In-Depth Notes on Pathogenic Gram Positive Bacteria - Streptococci

Chapter 19: Pathogenic Gram Positive Bacteria

General Information on Genus Streptococcus
  • Gram Positive Cocci: Organized in pairs or chains.
  • Catalase Negative: Distinction from other bacteria that are catalase positive.
  • Categorization Methods:
    • Hemolysis Classification: 1. Beta Hemolysis: Complete lysis of red blood cells (e.g., S. pyogenes).
    1. Alpha Hemolysis: Partial lysis; greenish coloration around colonies (e.g., S. pneumoniae).
    2. Gamma Hemolysis: No hemolysis observed.
    • Lancefield Classification: Based on carbohydrate composition of bacterial antigens in cell walls.
  • Growth Requirements: Some strains require CO2 for optimal growth; commonly found in the body rather than on the skin. Requires blood cultures for growth, with different strains presenting varied growth characteristics.
Hemolysis Types in Streptococcus
  • Beta Hemolysis: Clear zones due to complete RBC lysis.
  • Alpha Hemolysis: Green zone caused by partial lysis (e.g., Streptococcus pneumoniae).
  • Gamma Hemolysis: No observable change with no RBC lysis.
Lancefield Classification System
  • Developed by Rebecca Lancefield to classify streptococcal species.
  • Key Groups:
    • Group A: S. pyogenes
    • Group B: S. agalactiae
    • Group D: S. bovis
    • Group C, F, G: Other species, less commonly associated with human diseases.
Pathogenic Effects of Streptococcus pyogenes (Group A)
  • Diseases Associated with S. pyogenes:
    • Pharyngitis: Can progress to scarlet fever if erythrogenic toxin is produced; marked by characteristically red rash and tongue.
    • Impetigo/Pyoderma: Highly infectious skin condition, potentially evolving into erysipelas if lymph nodes are involved.
    • Necrotizing Fasciitis: Also called "flesh-eating strep"; characterized by deep tissue infection with large, dark blisters, high mortality (up to 50%).
    • Additional Complications: Include toxic shock, bacteremia, and multi-system organ failure with a mortality rate around 40%.
Long-term Consequences of Untreated S. pyogenes Infections
  • Rheumatic Heart Fever (RHF): Autoimmune response leads to heart disease, particularly in children globally.
  • Glomerulonephritis: Accumulation of antigen-antibody complexes in kidney glomeruli can block blood flow and result in permanent damage.
Diagnostic Tests for Streptococcus pyogenes
  • Rapid Strep Test: Up to 30% false negative; cultures recommended if negative.
  • Bacitracin Test: Useful for Strep Group A identification.
  • Latex Agglutination: Further identification in laboratory settings.
Treatment of Beta-Strep Group A Infections
  • First Line Treatment: Penicillin remains effective although documented risks of resistance exist with other antibiotics (e.g., cephalosporin, erythromycin).
  • Necrotizing Fasciitis Management: Surgical intervention necessary for dead tissue removal.
  • Individual Immunity: Antibodies from one strain do not provide protection against other strains.
Streptococcus agalactiae (Group B)
  • Impact on Newborns:
    • Colonizes maternal GI and urogenital tracts; can affect 60% of newborns if maternal antibodies are absent.
    • Early-Onset Infection (within 1 week): Acquired during passage through birth canal.
    • Late-Onset Infection (after 1 week): Related to caregiver handling.
    • Potential outcomes include sepsis, meningitis, pneumonia, with high risks of lasting neurological impairment after meningitis.
Prevention and Control Measures for Group B Streptococcus (GBS)
  • Screening Expectant Mothers: Culturing expected mothers as delivery approaches to identify GBS presence.
  • Intravenous Antibiotic Administration: Penicillin G is the drug of choice.
  • Vaccination Development: Vaccines targeting GBS are in progress reaching out specifically to women before pregnancy which has reduced early-onset infections by approximately 70%.
Identification of Group B Streptococcus
  • Beta-Hemolysis Indicator: Positive CAMP test enhances the beta hemolysis demonstration of Staphylococcus aureus.
  • Laboratory Tests: Include antigen B spot tests and latex agglutination.
Other Important Streptococcal Species

  • Streptococcus pneumoniae:

    • Diplococci in a lancet shape; significant cause of pneumonia, otitis media, sinusitis, and meningitis.
    • Penicillin resistance is noted in one-third of strains; treatment shifted to cephalosporins, and vaccines have been developed for children and adults to reduce prevalence.

  • S. mutans: Causes dental caries via sucrose, forming biofilms leading to potential endocarditis.

  • Streptococcus bovis: Found in the GI tract and associated with colon cancer; displays distinct lancefield Group D antigens.

  • Enterococcus: Known for its resilience under various conditions, pathogenic outside the gut, contributes to nosocomial infections; VRE poses a growing challenge in healthcare settings.