What are MRSA & CA-MRSA staph infections?


Methicillin-Resistant Staphylococcus aureus (MRSA) Infections

First seen in the early 1960s, MRSA differed from other strains of Staphylococcus aureus (S. aureus) because they had the genetic capacity to grow even when treated with certain antibiotics, such methicillin and cephalosporins. There are two main categories into which the different strains of MRSA are classified: health care-associated and community-associated.


  • HA-MRSA infections have greater antibiotic resistance than their community-associated counterparts. These infections are found in individuals who have recently been exposed to bacteria in a health care setting (including long-term care facilities). HA-MRSA, like other S. aureus strains, is thought to colonize primarily the external part of the nostrils.
  • Risk factors for HA-MRSA infections stem from the bacteria entering the skin or mucous membrane barrier through injuries, such as cuts, abrasions or burns. Hospital-based procedures, such as a surgical procedures, or insertion of an indwelling catheter or a breathing tube, put patients at higher risk. HA-MRSA is spread by contaminated hands, but it can also be transmitted through contact with contaminated surfaces.
  • HA-MRSA infections are associated with severe, invasive disease in hospitalized patients. Infections stemming from catheters can lead to bacteremia (bacterial invasion of the bloodstream), surgical procedures can cause surgical site infections, and patients who have been mechanically ventilated can develop ventilator-associated pneumonia.
  • IV-administered antibiotic therapy may be considered in cases of HA-MRSA, especially in patients with extensive soft tissue involvement, signs of systemic illness, diabetes, or a weakened immune system. Vancomycin is the most widely tested and used antibiotic to treat HA-MRSA, but linezolid and daptomycin are newer antibiotics that have been approved for MRSA treatment in the last decade. Both of these newer antibiotics have broad-spectrum activity (inhibits the growth of or kills a wide range of bacteria), and although they are expensive, some studies suggest that their use leads to shorter hospital stays and are therefore more cost-effective.


  • CA-MRSA infections typically begin in the community setting and are more responsive to antibiotic therapy. Infected individuals are less likely to have the risk factors for the hospital-associated variety. CA-MRSA colonization may be more common at sites other than the nose, including the underarms, groin, and rectum.
  • Risk factors for CA-MRSA include recent antibiotic use (particularly of cephalosporins and fluoroquinolones), men who have sex with men, injection drug use, close person-to-person contact, and sharing of a crowded athletic space or personal items, like towels.
  • CA-MRSA differs from HA-MRSA in that it releases Panton-Valentine leukocid (PVL), a toxic substance that destroys living cells. This toxin is commonly associated with soft tissue infections, such as cellulitis, folliculitis, and abscess formation. Cases of necrotizing pneumonia, which causes the death of lung tissue, have also been linked to PVL and CA-MRSA.
  • Antibiotic therapy is not necessary to treat the majority of skin abscesses – in many cases, incision and drainage is sufficient. However, antibiotic therapy should be considered if the patient:
    • shows signs and symptoms of systemic illness.
    • is diabetic or has a weakened immune system.
    • has an abscess that is surrounded by large cellulitis or in a location where total draining is difficult.
  • In general, there is not an increased need for antibiotics for CA-MRSA when compared with methicillin-sensitive staph infections.
  • Antibiotics that can be used in CA-MRSA include:
    • Vancomycin – antibiotic of choice
    • TMP-SMX (trimethoprim-sulfamethoxazole) – not indicated for use against S. aureus infections, but it is effective against most CA-MRSA strains
    • Clindamycin – higher rates of resistance in CA-MRSA
    • Tetracyclines (doxycycline or minocycline) – tetracyclines can cause photosensitivity (sensitivity of the skin to sunlight)
    • Quinolones – most CA-MRSA are resistant to quinolones


  • S. aureus has been estimated to colonize about one-third of the general population. Although it is considered to be a part of the normal human flora (the community of microorganisms that live on the skin), S. aureus colonization is known to raise the risk of infection after medical and surgical procedures. In addition to the nostrils, MRSA can colonize the armpits, hands, pharynx, and anogenital region. Research has found that 4% of health care workers are nasal carriers of MRSA.
  • There appears to be limited agreement by experts around decolonization (methods aimed at eliminating the germ from the skin and nose in the hope of preventing future infections), but many studies suggest that such an approach is of little benefit in controlling the spread of MRSA and it does not necessarily provide sustained elimination of MRSA carriage. Although strategies vary, decolonization of the nostrils is required to avoid recolonization. Treatment options include:
    • Mupirocin ointment (2%) twice daily for 7 days to eradicate nasal colonization. This should be used only if nasal cultures are positive for MRSA, because some evidence suggests that colonization with methicillin-sensitive strains of staph in the nose is protective against MRSA colonization.
    • Chlorhexidine washes (4%) to skin and hair for 7 days
    • Chlorhexidine spray or gargle (0-2%) to eliminate carriage in the pharynx