Ancef (Cefazolin) Is Not Effective Against MRSA Infections
Cefazolin (Ancef) is not effective against methicillin-resistant Staphylococcus aureus (MRSA) infections and should not be used for treating MRSA. 1, 2
Mechanism of MRSA Resistance to Cefazolin
- MRSA strains are uniformly resistant to cefazolin due to the acquisition of the mecA gene, which encodes a penicillin-binding protein (PBP2a) with significantly lower affinity for beta-lactams, including cefazolin 3
- This resistance mechanism allows cell-wall biosynthesis to continue even in the presence of typically inhibitory concentrations of beta-lactam antibiotics 3
- The FDA drug label for cefazolin explicitly states that "methicillin-resistant staphylococci are uniformly resistant to cefazolin" 1
Appropriate Antibiotics for MRSA Infections
For MRSA infections, the following antibiotics are recommended instead of cefazolin:
For Outpatient MRSA Skin and Soft Tissue Infections:
- Oral options include:
For Inpatient/Severe MRSA Infections:
- Intravenous options include:
Cefazolin's Role in Staphylococcal Infections
- Cefazolin is effective against methicillin-susceptible S. aureus (MSSA) and is often a preferred agent for MSSA infections 3
- For purulent skin and soft tissue infections likely due to S. aureus (when methicillin-susceptibility is known or highly likely), cefazolin is an appropriate choice 3
- Cefazolin is recommended for surgical site infections after surgery of the trunk or extremities when MRSA is not suspected 3
Special Considerations
- Recent research has explored novel delivery methods like ultrasonic drug dispersion (UD2) that can achieve extremely high tissue concentrations of cefazolin (1,300 mg/L), which demonstrated in vitro bactericidal activity against MRSA 4
- Some studies have investigated synergistic combinations of cefazolin with other antibiotics (like ertapenem) against MRSA, but these are experimental and not standard of care 5
- Despite these experimental approaches, standard dosing of cefazolin should not be used for MRSA infections in clinical practice 1, 2
Clinical Implications
- Using cefazolin for MRSA infections would likely result in treatment failure and potentially worse patient outcomes 2
- When empiric coverage for both MSSA and MRSA is needed before culture results are available, appropriate options include vancomycin, daptomycin, or linezolid, not cefazolin 3
- Once MRSA is identified, therapy should be promptly switched to an appropriate anti-MRSA agent 3
In conclusion, while cefazolin is an excellent choice for MSSA infections, it has no role in the treatment of MRSA infections in standard clinical practice.