Review
Community-associated meticillin-resistant Staphylococcus aureus: the case for a genotypic definition

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Summary

Background

New distinct strains of community-associated meticillin-resistant Staphylococcus aureus (CA-MRSA) have emerged as a cause of infection in previously healthy individuals in community settings. It is important to identify CA-MRSA for clinical management, epidemiological analysis, infection prevention and control, and regulatory reporting, but definitions and nomenclature of these strains are confused.

Aim

To review attempts to define CA-MRSA and propose a new definition.

Methods

Non-systematic review.

Findings

Epidemiological definitions were useful for differentiating CA-MRSA and healthcare-associated (HA)-MRSA strain types in the past. However, although HA-MRSA strain types are rarely transmitted in the community, CA-MRSA strains have started to be transmitted in healthcare facilities, so epidemiological definitions are breaking down. CA-MRSA are community strains of S. aureus that have acquired the meticillin resistance gene, mecA. They are distinct from HA-MRSA and should be defined genetically. This may be done by combining genotypic typing by multi-locus sequence or spa with analysis of the staphylococcal cassette chromosome mec. Carriage of Panton-Valentine leukocidin or antimicrobial susceptibility profiles can be useful indicators of CA-MRSA but should not be used for their definition.

Conclusion

For full assessment of their epidemiology, MRSA infections should be characterized as: (1) caused by HA- or CA-MRSA strain types; (2) acquired in community or healthcare settings; and (3) onset in the community or healthcare facility.

Introduction

The great majority of infections with meticillin-resistant Staphylococcus aureus (MRSA) have occurred as a result of cross-infection in hospitals and other healthcare facilities. Only a limited number of clonal lineages of MRSA have been involved, and these healthcare-associated strains (HA-MRSA) have rarely spread or caused infections amongst healthy individuals in the community or amongst healthcare workers.1, 2 However, MRSA infections in previously healthy individuals with no history of healthcare contact were noticed in the early 1990s in Australia and New Zealand, in the late 1990s in the USA, and in the early 2000s in Europe and elsewhere.1, 2, 3, 4, 5 Subsequent molecular analysis showed that these infections were being caused by new types of MRSA which appeared to be common community strains of meticillin-susceptible S. aureus that had acquired the mobile genetic element, the staphylococcal cassette chromosome mec (SCCmec) that encodes the meticillin resistance gene, mecA.6, 7

There is considerable confusion in the literature regarding the definition and nomenclature of these new MRSA types.8, 9 For example, they have been termed ‘community-associated’, ‘community-acquired’, ‘community-onset’, ‘community’, ‘true’ and ‘de-novo’ types of MRSA.8, 9, 10 The most common term used to describe these strains was initially ‘community-acquired MRSA’,3, 4 but this has been widely replaced by ‘community-associated MRSA’ (CA-MRSA), reflecting uncertainty as to whether the MRSA was acquired in hospital or in the community.9,11–13 However, ‘CA-MRSA’ is poorly defined and used in different ways by different authors.8 This review will discuss the limitations of an epidemiological definition of CA-MRSA, and propose practical ways to define CA-MRSA by genotype.

Section snippets

Definitions of CA-MRSA

CA-MRSA were traditionally regarded as MRSA strains causing infection in previously healthy young patients without prior healthcare contact, susceptible to most non-β-lactam antimicrobial agents, and carrying Panton-Valentine leukocidin (PVL) genes and SCCmec types IV or V (Table I).2, 4, 14 However, as the microbiology and epidemiology of CA-MRSA have evolved, traditional definitions has broken down.8

Practical definition of CA-MRSA

The introduction of a genotypic method into the definition of CA-MRSA adds extra cost, time and requirement for laboratory equipment, expertise and experience to define these strains. However, the costs of genotyping are decreasing, and the equipment and expertise required, particularly for spa typing, is within the capabilities of most clinical laboratories.30 Furthermore, collaboration with reference centres can be sought to provide genotyping. If this approach becomes more widely accepted

Recommendations and conclusion

At the current time, MRSA strains involved in outbreaks should either be typed by individual hospital laboratories or referred to reference laboratories in order to determine whether they are CA-MRSA or HA-MRSA strain types, because a wider group of patients and staff may be at risk and novel control strategies may be required for CA-MRSA.11, 13, 59 Novel strategies may include hospital staff screening, increased follow-up of cases with hospital and community onset to reduce household

Conflict of interest statements

JAO is employed part-time by Bioquell UK Ltd. GLF declares no potential conflict of interest.

Funding sources

None.

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