Methicillin-resistant Staphylococcus aureus
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Methicillin-resistant Staphylococcus aureus (MRSA) is a specific strain of the Staphylococcus aureus bacterium that has developed antibiotic resistance to all penicillins, including methicillin and other narrow-spectrum β-lactamase-resistant penicillin antibiotics. It was first discovered in the UK in 1961 and is now widespread, particularly in the hospital setting where it is commonly termed a superbug.
MRSA may also be known as oxacillin-resistant Staphylococcus aureus (ORSA) and multiple-resistant Staphylococcus aureus.
Non-methicillin resistant strains of S. aureus are sometimes called methicillin-susceptible Staphylococcus aureus (MSSA) if an explicit distinction must be made.
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Microbiology
Methicillin resistance arises by acquisition of a staphylococcal cassette chromosome SCCmec, and is conferred by the mecA gene. Expression of this gene yields PBP2a, a penicillin binding protein with reduced affinity for β-lactam rings (the primary active-site of the β-lactam antibiotics).
Some strains of S. aureus over-express β-lactamase and appear to be resistant to oxacillin and, rarely, methicillin despite being mecA-negative. They have slightly raised minimum inhibitory concentrations (MICs) and may thus be described as "minimally resistant". Other strains express modified PBPs (not PBP2) and exhibit varying degrees of β-lactam antibiotic resistance.
Clinical presentation and concerns
S. aureus most commonly colonises the anterior nares (the nostrils) although the respiratory tract, open wounds, intravenous catheters and urinary tract are also potential sites for infection.
MRSA infections are usually asymptomatic in healthy individuals and may last from a few weeks to many years. Patients with compromised immune systems are at significantly greater risk of a symptomatic secondary infection.
Treatment and infection control initiatives
Vancomycin and teicoplanin are glycopeptide antibiotics used to treat MRSA infections. Teicoplain is a structural congener of vancomycin that has a similar activity spectrum but a longer half-life (t1/2). Both drugs have low oral absorption thus are administered intravenously for systemic infections, with the exception of pseudomembranous colitis where vancomycin can be given by mouth for this GI tract infection.
Several new strains of MRSA have been found showing antibiotic resistance even to vancomycin and teicoplanin; those new evolutions of the MRSA bacteria are dubbed "vancomycin intermediate-resistant Staphylococcus aureus" (VISA). Linezolid, quinupristin/dalfopristin, daptomycin, tigecycline are more recent additions to the therapeutic arsenal, generally reserved for severe infections which do not respond to glycopeptides. Less severe infections may be treated by oral agents including: linezolid, rifampicin+fusidic acid, pristinamycin, co-trimoxazole (trimethoprim+sulfamethoxazole), doxycycline, and clindamycin.
At the end of August 2004, after a successful pilot scheme to tackle MRSA, the English National Health Service announced its Clean Your Hands campaign. Wards will be required to ensure that alcohol-based hand rubs are placed near to all beds so that staff can hand wash more regularly. It is thought that if this cuts infection by just 1% the plan will pay for itself many times over. Health care workers in the United States are reportedly largely neglecting the simple-yet-effective practice of hand-washing, despite the Centers for Disease Control and Prevention (CDC)'s report that hand-washing alone would save the lives of roughly 30,000 patients per year, not from MRSA alone, but from all nosocomial infections.
Recently, it has been observed that MRSA can replicate inside of Acanthamoeba, increasing MRSA numbers by 1000-fold [1]. Since Acanthamoeba can form cysts easily picked up by air currents, these organisms can spread MRSA via airborne routes. Hence, control of Acanthamoeba in the clinical environment may be advisable.
Epidemiology
Because cystic fibrosis patients are often treated with multiple antibiotics in hospital settings, they are often colonised with MRSA, potentially increasing the rate of life-threatening MRSA pneumonia in this group. The risk of cross-colonisation has led to increased use of isolation protocols among these patients.
In the US there are increasing reports of outbreaks of MRSA colonisation and infection through skin contact in locker rooms and gymnasiums, even among healthy populations. MRSA causes as many as 20% of Staphylococcus aureus infections in populations that use intravenous drugs. These out-of-hospital strains of MRSA, now designated as community-acquired, methicillin-resistant staph. aureus, or CAMRSA, are not only difficult to treat but are especially virulent. CAMRSA apparently did not evolve de novo in the community, but represents a hybrid between MRSA which escaped from the hospital environment and the once easily treatable community organisms. Most of the hybrid strains also acquired a virulence factor which makes their infections invade more aggressively, resulting in deep tissue infections following minor scrapes and cuts, and many cases of fatal pneumonia as well.
As of early 2005, the number of deaths in the United Kingdom attributed to MRSA have been estimated by various sources to lie in the area of 3000 per year.
Publications (a.o. [2], September 9, 2005) in The Netherlands report of transmission of MRSA bacteria from pigs to humans. During the summer of 2005 it was discovered that three pig farmers or their families were infected by MRSA bacteria that were also found on their pigs. Researchers from Radboud University Nijmegen are now investigating how widespread the MRSA bacteria is in pigs, and whether it will become characterised among the zoonoses.de:Methicillin-resistenter Staphylococcus aureus es:Staphylococcus aureus resistente a meticilina fi:MRSA he:MRSA ja:MRSA nl:Methicilline-resistente staphylococcus aureus sv:MRSA