Methicillin-resistant Staphylococcus Aureus
The emergence of methicillin-resistant S aureus (MRSA) was discovered shortly after methicillin was introduced in 1959. Containing and preventing outbreaks has become a huge problem in many hospitals (most notably in the UK); however, it is extremely difficult to control and resistance increases with the increasing use of antibiotics. For example, although vancomycin was once the drug of choice for treating MRSA, vancomycin-resistant strains have emerged. Administering mupirocin to the anterior nares of carriers of the bacteria has been thought to help control the spread of MRSA. Results, however, have been only marginally effective and now there are emergent strains of mupirocin-resistant MRSA. Although MRSA was initially primarily a nosocomial problem, there are increasing reports of outbreaks of community-associated MRSA (CA-MRSA).
Community-associated MRSA (CA-MRSA)
CA-MRSA frequently has different molecular and antimicrobial susceptibility characteristics than healthcare-associated MRSA. The groups predominantly affected by CA-MRSA include children in day care centers, Native American communities, athletic teams, military personnel, men who have sex with men (in Los Angeles and San Francisco), prison inmates and most recently, people in tattooing settings. Infection with CA-MRSA is frequently recurring in individuals and can spread within families.
Patients with CA-MRSA usually present with skin and soft tissue infections but may have developed necrotizing pneumonia, necrotizing fasciitis, or fatal septicemia, endocarditis or osteomyelitis. Adults may also present with otitis media/externa, toxic shock syndrome or bacteremia.
The Centers for Disease Control and Prevention recommends the following to prevent MRSA infections:
- Keep hands clean by washing thoroughly with soap and water or using an alcohol-based hand sanitizer
- Keep cuts and scrapes clean and covered with a bandage until healed
- Avoid contact with other people's wounds or bandages
- Avoid sharing personal items, such as towels, washcloths, razors, clothing or uniforms
Healthcare-associated MRSA (HA-MRSA)
The United States has a high rate of HA-MRSA both within hospitals and long-term care facilities. It is most frequently spread by health care workers, whose hands or gloves carry bacteria. Risk factors for HA-MRSA include prolonged hospitalization (greater than 14 days), antimicrobial therapy (primarily with cephalosporins and fluoroquinolones), being in an ICU or burn unit, hemodialysis, having a surgical site infection, and being in close proximity to a patient colonized or infected with MRSA. HA-MRSA is considered a source of CA-MRSA. Body sites most often affected by HA-MRSA include wound sites, skin and the bloodstream. Patients may also develop lower respiratory infections, or infections in the urinary tract.
The Society for Healthcare Epidemiology of America recommends the following for the prevention of HA-MRSA outbreaks:
- Implement a program of active surveillance cultures to identify patients colonized or infected with MRSA. Surveillance cultures of the anterior nares and open wounds are recommended for patients at high risk of MRSA colonization or infection
- Wear clean, non-sterile gloves when entering the patient's room; remove the gloves when leaving the patient's room
- Wear a gown when entering the room if substantial contact with the patient or environmental surfaces in the room is anticipated, or if the patient has wound drainage not contained by a dressing. Remove the gown before leaving the patient's room
- Upon removing gloves and gown, clean hands with an alcohol-based hand rub. However, if hands are visibly contaminated with blood or other proteinaceous materials, wash hands with an antimicrobial soap and water
Mupirocin-resistant MRSA
Mupirocin is frequently used to treat a variety of skin infections, including secondarily infected traumatic lesions, and is comparable to the use of systemic antibiotics in the case of some dermatoses, such as impetigo. Instances of mupirocin-resistant strains of MRSA are increasing. Therefore it is recommended that patients showing no response to mupirocin within 3 to 5 days be treated with a systemic antibiotic.
Treatment Strategies
Emerging resistance in MRSA strains makes infections extremely difficult to treat. Treatment options are limited to the groups of new quinolones, oxazolidinones, quinupristin—dalfopristin, various combinations of antibiotics, and new investigational compounds. Unfortunately, none of these agents has been clinically tested on a sufficient scale. Methicillin-susceptible S aureus (MSSA) is typically treated with a β-Lactam antibiotic, such as flucloxacillin or cloxacillin; a recent report found that a delay in switching from a β-Lactam antibiotic to appropriate MRSA therapy does not affect patient outcomes. For skin and soft tissue infections, the following guidelines were published in the Journal for Antimicrobial Chemotherapy:
- Tetracyclines should be more widely used in adults for treatment unless there is a risk of bacteremia or endocarditis
- If risk of bacteremia or endocarditis is high, glycopeptides or linezolids should be used
- In infections that have failed treatment with single active agents, combined use of rifampicin and fusidic acid, or glycopeptides and fusidic acid, or glycopeptides and rifampicin (where antibiotics remain active in vitro) is warranted
- In MRSA susceptible to erythromycin, clindamycin should be considered
- In severe IV infections, IV glycopeptide or linezolid should be used
