What is the recommended antibiotic coverage for cellulitis, colitis, and pneumonia?

Antibiotic Coverage for Cellulitis, Colitis, and Pneumonia

Cellulitis

For typical nonpurulent cellulitis, use beta-lactam monotherapy (cephalexin 500 mg four times daily or dicloxacillin 500 mg four times daily) for 5 days, extending only if symptoms have not improved—MRSA coverage is unnecessary in 96% of cases. 1, 2

First-Line Treatment Algorithm

• Assess for purulent features: Look for purulent drainage, exudate, abscess formation, or fluctuance 1, 2

• If nonpurulent cellulitis (no drainage, no abscess):

  • Use beta-lactam monotherapy: cephalexin 500 mg PO four times daily, dicloxacillin 500 mg PO four times daily, or amoxicillin 1, 2
  • Duration: 5 days if clinical improvement occurs; extend only if no improvement 3, 1
  • Success rate: 96% with beta-lactam alone 1, 2

• If purulent cellulitis (drainage, exudate, or abscess present):

  • Add MRSA coverage: clindamycin 300-450 mg PO every 6 hours (covers both streptococci and MRSA) 1, 2
  • Alternative: trimethoprim-sulfamethoxazole (TMP-SMZ) PLUS a beta-lactam, or doxycycline PLUS a beta-lactam 1, 2
  • Never use doxycycline or TMP-SMZ as monotherapy—unreliable streptococcal coverage 1, 2

When to Add MRSA Coverage

Add MRSA-active antibiotics ONLY when these specific risk factors are present 1, 2:

• Penetrating trauma or injection drug use
• Purulent drainage or exudate
• Known MRSA colonization or infection elsewhere
• Systemic inflammatory response syndrome (SIRS): fever >38°C, heart rate >90, respiratory rate >24
• Treatment failure with beta-lactam therapy

Hospitalized Patients with Cellulitis

• Uncomplicated cellulitis requiring hospitalization: IV cefazolin 1-2 g every 8 hours or oxacillin 2 g every 6 hours 1, 4

• Complicated cellulitis with MRSA risk: vancomycin 15-20 mg/kg IV every 8-12 hours (A-I evidence) 1, 4

  • Alternatives: linezolid 600 mg IV twice daily, daptomycin 4 mg/kg IV once daily, or clindamycin 600 mg IV every 8 hours (if local resistance <10%) 1, 4

• Severe cellulitis with systemic toxicity or suspected necrotizing fasciitis: vancomycin 15-20 mg/kg IV every 8-12 hours PLUS piperacillin-tazobactam 3.375-4.5 g IV every 6 hours 1, 4

  • Alternative combinations: linezolid PLUS piperacillin-tazobactam, or vancomycin PLUS a carbapenem 1, 4
  • Duration: 7-14 days for severe infections 1, 4

Critical Pitfalls to Avoid

• Do not reflexively add MRSA coverage simply because the patient is hospitalized—MRSA is uncommon in typical cellulitis even in high-prevalence settings 1, 2
• Do not continue ineffective antibiotics beyond 48 hours—reassess for necrotizing infection, MRSA, or misdiagnosis 1
• Always consider non-infectious mimics: lymphedema, venous stasis, deep vein thrombosis 5

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Colitis (Clostridioides difficile)

For C. difficile colitis, use oral vancomycin 125 mg four times daily for 10 days as first-line therapy; fidaxomicin is an alternative but more expensive. (General medical knowledge)

Treatment Approach

• Mild-to-moderate C. difficile infection: oral vancomycin 125 mg four times daily for 10 days (General medical knowledge)
• Severe C. difficile infection: oral vancomycin 125 mg four times daily PLUS IV metronidazole 500 mg every 8 hours (General medical knowledge)
• Fulminant colitis: oral vancomycin 500 mg four times daily PLUS IV metronidazole 500 mg every 8 hours, with surgical consultation (General medical knowledge)

Key Considerations

• Discontinue the inciting antibiotic if possible—all antibacterial agents can cause C. difficile-associated diarrhea (CDAD) 6
• CDAD can occur up to 2 months after antibiotic exposure 6
• If CDAD is suspected, discontinue ongoing antibiotics not directed against C. difficile 6

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Pneumonia (Community-Acquired)

For community-acquired pneumonia (CAP), treat for a minimum of 5 days, extending therapy only if clinical stability criteria are not met—this includes temperature ≤37.8°C, heart rate ≤100, respiratory rate ≤24, systolic BP ≥90 mmHg, oxygen saturation ≥90%, ability to eat, and normal mentation. 3

Outpatient Treatment Algorithm

Previously healthy, no recent antibiotics:

• Amoxicillin 1 g PO every 8 hours 3
• Alternative: doxycycline 100 mg PO twice daily 3

Comorbidities (COPD, diabetes, heart/liver/renal disease) OR recent antibiotic use:

• Combination therapy: amoxicillin/clavulanate 1.2 g IV/PO every 12 hours PLUS azithromycin 500 mg day 1, then 250 mg daily for 4 days 3
• Monotherapy alternative: levofloxacin 750 mg PO daily or moxifloxacin 400 mg PO daily 3

Hospitalized Patients (Non-ICU)

• Beta-lactam PLUS macrolide: ceftriaxone 1-2 g IV every 12 hours PLUS azithromycin 500 mg IV/PO daily 3
• Fluoroquinolone monotherapy: levofloxacin 750 mg IV/PO daily or moxifloxacin 400 mg IV/PO daily 3

Pathogen-Specific Treatment

Streptococcus pneumoniae (penicillin MIC <2):

• Preferred: penicillin G 2-3 million units IV every 4 hours, or amoxicillin 1 g PO every 8 hours 3
• Alternative: ceftriaxone 1-2 g IV every 12 hours, levofloxacin 750 mg IV/PO daily 3

Methicillin-susceptible Staphylococcus aureus:

• Preferred: oxacillin 2 g IV every 4-6 hours or cefazolin 2 g IV every 8 hours 3

Methicillin-resistant Staphylococcus aureus:

• Preferred: vancomycin 15-20 mg/kg IV every 8-12 hours PLUS rifampin 3
• Alternative: linezolid 600 mg PO/IV every 12 hours 3

Atypical pathogens (Mycoplasma, Chlamydophila):

• Mycoplasma: doxycycline 100 mg IV/PO twice daily for 7-14 days 3
• Chlamydophila: azithromycin 500 mg day 1, then 250 mg daily for 4 days 3

Legionella species:

• Preferred: levofloxacin 750 mg IV/PO daily or moxifloxacin 400 mg IV/PO daily 3

Treatment Duration and Clinical Stability

• Minimum duration: 5 days for all CAP 3
• Extend beyond 5 days only if clinical stability NOT achieved 3
• Clinical stability criteria (all must be met): temperature ≤37.8°C, heart rate ≤100 bpm, respiratory rate ≤24 breaths/min, systolic BP ≥90 mmHg, oxygen saturation ≥90%, ability to maintain oral intake, normal mental status 3

Special Populations

Pseudomonas aeruginosa risk factors (recent hospitalization, frequent antibiotics [>4 courses/year or within 3 months], severe COPD [FEV1 <30%], oral steroids >10 mg prednisone daily):

• Use antipseudomonal beta-lactam: ceftazidime 1-2 g IV every 8-12 hours, or piperacillin-tazobactam 4.5 g IV every 6 hours 3
• Consider dual antipseudomonal therapy initially, de-escalate to monotherapy once susceptibilities known 3

Critical Pitfalls

• Do not use azithromycin alone for hospitalized CAP—combination with beta-lactam is essential 3
• Do not default to 10-day courses—5 days is sufficient if clinical stability achieved 3
• Assess for clinical stability at day 5—this determines whether to extend therapy 3