The Key Difference: Acinetobacter baumannii Coverage
The primary organism covered by Unasyn (ampicillin-sulbactam) but NOT by Augmentin (amoxicillin-clavulanate) is Acinetobacter baumannii and other Acinetobacter species.
Why This Matters Clinically
Sulbactam has intrinsic antibacterial activity against Acinetobacter species that is independent of its beta-lactamase inhibitor function 1. This is a unique property not shared by clavulanate. Multiple studies demonstrate that ampicillin-sulbactam achieves clinical and microbiological cure rates comparable to or better than polymyxins for multidrug-resistant Acinetobacter infections, with significantly better safety profiles 1.
Evidence for Sulbactam's Unique Activity
- Sulbactam is recommended for directed therapy of A. baumannii at MIC ≤4 mg/L 1
- For severe infections, dosing of 9-12 g/day of sulbactam in 3 daily doses is recommended 1
- Research confirms ampicillin-sulbactam demonstrates significantly superior in vitro activity compared to amoxicillin-clavulanate against A. baumannii, with 86% of strains showing amoxicillin-clavulanate MIC >16/8 mcg/mL versus only 38% for ampicillin-sulbactam 2
Spectrum Similarities
Both agents cover similar organisms otherwise:
- Beta-lactamase producing Staphylococcus aureus (MSSA only)
- Beta-lactamase producing E. coli, Klebsiella, Proteus mirabilis
- Anaerobes including Bacteroides fragilis
- Enterobacter species
The FDA label for Unasyn specifically lists Acinetobacter calcoaceticus as a covered pathogen 3, while Augmentin has no such indication.
Clinical Implications
When to Choose Unasyn Over Augmentin:
- ICU-acquired infections where Acinetobacter is endemic or suspected 1
- Patients previously colonized with A. baumannii 1
- Ventilator-associated pneumonia in settings with known Acinetobacter prevalence 1
- Carbapenem-resistant Acinetobacter infections where sulbactam MIC ≤4 mg/L 1
Important Caveats:
- Neither agent covers Pseudomonas aeruginosa adequately
- Neither covers MRSA
- Increasing sulbactam resistance in A. baumannii has been observed over the past decade 1
- Susceptibility testing for Acinetobacter is unreliable with automated methods; Etest with MIC ≤4 mg/L is the accepted standard 1
- For optimal pharmacodynamics with Acinetobacter, use 4-hour infusions of sulbactam 1
Route of Administration Difference:
Unasyn is available IV/IM for severe infections 3, while both have oral formulations for less severe community-acquired infections. For Acinetobacter specifically, only parenteral ampicillin-sulbactam should be used 4, 5.