MRSa
MRSa
Disease History, Characteristics, and Transmission
Introduction
MRSa is an acronym for methicillin-resistant Staphylococcus aureus, which is a particular type (strain) of S. aureus. The bacterium is important because of its antibiotic resistance and because it can cause a number of severe diseases. One such disease is necrotizing fasciitis, more popularily known as “flesh-eating disease.” MRSa is also known as oxacillin-resistant S. aureus (oxacillin is another antibiotic) and multiple-resistant S. aureus.
Until the beginning of this century, MRSa was almost exclusively found in hospitals, because the tremendous antibiotic use in hospitals provided a powerful selection pressure, that is, an environment where only the most resiliant bacteria could survive. In 2007, the prevalence of MRSa in environments outside of the hospital is increasing. This form of the bacterium (whether it is different from the hospital form of MRSa is not known) has been designated as community associated-MRSa or CA-MRSa.
Disease History, Characteristics, and Transmission
MRSa is resistant to methicillin, a synthetic penicillin antibiotic. It is also resistant to all of the penicillin class of antibiotics. This wide range of resistance makes the bacterium hard to treat, since commonly used antibiotics are will not kill it.
MRSa has been evident almost as long as methicillin has been in use. Methicillin was introduced in 1959 to treat strains of S. aureus that had developed resistance to penicillin. By chemically altering the structure of penicillin, scientists were able to produce methicillin, and the penicillin-resistant S. aureus were killed by the newly synthesized antibiotic. But this beneficial effect did not last long. By 1961, MRSa were making a comeback, despite the use of methicillin in the United Kingdom (UK). Soon, reports of MRSa came from other countries in Europe, Japan, Australia, and North America. By 2005, thousands of hospital deaths in the UK were caused by MRSa, and the organism accounted for almost 50% of all hospital-acquired infections.
Methicillin resistance is caused by the presence of a gene (a section of genetic material that codes for the production of a protein or other compound) that codes for a protein that binds to the antibiotic and prevents the antibiotic from entering the bacteria. The S. aureus that is susceptible to methicillin does not have this gene. It is the transfer of this gene from one bacterium to another that has spread the resistance through populations of Staphylococcus around the globe.
The spread of MRSa has been aided by the fact that S. aureus is normally found in the environment. The bacterium is present in soil and in our bodies. Studies of the bacteria present in certain areas of the body have revealed that approximately 30% of healthy adults harbor S. aureus, including MRSa, on the surface of their skin or in other places, like their noses. In these environments, the bacterium is harmless. But, if MRSa gets into a wound and/or if a person's immune system is not functioning efficiently, illness can result.
MRSa is sometimes capable of causing necrotizing fasciitis, an extremely invasive disease that progresses rapidly. Sometimes amputation of the infected limb is the only way to save the patient's life. MRSa can also carry genes that code for the production of potent toxins. If these toxins get into the bloodstream, the resulting effects can be devastating to the body.
WORDS TO KNOW
COLONIZATION: Colonization is the process of occupation and increase in number of microorganisms at a specific site.
PATHOGEN: A disease causing agent, such as a bacteria, virus, fungus, etc.
RESISTANCE: Immunity developed within a species (especially bacteria) via evolution to an anti-biotic or other drug. For example, in bacteria, the acquisition of genetic mutations that render the bacteria invulnerable to the action of antibiotics.
SELECTION PRESSURE: Selection pressure refers to factors that influence the evolution of an organism. An example is the overuse of anti-biotics, which provides a selection pressure for the development of antibiotic resistance in bacteria.
Scope and Distribution
Since S. aureus has a worldwide distribution, it is not surprising that MRSa has a similar distribution. In the past, MRSa was usually found in hospitals and athletic facilities, since both are places where abrasions, cuts, and scrapes occur. In 2007, however, MRSa is becoming increasingly prevalent in the community, which raises the possibility that certain illnesses, such as necrotizing fasciitis, may become more common.
It is estimated that over 50 million people around the globe carry MRSa in their bodies. In the United States, about 32% of people are colonized with S. aureus in their noses. Colonization refers to bacteria (or other pathogens) that establish a presence on a tissue. Fewer than one percent of otherwise healthy individuals colonized with MRSa will develop a MRSa-related disease.
Having another infection can increase the likelihood of developing a MRSa infection. For example, individuals with cystic fibrosis often have recurring lung infections that require treatment with a number of different antibiotics. This situation increases the risk that MRSa will be able to gain a foothold in these patients.
Ominously, in February of 2007, an article appeared in Clinical & Infectious Disease detailing the person-toperson spread of CA-MRSa via sexual contact. This is the first time this route of transmission has been reported for MRSa.
Treatment and Prevention
Treating MRSa is challenging. Only a few antibiotics remain effective against the bacterium. One of these is vancomycin. It has the disadvantage of not being absorbed easily into the body; it cannot be given by mouth because there will be little active compound left by the time the antibiotic circulates through the bloodstream to the site of the infection. Rather, vancomycin must be given intravenously—via a needle inserted into a vein. This usually means that a person being treated must be hospitalized.
The search continues for new antibiotics that will be effective against MRSa. This research is literally a race against time. The development of new antibiotics must at least keep pace with the evolution of resistance by MRSa.
Another potential treatment option is called phage therapy. Phage is short for bacteriophage, which is a virus that specifically infects and forms new phage particles inside of a bacterium. The phage-bacterium association is specific—a certain type of phage infects a certain type of bacterium. In doing so, the phage ultimately destroys the bacterial cell. Scientists are experimenting with a phage that targets MRSa. If this technique proves successful, it would be a powerful treatment, since resistance to a phage does not typically develop.
Contact precautions, including handwashing, are critical in preventing MRSa infection. In a hospital, washing hands before and after caring for a patient is the most important method of preventing the spread of MRSa from patient to patient. Many hospitals now have alcohol-based hand cleansers in each room, sometimes right by each patient's bed. Washing with an alcohol-based wash takes only a few seconds, and, thus, is easier for busy health care providers to do. Moreover, MRSa is usually sensitive to alcohol. Compliance with handwashing precautions is surprisingly low. Surveys in the United States and Europe have confirmed that health care providers only wash their hands about half as much as is optimum for reducing the spread of infection. The Centers for Disease Control and Prevention has estimated that properly performed handwashing could save 30,000 lives a year that are currently lost due to hospital acquired infections, including MRSa infections.
IN CONTEXT: EXPERT ADVICE FOR REAL-WORLD QUESTIONS
Question: Can I get a staph or MRSA infection at my health club?
The Centers for Disease Control and Prevention (CDC) states, “In the outbreaks of MRSA, the environment has not played a significant role in the transmission of MRSA. MRSA is transmitted most frequently by direct skin-to-skin contact. You can protect yourself from infections by:
- practicing good hygiene (e.g., keeping your hands clean by washing with soap and water or using an alcohol-based hand rub and showering after working out);
- covering any open skin areas such as abrasions or cuts with a clean dry bandage;
- avoiding sharing personal items such as towels or razors;
- using a barrier (e.g., clothing or a towel) between your skin and shared equipment;
- and wiping surfaces of equipment with disinfectant before and after use.”
SOURCE: Centers for Disease Control and Prevention (CDC). February, 2007.
Impacts and Issues
Studies have indicated that a hospitalized patient who acquires MRSa is about five times more likely to die than a patient in the same hospital that does not carry the bacterium.
Variants of MRSa are appearing that are resistant even to vancomycin. These new forms of the bacterium, which are called vancomycin intermediate-resistant Staphylococcus aureus, are especially troublesome, as they can be treated with only a very few compounds. With time, further resistance could develop, and, if newer and more powerful (and likely more expensive) antibacterial agents have not been discovered and tested, there could be no means of combating the infections.
In 2006, researchers published a paper in Nature describing the development of a new antibiotic produced by a fungus. This antibiotic, called platensimycin, successfully treats MRSa infections, but further preclinical studies and human clinical trials are necessary before the drug can be approved for human use.
Community-acquired MRSa is a great concern. The organism tends to more aggressively invade tissues and produces a more severe infection than that produced by hospital-acquired MRSa, for reasons that are not yet clear. In addition, it has been discovered that MRSa can grow and divide inside another microscopic organism called Acanthamoeba. Acanthamoeba can become airborne and drift for a considerable distance on air currents. This may mean that MRSa is acquiring the ability to spread great distances, which would make treatment even harder.
See AlsoAntibiotic Resistance; Contact Precautions; Resistant Organisms.
BIBLIOGRAPHY
Books
Brunelle, Lynn, and Barbara Ravage. Bacteria. Milwaukee, WI: Gareth Stevens Publishing, 2003.
DiClaudio, Dennis. The Hypochondriac's Pocket Guide to Horrible Diseases You Probably Already Have. New York: Bloomsbury, 2005.
Roemmele, Jacqueline A., and Donna Batdorff. Surviving the Flesh-eating Bacteria: Understanding, Preventing, Treating, and Living with Necrotizing Fascitis. New York: Avery, 2003.
Periodicals
Cook, H., et al. “Heterosexual Transmission of Community-associated Methicillin-resistant Staphylococcus aureus.” Clinical & Infectious Disease (2007) 44: 410–413.
Wang, J., et al. “Platensimycin Is a Selective FabF Inhibitor with Potent Antibiotic Properties.” Nature (2006) 441: 358–363.
Brian Hoyle
Staphylococcus
MRSA
www.dh.gov.uk/en/Policyandguidance/Healthandsocialcaretopics/Healthcareacquiredinfection/Healthcareacquiredgeneralinformation/DH_4093113 A simple guide to MRSA from the Department of Health