Cloxacillin  Susceptibility Testing 

Cloxacillin is a narrow-spectrum beta-lactam antibiotic belonging to the penicillinase-resistant penicillins, primarily used for treating infections caused by methicillin-sensitive Staphylococcus aureus (MSSA). Due to its resistance to beta-lactamases, cloxacillin remains a key agent in staphylococcal infections. However, susceptibility testing is crucial to detect resistance mechanisms and guide treatment decisions.

Testing Methods

Cloxacillin susceptibility is assessed using standardized phenotypic and molecular techniques to ensure accurate determination of resistance:

• Agar Dilution: CLSI and EUCAST reference method for MIC determination in Staphylococcus spp..
• Broth Microdilution: Quantitative MIC determination for MSSA and coagulase-negative staphylococci (CoNS).
• E-test (Epsilometer Test): Gradient diffusion method to establish MIC values for clinical isolates.
• Disk Diffusion Assay: Interpretation of zone diameters according to CLSI/EUCAST breakpoints, commonly used in routine laboratories.
• Molecular Methods: PCR and sequencing detect mecA and mecC genes, which indicate methicillin resistance, rendering cloxacillin ineffective.

Clinical Applications

Cloxacillin susceptibility testing plays a crucial role in various clinical settings:

• MSSA Infections: Ensures efficacy in treating bloodstream infections, endocarditis, and osteomyelitis.
• Skin and Soft Tissue Infections (SSTIs): Differentiates between MSSA and methicillin-resistant Staphylococcus aureus (MRSA) for appropriate antibiotic selection.
• Bone and Joint Infections: Guides therapy for septic arthritis and prosthetic joint infections caused by Staphylococcus spp..
• Respiratory Tract Infections: Determines the effectiveness of cloxacillin in staphylococcal pneumonia cases.

Cloxacillin susceptibility testing is essential for differentiating MSSA from MRSA, guiding treatment in staphylococcal infections, and ensuring optimal antibiotic stewardship. Molecular methods such as mecA/mecC PCR provide rapid and definitive resistance detection, improving clinical outcomes and therapeutic efficacy.