Does Cephalexin Treat Staphylococcus?
Yes, cephalexin is effective for treating methicillin-susceptible Staphylococcus aureus (MSSA) infections and is specifically indicated by the FDA for skin and soft tissue infections, bone infections, and other conditions caused by S. aureus. 1
FDA-Approved Indications
The FDA label explicitly lists Staphylococcus aureus as a susceptible organism for which cephalexin is indicated, including:
- Skin and skin structure infections caused by S. aureus
- Bone infections caused by S. aureus
- Otitis media due to S. aureus 1
The drug label confirms that cephalexin has been shown to be active against S. aureus (including penicillinase-producing strains) both in vitro and in clinical infections 1.
Critical Limitation: MRSA Resistance
Cephalexin does NOT work against methicillin-resistant Staphylococcus aureus (MRSA). The FDA label explicitly states that methicillin-resistant staphylococci are resistant to cephalosporins, including cephalexin 1. This is a crucial clinical distinction.
Guideline-Based Recommendations
For Skin and Soft Tissue Infections
The 2014 IDSA guidelines recommend cephalexin as an appropriate oral agent for MSSA infections 2:
- For impetigo (Staphylococcus and Streptococcus): Cephalexin 250 mg four times daily (adults) or 25-50 mg/kg/day in 3-4 divided doses (children)
- For MSSA SSTI: Cephalexin 500 mg four times daily (adults) or 25-50 mg/kg/day in 4 divided doses (children)
- The guidelines note cephalexin is preferred for penicillin-allergic patients (except those with immediate hypersensitivity reactions) and highlight its advantage in pediatrics due to suspension availability and less frequent dosing requirements 2
For Recurrent S. aureus Infections
The IDSA guidelines recommend considering a 5-day decolonization regimen that includes intranasal mupirocin, daily chlorhexidine washes, and daily decontamination of personal items for recurrent S. aureus infections 2.
Dosing Considerations Based on Recent Evidence
Recent pharmacokinetic/pharmacodynamic research suggests that traditional four-times-daily dosing may not always be necessary:
- For mild-moderate MSSA infections: Three-times-daily dosing at 15-25 mg/kg may achieve adequate pharmacodynamic targets 3
- For more serious MSSA infections (MIC = 4 mg/L): Higher doses (45 mg/kg three times daily or 80 mg/kg twice daily) may be required 3
- For severe infections like vertebral osteomyelitis: Cephalexin 1,500-2,000 mg/day has shown 87% treatment success when used to complete antibiotic courses after initial IV therapy 4
Clinical Pearls and Pitfalls
Always obtain cultures before starting therapy to confirm susceptibility, as the guidelines emphasize that culture and susceptibility tests should be initiated prior to and during therapy 1.
Common pitfall: Prescribing cephalexin for community-acquired skin infections without considering local MRSA prevalence. In areas with high MRSA rates or in patients with risk factors (injection drug use, prior MRSA infection, nasal colonization), empiric coverage for MRSA is needed instead 2.
Duration of therapy: For uncomplicated cellulitis, 5 days may be sufficient, though treatment should be extended if infection has not improved 2. For serious infections like osteomyelitis, much longer courses (median 86 days total in one study) may be required 4.