MRSA Diagnosis: 5 Easy Tests to Remember

Phenotypic Methods for MRSA Detection

Cefoxitin and Oxacillin Disc Diffusion Tests

Just to be clear, we should be talking of “Methicillin” instead of Cefoxitin and/or Oxacillin. But Methicillin is no longer produced. We use Cefoxitin and/or Oxacillin to classify a Staph aureus as an MRSA or MSSA.

Logically, we should be calling it CRSA( Cefoxitin-Resistant Staphylococcus Aureus) or ORSA( Oxacillin-Resistant Staphylococcus Aureus) and not MRSA, but no, we are stuck with the old name “Methicillin.”

Disc diffusion tests are widely used for initial MRSA screening. In short, many laboratories in poorly funded hospitals hardly go beyond this stage in MRSA screening or diagnosis.

However, these tests involve placing antibiotic-impregnated discs on agar plates inoculated with bacterial samples(Staphylococcus aureus). The size of the inhibition zone around the disc indicates the bacteria’s susceptibility to the antibiotic.

However, cefoxitin is generally considered better than oxacillin for MRSA disc diffusion tests. Cefoxitin is a more effective inducer of the mecA gene, which is responsible for methicillin resistance in Staphylococcus aureus. This makes cefoxitin tests easier to read and interpret, providing clearer endpoints. The Clinical Laboratory Standards Institute (CLSI) recommends cefoxitin for the same reason.

A zone diameter of ≤21 mm for 30ug cefoxitin indicates potential MRSA.

The second one is …

Minimum Inhibitory Concentration (MIC) Test

The MIC test determines the lowest concentration of an antibiotic that inhibits visible bacterial growth.

For MRSA detection, oxacillin or cefoxitin MIC tests are commonly used. An oxacillin MIC of ≥2 mg/L or a cefoxitin MIC of ≥4 mg/L suggests MRSA according to The European Committee on Antimicrobial Susceptibility Testing (EUCAST)

Third one is …

Chromogenic Selective MRSA Medium

Chromogenic media are specialized agar plates that contain chromogenic substrates (colour) and selective agents.

These media allow for direct identification of MRSA based on colony color, typically within 24-48 hours of incubation. This method is particularly useful for screening MRSA in hospitals with large samples and  high-risk patients or in outbreak situations. Nevertheless, some labs use them routinely irrespective of their sample or patient risk profile. But the media is expensive, I must add that for your information.

The fourth one is ….

Molecular Methods

mecA Gene Detection by PCR

The mecA gene is responsible for methicillin resistance in Staphylococcus aureus. Polymerase Chain Reaction (PCR) can rapidly detect this gene, providing results within hours. This molecular method is highly sensitive and specific for MRSA identification. I must say it is not just mecA, there is also mecC gene which is responsible for MRSA. We will discuss the genes in a separate blog posts. One of the most common PCR machine that can be used to detect MRSA gene(mecA/C) is GeneXpert machine.

PCR can be performed on various sample types, including blood cultures, wound swabs, and nasal specimens. Some advanced PCR techniques can simultaneously detect multiple genetic markers, including mecA and S. aureus-specific genes, enhancing diagnostic accuracy. There are literally thousands of PCR machines with their specific target genes built into their platforms.

Immunochromatographic Membrane Assays

Rapid immunoassays, such as those using anti-PBP2a monoclonal antibodies, offer quick MRSA detection. These tests identify the PBP2a protein, which is encoded by the mecA gene. Results are typically available within 15-20 minutes, making them valuable for point-of-care testing.

For clarification, Penicillin-Binding Protein 2a(PBP2a), is a protein found in Methicillin-resistant Staphylococcus aureus. This protein is encoded by the mecA gene and is a key factor in the bacteria’s resistance to β-lactam antibiotics, such as methicillin. This means that ordinary Staphylococcus aureus do not have the PBP2a protein.

Clinical Sample Collection

Nasal Swab Testing

Nasal swabs are the most common method for MRSA screening. The anterior nares are the primary colonization site for S. aureus. Proper technique involves inserting a sterile swab into the anterior portion of the naris and rotating it against the nasal mucosa and then sent to the lab.

Blood Culture and Urine Testing

For suspected systemic MRSA infections, blood cultures are essential. Two to three sets of blood cultures are collected from different venipuncture sites before initiating antibiotic therapy.

Urine testing for MRSA is less common but may be necessary for patients with suspected urinary tract infections. Clean-catch midstream urine samples or catheter specimens are preferred.

Wound Sample Collection

For skin and soft tissue infections, wound swabs or tissue samples are collected. The wound should be cleaned with sterile saline before sampling to reduce contamination. Deep tissue samples or aspirates are preferable to surface swabs for accurate diagnosis.

Laboratory Guidelines and Interpretive Criteria

Healthcare professionals usually follow current laboratory guidelines, such as those provided by the Clinical and Laboratory Standards Institute (CLSI) or the European Committee on Antimicrobial Susceptibility Testing (EUCAST,) for MRSA testing and interpretation. These guidelines offer standardized methods and breakpoints for interpreting test results.

Interpretive criteria may vary slightly between guidelines, so it’s essential to use consistent standards within a healthcare facility to ensure accurate diagnosis and appropriate treatment decisions.

High-Risk Specialties and Local Risk Assessments

Certain medical specialties, such as intensive care units, burn units, and dialysis centers, are at higher risk for MRSA transmission. These areas often require more frequent MRSA screening (weekly) and stringent infection control measures.

To conclude, accurate MRSA diagnosis requires a multifaceted approach, combining phenotypic and molecular methods with proper sample collection techniques. By integrating these various diagnostic tools and following current guidelines, healthcare professionals can effectively identify MRSA infections, guide appropriate treatment, and implement necessary infection control measures. This comprehensive approach is essential in the ongoing fight against antibiotic-resistant infections and the promotion of patient safety in healthcare settings.

…the end!