Quantitative disk diffusion as a convenient method for determining minimum inhibitory concentrations of oxacillin for...

Area of Science:

• Microbiology
• Clinical Diagnostics
• Antimicrobial Resistance

Background:

• Staphylococcal infections pose a significant public health threat.
• Accurate detection of oxacillin resistance in Staphylococcus strains is crucial for effective treatment.
• Existing phenotypic methods may fail to detect certain resistance mechanisms, such as blocked mecA gene expression.

Purpose of the Study:

• To evaluate a novel quantitative disk diffusion (DD) method for determining oxacillin susceptibility in Staphylococcus aureus and coagulase-negative staphylococci (CoNS).
• To compare the quantitative DD method with established phenotypic methods (agar dilution, cefoxitin disk diffusion) and molecular detection (mecA gene PCR).

Main Methods:

• Evaluated 58 Staphylococcus aureus and 58 CoNS strains.
• Employed a novel quantitative disk diffusion (DD) method.
• Compared results with agar dilution (AD) for oxacillin, cefoxitin disk diffusion, and mecA gene detection by PCR.

Main Results:

• Quantitative DD method's Minimum Inhibitory Concentrations (MICs) were equivalent to AD method's MICs for both S. aureus (p=0.99) and CoNS (p=0.97).
• Identified 8 strains (5 S. aureus, 3 CoNS) with incongruent results due to blocked mecA gene expression.
• The quantitative DD method detected oxacillin resistance in strains with low-level or heterogeneous resistance where other methods failed.

Conclusions:

• The quantitative disk diffusion method provides accurate and reliable identification of oxacillin-resistant staphylococci.
• This cost-effective and accessible method can be implemented in any laboratory setting.
• It offers superior detection capabilities for challenging resistance profiles, including those with blocked mecA gene expression.