What bacteria does Augmentin (amoxicillin-clavulanate) cover that Keflex (cephalexin) does not, particularly in patients with suspected bacterial infections and no known allergies to penicillin or cephalosporins?

Augmentin's Additional Coverage Beyond Keflex

Augmentin (amoxicillin-clavulanate) covers β-lactamase-producing bacteria that Keflex (cephalexin) does not, specifically β-lactamase-producing strains of Haemophilus influenzae, Moraxella catarrhalis, and certain Escherichia coli and Klebsiella species. 1, 2

Key Bacterial Coverage Differences

β-Lactamase-Producing Organisms

• Augmentin provides coverage against β-lactamase-producing Haemophilus influenzae and Moraxella catarrhalis (100% susceptibility), which are common respiratory pathogens that Keflex cannot reliably treat 1, 2
• The clavulanate component preserves amoxicillin's activity by inhibiting β-lactamases, while Keflex remains vulnerable to these enzymes 1, 3
• β-lactamase-producing strains of H. influenzae have become increasingly prevalent, with Augmentin specifically developed to address this resistance mechanism 2, 4

Gram-Negative Enterobacteriaceae

• Augmentin demonstrates significantly greater activity than cephalexin against Klebsiella pneumoniae, Citrobacter diversus, Proteus vulgaris, and approximately one-third of E. coli strains 5
• This enhanced coverage is particularly relevant for urinary tract infections caused by multiply-resistant bacteria 6

Anaerobic Coverage

• Augmentin provides superior anaerobic coverage compared to Keflex, making it the preferred agent for animal and human bite wounds where Pasteurella multocida and oral anaerobes are common 1
• For animal bites, Augmentin is specifically recommended because agents like cephalexin have poor activity against P. multocida 1

Clinical Scenarios Where Augmentin is Preferred

Respiratory Tract Infections

• Augmentin is recommended for acute bacterial rhinosinusitis when β-lactamase-producing organisms or penicillin-resistant S. pneumoniae are suspected 1
• High-dose formulations (90/6.4 mg/kg/day for children, 2000/125 mg twice daily for adults) provide enhanced coverage against drug-resistant S. pneumoniae 1, 2

Bite Wounds

• Augmentin is the oral treatment of choice for both animal and human bites (ampicillin-sulbactam for IV therapy) 1
• Keflex, erythromycin, and clindamycin should be avoided due to poor activity against P. multocida 1

Chronic Streptococcal Carriers

• Augmentin (40 mg amoxicillin/kg/day in three doses for 10 days) is recommended for eradicating chronic Group A Streptococcus carriage when treatment is indicated 1
• This is a specific indication where Keflex is not recommended 1

Important Caveats

When Keflex Remains Appropriate

• For simple cellulitis or erysipelas without risk factors for β-lactamase-producing organisms, Keflex (cephalexin) remains an appropriate first-line choice 1
• Keflex is specifically recommended for Group A streptococcal pharyngitis in penicillin-allergic patients (avoiding those with immediate hypersensitivity) 1

Resistance Considerations

• Cephalosporins like Keflex are inherently less active than amoxicillin against S. pneumoniae, with baseline MICs fourfold higher 1
• The physicochemical properties of oral cephalosporins limit achievable concentrations regardless of dose, making them less suitable when S. pneumoniae is the primary pathogen 1

Gastrointestinal Tolerability

• Augmentin causes more gastrointestinal side effects (especially nausea and diarrhea) compared to Keflex, though most patients complete therapy 4, 6
• The clavulanate component is primarily responsible for these adverse effects 1, 6