What is the initial treatment for a suspected bacterial or fungal skin infection?

Initial Treatment for Suspected Bacterial or Fungal Skin Infection

For suspected bacterial skin infections in immunocompetent patients, initiate oral beta-lactam antibiotics (cephalexin 500mg three times daily or amoxicillin-clavulanate 500-875mg twice daily) for mild-to-moderate infections, while simple abscesses require only incision and drainage without antibiotics unless systemic signs are present. 1, 2

Bacterial Skin Infections

Mild-to-Moderate Infections (Cellulitis, Erysipela, Impetigo)

First-line oral therapy:

• Cephalexin 500mg three times daily for streptococcal and staphylococcal infections in areas where community-acquired MRSA (CA-MRSA) is not prevalent 1, 2, 3
• Amoxicillin-clavulanate 500-875mg twice daily provides adequate Gram-positive coverage 1
• Dicloxacillin or cefuroxime are alternative beta-lactam options 1

Duration: 7-14 days depending on clinical response 4, 1

MRSA Suspected or Confirmed

If CA-MRSA is prevalent in your area or suspected based on clinical features (purulent drainage, abscess formation, treatment failure):

• Avoid beta-lactams as they will be ineffective 1
• Consider clindamycin, trimethoprim-sulfamethoxazole, or doxycycline for outpatient oral therapy 4
• For severe infections requiring parenteral therapy: vancomycin, linezolid, or daptomycin 4, 1

Abscesses and Furuncles

Incision and drainage is the primary treatment without antibiotics for simple, uncomplicated abscesses 1

Add antibiotics only if:

• Systemic signs of infection present (fever, tachycardia, hypotension) 1
• Immunocompromised patient 1
• Incomplete source control 1
• Significant surrounding cellulitis 1
• Complex abscess (perianal, perirectal, injection drug use sites) requiring broad-spectrum coverage for Gram-positives, Gram-negatives, and anaerobes 1

Severe/Complicated Infections

Parenteral broad-spectrum therapy is required for:

• Systemic toxicity or hemodynamic instability 5
• Deep tissue involvement or necrotizing infections 1
• Immunocompromised hosts 4

Empiric regimens:

• Ampicillin-sulbactam 1.5-3g IV every 6-8 hours 4, 5
• Piperacillin-tazobactam 3.37g IV every 6-8 hours 4, 1
• Vancomycin plus piperacillin-tazobactam or a carbapenem for MRSA coverage with Gram-negative/anaerobic activity 1, 5

Bite Wounds (Animal or Human)

Prophylactic antibiotics are indicated given infection rates of 30-50% for cat bites, 5-25% for dog bites, and 20-25% for human bites 1

Preferred regimen:

• Amoxicillin-clavulanate 500-875mg twice daily orally 4, 1
• Ampicillin-sulbactam 1.5-3g IV every 6-8 hours for severe infections 4
• Covers Pasteurella multocida (animal bites), streptococci, staphylococci, and anaerobes 4

Duration: 7-10 days 4

Fungal Skin Infections

Superficial Dermatophyte Infections (Tinea Corporis, Cruris, Pedis)

Topical antifungals are first-line for localized infections 6, 7

Oral therapy is indicated when:

• Extensive involvement 6, 8
• Topical therapy fails 6
• Immunocompromised patient 6

Oral regimens:

• Terbinafine 250mg daily for 1-2 weeks (tinea corporis/cruris) or 2 weeks (tinea pedis) 8
• Itraconazole 100mg daily for 2 weeks or 200mg daily for 7 days 8
• Fluconazole 150mg once weekly for 2-3 weeks 8

Pityriasis Versicolor (Malassezia furfur)

Oral options:

• Fluconazole 400mg single dose 8
• Itraconazole 200mg daily for 5-7 days 8
• Terbinafine is ineffective for this organism 8

Candidal Skin Infections

Topical nystatin or azoles for localized infections 6

Systemic therapy for immunocompromised patients:

• Fluconazole 100-400mg daily (note: Candida krusei and Candida glabrata are resistant) 4
• Echinocandin for severe infections or resistant species 4
• Duration: 2 weeks after resolution of skin lesions 4

Critical Pitfalls to Avoid

• Do not use beta-lactams empirically in areas with high CA-MRSA prevalence without considering MRSA-active alternatives 1
• Do not prescribe antibiotics for simple abscesses that can be adequately drained 1
• Do not use terbinafine for pityriasis versicolor—it is ineffective against Malassezia 8
• Do not delay surgical debridement in necrotizing infections; antibiotics alone are insufficient 1, 5
• Do not use high-pressure irrigation in wounds as this spreads bacteria into deeper tissue planes 1