Microbiology

Methicillin-resistant Staphylococcus aureus (MRSA)

Methicillin-resistant Staphylococcus aureus (MRSA)

Classification: Bacterium
Also known as: Oxacillin-resistant Staphylococcus aureus (ORSA), Methicillin-resistant S. aureus (MRSA)

Microbiology: MRSA is a Gram-positive coccus that is catalase positive and oxidase negative with a characteristic ‘bunch of grapes’ appearance under the microscope due to the activity of the coagulase enzyme. MRSA is S. aureus that has acquired the meticillin resistance gene, mecA, on a mobile genetic element called the Staphylococcal Cassette Chromosome mec (SCCmec).1


Patients found to be infected or colonised will be usually isolated either in single rooms or in cohorts of other patients who are already colonised or infected with MRSA to prevent the further spread of MRSA. Healthcare workers treating isolated patients should take special precautions including the use of gowns and gloves to prevent further spread.

MRSA is shed into the hospital environmental and can survive for extended periods.11 13 Admission to a room previously occupied by a patient with MRSA is a risk factor for MRSA acquisition.6 Thus, effective decontamination of the environment is important for the control of MRSA.9 10 Bioquell hydrogen peroxide vapour (HPV) has been shown to be more effective than conventional cleaning for the decontamination of hospital surfaces contaminated with MRSA13 14 and the use of HPV has been useful in bringing MRSA outbreaks under control.15 16

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Biology

Habitat and transmission:
S. aureus colonises the skin of approximately 1/3 of humans, commonly in the nose, throat and various skin sites. MRSA is also a human coloniser, although rates of carriage outside of hospitals are approximately 1%.2 MRSA can also be an animal coloniser.3 MRSA is usually spread by direct person to person contact, although indirect transmission via contaminated environmental surfaces also occurs.46
 
Treatment and antibiotic resistance:
MRSA infections are difficult to treat because meticillin is the best therapeutic option for S. aureus infection. To compound matters, MRSA in hospitals is usually resistant to a wide range of antibiotics in addition to meticillin and the other ß-lactam antibiotics. Vancomycin remains the mainstay of treatment for MRSA. Thus, the emergence of vancomycin-resistant S. Aureus(VRSA) is a serious concern. Fortunately, only a handful of VRSA cases have been reported.7 However, the emergence of lower level vancomycin-resistance (vancomycin-intermediate S. aureus, VISA) or a decrease in susceptibility to vancomycin in the susceptible range (‘MIC creep’) are likely to become more problematic in coming years.8

Prevention and control:
Although strategies for the prevention and control of MRSA in hospitals vary between different countries and to a lesser extent between hospitals, prevention and control is achieved through detection and isolation of colonised or infected individuals.9 10 Depending on the type of procedure being conducted, a patient group may be routinely screened on admission to the hospital to detect colonisation or have swabs taken from clinical specimens such as wound or line tips if an MRSA infection is suspected. In the UK, all hospital admissions are screened for MRSA.

Symptoms/Effects

Disease and symptoms:

 

MRSA can cause a range of diseases in humans, including skin and soft tissue infections and life-threatening invasive infections such as bacteraemia. MRSA usually only causes infections in hospitalised patients, although community associated MRSA is emerging.17

Technical

Technical

1. Deurenberg RH, Stobberingh EE. The molecular evolution of hospital- and community associated methicillin-resistant Staphylococcus aureus. Curr Mol Med 2009; 9: 100-115.
2. Grundmann H, Tami A, Hori S, Halwani M, Slack R. Nottingham Staphylococcus aureus population study: prevalence of MRSA among elderly people in the community. BMJ 2002; 324:  1365-1366.
3. Wassenberg MW, Bootsma MC, Troelstra A, Kluytmans JA, Bonten MJ. Transmissibility of livestock-associated methicillin-resistant
Staphylococcus aureus (ST398) in Dutch hospitals. Clin Microbiol Infect 2011;17: 316-319.
4. Hardy KJ, Oppenheim BA, Gossain S, Gao F, Hawkey PM. A study of the relationship between environmental contamination with methicillinresistant Staphylococcus aureus (MRSA) and patients’ acquisition of MRSA. Infect Control Hosp Epidemiol 2006; 27: 127-132.
5. Dancer SJ. The role of environmental cleaning in the control of hospital-acquired infection. J Hosp Infect 2009; 73: 378-385.
6. Huang SS, Datta R, Platt R. Risk of acquiring antibiotic-resistant bacteria from prior room occupants. Arch Intern Med 2006; 166:1945-1951.
7. Tenover FC, Weigel LM, Appelbaum PC et al. Vancomycin-resistant Staphylococcus aureus isolate from a patient in Pennsylvania.  Antimicrob Agents Chemother 2004; 48: 275-280.
8. Gould IM. Clinical relevance of increasing glycopeptide MICs against Staphylococcus aureus. Int J Antimicrob Agents 2008; 31 Suppl 2: 1-9.
9. Coia JE, Duckworth GJ, Edwards DI et al. Guidelines for the control and prevention of meticillin-resistant Staphylococcus aureus (MRSA) in healthcare facilities. J Hosp Infect 2006; 63: 1-44.
10. Muto CA, Jernigan JA, Ostrowsky BE et al. SHEA guideline for preventing nosocomial transmission of multidrug-resistant strains
of Staphylococcus aureus and enterococcus. Infect Control Hosp Epidemiol. 2003; 24: 362-386.
11. Kramer A, Schwebke I, Kampf G. How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infect Dis 2006; 6: 130.
12. Dancer SJ. Importance of the environment in meticillin-resistant Staphylococcus aureus acquisition: the case for hospital cleaning.  Lancet Infect Dis 2008; 8: 101-113.
13. French GL, Otter JA, Shannon KP, Adams NM, Watling D, Parks MJ. Tackling contamination of the hospital environment by methicillin-resistant Staphylococcus aureus (MRSA): a comparison between conventional terminal cleaning and hydrogen peroxide vapour decontamination. J Hosp Infect 2004; 57: 31-37.
14. Hardy KJ, Gossain S, Henderson N et al. Rapid recontamination with MRSA of the environment of an intensive care unit after decontamination with hydrogen peroxide vapour. J Hosp Infect 2007; 66: 360-368.
15. Jeanes A, Rao G, Osman M, Merrick P. Eradication of persistent environmental MRSA. J Hosp Infect 2005; 61: 85-86.
16. Dryden M, Parnaby R, Dailly S et al. Hydrogen peroxide vapour decontamination in the control of a polyclonal meticillin-resistant Staphylococcus aureus outbreak on a surgical ward. J Hosp Infect 2008; 68: 190-192.
17. Otter JA, French GL. Molecular epidemiology of communityassociated meticillin-resistant Staphylococcus aureus in Europe.  Lancet Infect Dis 2010; 10: 227-239.

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