• gram positive bacteria

• arranged in grape-like clusters

• 3 species:

  • Staphylococcus aureus

  • Staphylococcus epidermidis

  • Staphylococcus saprophyticus

Staphylococcus Aureus

• Pathogenic

• Catalase positive

• Coagulase positive

• Mannitol positive

• Hemolysin of red blood cells

Morphology

• gram positive cocci

• arranged in grape-like clusters

• non-motile

• non-sporing

Cultural Characters

• Faculative anaerobes

• grow on:

  • nutrient agar, producing golden yellow colonies

  • beta hemolytic colonies on blood agar

  • mannitol fermentation

Epidemiology

• carried by healthy individuals on the skins and mucous membrane

• carriers serve as a source of infection by:

  • direct contact

  • food

Pathogenesis

Pathogenic virulence factors: genetic, biochemical or structural features that enable an organism to produce disease.

• Clinical outcome of an illness depends on the virulence of the pathogen and the opposing effectiveness of the host defense mechanisms

S. aureus many potential virulence factors

1. Cell wall virulence factors:

   1. Protein A:

      • major component of cell wall

      • binds to Fc moiety of IgG, exerting an antiopsonin effect

   2. Fibronectin-binding protein (FnBP):

      • promote binding to mucosal cells and tissue matrices

2. Cytolytic exotoxins:

   • Hemolysins: α, β, γ, and δ toxins attack mammalian cell (including red blood cell) membrane

3. Superantigen exotoxins:

   • affinity for T cell receptor - MHC Class II antigen complex

   • stimulate enhanced T lymphocyte response

     • as many as 20% of T cells respond as compared to 0.01% responding to usual processed antigens

       • major T cell activation can cause toxic shock syndrome

   1. Enterotoxins:

      • heat stable

      • resistant to the action of gut enzymes

      • cause diarrhea and vomiting associated with staphylococcal food poisoning

   2. Toxic shock syndrome toxin (TSST-1):

      • classic cause of toxic shock syndrome

        Manifestation:

        • fever

        • diffuse macular rash

        • shock

        • multisystem involvement

        • renal failure

        • heart failure

   3. Exfoliatin (exfoliative toxin, ET):

      • superantigen

      • causes scalded skin syndrome in children

Clinical Significance

1. Localized skin infections:

   • e.g. common sty (external hordeolum), furuncles (boils), carbuncles, impetigo

2. Deep, localized skin infections:

   • e.g. acute and chronic infection of bone marrow, acute infection of bone space in children (septic joint)

3. Acute endocarditis:

   • associated with intravenous drug abuse, caused by needles contaminated with S. aureus

4. Septicemia

5. Pneumonia

6. Nosocomial infections

7. Toxinoses: diseases caused by the action of a toxin, frequently when the organism that secreted the toxin is undetectable

   1. Toxic shock syndrome

   2. Staphylococcal gastroenteritis

   3. Scalded skin syndrome

Treatment

Hospital-Acquired Methicillin-resistant S. aureus (MRSA)

• vancomycin

Community-aquired MRSA (CA-MRSA)

• ciprofloxacin

• clindamycin

• erythromycin

• gentamicin

• rifampin

• tetracycline

• trimethoprim-sulfamehoxazole

Vancomycin Resistance

• quinupristin-dalfopristin

• linezolid

• daptomycin

Prevention

• no effective vaccine

• infection control procedures are important

Staphylococcus Epidermidis

• Epidermis of skin and mucous membranes

• Commensal bacteria

• Catalase positive

• Coagulase negative

• Novobiocin sensitive

• Causes infections on top of prosthetic devices

  • e.g.: prosthetic valves, artificial joints, intravenous catheters

• Drug of choice: Vancomycin

Staphylococcus Saprophyticus

• Saprophytic

• Opportunistic pathogen

• Catalase positive

• Coagulase negative

• Novobiocin resistant

• Causes urinary tract infection in sexually active young women

• causes cystitis