From the Guidelines
For infections caused by Staphylococcus mecA/C, vancomycin is the recommended first-line treatment, given its effectiveness in targeting the altered penicillin-binding proteins encoded by these genes. The mecA and mecC genes confer methicillin resistance in Staphylococcus bacteria, most notably in Staphylococcus aureus, creating MRSA (Methicillin-Resistant Staphylococcus aureus) 1. These genes encode altered penicillin-binding proteins (specifically PBP2a) that have low affinity for beta-lactam antibiotics, making the bacteria resistant to methicillin, oxacillin, and all other beta-lactam antibiotics including penicillins, cephalosporins, and carbapenems.
Key Considerations for Treatment
- Vancomycin is typically administered at doses of 15-20 mg/kg IV every 8-12 hours (not to exceed 2g per dose) for serious infections, with target trough levels of 15-20 μg/mL 1.
- Alternative treatments include linezolid (600 mg IV/oral twice daily), daptomycin (6-10 mg/kg IV once daily), or trimethoprim-sulfamethoxazole (5 mg/kg IV/oral of the trimethoprim component twice daily) for less severe infections.
- Treatment duration typically ranges from 7-14 days depending on infection site and severity.
- Rapid identification of mecA/C genes through molecular testing is crucial for appropriate antibiotic selection, as these infections will not respond to standard beta-lactam therapy and delayed effective treatment can lead to treatment failure and increased mortality 1.
Populations at Increased Risk
Populations at increased risk for CA-MRSA include:
- Children < 2 years old
- Athletes (mainly contact-sport participants)
- Injection drug users
- Homosexual males
- Military personnel
- Inmates of correctional facilities, residential homes, or shelters
- Vets, pet owners, and pig farmers
- Patients with post-flu-like illness and/or severe pneumonia
- Patients with concurrent SSTI
- History of colonization or recent infection with CA-MRSA
- History of antibiotic consumption in the previous year, particularly quinolones, or macrolides 1.
Pathogen-Directed Therapy
If risk factors for Staphylococcus aureus infection, including community-acquired methicillin-resistant S. aureus, are present, vancomycin (possibly in combination with clindamycin) or linezolid alone should be added to the antibiotic regimen 1.
From the FDA Drug Label
Staphylococcus aureus (MRSA) 87.0% (208/239) Staphylococcus aureus (MSSA) 82.0% (132/161)
The FDA drug label does not answer the question about Staphylococcus mecA/C.
From the Research
Staphylococcus mecA/C Overview
- Staphylococcus aureus is a common cause of infections, with many strains resistant to penicillin and methicillin-resistant Staphylococcus aureus (MRSA) being a significant concern in hospitals and communities 2.
- The mecA gene is responsible for methicillin resistance in S. aureus, and the mecC gene is a variant of mecA that also confers resistance to beta-lactam antibiotics 3.
Treatment of MRSA Infections
- Serious MRSA infections should be treated with parenteral vancomycin or teicoplanin, while less serious infections can be treated with lincosamides or cotrimoxazole 2.
- New antibiotics such as linezolid and quinupristin/dalfopristin have good antistaphylococcal activity but are reserved for patients who fail on or are intolerant of conventional therapy 2.
- Linezolid has been shown to be effective and safe in treating MRSA bacteremia, with comparable outcomes to vancomycin, teicoplanin, or daptomycin 4.
Mechanisms of Resistance
- The staphylococcal chromosomal cassette mec (SCCmec) carries the mecA or mecC gene, which encodes for a novel penicillin-binding protein (PBP2a) that confers resistance to beta-lactam antibiotics 3.
- Vancomycin-resistant S. aureus (VRSA) strains can carry the enterococcal vanA gene complex, which interacts with the mecA-based resistance mechanism to produce high-level resistance to vancomycin 5.
- Vancomycin tolerance in MRSA can be influenced by the expression of genes such as vraSR, dltA, and mprF, which can be induced by subinhibitory antibiotic exposures 6.