Daptomycin Susceptibility Testing 

Daptomycin is a cyclic lipopeptide antibiotic with potent activity against Gram-positive pathogens, including methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enterococcus (VRE), and Streptococcus spp. It is used primarily for complicated skin and soft tissue infections, bacteremia, and endocarditis. Due to the emergence of resistance, susceptibility testing is critical for guiding therapy and optimizing patient outcomes.

Testing Methods

Daptomycin susceptibility is determined using standardized phenotypic and molecular techniques to ensure accurate resistance detection:

• Broth Microdilution (BMD): CLSI and EUCAST gold-standard method; determines MIC values for Staphylococcus spp., Streptococcus spp., and Enterococcus spp..
• Agar Dilution: Less commonly used but effective for precise MIC determination.
• E-test (Epsilometer Test): Gradient diffusion method used in clinical microbiology labs for daptomycin MIC determination.
• Disk Diffusion Assay: Not recommended for daptomycin due to poor correlation with MIC values.
• Molecular Methods (PCR, Whole Genome Sequencing): Detects mutations in mprF, liaFSR, and yycFG, which contribute to daptomycin resistance in Gram-positive bacteria.

Clinical Applications

Daptomycin susceptibility testing is crucial in Gram-positive infections, particularly in the following cases:

• Complicated Skin and Soft Tissue Infections (cSSTIs): Ensures effective treatment of MRSA and VRE-related infections.
• Osteomyelitis and Bone Infections: Important for prolonged therapy and resistance monitoring.
• Endocarditis and Bacteremia: Evaluates susceptibility in enterococcal and staphylococcal bloodstream infections.
• Prosthetic Joint Infections (PJI): Helps guide treatment strategies in persistent Gram-positive infections.

Daptomycin susceptibility testing is essential for Gram-positive bacterial infections, particularly MRSA, VRE, and Streptococcus spp.. Broth microdilution remains the gold standard, while molecular methods provide rapid resistance detection, supporting antimicrobial stewardship and infection control efforts.