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Updated: Dec 3 2020

Staphylococcus aureus

  • Snapshot
    • A 23-year-old woman presents to the emergency department with fever, chills, and watery diarrhea. Her symptoms began approximately 1 day after menstruation began. Her temperature is 102°F (38.9°C), blood pressure is 75/50 mmHg, pulse is 125/min, and respirations are 20/min. Physical examination is notable for a diffuse erythematous rash and desquamation of the palms and soles. 
  • Introduction
    • Classification
      • gram-positive cocci in clusters
    • Microbiology
      • properties
        • facultative anaerobes
        • catalase positive
          • neutralizes its own hydrogen peroxide, which results in the hosts phagocytes inability to produce reactive oxygen species to combat infection (especially in patient's with chronic granulomatous disease )
        • coagulase positive
          • coagulase activates prothrombin, leading to clotting
            • leads to fibrin formation around this organism
          • differentiates S. aureus from S. epidermidis and S. saprophytic
        • β-hemolytic
          • complete hemolysis of red blood cells on an agar plate
        • appears gold in sheep blood agar
        • protein A
          • binds to the Fc region of the IgG antibody
            • this prevents opsonization and phagocytosis
        • hemolysins
          • damages red blood cells, platelets, neutrophils, and macrophages
        • leukocidins
          • damages white blood cells
        • penicillinase
          • a secreted form of β-lactamase, which makes S. aureus resistant to penicillins
        • toxins
          • exfoliative toxin
            • causes skin sloughing, as seen in staphylococcal scalded skin syndrome
          • heat-stable enterotoxin
            • leads to food poisoning
          • toxic shock syndrome toxin (TSST)
            • leads to staphylococcal toxic shock syndrome
  • Diseases
      • S. aureus Associated Diseases
      • Disease
      • Clinical Presentation
      • Comments
      • Toxic shock syndrome
      • Symptoms
        • nausea and vomiting
        • watery diarrhea
      • Physical exam
        • fever
        • diffuse erythematous rash
        • palm and sole desquamation
        • hypotension
      • Can be caused by leaving tampons in place for a long period of time
        • stimulates toxic shock syndrome toxin-1 (TSST-1) release, penetrating the vaginal mucosa, and cross-linking the β region of the T-cell receptor to MHC class II
          • leads to an overwhelming release ofIL-1 and IL-2IFN-γTNF-α
      • Treatment
        • source control
          • removing the tampon or surgical suture that enabled the production of TSST-1
        • antibiotics
          • the choice depends on drug sensitivity testing
          • will not cure the disease but may help as it can eliminate TSST-1 producing S. aureus
      • Staphylococcal skin syndrome
      • Physical exam
        • fever
        • erythematous rash with skin sloughing
      • Exfoliative toxin destroys keratinocyte attachments to the stratum granulosum
      • Typically seen in newborns, children, and adults with renal insufficiency
      • Gastroenteritis
      • Symptoms
        • nausea
        • vomiting
        • abdominal pain
      • Physical exam
        • watery diarrhea
      • Staphylococci can produce exotoxin as it grows in food
        • ingested pre-formed toxin causes intestinal peristalsis, resulting in
          • nausea
          • vomiting
          • abdominal pain
          • watery diarrhea
      • Pneumonia
      • Physical exam
        • fevers
        • chills
      • Typically seen as a superinfection after an influenza upper respiratory infection
        • results in a lobar consolidation and lung parenchymal cavitations
      • Osteomyelitis
      • Physical exam
        • fever
        • warm and swollen tissue over bone
      • S. aureus spreads to the bone hematogenously
      • Most common cause of osteomyelitis overall 
      • Acute endocarditis 
      • Symptoms
        • chills
        • myalgias
      • Physical exam
        • fever
      • There is rapid vegetation growth on the heart valve, which can cause
        • valvular destruction
        • embolism to the brain leading to stroke with left-heart valvular involvement
        • embolism to the lung
          • with right-heart valvular involvement
            • more common in intravenous drug users
      • Septic arthritis
      • Symptom
        • joint pain
      • Physical exam
        • inflamed joint with decreased range of motion
      • Caused by S. aureus invasion into the synovial membrane
      • Joint aspiration can demonstrate
        • yellow and turbid synovial fluid
        • prominent amount of neutrophils (> 100,000 CFU/mL)
        • positive Gram stain (gram-positive cocci in clusters)
      • Skin infections
      • Impetigo 
        • physical exam
          • small vesicles or pustules that crust over into honey-colored lesions
            • typically appear in the face and especially around the mouth
      • Cellulitis
        • physical exam
          • fever
          • the affected area is erythematous, warm, and tender to palpation
      • Other skin infections include
        • local abscess
          • pus collection
        • furuncles
          • infection of a hair follicle
        • carbuncles
          • a cluster of furuncles
      • Some skin infections can be caused by Streptococcus pyogenes or S. aureus; therefore, these skin infections should be treated with penicillinase-resistant penicillins (e.g., dicloxacillin)
    • Staphylococci can produce exotoxin as it grows in food
      • ingested pre-formed toxin causes intestinal peristalsis, resulting in
        • nausea
        • vomiting
        • abdominal pain
        • watery diarrhea
  • Methicillin-Resistant S. aureus (MRSA)
    • Introduction
      • most staphylococci are penicillin resistant due to their penicillinase
        • to combat this, a number of penicillinase-resistant penicillins were developed (e.g., methicillin and nafcillin)
      • MRSA is a strain of S. aureus that has acquired resistance against penicillinase-resistant penicillins (thus being methicillin-resistant)
        • due to altered penicillin-binding protein
    • Epidemiology
      • incidence
        • typically appears in the hospital setting
    • Microbiology
      • transmission
        • via health care workers
    • Treatment
      • medical
        • vancomycin
          • indication
            • the drug of choice for MRSA
        • linezolid
          • indication
            • can be used to treat vancomycin-resistant S. aureus (VRSA)
  • Treatment
    • Medical
      • penicillinase-resistant penicillins
        • indication
          • drug of choice for organisms sensitive to these drugs
        • medications
          • nafcillin
          • oxacillin
          • dicloxacillin
      • vancomycin
        • indication
          • MRSA
      • linezolid
        • indication
          • VRSA
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