Amoxicillin is Not Appropriate for Staphylococcus aureus Pharyngitis
Amoxicillin should not be used to treat Staphylococcus aureus throat infections because S. aureus is not susceptible to amoxicillin due to widespread penicillinase (β-lactamase) production, rendering amoxicillin ineffective. 1
Why Amoxicillin Fails Against S. aureus
The FDA label for amoxicillin explicitly states it is indicated ONLY for β-lactamase–negative isolates of Staphylococcus species, meaning it will not work against the vast majority of S. aureus strains that produce penicillinase 1
Most S. aureus strains (>90%) produce β-lactamase enzymes that destroy amoxicillin, making standard penicillins and amoxicillin ineffective 2
S. aureus pharyngitis is exceedingly rare as a primary pathogen in throat infections—Group A Streptococcus causes approximately 10% of adult pharyngitis cases, while S. aureus is "a very infrequent cause of acute bacterial sinusitis in children" and similarly uncommon in pharyngitis 3, 4
Appropriate Treatment for Confirmed S. aureus Throat Infection
For Methicillin-Susceptible S. aureus (MSSA)
Dicloxacillin 500 mg four times daily orally for adults is the oral agent of choice for methicillin-susceptible strains 3
Cephalexin 500 mg four times daily orally for adults (25–50 mg/kg/day in 4 divided doses for children) is an alternative for penicillin-allergic patients except those with immediate hypersensitivity reactions 3
Clindamycin 300–450 mg four times daily orally for adults (20 mg/kg/day in 3 divided doses for children) is appropriate for patients with penicillin allergy 3
For Methicillin-Resistant S. aureus (MRSA)
Trimethoprim-sulfamethoxazole 1–2 double-strength tablets twice daily orally for adults (8–12 mg/kg based on trimethoprim component in 2 divided doses for children) is a first-line oral option 3
Clindamycin 300–450 mg three to four times daily orally for adults (25–40 mg/kg/day in 3 divided doses for children), though potential for inducible resistance exists in erythromycin-resistant strains 3
Doxycycline 100 mg twice daily orally for adults (not recommended for children <8 years) is bacteriostatic with limited recent clinical experience 3
Linezolid 600 mg twice daily orally for adults (10 mg/kg every 12 hours for children <12 years) is bacteriostatic with no cross-resistance but expensive 3
Critical Diagnostic Considerations
Confirm the diagnosis with throat culture and susceptibility testing before treating for S. aureus pharyngitis, as this organism is an uncommon cause of primary pharyngeal infection 3, 4
Consider whether the patient is a chronic S. aureus carrier experiencing concurrent viral pharyngitis rather than true S. aureus infection, as carriers generally do not require treatment 5
S. aureus is a significant pathogen in complications of sinusitis (orbital and intracranial complications) but rarely causes uncomplicated pharyngitis, so evaluate for deeper infection if S. aureus is isolated 3
Common Pitfall to Avoid
- Never prescribe amoxicillin or amoxicillin-clavulanate as monotherapy for suspected or confirmed S. aureus infection—even the addition of clavulanate (as in Augmentin) at standard doses may not provide adequate β-lactamase inhibition for all S. aureus strains, and penicillinase-resistant penicillins or alternative agents are required 3, 2