Staphylococcus aureus, MRSA, MSSA, Bacterial strain, organism, antimicrobial therapy,
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Staphylococcus aureus

Background:

>Staphylococcus aureus
     1]  MSSA (methicillin susceptible strains)
     2]  MRSA (methicillin resistant Staph aureus)

Staphylococcus aureus:
  • (facultative anaerobic Gram-positive coccal bacterium;  catalase-positive)
  • -Usually found in normal skin flora, anterior nares of the nasal passages, and respiratory tract.
  • -Frequent contributor to skin and respiratory infections.
  • -Antibiotic-resistant forms represent a growing problem (MRSA).
  • -Most common staphylococcal species to cause Staph infections.
  • -Presence of S. aureus does not always indicate infection.

 

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Methicillin-resistant Staphylococcus aureus (MRSA):
  • MRSA: any strain of S. aureus that has developed, through the process of natural selection, resistance to beta-lactam antibiotics, which include the penicillins (methicillin, dicloxacillin, nafcillin, oxacillin, etc.) and the cephalosporins.  Note:  Not more intrinsically virulent than other strains of Staphylococcus aureus, just more difficult to treat.
  • MSSA:  strains that are still sensitive to standard antibiotics.
  • S. aureus most commonly colonizes the anterior nares (the nostrils). The rest of the respiratory tract, open wounds, intravenous catheters, and the urinary tract are also potential sites for infection.
  • MRSA is especially troublesome in hospitals, prisons and nursing homes, where patients with open wounds, invasive devices, and weakened immune systems are at greater risk of infection than the general public.
  • The initial presentation of MRSA is small red bumps that resemble pimples, spider bites, or boils; they may be accompanied by fever and, occasionally, rashes. Within a few days, the bumps become larger and more painful; they eventually open into deep, pus-filled boils.
  • Other abbreviations:  CA-MRSA:  community-associated MRSA.  HA-MRSA: hospital acquired.
  • People are very commonly colonized with CA-MRSA and are completely asymptomatic. The most common manifestations of CA-MRSA are simple skin infections, such as impetigo, boils, abscesses, folliculitis, and cellulitis.
  • Individuals at greatest risk:
    • People with weak immune systems (HIV/AIDS, lupus, or cancer sufferers; transplant recipients, severe asthmatics, etc.)
    • Diabetics
    • Intravenous drug users
    • Users of quinolone antibiotics
    • Young children and the elderly
    • College students living in dormitories
    • People staying or working in a health care facility for an extended period of time
    • People who spend time in confined spaces with other people, including occupants of homeless shelters and warming centers, prison inmates, military recruits in basic training, etc.
    • Veterinarians, livestock handlers, and pet owners.
  • Healthcare provider-to-patient transfer is common, especially when healthcare providers move from patient to patient without performing necessary hand-washing techniques between patients.
  • Restricting antibiotic use:   Glycopeptides, cephalosporins and in particular quinolones are associated with an increased risk of colonisation of MRSA. Reducing use of antibiotic classes that promote MRSA colonisation, especially fluoroquinolones, is recommended in current guidelines.
  • Source: http://en.wikipedia.org/wiki/Methicillin-resistant_Staphylococcus_aureus

 


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Therapy:

Important considerations:  The choice of an agent should be based on local antimicrobial sensitivities, site of infection, cost, and comorbid conditions.   Generally, the most common agents/regimens are listed first.  Listed dosages may need to be adjusted for renal dysfunction.

Staphylococcus aureus Methicillin susceptible (MSSA):
  1. Dicloxacillin 500mg orally four times daily
  2. Clindamycin  600mg IV every 6 or 8 hours or 300mg orally tid or qid
  3. Bactrim DS (TMP-SMX)  po bid
  4. Cephalexin 500 mg po qid
  5. Nafcillin 1-2 grams IVPB q4-6 hours
  6. Cefazolin 1 to 2 g IV every eight hours
  7. Vancomycin 1 gram  ivpb q12h (patient-specific dosing required) 


Staphylococcus aureus Methicillin-resistant (MRSA):
  1. LOW-MIC, high susceptibility: clindamycin, Bactrim (TMP-SMX),  or doxycycline
  2. Vancomycin - (patient-specific regimen - trough goal 15-20 mcg/ml)
  3. Linezolid 600 mg orally or IV q12h
  4. Daptomycin 6 mg/kg IV once daily.    - NOT FOR PNEUMONIA
  5. Ceftaroline  600mg IV q12h
  6. Quinupristin-Dalfopristin 7.5 mg/kg IV q8h
  7. Telavancin 10 mg/kg IV every 24hours




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