Streptococci

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.

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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).

 

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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.

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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.

 

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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.

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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.

 

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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:

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

 

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Antibiotic Resistance

Common Antibiotics Used Against Streptococci

• Penicillin

• Amoxicillin

• Cephalosporins

• Macrolides (Erythromycin, Azithromycin)

• Tetracyclines

• Vancomycin (for severe infections)

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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