Streptococci

Streptococci are a diverse and clinically significant group of bacteria responsible for various infections, from mild to life-threatening. Understanding their characteristics, pathogenic mechanisms, and laboratory identification methods is crucial for effective diagnosis and treatment. Ongoing research and surveillance are essential to manage antibiotic resistance and new pathogenic strains’ emergence effectively.

Table of contents

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

Specimen Collection: Depending on the suspected infection, specimens can be obtained from throat swabs, blood, wounds, or other infected tissues.
Microscopic Examination:
- A Gram stain of the specimen reveals Gram-positive cocci in chains or pairs.
Culture Techniques:
- Inoculate appropriate media (blood agar, selective media) and incubate at optimal conditions (35-37°C for 24-48 hours).
Biochemical Testing:
- Perform catalase, hemolysis, and Lancefield classification tests to identify species.
- Based on the initial findings, conduct additional tests (e.g., bacitracin sensitivity, Camp test).
Molecular Methods:
- PCR and other nucleic acid amplification techniques can rapidly identify and type streptococcal species, especially in outbreak investigations or severe infections.

Antibiotic Resistance

Emergence of Resistance: Some species, particularly enterococci, have developed resistance to multiple antibiotics, including vancomycin (VRE).
Penicillin Resistance: While many streptococci remain sensitive to penicillin, some strains (e.g., certain S. pneumoniae) show decreased susceptibility, complicating treatment.