Pathogenic Gram Positive Bacteria - In Depth Notes

Chapter 19: Pathogenic Gram Positive Bacteria

Gram Positive Cocci
  • Genus Streptococcus
    • Gram Positive Cocci typically found in pairs or chains.
    • Catalase negative (do not produce catalase enzyme).
    • Categorized based on hemolysis patterns or the Lancefield classification system.
    • Some species require CO2 for growth; all are enhanced by it (more common inside the human body).
    • Usually require blood for growth.
Hemolysis Patterns

  • Beta (β) Hemolysis:

    • Complete lysis of RBCs, producing a clear zone around colonies.
    • Example: Group A Streptococcus (S. pyogenes).

  • Alpha (α) Hemolysis:

    • Partial lysis of RBCs, resulting in a green zone.
    • Example: Streptococcus pneumoniae.

  • Gamma (γ) Hemolysis:

    • No lysis of RBCs, resulting in no visible zone.
Lancefield Classification
  • Developed by Rebecca Lancefield, categorizes streptococci based on carbohydrate composition of bacterial antigens on their cell walls.
    • Group A: S. pyogenes
    • Group B: S. agalactiae
    • Group D: S. bovis
    • Other groups (C, F, G) are less commonly associated with human disease.
Clinical Manifestations of Streptococcus pyogenes (Group A)

  • Diseases caused:

    • Pharyngitis (Strep Throat): Can progress to scarlet fever if erythrogenic toxin is present, characterized by a rash and red tongue.
    • Pyoderma (Impetigo): Highly infectious skin infection.
    • Toxic Shock Syndrome: Can lead to bacteremia and multisystem failure, with a 40% fatality rate.
    • Necrotizing Fasciitis: Known as “flesh-eating disease,” with a 50% fatality rate.

  • Untreated infections can lead to complications:

    • Autoimmune response leading to conditions like rheumatic heart fever (RHF) and glomerulonephritis.
Diagnosis of Group A Infections
  • Rapid Strep Test: Up to 30% false negative rate.
  • Bacitracin test: Reference test for negative results for Strep Group A; culture if negative.
  • Latex Agglutination Test: Identification of Strep Group A in the lab.
Treatment of Group A Infections
  • Sensitive to Penicillin.
  • Alternatives for penicillin-allergic patients: cephalosporin or erythromycin.
  • Topical Bacitracin effective for Group A impetigo.
  • Necrotizing fasciitis requires surgical removal of necrotic tissue.
Beta-Hemolytic Streptococcus Agalactiae (Group B)
  • Primarily affects newborns; colonizes gastrointestinal, urinary, and genital tracts.
  • 60% of newborns may be exposed; disease occurs if maternal antibodies are not protective.
  • Early-onset Infection: Develops within 1 week after birth due to passage through birth canal.
  • Late-onset Infection: Occurs after 1 week, usually from caretakers.
  • Can cause severe outcomes such as sepsis and meningitis, leading to neurological impairment.
Prevention of Early-Onset Group B Disease
  • Expectant mothers are cultured for Group B before delivery. If positive, IV Penicillin G administered during labor.
  • Early-onset infections reduced by 70% with this practice. (BSGB) vaccine in development for women pre-pregnancy.
Beta-Strep Group B Identification
  • CAMP Test: Enhances beta-hemolysis of S. aureus; used for lab identification.
  • Antigen Spot Test and Latex Agglutination Tests for various Streptococcus groups.
Non-Beta-Hemolytic Streptococci
  • Includes:
    • Streptococcus pneumoniae (Alpha-hemolytic)
    • Viridans Streptococci (especially S. mutans)
    • Streptococcus bovis (Alpha-hemolytic)
    • Enterococcus (Gamma-hemolytic)
Streptococcus pneumoniae
  • Typically found as lancet-shaped diplococci; capnophilic, sensitive to Optochin and bile.
  • Causes 85% of pneumonia cases, along with sinusitis and otitis media.
  • Treatment:
    • Penicillin was once the drug of choice, but 1/3 of strains are now resistant.
    • Alternatives: cephalosporins, erythromycin, chloramphenicol.
    • Vaccination protocols in place for young children since 2000.
Streptococcus mutans
  • Normal flora in upper respiratory, gastrointestinal, and genital tracts.
  • Causes dental caries when sucrose is present due to dextran production, leading to biofilm formation.
Streptococcus bovis
  • Found in the gastrointestinal tract; associated with colon cancer. Has Lancefield Group D antigen, resistant to bile.
Enterococcus
  • Lives in the intestinal tract and vagina; hardy organism capable of surviving low pH and high salt concentrations.
  • Indicator of water quality; resistant to multiple antibiotics, with increasing incidents of Vancomycin Resistant Enterococcus (VRE).
  • Also known for causing nosocomial infections.