Methicillin-resistant Staphylococcus aureus
MRSA stands for Methicillin-resistant Staphylococcus aureus. Methicillin resistance equates with flucloxacillin resistance. As with Methicillin-sensitive staphylococcus aureus (MSSA), different strains exist, carrying different genes.
Traditionally MRSA was found in institutions and the elderly, but now can be seen frequently in the young and healthy, causing the same infections that MSSA causes eg skin/soft tissue. It can also be responsible for rarer, more severe diseases eg necrotizing fasciitis. The US Center for Disease Control details criteria for distinguishing hospital acquired and community acquired MRSA infections - community acquired strains are typically SCCmec type IV, which are sensitive to most non-beta lactam antibiotics, but on the other hand is associated with Panton Valentine Leucocidin (PVL, a cytotoxin associated with necrotizing disease). But again, this distinction is becoming less clear with strains associated with community acquired infection becoming more frequent in hospital acquired cases, and having variable levels of non-beta lactam antibiotic resistance. 
In itself, antibiotic resistance may not translate to increased virulence and pathogenicity - it may just make it harder to treat. Studies have shown that after correcting for other factors eg age and co-morbidity, mortality is not significantly different. However, one important factor is use of inappropriate antibiotics, which of course is more likely with MRSA. Furthermore, in the US many MRSA outbreaks are caused by the USA300 clone, which carries a number of genes (in common with Methicillin sensitive staphylococcus aureus) eg PVL, ACME which are associated with enhanced pathogenicity.
- MRSA has been shown to survive on sterile packaging for at least 6 months 
- Basic simple infection control like hand washing works.
- MRSA prevalence in hospitals is associated with macrolide and 3rd generation cephalosporin use.
- Alcohol hand rub reduces its transmission and hospitals which have introduced a policy of using this between patient contacts reduce their MRSA rate.
- Isolation and screening work but may be impracticable in emergency admissions or in hospitals with near 100% occupancy.
- Hospitals which have limited the use of quinolones such as ciprofloxacin can reduce their MRSA rate.
- Spread of the EMRSA-16 (ST36-II) strain in the UK over many decades has been able to be mapped from tertiary centres to secondary hospitals implicating patient referrals as an important cause of nationwide transmission
Community Acquired MRSA
- Total community burden of MRSA is greater than hospital burden
- Most community acquired MRSA is after hospital contact
- Much MRSA is reintroduced into hospitals from the community
- Care home residents have a higher community risk
- UK carriage rates approaching 5% in trauma admissionsand 17% in care homes
- Association with hospital admission in last 12 months
- Association with multibed rooms and room mate with MRSA
- Association with physical dependency
- Association with urinary catheter being present
TreatmentCommunity acquired MRSA tends to have a narrower resistance profile than hospital acquired MRSA, because survival in the community, where there is less selection pressure from antibiotic use, depends on a small, "low maintenance" that is not disadvantageous to survival compared with MSSA.
Clindamycin, Co-trimoxazole and Rifampicin are effective against many community acquired strains, and can all be given orally. Doxycycline is effective in skin infections! Linezolid has an oral as well as an IV form. For intravenous therapy, vancomycin and teicoplanin are effective, although vancomycin resistance has been described since 2002.
Eradication of nasal colonization eg with 72 hour mupirocin works, but recolonization usually occurs within a relatively short time, and the Cochrane review did not find much evidence in favour. Use of mupirocin to prevent infection in endemic settings eg dialysis centres has shown conflicting results although metanalysis suggests benefit. Emergence of mupirocin resistance is a major concern. Combined treatment is effective. A recent RCT of 2% chlorhexidine gluconate washes, 2% mupirocin ointment intranasally, oral rifampin and doxycycline for 7 days vs no treatment cleared carriage in 74% cf 32% at 3 months of follow-up, with significant benefit persisting at 8 months (54% of those treated remained culture negative). On multivariable analysis, having a mupirocin-resistant isolate was associated with treatment failure (relative risk= 9.4). Mupirocin resistance emerged in only 5% of follow-up isolates.  Other control measures include a combination of active surveillance cultures of high risk patients, improved health care worker hand hygiene, consistent use of contact precautions for colonized/infected patients, and directed treatment of health care workers implicated in transmission.  
- ↑ Deurenberg RH, Vink C, Kalenic S, Friedrich AW, Bruggeman CA, Stobberingh EE. The molecular evolution of methicillin-resistant Staphylococcus aureus. Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases 2007;13(3):222-35. (Direct link – subscription may be required.)
- ↑ Dietze B, Rath A, Wendt C, Martiny H. Survival of MRSA on sterile goods packaging. The Journal of hospital infection 2001;49(4):255-61. (Direct link – subscription may be required.)
- ↑ MacKenzie FM, Bruce J, Struelens MJ, Goossens H, Mollison J, Gould IM, et al. Antimicrobial drug use and infection control practices associated with the prevalence of methicillin-resistant Staphylococcus aureus in European hospitals. Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases 2007;13(3):269-76. (Direct link – subscription may be required.)
- ↑ Cook PP, Catrou P, Gooch M, Holbert D. Effect of reduction in ciprofloxacin use on prevalence of meticillin-resistant Staphylococcus aureus rates within individual units of a tertiary care hospital. The Journal of hospital infection 2006;64(4):348-51. (Direct link – subscription may be required.)
- ↑ McAdam PR, Templeton KE, Edwards GF, Holden MT, Feil EJ, Aanensen DM, Bargawi HJ, Spratt BG, Bentley SD, Parkhill J, Enright MC, Holmes A, Girvan EK, Godfrey PA, Feldgarden M, Kearns AM, Rambaut A, Robinson DA, Fitzgerald JR. Molecular tracing of the emergence, adaptation, and transmission of hospital-associated methicillin-resistant Staphylococcus aureus. Proceedings of the National Academy of Sciences of the United States of America. 2012 May 14.(Epub ahead of print) (Link to article – subscription may be required.)
- ↑ a b c d e Suetens C, Niclaes L, Jans B, Verhaegen J, Schuermans A, Van Eldere J, et al. Determinants of methicillin-resistant Staphylococcus aureus carriage in nursing homes. Age and ageing 2007;36(3):327-30. (Direct link – subscription may be required.)
- ↑ Nixon M, Jackson B, Varghese P, Jenkins D, Taylor G. Methicillin-resistant Staphylococcus aureus on orthopaedic wards: incidence, spread, mortality, cost and control. The Journal of bone and joint surgery. British volume 2006;88(6):812-7. (Direct link – subscription may be required.)
- ↑ Fraise AP, Mitchell K, O'Brien SJ, Oldfield K, Wise R. Methicillin-resistant Staphylococcus aureus (MRSA) in nursing homes in a major UK city: an anonymized point prevalence survey. Epidemiology and infection 1997;118(1):1-5.
- ↑ Simor AE, Phillips E, McGeer A, Konvalinka A, Loeb M, Devlin HR, et al. Randomized controlled trial of chlorhexidine gluconate for washing, intranasal mupirocin, and rifampin and doxycycline versus no treatment for the eradication of methicillin-resistant Staphylococcus aureus colonization. Clinical infectious diseases : an official publication of the Infectious Diseases Society of America 2007;44(2):178-85. (Direct link – subscription may be required.)
- ↑ Chen SF. Staphylococcus aureus decolonization. The Pediatric infectious disease journal 2005;24(1):79-80.
- ↑ Harbarth S. Control of endemic methicillin-resistant Staphylococcus aureus-recent advances and future challenges. Clinical microbiology and infection : the official publication of the European Society of Clinical Microbiology and Infectious Diseases 2006;12(12):1154-62. (Direct link – subscription may be required.)