Methicillin-resistant Staphylococcus aureus (MRSA)

Article By:

Pierce, Marcia M. Department of Biological Sciences, Eastern Kentucky University, Richmond, Kentucky.

Last reviewed:May 2020

DOI:https://doi.org/10.1036/1097-8542.421540

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• Methicillin-resistant Staphylococcus aureus (MRSA), published June 2016:Download PDF Get Adobe Acrobat Reader

Strains of Staphylococcus aureus bacteria that have become resistant to methicillin and other beta-lactam antibiotics. Staphylococcus aureus is a Gram-positive coccus or spherical bacterial species (that is, it retains a violet color when Gram's stain is applied). It is carried in the nasopharynx of approximately 20% of the population, who are known as persistent carriers. Some strains of the species are resistant to methicillin, which is a member of the beta-lactam family of antibiotics, and the strains are referred to as methicillin-resistant Staphylococcus aureus (MRSA; Fig. 1). Often, the term staph infection is used to describe MRSA disease, but this is a misnomer. Although MRSA infections are caused by staphylococci, not all staph infections are caused by the highly resistant strains of S. aureus. Informally, MRSA is considered a "superbug," meaning that it carries resistance to multiple types of antibiotics. Worldwide, MRSA is a serious medical condition and often leads to death. In the United States, for example, there are more than 1 million cases of MRSA infections in hospitals annually. Of these cases, approximately 120,000 are septicemias, in which an infection has spread to the bloodstream. See also: Bacteria; Human susceptibility to Staphylococcus aureus; Infectious disease; Microbiology; Nosocomial disease; Staphylococcus

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