Classifying MRSA—A Modest Proposal

Abstract & Commentary

By Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford, Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.

The rapidly evolving epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) colonization and infection has resulted in ambiguity in the designation of individual instances as either community or health care associated. This essay represents a proposal for a telegraphic system of unambiguous description of isolates. Please note, however, that this proposal was quickly rejected by 2 journals, and should be judged accordingly.

The CDC classification of MRSA as being of community origin (CA-MRSA) depends upon diagnosis of infection or colonization in an outpatient, or on a positive culture obtained in the first 48 hours of hospitalization, in the absence of factors indicating the possibility of acquisition in the healthcare setting (see Table 1).1 Strains of MRSA have also been classified as being hospital or community acquired based upon their SCCmec gene type, with SCCmec type IV and SCCmec type V considered as indicative of an isolate being CA-MRSA.2 In the absence of molecular characterization of SCCmec, some authors have used antibiotic susceptibility phenotype as a surrogate to suggest community or hospital origin.

Unfortunately, these designations have not kept pace with the evolution of the epidemiology of MRSA. Strains considered of hospital origin may be acquired in the community, and vice versa.3,4 As a consequence, the use of the terms, HA-MRSA and CA-MRSA, are often ambiguous in their meaning. There is an important need for a more precise and unambiguous method of communicating the nature of the isolate or infection. The following is a suggestion whose purpose is to provide improved clarity in communications regarding MRSA.

Assignment as hospital-acquired MRSA (HA-MRSA) or CA-MRSA is made by the CDC epidemiologic definition (EPI), by molecular characterization of SCCmec (MOL), or both (EPI/MOL). The molecular classification may be expanded to make it more precise when further information is available; (eg, CAMOL(SCCmecIV)-MRSA or CAMOL(USA300)-MRSA). In cases in which the molecular type is inferred from the phenotypic expression of antibiotic susceptibility, the designation (PHE) may substitute for (MOL). This system allows for an accurate telegraphic description of isolates whose classification may differ depending upon the means utilized (eg, CA-MRSA by molecular methods and HA-MRSA by the CDC epidemiological definition, yielding the designation CAMOLHAEPI-MRSA). Excluding the use of the phenotypic designation by antibiotic susceptibility pattern, the resultant possibilities are as seen in Table 2.

The dynamic nature of the evolving epidemiology of MRSA, however, may eventually render the distinctions between the settings in which this organism is acquired irrelevant. In the meantime, however, this proposed classification is intended to help remove much of the ambiguity in communications regarding HA-MRSA and CA-MRSA.


  2. Deresinski S. Methicillin-Resistant Staphylococcus aureus: An Evolutionary, Epidemiologic, and Therapeutic Odyssey. Clin Infect Dis. 2005;40:562-573.
  3. Carleton HA, et al. Community-Adapted Methicillin-Resistant Staphylococcus aureus (MRSA): Population Dynamics of an Expanding Community Reservoir of MRSA. J Infect Dis. 2004;190:1730-1738.
  4. Seybold U, et al. Emergence of Community-Associated Methicillin-Resistant Staphylococcus aureus USA300 Genotype as a Major Cause of Health Care-Associated Blood Stream Infections. Clin Infect Dis. 2006;42:647-656.