Augmentin's Additional Bacterial Coverage Beyond Cefalexin
Augmentin covers β-lactamase-producing Haemophilus influenzae and Moraxella catarrhalis, as well as oral anaerobes like Bacteroides species and Fusobacterium, which cefalexin does not reliably cover. 1, 2
Key Bacterial Coverage Differences
β-Lactamase-Producing Organisms
The clavulanate component in Augmentin inactivates β-lactamases, preserving amoxicillin's activity against β-lactamase-producing strains of H. influenzae, M. catarrhalis, Staphylococcus aureus, and Klebsiella species. 2, 3 Cefalexin remains vulnerable to these enzymes and cannot reliably treat these infections. 1
All strains of Klebsiella pneumoniae produce β-lactamases and are covered by Augmentin but not by cefalexin. 2
Approximately 30-40% of H. influenzae and the majority of M. catarrhalis strains produce β-lactamases, making Augmentin essential for respiratory tract infections where these pathogens are suspected. 3
Anaerobic Coverage
Augmentin provides coverage against anaerobic bacteria including Bacteroides fragilis, Fusobacterium species, and Peptostreptococcus species (both β-lactamase and non-β-lactamase-producing strains). 2 Cefalexin has no reliable anaerobic coverage. 4
This anaerobic coverage makes Augmentin the preferred agent for bite wounds (animal and human), where Pasteurella multocida and oral anaerobes are common pathogens. 1
Gram-Negative Coverage
- Augmentin covers Enterobacter species (particularly in urinary tract infections), Eikenella corrodens, and Neisseria gonorrhoeae. 2 Cefalexin's FDA-approved indications are limited to E. coli, Proteus mirabilis, and Klebsiella pneumoniae in the genitourinary tract. 4
Clinical Scenarios Where This Matters
Respiratory Tract Infections
For acute bacterial rhinosinusitis, Augmentin is recommended when β-lactamase-producing H. influenzae or M. catarrhalis are suspected, particularly in patients with recent antibiotic exposure. 5, 1 Cefalexin is not recommended for these infections.
The predicted clinical efficacy of high-dose amoxicillin/clavulanate for acute bacterial rhinosinusitis in adults is 90-92%, compared to 65-66% for first-generation cephalosporins like cefalexin. 5
Bite Wounds
- Augmentin is the oral treatment of choice for both animal and human bites due to coverage of Pasteurella multocida and oral anaerobes. 1 Cefalexin lacks this coverage and should not be used for bite wounds.
Diabetic Foot Infections
- For mild diabetic foot infections with recent antibiotic exposure, Augmentin provides coverage against both gram-positive cocci and gram-negative rods including β-lactamase producers. 5 Cefalexin is only recommended for mild infections without complicating features. 5
Important Caveats
When Cefalexin Remains Appropriate
For simple cellulitis, erysipelas, or uncomplicated skin and soft tissue infections caused by Staphylococcus aureus or Streptococcus pyogenes without risk factors for β-lactamase production, cefalexin remains an appropriate first-line choice. 5, 1, 4
Cefalexin is FDA-approved for respiratory tract infections caused by Streptococcus pneumoniae and Streptococcus pyogenes, where β-lactamase production is not a concern. 4
Resistance Considerations
Among viridans group streptococci, resistance rates are 15% for amoxicillin and 96% for cefalexin, highlighting cefalexin's inferior activity against streptococcal species. 5
Cephalosporins like cefalexin are inherently less active than amoxicillin against S. pneumoniae, with baseline MICs fourfold higher. 1 This makes Augmentin superior for pneumococcal infections, even those without β-lactamase production.