Reliability of the MS-2 system in detecting methicillin-resistant Staphylococcus aureus

Area of Science:

• Clinical Microbiology
• Infectious Diseases
• Diagnostic Technology

Background:

• Automated antimicrobial susceptibility testing (AST) systems aim for rapid results.
• Short incubation periods in some automated systems may compromise detection of slowly growing resistant organisms.
• Discrepancies were observed with the MS-2 system for methicillin-resistant Staphylococcus aureus (MRSA) isolates.

Purpose of the Study:

• To evaluate the reliability of the MS-2 automated system for antimicrobial susceptibility testing of methicillin-resistant Staphylococcus aureus (MRSA).
• To compare the performance of the MS-2 system against standard methods like Kirby-Bauer disk diffusion and agar dilution for MRSA detection.

Main Methods:

• Susceptibility testing of 75 MRSA isolates using the MS-2 system and Kirby-Bauer disk diffusion.
• Determination of minimum inhibitory concentrations (MICs) for methicillin, oxacillin, and cephalothin for 33 isolates via agar dilution.
• Comparative analysis of results between the MS-2 system and reference methods.

Main Results:

• Low agreement between MS-2 and disk diffusion for methicillin (47%) and cephalothin (15%).
• High agreement (>93%) for other antimicrobial agents tested.
• MS-2 showed only 49% agreement with agar dilution for methicillin resistance detection.
• Inconsistent results were reported by the MS-2 system for individual MRSA isolates and across multiple tests.

Conclusions:

• The MS-2 automated system demonstrates unreliability in detecting methicillin and cephalothin resistance in Staphylococcus aureus.
• Standardized methods remain crucial for accurate susceptibility testing of MRSA, particularly for beta-lactam antibiotics.
• Clinical laboratories should exercise caution when relying on the MS-2 system for MRSA susceptibility profiling.