What antibiotics are used to treat acute bacterial infections?

Antibiotics for Acute Bacterial Infections

For most acute bacterial respiratory tract infections in adults, amoxicillin or amoxicillin-clavulanate should be first-line therapy, with respiratory fluoroquinolones (levofloxacin, moxifloxacin) or ceftriaxone reserved for moderate-to-severe disease, recent antibiotic exposure, or treatment failure. 1

Community-Acquired Pneumonia (CAP)

Mild Disease (Outpatient, No Recent Antibiotics)

• Amoxicillin 1.5-4 g/day is the reference treatment for pneumococcal pneumonia 1
• In children <3 years: Amoxicillin 80-100 mg/kg/day in three divided doses 1
• In children >3 years with suspected atypical pathogens (Mycoplasma, Chlamydophila): macrolide therapy is reasonable 1
• Alternative options: cefpodoxime proxetil, cefuroxime axetil, or cefdinir 1

Moderate-to-Severe Disease or Recent Antibiotic Use

• Respiratory fluoroquinolone (levofloxacin 750 mg daily, moxifloxacin, or gatifloxacin) 1, 2
• High-dose amoxicillin-clavulanate (4 g/250 mg per day in adults; 90 mg/6.4 mg per kg per day in children) 1
• Ceftriaxone 1-2 g/day for 5 days (parenteral) 1
• Combination therapy: β-lactam plus macrolide (clarithromycin preferred over erythromycin due to fewer adverse events) 1

Critical caveat: The WHO 2024 guidelines note that azithromycin should be avoided due to increased cardiovascular risk, and fluoroquinolones carry risks of tendon, muscle, joint, nerve, and CNS adverse events 1. Reserve fluoroquinolones for patients who have failed other regimens or have β-lactam allergies 1.

Acute Bacterial Rhinosinusitis (ABRS)

Adults - Mild Disease (No Recent Antibiotics)

• Amoxicillin-clavulanate 1.75-4 g/250 mg per day (calculated efficacy 90-91%) 1
• Amoxicillin 1.5-4 g/day (calculated efficacy 87-88%) 1
• Alternatives: cefpodoxime proxetil, cefuroxime axetil, or cefdinir 1

Adults - Moderate Disease or Recent Antibiotic Use

• Respiratory fluoroquinolone (levofloxacin, moxifloxacin, gatifloxacin) - calculated efficacy 92% 1
• High-dose amoxicillin-clavulanate (4 g/250 mg per day) 1
• Ceftriaxone 1-2 g/day for 5 days 1

Children

• High-dose amoxicillin-clavulanate (90 mg/6.4 mg per kg per day) - calculated efficacy 91-92% 1
• High-dose amoxicillin (90 mg/kg per day) - calculated efficacy 86-87% 1
• For β-lactam allergic children: TMP/SMX, azithromycin, clarithromycin (though bacterial failure rates of 20-25% are possible) 1

Reassess after 72 hours: If no improvement, switch to alternative therapy or reevaluate the patient 1

Acute Exacerbation of Chronic Bronchitis

First-Line (Infrequent Exacerbations, FEV1 >35%)

• Amoxicillin is the reference compound 1
• Alternatives: first-generation cephalosporins, macrolides, pristinamycin, or doxycycline 1
• Indication: At least 2 of 3 Anthonisen criteria (increased dyspnea, sputum volume, sputum purulence) 1

Second-Line (Frequent Exacerbations ≥4/year or FEV1 <35%)

• Amoxicillin-clavulanate is the reference antibiotic 1
• Alternatives: cefuroxime-axetil, cefpodoxime-proxetil, cefotiam-hexetil, or respiratory fluoroquinolones (levofloxacin, moxifloxacin) 1
• Avoid: Ciprofloxacin and cefixime (inadequate pneumococcal coverage) unless Pseudomonas is suspected 1

Urinary Tract Infections

Uncomplicated Cystitis (Women)

• Nitrofurantoin (first choice) 1
• Trimethoprim-sulfamethoxazole 1
• Alternatives: fosfomycin, pivmecillinam, or amoxicillin-clavulanate 1

Acute Pyelonephritis

• Ceftriaxone or ciprofloxacin for empiric therapy 1
• Levofloxacin 750 mg daily for 5-10 days is FDA-approved and effective 2, 3
• Important resistance consideration: A 2021 Iranian study found ceftriaxone superior to levofloxacin for microbiological eradication (68.7% vs 21.4%), with high fluoroquinolone resistance rates 4. In France (2014), approximately 10% of community E. coli and 18% of hospital E. coli were fluoroquinolone-resistant 5

Complicated UTI

• Levofloxacin 750 mg daily for 5 days (mild-moderate) or 10 days (more severe) 2
• Alternatives based on susceptibility: amoxicillin-clavulanate, ceftazidime plus ampicillin, or aminoglycoside plus ampicillin 1

Skin and Soft Tissue Infections

Uncomplicated

• Levofloxacin 500 mg daily for 7-10 days (targets methicillin-susceptible S. aureus and S. pyogenes) 2, 6, 3

Complicated

• Levofloxacin 750 mg daily for 7-14 days (IV and/or oral) 2, 3
• Covers methicillin-susceptible S. aureus, Enterococcus faecalis, S. pyogenes, and Proteus mirabilis 2

Key Resistance and Safety Considerations

Fluoroquinolone stewardship: The widespread use of respiratory fluoroquinolones for mild disease promotes resistance across multiple organisms 1. Reserve for moderate-severe infections, treatment failures, or β-lactam allergies 1.

Penicillin-resistant S. pneumoniae: High-dose amoxicillin (4 g/day in adults, 90 mg/kg/day in children) or amoxicillin-clavulanate provides adequate coverage 1. Levofloxacin maintains activity against multi-drug resistant S. pneumoniae (MDRSP) with <1% resistance in the US 2, 3.

Duration: Pneumococcal pneumonia requires 10 days of β-lactam therapy; atypical pneumonia requires ≥14 days of macrolide therapy 1. Short-course levofloxacin 750 mg for 5 days is equivalent to 500 mg for 10 days in CAP 3.