Characteristics, Detection and Typing Methods of Community-Acquired Methicillin-Resistant Staphylococcus aureus (CA-MRSA) — A Review
Last reviewed: March 2026
Key Findings
- Community-acquired MRSA (CA-MRSA) has emerged as a cause of skin and soft tissue infections in otherwise healthy individuals, distinct from the hospital-acquired strains that dominated for decades.
- CA-MRSA strains are characterised by susceptibility to most non-beta-lactam antimicrobials, carriage of genes for Panton-Valentine leukocidin (PVL) toxin, and smaller staphylococcal cassette chromosome mec (SCCmec) types IV or V.
- Molecular typing methods including PFGE, MLST, SCCmec typing, and spa typing are essential for tracking CA-MRSA transmission and epidemiology.
- Close personal contact is the suspected primary mode of CA-MRSA transmission, leading to skin or nasal colonisation followed by active infection.
Background and Context
Staphylococcus aureus is a Gram-positive coccal bacterium that colonises the skin and mucous membranes — particularly the anterior nares — of approximately 20 to 30% of the healthy human population. While colonisation is typically asymptomatic, S. aureus is an opportunistic pathogen capable of causing a wide spectrum of infections, from minor skin and soft tissue conditions (boils, impetigo, wound infections) to life-threatening invasive diseases including bacteraemia, endocarditis, pneumonia, and toxic shock syndrome.
Methicillin-resistant Staphylococcus aureus (MRSA) has been a major healthcare concern since its initial identification in the United Kingdom in 1961, shortly after the introduction of methicillin as a therapeutic agent against penicillinase-producing staphylococci. Methicillin resistance is conferred by the mecA gene, which encodes an altered penicillin-binding protein (PBP2a) with reduced affinity for beta-lactam antibiotics. The mecA gene is carried on a mobile genetic element called the staphylococcal cassette chromosome mec (SCCmec), which integrates into the S. aureus chromosome.
For over four decades, MRSA was predominantly a healthcare-associated pathogen (HA-MRSA), causing nosocomial infections in hospitalised patients with identifiable risk factors such as recent surgery, indwelling medical devices, prolonged hospital stays, or residence in long-term care facilities. HA-MRSA strains typically carry larger SCCmec types (I, II, or III) that harbour additional resistance determinants, conferring multidrug resistance to multiple antibiotic classes.
The emergence of CA-MRSA in the late 1990s and early 2000s represented a paradigm shift. These strains caused infections in previously healthy individuals in the community who lacked traditional risk factors for MRSA infection. Genotypically and phenotypically distinct from HA-MRSA, CA-MRSA strains are characterised by smaller SCCmec cassettes (primarily types IV and V), susceptibility to most non-beta-lactam antimicrobials (making them “pauci-resistant”), and frequent carriage of the Panton-Valentine leukocidin (PVL) gene — a pore-forming toxin associated with tissue necrosis and severe skin and soft tissue infections.
CA-MRSA in the Malaysian Context
The Malaysian National Surveillance on Antimicrobial Resistance (NSAR) has documented MRSA prevalence rates ranging from approximately 17% to 28% of all S. aureus isolates across government and university hospitals between 1999 and 2017. While the majority of these isolates represent HA-MRSA, the identification of CA-MRSA cases has been documented at several Malaysian centres.
A study at Universiti Kebangsaan Malaysia Medical Centre identified five CA-MRSA cases during a 12-month surveillance period in 2009. All cases presented as skin and soft tissue infections, including diabetic foot with gangrene, infected scalp haematoma, philtrum abscess in a healthcare worker, thrombophlebitis complicated by abscess, and infected bedsore. All isolates were confirmed as MRSA by detection of the mecA gene, carried SCCmec type IV (except one case), and were positive for the PVL gene. The antibiotic susceptibility profile showed resistance limited to penicillin and oxacillin, with sensitivity retained to ciprofloxacin, erythromycin, clindamycin, gentamicin, trimethoprim-sulfamethoxazole, and rifampicin.
Molecular epidemiological studies of MRSA in Malaysia between 2002 and 2020 have documented considerable clonal diversity, identifying 36 different sequence types (STs). Among HA-MRSA, the dominant clone has shifted over time from ST239-t037-SCCmec III (the pandemic “Hungarian/Brazilian” clone) to ST22-t032-SCCmec IV. Among CA-MRSA, ST30, ST772, ST6, and ST22 have been repeatedly detected, though no single CA-MRSA strain has become predominant in the Malaysian community.
Molecular Typing Methodologies
The review article examined the principal molecular typing methods used to characterise CA-MRSA strains and track their transmission in community settings. Each method offers distinct advantages and limitations in terms of discriminatory power, reproducibility, cost, rapidity, and portability of results.
Comparison of MRSA Molecular Typing Methods
| Method | Target | Strengths | Limitations |
|---|---|---|---|
| PFGE | Whole chromosome restriction pattern | High discriminatory power; considered “gold standard” | Time-consuming; results not easily portable between labs |
| MLST | Seven housekeeping gene sequences | Highly reproducible; results portable via public databases | Lower resolution for closely related strains; more expensive |
| SCCmec typing | Mobile cassette carrying mecA | Directly relevant to resistance mechanism; distinguishes HA/CA | Limited to SCC element; multiple subtypes complicate interpretation |
| spa typing | Variable repeat region of protein A gene | Rapid; good discriminatory power; single-locus simplicity | Unilocus typing may miss wider genomic context |
The review recommended that combination approaches using multiple typing methods provide the most comprehensive characterisation of CA-MRSA strains, as each method captures different aspects of the bacterial genome. Pulsed-field gel electrophoresis (PFGE) remains the reference standard for outbreak investigation, while MLST provides the framework for global epidemiological comparison through standardised sequence type nomenclature.
Public Health Implications
The emergence of CA-MRSA poses distinct challenges for infection control and antimicrobial stewardship. Unlike HA-MRSA, which can be targeted through hospital-based surveillance and infection prevention measures, CA-MRSA transmission occurs in community settings — schools, sports facilities, military barracks, prisons, and households — making containment more difficult. The clinical features of CA-MRSA infections are often indistinguishable from those caused by methicillin-susceptible S. aureus, complicating empirical antibiotic selection.
In Malaysia, continued surveillance combining phenotypic antibiotic susceptibility testing with molecular characterisation is essential for detecting emerging CA-MRSA clones, monitoring for the acquisition of additional resistance determinants (particularly vancomycin resistance, which has been documented in neighbouring Indonesia), and tracking the potential convergence of CA-MRSA and HA-MRSA lineages through horizontal gene transfer.
Limitations
As a review article, this paper synthesised existing literature and did not generate new primary data. The rapidly evolving nature of MRSA molecular epidemiology means that findings from the review period may not reflect the current circulating strains. The available Malaysian data on CA-MRSA is predominantly from hospital-based surveillance, which may underestimate the true community burden, as many CA-MRSA skin infections are managed in primary care or self-treated without microbiological investigation.
Content shared under CC BY-NC 4.0 licence. © Malaysian Journal of Public Health Medicine.