5. Catalase Test

  • Catalase negative → confirms Streptococci

  • Differentiates from Staphylococci (catalase positive)

6. Biochemical Tests (Important for MLT Exams)

For Beta-hemolytic Streptococci

  • Bacitracin sensitivity

    • Sensitive → Streptococcus pyogenes (Group A)

  • CAMP test

    • Positive → Streptococcus agalactiae (Group B)

  • PYR test

    • Positive → Group A streptococci

For Alpha-hemolytic Streptococci

  • Optochin sensitivity

    • Sensitive → Streptococcus pneumoniae

  • Bile solubility test

    • Positive → S. pneumoniae

For Enterococci

  • Bile esculin test – Positive

  • Growth in 6.5% NaCl – Positive

7. Serological Tests

Used mainly for post-streptococcal complications:

  • ASO (Antistreptolysin-O) test

  • Anti-DNase B test

Useful in diagnosing rheumatic fever and glomerulonephritis

8. Antigen Detection Tests

  • Rapid antigen detection tests (RADT) from throat swabs

  • Useful for quick diagnosis of Group A streptococcal pharyngitis

9. Molecular Methods (PG Level)

  • PCR-based assays

  • High sensitivity and specificity

  • Used in reference and research laboratories

10. Antibiotic Sensitivity Testing

  • Performed using Kirby–Bauer disk diffusion method

  • Guides appropriate antimicrobial therapy

  • Important due to emerging resistance

 

Antibiotic Resistance

Common Antibiotics Used Against Streptococci

  • Penicillin

  • Amoxicillin

  • Cephalosporins

  • Macrolides (Erythromycin, Azithromycin)

  • Tetracyclines

  • Vancomycin (for severe infections)

Antibiotic Resistance Pattern in Streptococci

1. Penicillin Resistance

  • Traditionally, Streptococci are penicillin-sensitive

  • Resistance (especially in Streptococcus pneumoniae) occurs due to:

    • Alteration of Penicillin-Binding Proteins (PBPs)

  • Results in reduced binding of penicillin, not enzyme destruction

Important exam point:
Streptococci do not produce beta-lactamase

2. Macrolide Resistance (Erythromycin, Azithromycin)

Common in:

  • Streptococcus pyogenes

  • Streptococcus pneumoniae

Mechanisms:

  • Target site modification (methylation of ribosomal RNA)

  • Efflux pumps expelling antibiotic from the cell

3. Tetracycline Resistance

  • Occurs due to:

    • Efflux pumps

    • Ribosomal protection proteins

  • Commonly plasmid-mediated

4. Vancomycin Resistance

  • Rare in streptococci

  • More common in Enterococci

  • Resistance occurs due to:

    • Altered cell wall precursors