Group B Streptococci Overview

Group B Streptococci

• May be carried in the throat.
• Important cause of neonatal sepsis.
• Colonization in neonates usually occurs via the mother's genital tract.
• Risk factors for neonatal colonization:
  • Maternal colonization.
  • Premature delivery or premature rupture of membranes.
  • Prolonged labor.
  • Low birth rate.
  • Intra-partum fever.

Group B Streptococci: Irish Epidemiology

• Started collecting data in Ireland in 2019.
• Number of isolates increased from 176 in 2019 to 102 in 2020.
• Percent of isolates with Erythromycin resistance increased from 41.9% to 31.9% in 2020.

Group B Streptococci: Virulence Mechanisms

• Have different serotypes based on capsular polysaccharides.
• Types Ia, III to V are most commonly associated with colonization and disease.
• Produce haemolysins, hyaluronidase, and surface proteins (adhesins).

Other Beta-Hemolytic Streptococci

• Can cause similar diseases as Group A streptococcus but without the immunological complications.
• Group C streptococci can cause:
  • Puerperal fever.
  • Tonsillitis.
  • Wound sepsis.
• Group G streptococci can cause:
  • Upper respiratory tract infections.
  • Endocarditis.

Alpha-Hemolytic Streptococci

• Include "viridans" streptococci and Streptococcus pneumonia.

"Viridans" Streptococci

• Commonly found in the oropharynx, Gastrointestinal tract, and Genitourinary tract.
• Most lack Lancefield antigens except S.bovis (Group D).
• Common "viridans" streptococci include:
  • S.sanguis
  • S.mitis
  • S.mutans.
• S.bovis (Group D) can cause infective endocarditis.
• S.mutans can cause dental caries.

Hemolytic Patterns of Streptococci

• Alpha-hemolytic: Reduce Hb and cause a greenish discoloration of blood agar.
• Beta-hemolytic: Lyse blood cells and cause complete clearing of blood agar.
• Gamma-hemolytic: No change in blood agar.

Lancefield Classification of Streptococci

• A serological system for grouping streptococci.
• Based on specific antigens present in their cell walls.
• Groups A-G are of the most clinical significance.
• Basis of test: antibody/antigen reaction.
• Positive test: agglutination.

Clinical Case 1

• A 10 year old male presents to his GP with a sore throat.

Clinical Case 1: Examination

• Examination shows swollen, erythematous tonsils with exudate.
• Cervical lymph nodes are swollen and tender.

Clinical Case 1: Likely Diagnosis

• Pharyngitis.

Clinical Case 1: Throat Swab Culture

• Gram-positive cocci in chains.

Clinical Case 1: Cause of Infection

• Streptococcus pyogenes.

Streptococcus pyogenes (Group A Beta-Hemolytic)

• Possesses the Lancefield group A antigen.
• Commonly colonizes oropharynx of children and young adults.
• Colonization is transient.
• Influenced by acquired immunity and competition from other organisms in the oropharynx.
• Major cause of bacterial pharyngitis.

Group A Streptococci: Irish Epidemiology

• Invasive group A strep is a notifiable disease.

Group A Beta-Hemolytic Streptococci: Virulence Mechanisms

• Structural components include:
  • Capsule: resists phagocytosis.
  • Cell wall: peptidoglycan can activate the alternative complement pathway.
• M-proteins:
  • Over 80 types, mediate attachment to cells, anti-phagocytic.
  • Major virulence factors - some types associated with greater severity of disease.
• M-like proteins: bind IgG and IgM.
• Cytolysins:
  • Streptolysin O/S: exotoxins.
  • Streptolysin S: exotoxins.
• Hyaluronidase: tissue destruction, allowing the spread of infection.
• Leucocidin: produced by both antigen-presenting cells and T lymphocytes.
• Hemolysins.
• Streptokinase.

Pathogenesis of Group A Beta-Hemolytic Streptococci

• Droplet spread.
• Direct contact.
• Colonizes the oropharynx.
• Invades epithelial cells.
• M-proteins and capsule aid in evading the immune system.
• Exotoxins, haemolysins, hyaluronidase, leucocidin, and streptokinase cause damage to host cells.

Clinical Infections of Group A Beta-Hemolytic Streptococci

• Suppurative infections:
  • Pharyngitis.
  • Scarlet fever.
  • Erysipelas, cellulitis, necrotizing fasciitis.
  • Intra-abdominal infections.
• Prolonged hospitalization, especially in the ICU.

Infective Endocarditis

• Turbulent blood flow through the heart provides a surface for bacteria to attach.
• Bacteria enter the blood usually after a procedure that damages epithelial barriers.
• Bacteria attach to damaged valves and form vegetations.

Anaerobic Streptococcus (Peptostreptococcus)

• Over 25% of anaerobes found in clinical specimens.
• Colonize the oral cavity, GI tract, GU tract, and skin.
• Infections include:
  • Aspiration pneumonia.
  • Sinusitis and brain abscess.
  • Intra-abdominal abscesses.
  • Pelvic infections.

General Aspects of Diagnosis of Streptococcal Infections

• Clinical suspicion based on clinical features.
• Appropriate samples are sent to the laboratory based on the site of infection:
  • Blood (if invasive disease is suspected).
  • CSF (meningitis).
  • Urine.
  • Throat swab (pharyngitis).
• Laboratory tests:
  • Gram stain on sterile site sample (blood, CSF, pus).
  • Culture (takes 24-48 hours): samples incubated aerobically and anaerobically.
    • Blood agar (for haemolysis).
    • MacConkey agar (for enterococci).

Laboratory Diagnosis of Streptococcal Infections

• Identification:
  • Lancefield grouping.
  • Bacitracin susceptibility (GAS).
  • Optochin sensitivity (pneumococcus).
  • Bile solubility (enterococci).
• Serology: detect a recent Group A infection in suspected rheumatic fever and glomerulonephritis.
• PCR: blood, CSF.
• Urinary antigen (for pneumococcus).

Streptococcus pyogenes Lab Diagnosis

• Gram-positive cocci in chains.
• Beta-hemolytic.
• Catalase-negative.
• Group A.
• Bacitracin susceptible.