Hemolytic bacteria

α-hemolytic bacteria

Partial oxidation of hemoglobin → greenish or brownish color without clearing around growth on blood agar.
Include
- Streptococcus pneumoniae
- Viridans streptococci.

Complete lysis of RBCs → pale/clear area surrounding colony on blood agar.
Include
- Staphylococcus aureus
- Streptococcus pyogenes (group A strep)
- Streptococcus agalactiae (group B strep)
- Listeria monocytogenes.

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All staphylococci are catalase positive.

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Grape-like cocci in clusters

 Grape-like cocci in clusters

Proteins
- Protein A→ binds Fc-IgG, inhibiting complement activation and phagocytosis.
- Clumping factor A→ binds to fibrinogen → platelet activation, aggregation, and blood clumping→ Forms fibrin clot around itself→ abscess.
Enzymes
- Coagulase→ protection against phagocytosis
Toxins
- Exfoliative toxin
- Superantigens
  - Enterotoxin B (heat stable)→ not destroyed by cooking
    
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    Can be prevented by proper refrigerator storage or washing hands.
    
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  - **Toxic shock syndrome toxin-1 (TSST-1)**→ Superantigen that binds to MHC II and T-cell receptor, resulting in polyclonal T-cell activation and dramatic cytokine release.

Inflammatory diseases
- Skin infections (e.g., cellulitis, impetigo)
- Organ abscesses
- Pneumonia→ often after influenza virus infection.
- Infective endocarditis
  - Right sided tricuspid endocarditis in IVDU
  - Differentiate from candida endocarditis by HIV testing
- Septic arthritis
- Osteomyelitis
Toxin-mediated disease
- Scalded skin syndrome
- Staphylococcal toxic shock syndrome (TSS)
  - Causative Toxin→ Staphylococcal toxic shock syndrome (TSS)
  - Symptoms→ Fever, vomiting, diarrhea, rash, desquamation, shock, end-organ failure.
  - Lab values→ ↑ AST, ↑ ALT, ↑ bilirubin.
  - Causes→ Prolonged use of vaginal tampons or nasal packing.
    
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    Streptococcus pyogenes TSS (a toxic shock–like syndrome associated with painful skin infection).
    
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- Rapid-onset food poisoning
  - Causative Toxin→ Ingestion of preformed enterotoxin B
  - Presentation
    - Rapid onset → short incubation period (2–6 hr)
    - Nonbloody diarrhea and emesis (Vomiting predominant)

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MRSA (methicillin-resistant S aureus)

Important cause of serious healthcare-associated and community-acquired infections.
β-lactams resistance due to altered penicillin-binding proteins (encoded by mecA gene).
Some strains release Panton-Valentine Leukocidin (PVL)→ a protease that kills leukocytes and causes necrosis, are most likely to cause skin or soft-tissue abscess and invasive disease (eg, necrotizing pneumonia) </aside>

Secrete adherent biofilms (extracellular polysaccharide matrix) which infects
- Prosthetic devices (eg, hip implant, heart valve)
- **IV catheters (hemodialysis)**→ osteomyelitis