Demystifying MRSA

Drug-Resistant Bacteria Threatens HIV Community

By now you have probably heard of something called "MRSA infections" or "staph infections" from the news, or a friend. There have been outbreaks of skin infections caused by bacteria called MRSA in certain communities such as athletes, men who have sex with men, and HIV-positive individuals. MRSA can cause a wide variety of infections ranging from mild skin infections to serious infections such as pneumonia or sepsis. Patients with HIV are especially at risk due to their weakened immune systems.

Before we get into too many details, let's go over some basics.

What is MRSA?

MRSA stands for "methicillin-resistant Staphylococcus aureus." Staphylococcus aureus (also known as "staph") is a bacteria commonly carried on the skin or in the nose of healthy people. Staphylococcus aureus is carried in the nose by 10 to 40% of people in the community and the hospital environment.1,2 When it is carried on the skin, it is not considered an infection because it is part of what is normally found on the skin and is not causing any symptoms.

MRSA is a type of staph bacteria that is resistant to a class of antibiotics called beta-lactams. Beta-lactam antibiotics include methicillin, penicillin, amoxicillin, etc. A smaller number of people carry MRSA in their nose and skin (approximately 1%) than those carrying Staphylococcus aureus.1

The History of MRSA

Staphylococcus aureus resistance was first described shortly after the introduction of penicillin in the 1940s. MRSA was reported one year after the introduction of the antibiotic methicillin in 1961. Today MRSA is a common hospital-acquired infection and it accounts for more than 50% of infections in the intensive care units (ICU).3

Up until several years ago, it was mainly considered an infection related to being hospitalized. The first cases of MRSA in the community were reported in the 1980s, but these occurred in patients who had contact with healthcare services. During the past decade, MRSA infections were occurring in healthy patients without a recent hospitalization.3

What Is the Difference Between Hospital-Acquired MRSA (HA-MRSA) and Community-Associated MRSA (CA-MRSA)?

There are several main differences between hospital-acquired MRSA and community- acquired MRSA:

  1. CA-MRSA is less frequently associated with hospitalization whereas HA-MRSA is more frequently associated with a hospital stay.

  2. CA-MRSA has less drug resistance, so more classes of antibiotics can be used against it as compared to HA-MRSA.

  3. CA-MRSA contains a toxin that can cause tissue death and destroys white blood cells, and HA-MRSA does not usually have this.

A special test looking at the genetics of the bacteria can be done to distinguish between HA-MRSA and CA-MRSA, but this is not done commonly so it is based on clinical findings and the definition provided below.4

The definition of CA-MRSA is the following: 1

  1. Diagnosis of MRSA was made at the clinic or within the first 48 hours of admission to a hospital

  2. No previous infection with MRSA

  3. No previous history within the last year of being hospitalized, in a nursing home, or hospice, or receiving dialysis or surgery

  4. No permanent medical devices that pass through the skin into the body (such as catheters) present

This definition is important for the doctors and nurses taking care of you so they can treat you appropriately.

How Do I Know if I Am Infected?

Well, before we go into detail about infections with MRSA, it is important to distinguish between colonization and infection.

Colonization (or being a carrier) with MRSA or staph bacteria does not mean you are infected. Bacteria normally live on our skin without causing any symptoms. Your skin itself serves as a blockade, not allowing for bacteria to enter your body. If the skin is cut or broken, the bacteria are given the opportunity to infect and it may cause an infection. It is important to remember that just because you are colonized with MRSA doesn't always mean you are at risk for infection.5 People can be colonized in the nose (most commonly), the throat, the armpits, and the genital area. In 2001 and 2002 in the United States, it was estimated that 32% of non-hospitalized individuals were colonized with Staph bacteria and 0.8% were colonized with MRSA. 5

There are certain groups of people that are at higher risk for being colonized or carriers of MRSA. These include HIV-infected individuals, injection drug users, patients with abscesses, and those recently hospitalized. 6

Who Is at Risk for CA-MRSA Infections?

There are several groups that have been identified that have a higher risk for CA-MRSA infections, and outbreaks (or clusters) of CA-MRSA infections have occurred among many of them. From these outbreaks, we can learn who is at risk for infection and target these people with prevention education.

Alaska Natives4: There have been outbreaks of CA-MRSA skin infections associated with prior antibiotic use.

Native Americans4: At an Indian Health Service facility in the Midwest, the number of MRSA infections increased over an eight-year period. Low socio-economic class, crowded living conditions, and limited access to healthcare contributed to the high rate of skin infections.

Pacific Islanders4: A disproportionate percentage of patients infected with CA-MRSA at four healthcare centers in Hawaii were Pacific Islanders.

Prisoners4: In Georgia, California, and Texas, an increase of infections due to CA-MRSA among prisoners was associated with poor hygiene, limited access to medical care, and inadequate wound care supplies and staff. At many prison facilities, the inmates took care of their own wounds and boils as well as shared personal items.

Athletes4: Outbreaks of infection among wrestling, football, and fencing players have been reported. Most commonly the spread of the MRSA infection was caused by abrasions and cuts associated with the sport and the equipment and the physical contact. An outbreak of CA-MRSA infection even occurred in a professional football team!

Military personnel4: There have been several reports of CA-MRSA skin infections among military recruits and soldiers.

Children7,8: Some studies have indicated that children with CA-MRSA infections are increasing. Attending day-care may be a risk factor for some of these patients.

Men who have sex with men9: In Los Angeles, CA-MRSA skin infections in HIV-positive men who have sex with men were associated with high-risk sex and drug-using behavior and with environmental exposures. Immune status was not a risk factor but other studies indicate that the immune system may increase risk for infection.

HIV-positive individuals1,10: Outbreaks of CA-MRSA infections have been described in men who have sex with men (the majority of whom were HIV-infected) around the country in Los Angeles, San Francisco, San Diego, and New York. From the San Diego outbreak, researchers found that there were certain factors that increased risk for CA-MRSA infections, such as patients with HIV who acquired the infection via men who have sex with men or injection drug use. Also, patients with CD4 counts less than 50, higher viral loads, and absence of Bactrim therapy for prevention of PCP pneumonia were risk factors for CA-MRSA infections. Patients with HIV (or others with weakened immune systems) may be at risk for more severe illness if they get infected with MRSA.

Injection drug users: See above.

Many of the outbreaks listed above are associated with the spread of CA-MRSA infections via close skin to skin contact, openings in the skin (such as cuts), contact with contaminated items, crowded living conditions, and poor hygiene.

What Type of Infections Does CA-MRSA Cause?

"Staph" bacteria are one of the most common causes of skin infections in the United States. The majority of MRSA infections occur in patients in the hospital, but it is becoming more common in the community.1

Most commonly, CA-MRSA causes skin infections that may look like a pimple or boil. The skin can be red, swollen, and painful and can have pus in it. Patients often say it looks a spider bite. Staph infections can also cause more serious infections such as blood stream infections, urinary tract infections, or pneumonia. If you think you have a staph infection, it is important to contact your healthcare provider.1

How Are These Infections Treated?

Skin infections caused by MRSA, such as boils and abscesses, may be treated by incision and drainage of the wound by your healthcare provider. Antibiotics may also be used along with draining the wound. Often times, a biopsy of the skin or drainage from the infected site are sent to a microbiology lab in order to determine which antibiotic will be effective against the bacteria. For the HA-MRSA infections, a limited number of antibiotics are effective, so an intravenous antibiotic (an antibiotic given through your veins) called vancomycin can be used. CA-MRSA has unique susceptibilities in that more oral antibiotics are effective against it. To make things more complicated, CA-MRSA, like all bacteria, can be different strains in which varying antibiotics will be effective. For example, the strain of CA-MRSA that infects one person may be susceptible to antibiotic A and not to antibiotic B, but the strain that infects someone else might be cured with antibiotic B and not A. This is an important reminder that having your healthcare provider get a culture (or biopsy) is important for effective treatment.1,5

Which Antibiotics Can Be Used Against CA-MRSA?

There are several oral antibiotics that can be used against CA-MRSA. Luckily, Bactrim (generic name trimethroprim/sulfamethoxazole) is effective in those bacteria that are susceptible. This is the same drug that is used for PCP prevention in patients with CD4 counts less than 200 cells, but a higher dose is needed to treat the skin infections. Clindamycin is another antibiotic that can be used, but some strains of CA-MRSA may be resistant to it. It is a good alternative for those who may be allergic to "sulfa" antibiotics like Bactrim. Another group of antibiotics that can be used include doxycycline, minocycline, and tetracycline. Lastly, oral linezolid can be used, but it can be more expensive and reserved for special circumstances. Sometimes rifampin (a well-known tuberculosis drug) can be used in combination with another oral antibiotic for treatment. If a serious infection occurs and hospitalization is required, intravenous antibiotics may be needed, such as vancomycin, daptomycin, quinapristin/dalfopristin, linezolid, or tigecycline, or the intravenous forms of Bactrim, clindamycin, or doxycycline.3

How Do I Prevent CA-MRSA Skin Infections?

As mentioned earlier, CA-MRSA is transmitted from person to person via contaminated hands, and sharing towels, clothing, sports equipment or personal hygiene items (such as razors). Other factors contributing to transmission include skin-to-skin contact (for example during contact sports), crowded living conditions, and poor hygiene. So in order to prevent these infections follow these simple steps:

  1. Keep hands clean by washing them thoroughly with soap and water. (You can also use alcohol-based hand sanitizers.)

  2. Keep cuts and scrapes clean and covered with a bandage until fully healed.

  3. Avoid contact with other people's wounds.

  4. Avoid sharing personal items such as towels, razors, or sporting equipment without proper disinfection.

The above principles also apply to people with staph infections who do not want to infect others.1

What Happens if the Infection Comes Back?

Sometimes the staph or MRSA skin infection comes back after it is cured. To prevent this, it is important to follow the treatment plan given by your healthcare provider. It is important to take the antibiotics (if prescribed) until they are finished even if your wound looks better after several days. It is also important to follow the prevention steps outlined above. Contact your healthcare provider if the infection is not getting better in a few days.1

What Do I Do if I Think I Have a MRSA Skin Infection?

Contact your healthcare provider if you think you may have a skin infection.
Although some of the above information about CA-MRSA infections seems scary, it is important to follow the simple prevention steps outlined above and contact your healthcare provider if you think you may have an infection.

For more information go to the Centers for Disease Control and Prevention Web site:

Rupali Jain, Pharm.D., BCPS, is a Clinical Associate Professor at the University of Illinois at Chicago Medical Center, specializing in HIV care pharmacy. She also updated Positively Aware's 2007 Annual HIV Drug Guide. She thanks Dr. Mandavi Kulkarni for her review of this article and helpful comments.


  1. Centers for Disease Control and Prevention. Community-Associated MRSA Information for the Public.

  2. Mandell, Bennett, Dolin. Principles and Practice of Infectious Diseases, 6th edition. Ch. 192: Staphylococcus aureus.

  3. Maltezou HC, Giamarellou. Community-acquired methicillin-resistant Staphylococcus aureus infections. Int J Antimicrob Agents 2006; 27: 87-96.

  4. Weber JT. Community-Associated Methicillin-Resistant Staphylococcus Aureus. Clin Infect Dis 2005; 41:S269-72.

  5. Centers for Disease Control and Prevention. Strategies for Clinical Management of MRSA in the Community: Summary of an Experts' Meeting Convened by the Centers for Disease Control and Prevention. March 2006.

  6. Pan ES, Diep BA, Charlebois ED, et al. Population Dynamics of Nasal Strains of Methicillin-Resistant Staphylococcus aureus -- and Their Relation to Community-Associated Disease Activity. J Infect Dis 2005; 192: 811-8.

  7. Fridkin SK, Hageman JC, Morrison M, et al. Methicillin-Resistant Staphylococcus aureus Disease in Three Communities. N Engl J Med 2005; 352:1436-44.

  1. Herold BC, Immergluck LC, Maranan MC, et al. Community-Acquired Methicillin-Resistant Staphylococcus aureus in Children With no Identified Predisposing Risk. JAMA 1998; 279:593-8.

  2. Lee NE, Taylor MM, Bancroft E, et al. Risk factors of Community-Associated Methicillin-resistant staphylococcus aureus skin infections among HIV-positive men who have sex with men. Clin Infect Dis 2005; 40:1529-34.

  3. Mathews WC, Caperna JC, Barber RE, et al. Incidence of and Risk Factors for Clinically Significant Methicillin-Resistant Staphylococcus aureus Infection in a Cohort of HIV-infected Adults. J Acquired Immune Defic Syndr 2005; 40: 155-160.

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