Methicillin-resistant Staphylococcus aureus
What is methicillin-resistant Staphylococcus aureus?
MRSA is the term used for bacteria of the Staphylococcus aureus group that are resistant to the usual antibiotics used in the treatment of infections with such organisms. Traditionally MRSA stood for methicillin (meticillin) resistance but the term increasingly refers to a multi-drug resistant group. Such bacteria often have resistance to many antibiotics traditionally used against S. aureus.
MRSA now is usually categorised into two types.
- Hospital acquired (HA) MRSA
- Community acquired (CA) MRSA
What causes MRSA?
Resistance to methicillin is due to the presence of the mec gene, situated on Staphylococcal Cassette Chromosome mec (SCCmec). This alters the site at which methicillin binds to kill the organism. Hence, methicillin is not able to effectively bind to the bacteria. CA-MRSA is similar phenotypically and genotypically to HA-MRSA. CA-MRSA harbours SCCmec type IV, V or VII. However, the distinction between the two types is becoming less over time.
Infections caused by MRSA are the same as other staphylococcal infections because the organism itself is not any more virulent (or infectious) than usual type S aureus.
Like other S aureus, MRSA can colonise the skin and body of an individual without causing sickness, and in this way it can be passed on to other individuals unknowingly. Problems arise in the treatment of overt infections with MRSA because antibiotic choice is very limited.
Where is MRSA found?
MRSA is found worldwide, predominantly in hospitals and institutions such as nursing homes, where it is referred by the name hospital-acquired MRSA (HA-MRSA). Less commonly, MRSA is found in the general community (CA-MRSA).
There are three main reservoirs (and hence sources of spread and infection) for MRSA in hospital and institutions: staff, patients and inanimate objects such as beds, linen and utensils. By far the most important reservoir is patients, who may be colonised with MRSA without evidence of infection.
The usual sites of colonisation with MRSA are:
Most health professionals that are colonised with MRSA do not develop infection and many spontaneously clear the organism without treatment. Once colonisation has been present for more than three months, it becomes much more difficult to clear.
Patients, however, have a 30–60% risk of infection following colonisation. This is probably due to factors related to the illness for which they are hospitalised, which impair their ability to clear or control colonisation with the organism.
Most MRSA infections occur in wounds (eg surgical wounds), skin (eg intravenous access sites), or in the bloodstream. Mortality from these infections is not significantly different from those seen with usual type S aureus infections.
Community acquired MRSA infections mostly occur:
- In overcrowded places, due to frequent contact of skin and to sharing things. Examples include sports clubs, nurseries, schools and military barracks
- In people with a draining cut or sore or that are carriers of MRSA
- In people who undergo tattooing or body piercing.
Risk factors for severe skin infections with MRSA include:
- Diabetes with obesity
- Prior antibiotics (within 90 days)
- Chronic kidney disease/hemodialysis
- Intravenous drug use
- Previous MRSA exposure/infection (within 12 months)
- Previous hospitalisation (12 months)
- Human immunodeficiency virus infection or AIDS.
What are the clinical features of MRSA?
Hospital acquired MRSA presents as:
- Skin infections: abscesses, furuncles and carbuncles, impetigo and cellulitis
- Wound infection: trauma and surgical sites.
CA-MRSA mainly presents with:
Complications of MRSA
If untreated, MRSA can lead to sepsis with rash, headaches, muscle aches, chills, fever, chest pain and shortness of breath, and in some cases, the death of the patient. This is more common in HA-MRSA than CA-MRSA.
How is MRSA diagnosed?
The standard method to diagnose MRSA is by culture and antibiotic sensitivity testing of Staphylococcus aureus bacteria from the infected site.
- Testing for antibiotic susceptibility guides treatment
- PCR testing may also be used for screening for MRSA
What is the treatment for MRSA?
The following steps are used for treatment of carriers of MRSA:
- Topical application of an antibiotic ointment such as mupirocin or fusidic acid to the nostrils, 2–3 times per day for 3–5 days.
- Antibacterial soaps and hand rubs.
Treatment of active infection involves drainage of pus from furuncles and abscesses, and antibiotics.
- The antibiotic of choice for an infected inpatient is vancomycin given intravenously.
- Daptomycin is an alternative IV antibiotic.
- Oral clindamycin may be used in minor soft tissue infections in outpatients.
These antibiotics are no better than flucloxacillin in the treatment of usual type S aureus, but are much more effective in MRSA infections.
Other antibiotics are less effective and are used if there is resistance to vancomycin/clindamycin or in case of adverse reactions to these drugs.
In life-threatening infections such as infective endocarditis, multiple antibiotics are often prescribed simultaneously (eg vancomycin plus an aminoglycoside plus rifampicin).
How can MRSA be prevented?
In hospitals, patients who have been transferred from another hospital or institution should have swabs taken on admission to screen for MRSA colonisation or infection.
Common sites for swab collection are nostrils, armpits, groins, genital region and any areas of broken skin (eg surgical wounds, ulcers, sores).
New or transferring hospital staff are also screened. The results of swabs take a few days to be reported.
If an inpatient is found to have MRSA colonisation or infection:
- He or she should be isolated from unnecessary contact with staff and other patients in a single room, or share a room with other patients who have MRSA.
- Linen and clothing should be carefully sterilised.
- Barrier precautions should be taken by staff and visitors (gloves and gowns).
The above precautions should be strictly enforced until repeat swabs from the patient are negative for MRSA. This may take some weeks. Staff found to be colonised with MRSA should be removed from patient contact.
The following basic hygiene practice can help lower the incidence of CA-MRSA:
- Wash hands with antibacterial soap or hand rub/sanitiser
- Avoid sharing personal items
- Cover all wound sites.
Concerns about MRSA in the future
There is growing concern about MRSA infections. They appear to be increasing in frequency and displaying resistance to a wider range of antibiotics.
Of particular concern are the VISA strains of MRSA (vancomycin intermediate susceptibility S aureus). These are beginning to develop resistance to vancomycin, which is currently the most effective antibiotic against MRSA. This new resistance has arisen because another species of bacteria, called enterococci, relatively commonly express vancomycin resistance. In the laboratory, enterococci are capable of transferring the gene for vancomycin resistance to S aureus.
Newer antibiotics such as linezolid and synercid look promising for treatment of infections not responding to vancomycin. Many newer drugs including glycopeptides (dalbavancin, oritavancin and telavancin), anti-MRSA beta lactams (ceftobiprole) and diaminopyrimidines (iclaprim) are being tested for use against MSRA.