What antibiotic should be given to a patient with a bacterial infection?

Antibiotic Selection for Bacterial Infections

The choice of antibiotic depends critically on the specific infection site and severity, but for most common bacterial infections requiring empiric therapy, amoxicillin or amoxicillin-clavulanate should be first-line for mild community infections, while piperacillin-tazobactam or a respiratory fluoroquinolone should be used for hospitalized patients with moderate-to-severe disease. 1

Respiratory Tract Infections

Community-Acquired Pneumonia (Outpatient)

• For adults with mild CAP treated as outpatients: Use a respiratory fluoroquinolone (levofloxacin 750 mg daily, moxifloxacin, or gatifloxacin) OR a β-lactam (amoxicillin-clavulanate 4g/250mg daily, ceftriaxone, or cefotaxime) plus a macrolide (azithromycin or clarithromycin). 2
• The respiratory fluoroquinolones achieve 90-92% predicted clinical efficacy, making them among the most effective options. 2
• For pediatric patients <3 years: Amoxicillin 80-100 mg/kg/day in three divided doses is the recommended first-line agent. 1
• For children >3 years: Amoxicillin for suspected pneumococcal infection; macrolides for suspected atypical pathogens (Mycoplasma, Chlamydophila). 1

Community-Acquired Pneumonia (Hospitalized, Non-ICU)

• Use either: A respiratory fluoroquinolone alone (strong recommendation) OR a β-lactam (cefotaxime, ceftriaxone, or ampicillin) plus a macrolide. 2
• For penicillin-allergic patients, use a respiratory fluoroquinolone plus aztreonam. 2

Community-Acquired Pneumonia (ICU)

• Mandatory regimen: A β-lactam (cefotaxime, ceftriaxone, or ampicillin-sulbactam) PLUS either azithromycin OR a fluoroquinolone. 2
• For suspected Pseudomonas: Use an antipseudomonal β-lactam (piperacillin-tazobactam, cefepime, imipenem, or meropenem) plus either ciprofloxacin or levofloxacin 750 mg. 2
• For suspected MRSA: Add vancomycin 15 mg/kg IV every 8-12 hours (target trough 15-20 mg/mL) or linezolid 600 mg every 12 hours. 2, 3

Acute Bacterial Sinusitis

For adults with mild disease (no recent antibiotics):

• Amoxicillin-clavulanate (1.75-4g/250mg daily), amoxicillin (1.5-4g daily), cefpodoxime, cefuroxime, or cefdinir achieve 83-92% predicted efficacy. 2
• For β-lactam allergies: TMP-SMX, doxycycline, or azithromycin are acceptable but have 20-25% bacteriologic failure rates. 2

For adults with moderate disease or recent antibiotic use (within 4-6 weeks):

• Respiratory fluoroquinolone (gatifloxacin, levofloxacin, moxifloxacin) OR high-dose amoxicillin-clavulanate (4g/250mg daily) achieve 90-92% efficacy. 2
• Alternative: Ceftriaxone 1-2g IV daily for 5 days. 2

For children with mild disease:

• High-dose amoxicillin-clavulanate (90 mg/6.4 mg per kg daily), amoxicillin (90 mg/kg daily), cefpodoxime, cefuroxime, or cefdinir achieve 82-92% efficacy. 2

Hospital-Acquired Pneumonia (Non-Ventilator)

For patients NOT at high mortality risk and no MRSA risk factors:

• Monotherapy with piperacillin-tazobactam 4.5g IV every 6 hours, cefepime 2g IV every 8 hours, levofloxacin 750 mg IV daily, imipenem 500 mg IV every 6 hours, or meropenem 1g IV every 8 hours. 2

For high mortality risk (ventilatory support needed, septic shock) OR recent IV antibiotics (within 90 days):

• Dual therapy required: Choose TWO agents from different classes (avoid two β-lactams): One antipseudomonal β-lactam PLUS either a fluoroquinolone (levofloxacin 750 mg or ciprofloxacin 400 mg every 8 hours) OR an aminoglycoside (amikacin 15-20 mg/kg daily, gentamicin 5-7 mg/kg daily). 2
• PLUS MRSA coverage: Vancomycin 15 mg/kg IV every 8-12 hours (consider 25-30 mg/kg loading dose for severe illness, target trough 15-20 mg/mL) OR linezolid 600 mg IV every 12 hours. 2, 3

Febrile Neutropenia

For ambulatory low-risk patients:

• Amoxicillin-clavulanate combined with ciprofloxacin. 2

For all other patients (hospitalized, high-risk):

• First-line: Piperacillin-tazobactam as monotherapy. 2
• Add vancomycin or linezolid if high suspicion for central line infection, septic shock, or high local MRSA prevalence. 2
• Add meropenem, aminoglycosides, or both in settings with high prevalence of extended-spectrum β-lactamase-producing Enterobacterales. 2

Skin and Soft Tissue Infections

For impetigo:

• Dicloxacillin, cephalexin, clindamycin, or amoxicillin-clavulanate. 1

For purulent skin infections:

• Dicloxacillin, cefazolin, cephalexin, clindamycin, doxycycline, or trimethoprim-sulfamethoxazole. 1

For MRSA skin infections (outpatient):

• Trimethoprim-sulfamethoxazole, doxycycline, minocycline, or linezolid. 1

For necrotizing fasciitis:

• Vancomycin or linezolid PLUS piperacillin-tazobactam or a carbapenem OR ceftriaxone plus metronidazole. 1

Diabetic Foot Infections

For mild infections (oral therapy):

• Dicloxacillin, clindamycin, cephalexin, levofloxacin, or amoxicillin-clavulanate. 1

For moderate-to-severe infections (parenteral therapy):

• Levofloxacin, cefoxitin, ceftriaxone, ampicillin-sulbactam, moxifloxacin, ertapenem, or imipenem-cilastatin. 1
• Add MRSA coverage if indicated: Linezolid, daptomycin 4-6 mg/kg IV daily, or vancomycin. 1, 3

Sexually Transmitted Infections

For urethritis/cervicitis due to Chlamydia trachomatis:

• Azithromycin single 1g oral dose is highly effective and offers superior compliance compared to multi-day regimens. 4, 5

For gonorrhea (Neisseria gonorrhoeae):

• Azithromycin is FDA-approved but should be combined with appropriate cephalosporin therapy per current CDC guidelines. 4

Anthrax (Bioterrorism)

For inhalational anthrax:

• Adults: Ciprofloxacin 400 mg IV every 12 hours OR doxycycline 100 mg IV every 12 hours PLUS one or two additional antimicrobials, continued for 60 days total. 2
• Children: Ciprofloxacin 10-15 mg/kg IV every 12 hours (max 1g/day) OR doxycycline (dose based on age/weight) PLUS one or two additional agents. 2

For cutaneous anthrax:

• Adults: Ciprofloxacin 500 mg PO twice daily OR doxycycline 100 mg PO twice daily for 60 days. 2

Critical Dosing Considerations

• Vancomycin requires therapeutic drug monitoring: Target trough levels of 15-20 mg/mL for serious infections; use loading dose of 25-30 mg/kg for severe illness. 2, 3
• Linezolid requires no dose adjustment for renal or hepatic impairment, making it advantageous in critically ill patients. 3
• Extended infusions of β-lactams (piperacillin-tazobactam, cefepime) optimize time above MIC for better outcomes in severe infections. 3
• Switch to oral therapy when hemodynamically stable, clinically improving, able to ingest medications, and afebrile for 48-72 hours. 2

Common Pitfalls to Avoid

• Do not use macrolide monotherapy for pneumonia when local S. pneumoniae resistance exceeds 25%, as bacteriologic failure rates of 20-25% are expected. 2
• Avoid cefepime when other options are available due to higher mortality risk compared to piperacillin-tazobactam (RR 1.39-1.44). 2
• Do not rely on azithromycin alone for severe pneumonia requiring hospitalization, as low serum concentrations may allow breakthrough bacteremia. 5, 6
• Always adjust therapy based on culture results when available rather than continuing empiric broad-spectrum coverage unnecessarily. 1
• Ensure minimum 5-day treatment duration for CAP with clinical stability (afebrile 48-72 hours, no more than one sign of instability) before discontinuation. 2