mecA-positive Staphylococcus aureus with low-level oxacillin MIC in Taiwan

Area of Science:

• Microbiology
• Clinical Diagnostics
• Infectious Diseases

Background:

• Methicillin-resistant Staphylococcus aureus (MRSA) is a significant public health concern.
• The mecA gene is the primary determinant for methicillin resistance.
• Certain community-associated MRSA (C-MRSA) strains, particularly the Taiwan clone (CC59), can exhibit low oxacillin Minimum Inhibitory Concentration (MIC) values, falling within susceptible ranges.

Purpose of the Study:

• To investigate the challenges in accurately identifying MRSA strains with borderline oxacillin MICs.
• To evaluate the performance of different diagnostic methods in detecting MRSA in the presence of low-level oxacillin resistance.
• To assess the prevalence and characteristics of MRSA isolates with low oxacillin MICs belonging to the CC59 clone.

Main Methods:

• Initial screening of isolates with low oxacillin MICs (1-2 μg/ml) using Sensititre broth microdilution.
• Genotypic confirmation of mecA presence in suspected MRSA isolates.
• Retesting of all identified isolates using Sensititre, Vitek II, and disk diffusion (DD) methods.
• Evaluation of cefoxitin's utility in MRSA detection.

Main Results:

• A significant proportion of isolates initially presumed susceptible (MSSA) were found to be mecA-positive (57.1% for MIC 2 μg/ml, 3.3% for MIC 1 μg/ml).
• The mecA-positive, low-oxacillin-MIC isolates predominantly belonged to the Taiwan C-MRSA clone (CC59), often carrying SCCmec type V and the pvl gene.
• Diagnostic methods showed variable sensitivity: Sensititre (48.3%), Vitek II (46.6%), and DD (89.6%) for oxacillin MICs. Cefoxitin improved detection but still resulted in misidentification (12.1% to 5.2%).

Conclusions:

• Accurate identification of MRSA with borderline oxacillin MICs, especially within the CC59:SCCmec V clone, remains a diagnostic challenge.
• The misidentification of these MRSA strains may contribute to their undetected spread in healthcare and community settings.
• Findings are relevant globally due to the international spread of this clone and similar resistance patterns in other C-MRSA lineages, necessitating further research into clinical impact.