Methicillin-Resistant Staphylococcus Aureus (MRSA)

Staphylococcus aureus, often referred to as “staph”, is a common type of bacterium that is found in about 25 to 30 percent of healthy people, primarily on the skin or in the nose. Most of these individuals are colonized by the staph bacteria, meaning that the bacteria are present but are not causing disease. Some people become infected with staph bacteria indicating that the bacteria are present and cause disease. Although staph bacteria do not usually cause infection, they can bring about disease if they penetrate through a break in the skin or through mucous membranes.

Staph bacteria are one of the most common causes of skin infections and sometimes produce relatively minor skin infections such as pimples and boils. However, they can cause more serious illnesses such as surgical wound infections, bloodstream infections, bone infections, and pneumonia.

In the past few decades, a more dangerous form of staph has emerged. This form is known as methicillin-resistant Staphylococcus aureus and is usually referred to by the acronym MRSA. What sets MRSA apart is that it is resistant to an entire class of antibiotics called beta-lactams. This group of antibiotics includes methicillin, and the more commonly prescribed penicillin, amoxicillin, and oxacillin among others.

MRSA is categorized by the setting in which it is acquired. The first type, healthcare-acquired MRSA (HA-MRSA), has been recognized since the 1960s. Strains of staph were identified in patients in hospital and healthcare facilities that were resistant to methicillin (resistance to penicillin had occurred even earlier). The incidence of this infection has been increasing over time, with patients who have had surgery, medical devices implanted, or weakened immune systems being particularly at risk.

Of greater concern is a second type of MRSA which appeared in the 1990s and is known as community-acquired MRSA (CA-MRSA). CA-MRSA occurs outside of hospital settings and usually manifests itself as a skin infection in an otherwise healthy individual. CA-MRSA can develop into a more serious, life-threatening illness. CA-MRSA tends to occur under conditions where people are in prolonged physical proximity, such as in childcare and long-term care facilities, and in soldiers, prisoners, athletes involved in skin-to-skin contact sports such as wrestling, and in individuals sharing personal items such as towels. Unlike HA-MRSA, the source of infection for CA-MRSA is often difficult to identify.

CA-MRSA usually enters the body though a cut or scrape. The first sign of infection is commonly described as resembling a spider bite – a spot on the skin that is red, swollen, and painful. The site may produce pus. Infrequently, CA-MRSA infection can progress to a more serious disease, such as bloodstream infection or pneumonia. CA-MRSA can, in rare cases, lead to death. Highly publicized accounts of the deaths of at least three students from CA-MRSA in late 2007 prompted concern among students, parents, and school officials. The best defense against MRSA is to maintain good hygiene, including frequent and thorough hand washing, and to avoid the sharing of personal care items.

HA-MRSA and CA-MRSA differ at the genetic level and have distinct biologic properties. These differences suggest that CA-MRSA strains may spread more easily from person to person or cause more skin disease than HA-MRSA. At least three different strains of staph that cause CA-MRSA infections have been identified in the United States. Scientists are working towards understanding the differences between these strains and determining why certain people become infected and seriously ill.

The incidence of disease caused by MRSA bacteria is increasing worldwide. Thirty years ago, MRSA accounted for 2 percent of staph infections. By 2003, 64 percent of staph infections were caused by MRSA. According to a report by the Centers for Disease Control and Prevention (CDC) in the United States in 2005, more 94,000 people developed life-threatening infections caused by MRSA; nearly 19,000 people died during hospital stays related to these MRSA infections. The majority of MRSA cases, 85 percent, were associated with healthcare facilities, while approximately 14 percent occurred in individuals with no known exposure to healthcare.

The staph bacterium continues to evolve and is beginning to show resistance to additional antibiotics. In 2002 the first staph strains were found that are resistant to vancomycin, an antibiotic that is one of the few available treatments used as a last resort against MRSA. Although vancomycin-resistant staph strains are currently still quite rare, it is feared that these strains will become more widespread over time and further reduce the limited number of antibiotics that are effective against MRSA.

The rising problem of resistance of staph bacteria to methicillin and other antibiotics is part of a larger issue that greatly concerns healthcare professionals. The emergence of antimicrobial-resistant organisms is making it more difficult to treat a variety of infectious diseases. Besides MRSA, the treatment of other diseases complicated by drug resistance include HIV, tuberculosis, influenza, and malaria.

Drug resistance occurs because microbes, such as staph bacteria, need to reproduce to ensure their survival. When this ability is threatened, as when they are exposed to antibiotics, microbes adapt and evolve to overcome the block to their reproduction. This can occur naturally, and microbes become genetically altered in ways which allow them to survive in the presence of antimicrobial drugs. However, drug resistance adaptations can be accelerated by human actions, particularly by the overuse and inappropriate use of antibiotics. The escalating use of antimicrobials in humans, animals, and agriculture is increasing the problem of drug resistance.

The consequences of antimicrobial resistance pose a significant concern to scientists and medical professionals. Infection with drug-resistant organisms can lead to increased and longer hospital stays, more complicated treatment, more deaths, and higher healthcare costs.