Is Staphylococcus epidermidis with mecA detected susceptible to Invanz (ertapenem)?

Staphylococcus epidermidis with mecA Gene Is Not Susceptible to Ertapenem

Staphylococcus epidermidis with mecA gene detected is NOT susceptible to Invanz (ertapenem), and alternative antimicrobial therapy should be used. The presence of the mecA gene confers resistance to all beta-lactam antibiotics, including carbapenems like ertapenem.

Understanding mecA Gene and Its Implications

The mecA gene is the genetic determinant for methicillin resistance in staphylococci. When detected:

• It encodes for penicillin-binding protein 2a (PBP 2a), which has reduced affinity for all beta-lactam antibiotics 1
• Strains carrying mecA are resistant to all commercially available beta-lactams, including penicillins, cephalosporins, and carbapenems 1
• The presence of mecA is considered the gold standard for defining methicillin resistance in staphylococci 2

Appropriate Antimicrobial Options for S. epidermidis with mecA

For methicillin-resistant S. epidermidis (MRSE) infections, the following antimicrobials are recommended:

1. Vancomycin - First-line therapy for MRSE infections 3
2. Linezolid - Alternative with excellent activity against MRSE 4
3. Daptomycin - Effective against methicillin-resistant staphylococci 3

Susceptibility Testing Considerations

When evaluating S. epidermidis with mecA:

• PCR detection of mecA has 100% sensitivity and specificity for determining methicillin resistance 2
• Phenotypic testing methods (disk diffusion, E-test) may occasionally yield false results due to heterogeneous expression of resistance 5
• According to CLSI guidelines, molecular detection of mecA is definitive for determining resistance, regardless of phenotypic test results 3

Clinical Implications

The detection of mecA in S. epidermidis has important clinical implications:

• MRSE can serve as a reservoir for transferring mecA to other staphylococcal species, including S. aureus 4
• Infections with MRSE are associated with higher treatment failure rates when inappropriate antimicrobials are used
• Vancomycin remains the drug of choice for serious MRSE infections 1

Common Pitfalls to Avoid

1. Do not rely solely on phenotypic susceptibility testing - Some MRSE strains may appear susceptible to beta-lactams in vitro despite harboring mecA
2. Do not use beta-lactams despite apparent susceptibility - The heterogeneous expression of resistance may lead to treatment failure
3. Do not underestimate the clinical significance - S. epidermidis is often dismissed as a contaminant, but MRSE can cause serious infections, especially in immunocompromised patients and those with indwelling devices

In conclusion, the detection of mecA gene in S. epidermidis definitively indicates resistance to all beta-lactam antibiotics, including ertapenem. Treatment should be guided by this molecular finding rather than phenotypic susceptibility results.