Staphylococcus aureus

https://upload.medbullets.com/topic/104183/images/250px-staphylococcus_aureus_gram.jpg — **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.· (Toxic shock syndrome)

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	Symptomsnausea and vomitingwatery diarrheaPhysical examfeverdiffuse erythematous rashpalm and sole desquamationhypotension	Can be caused by leaving tampons in place for a long period of timestimulates 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 IIleads to an overwhelming release ofIL-1 and IL-2IFN-γTNF-αTreatmentsource controlremoving the tampon or surgical suture that enabled the production of TSST-1antibioticsthe choice depends on drug sensitivity testingwill not cure the disease but may help as it can eliminate TSST-1 producing S. aureus
Staphylococcal skin syndrome	Physical examfevererythematous rash with skin sloughing	Exfoliative toxin destroys keratinocyte attachments to the stratum granulosumTypically seen in newborns, children, and adults with renal insufficiency
Gastroenteritis	Symptomsnauseavomitingabdominal painPhysical examwatery diarrhea	Staphylococci can produce exotoxin as it grows in foodingested pre-formed toxin causes intestinal peristalsis, resulting innauseavomitingabdominal painwatery diarrhea
Pneumonia	Physical examfeverschills	Typically seen as a superinfection after an influenza upper respiratory infectionresults in a lobar consolidation and lung parenchymal cavitations
Osteomyelitis	Physical examfeverwarm and swollen tissue over bone	S. aureus spreads to the bone hematogenouslyMost common cause of osteomyelitis overall
Acute endocarditis	SymptomschillsmyalgiasPhysical examfever	There is rapid vegetation growth on the heart valve, which can causevalvular destructionembolism to the brain leading to stroke with left-heart valvular involvementembolism to the lungwith right-heart valvular involvementmore common in intravenous drug users
Septic arthritis	Symptomjoint painPhysical examinflamed joint with decreased range of motion	Caused by S. aureus invasion into the synovial membraneJoint aspiration can demonstrateyellow and turbid synovial fluidprominent amount of neutrophils (> 100,000 CFU/mL)positive Gram stain (gram-positive cocci in clusters)
Skin infections	Impetigo physical examsmall vesicles or pustules that crust over into honey-colored lesionstypically appear in the face and especially around the mouthCellulitisphysical examfeverthe affected area is erythematous, warm, and tender to palpationOther skin infections includelocal abscesspus collectionfurunclesinfection of a hair folliclecarbunclesa 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

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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