What is the difference in spectrum of activity between cefazolin (Ancef) and Clavulin (amoxicillin-clavulanate)?

Spectrum Comparison: Cefazolin vs Clavulin (Amoxicillin-Clavulanate)

Clavulin (amoxicillin-clavulanate) has broader antimicrobial coverage than cefazolin, particularly against anaerobes and beta-lactamase producing organisms, making it more appropriate for polymicrobial infections involving gram-negative and anaerobic bacteria. 1

Cefazolin Spectrum

Gram-Positive Coverage:

• Excellent activity against methicillin-sensitive Staphylococcus aureus (MSSA) - this is actually cefazolin's primary strength 2, 3
• Good coverage of beta-lactam susceptible streptococci 1
• No enterococcal coverage 1, 4

Gram-Negative Coverage:

• Limited to basic enteric gram-negative organisms like E. coli in community settings 1
• Narrower gram-negative spectrum compared to third-generation cephalosporins 2

Anaerobic Coverage:

• No anaerobic coverage - requires combination with metronidazole for infections involving Bacteroides fragilis and other obligate anaerobes 1

Clavulin (Amoxicillin-Clavulanate) Spectrum

Gram-Positive Coverage:

• Covers MSSA and streptococci 1
• No MRSA coverage 3
• Limited enterococcal activity (ampicillin-susceptible strains only) 4

Gram-Negative Coverage:

• Broader than cefazolin due to clavulanate's beta-lactamase inhibition 5
• Covers beta-lactamase producing E. coli, Klebsiella, and other Enterobacteriaceae 1
• Important caveat: Increasing resistance of E. coli to ampicillin/sulbactam combinations has been reported, requiring review of local susceptibility patterns before use 1

Anaerobic Coverage:

• Excellent anaerobic coverage including Bacteroides fragilis - this is a major advantage over cefazolin 1
• Approximately 70% susceptibility rate for B. fragilis to amoxicillin-clavulanate 1

Clinical Application Differences

When Cefazolin is Preferred:

• MSSA skin and soft tissue infections where narrower spectrum is adequate 2, 3
• Surgical prophylaxis in clean or clean-contaminated procedures 5
• Community-acquired infections from proximal GI sources (stomach, duodenum, biliary system) where anaerobes are less likely 1

When Clavulin is Preferred:

• Mild-to-moderate community-acquired intra-abdominal infections requiring single-agent therapy with anaerobic coverage 1
• Polymicrobial skin and soft tissue infections 6
• Diabetic foot infections (moderate severity) where gram-negative and anaerobic coverage is needed 1
• Distal small bowel or colon-derived infections where B. fragilis is commonly present 1

Key Clinical Pitfalls

Resistance Considerations:

• Clavulin's broader spectrum increases risk of antimicrobial resistance and collateral damage to normal flora 2
• Local resistance patterns must be reviewed before using amoxicillin-clavulanate, as E. coli resistance is increasing 1
• B. fragilis resistance to clavulanate-containing regimens is approximately 30%, which may require alternative therapy 1

Cost and Toxicity:

• Cefazolin is generally less expensive and has narrower spectrum, making it preferable when adequate 1
• Amoxicillin-clavulanate is associated with higher rates of drug-induced liver injury (DILI) compared to cefazolin 1

Combination Therapy Requirements:

• Cefazolin requires metronidazole addition for anaerobic coverage in intra-abdominal infections 1
• Clavulin can be used as monotherapy for mild-to-moderate community-acquired infections with anaerobic involvement 1