By JP Saleeby, MD
Methicillin-resistant Staphylococcus aureus (MRSA) is in the news a lot these days and on the minds of most healthcare workers in recent years and for good reason. MRSA is a type of bacteria that has acquired resistance to a number of commonly used antibiotics and is becoming a problem bug to treat. Physicians and hospitals are taking action to help stop the spread of this bacterium by ever more aggressive means.
Staphylococcus aureus (or Staph) is a bacterium which causes a multitude of infections in humans. A very common bacteria present on our skin, S. aureus is classified as a gram-positive bacteria with the appearance of a cluster-of-grapes under a microscope. It is distributed worldwide and as many as 11% - 40% of the population is estimated to be colonized with this bug. In the
S. aureus can become a “super bug” when it mutates and becomes resistant to antibiotics. Bacteria when exposed to certain antimicrobials develop ways to protect itself from them eventually; when this happens we refer to these bacteria as “super bugs” or resistant organisms. In 1959 the antibiotic methicillin (which is a relative of penicillin) was introduced to fight skin infections. Clinicians noticed a number of isolated cases of S. aureus that were resistant to this antibiotic within two years of its use, thus the name methicillin-resistant. However, MRSA shows resistance to many other antibiotics besides the now obsolete methicillin.
There are two classifications of MRSA. Those found in hospitals (HA-MRSA or hospital acquired MRSA) and those found in the community (CA-MRSA or community acquired MRSA). Twenty years ago MRSA was almost unheard of in the community and one would be limited to see cases in hospitalized or ICU patients, today it is a different story. Cultures of ICU patients in 1974 showed only a 2% rate of HA-MRSA. The number has risen to 64% (in surveys from 2004), and probably higher today in our ICU patients. As many as 59% of all skin and soft tissue infections are caused by MRSA today resulting in 94,000 patients each year presenting with serious invasive infections. There are reported approximately 19,000 deaths resulting from this bacterium (a rate higher than deaths due to HIV). Only 14% of these deaths are associated with the lesser virulent CA-MRSA, while HA-MRSA accounts for 86% of deaths.
The bacterium S. aureus causes infections and disease by several mechanisms. They produce proteolytic enzymes (that breakdown tissue proteins and cause pus), enterotoxins that are responsible for vomiting and diarrhea, exfoliative toxins that cause the skin to blister, and exotoxins (TSST-1 for example is a protein implicated in toxic shock syndrome, a very bad and lethal infectious process). When you add antibiotic resistance to the picture, you have a very dangerous pathogen. When mutation occurs in the bacterium changes take place at a genetic or molecular level. When a bacterium acquires resistance to an antibiotic it passes this genetic information to other bacterium. There are at least five types of genes involved. The genes denoted as SCCmec I-V have been isolated. Those S. aureus with genes I-III are associated with the more virulent HA-MRSA, and genes IV & V linked to CA-MRSA. HA-MRSA is the nastier of the two bugs with more drug resistance.
MRSA is transmitted by direct contact (through contact with skin and/or body fluids) and by indirect contact (through towels, toys, diapers, and fomites such as door knobs & counter tops). There are those individuals referred to as carriers who show no symptoms of infection, yet harbor this bacterium. Statistics indicate CA-MRSA is the predominant type found in the population and it can be cultured by a simple screening exam using nasal swabs. Hospitals nationwide will soon implement screening tests for all high-risk patients or those admitted to the ICU by non-invasive nasal swab screening tests to help identify these patients. Screening is a means to help eradicate or lessen the death rates associated with MRSA.
Those at greater risk for harboring this bacterium are those who play contact sports, share towels and toiletries, those who have suppressed immune systems (people with cancer, on immunosuppressant drugs, chemotherapy and HIV (+) patients). Additionally, those who live in unsanitary conditions and the very young or old are higher risk. Healthcare workers are also at increased risk because of their working environment.
While the symptoms of infection are very protean, they do have some commonality. Pus producing skin infections like boils, abscesses and impetigo are most often found to contain MRSA. In the emergency room we often see MRSA infections that are mistaken for “spider bites.” The dermatological presentation of an early MRSA infection is very similar to many insect bites. When a superficial MRSA infection becomes more complex, by expanding and spreading into deeper tissues, colonizing implantable devices (artificial joints, replacement heart valves, etc.) then mortality rises. Symptoms such as high fever, chills, weakness and mental deterioration as well as a drop in blood pressure ensue and result in a condition called sepsis. Mortality rates among septic patients are extremely high.
The only correct way to diagnose MRSA is by culture. Samples from the skin (or in conditions such as pneumonia where blood cultures are drawn) are plated on Baird-Parker agar plates and cultured in an incubator. S. aureus that produces a (+) coagulase test (coagulase is an enzyme that some bacteria produce that allow the bacteria to clot blood) are then subjected to methicillin. Those bacteria that survive are labeled MRSA. Further evaluation of resistance and sensitivity to different antibiotics is accomplished by placing Kirby-Bauer discs on the agar plates and checking for antibiotic susceptibility. This microbiological culture process can take several days.
Treatment of simple topical MRSA infections is accomplished by topical mupirocin (Bactroban ®) cream. For those more serious infections of the deeper tissues oral and IV antibiotics must be used. Sometimes two or more are needed. Drugs like Septra ® (TMP-SMZ), doxycycline, vancomycin, linezolid and rifampin are used. Patients with severe infections are usually hospitalized for intravenous antibiotic therapy. Drainage of the infected tissue and often removal of infected implanted appliances are often required.
A ways to help avoid MRSA infection is by good hygiene practice (washing with warm soapy water upon contact with high risk individuals, using antibacterial hand cleansers, etc.) While the National Institute of Health reports that MRSA pneumonia and blood poisoning (sepsis) have a high death rate, people in general good health with CA-MRSA recover in almost every case.
Davis, C., Stoppler, M. MRSA Infection, eMedicineHealth.com
Kuehnert, MJ, et. al. Prevalence of Staphylococcus aureus nasal colonization in the United States, 2001-2002. J Infect Dis. 2006 Jan 15;193(2):172-9.