What types of urinary tract infection (UTI) bacteria can amoxicillin treat?

Amoxicillin Coverage for UTI Bacteria

Amoxicillin alone has limited utility in modern UTI treatment due to high resistance rates (median 75% of E. coli isolates globally), and should NOT be used for empiric therapy; however, it remains effective against β-lactamase-negative strains of E. coli, Proteus mirabilis, and Enterococcus faecalis when susceptibility is confirmed. 1, 2

Bacteria Covered by Amoxicillin (When Susceptible)

Gram-Negative Organisms (β-lactamase-negative only):

• Escherichia coli - Only non-β-lactamase-producing strains 2
• Proteus mirabilis - Only non-β-lactamase-producing strains 2

Gram-Positive Organisms:

• Enterococcus faecalis - Remains susceptible in many cases 2
• Streptococcus agalactiae (Group B Strep) - Highly susceptible; amoxicillin is preferred treatment 3

Critical Resistance Considerations

The WHO removed amoxicillin from recommended empiric UTI treatment in 2021 after GLASS surveillance data showed 75% median resistance among E. coli urinary isolates (range 45-100% across 22 countries). 1 This makes amoxicillin unsuitable for empiric therapy without culture confirmation.

When Amoxicillin May Still Be Used:

• Culture-directed therapy only - After susceptibility testing confirms β-lactamase-negative organisms 2
• Enterococcal UTIs - Including vancomycin-resistant enterococcus (VRE) when ampicillin-resistant, using high-dose amoxicillin 500 mg PO/IV every 8 hours to achieve sufficient urinary concentrations 1
• Group B Streptococcus UTIs - Amoxicillin-clavulanic acid 875/125 mg PO twice daily provides broader coverage if mixed infection suspected 3

Amoxicillin-Clavulanic Acid: Superior Alternative

Amoxicillin-clavulanic acid is recommended as first-line empiric therapy for lower UTIs because the clavulanic acid component overcomes β-lactamase resistance. 1 This combination achieved 85% cure rates versus only 25% with amoxicillin alone in penicillin-resistant UTIs. 4

Coverage of Amoxicillin-Clavulanic Acid:

• E. coli - Including many β-lactamase-producing strains 1, 5
• Klebsiella species - Covered by combination 1
• Proteus species - Covered by combination 1
• Enterococcus species - Maintained coverage 1

Practical Clinical Algorithm

For Empiric Treatment (No Culture Available):

1. DO NOT use amoxicillin alone - Resistance rates too high 1
2. Use amoxicillin-clavulanic acid, nitrofurantoin, or trimethoprim-sulfamethoxazole instead as first-line options 1

For Culture-Directed Treatment:

1. If susceptibility shows β-lactamase-negative E. coli or Proteus - Amoxicillin 250-500 mg three times daily for 7 days acceptable 2, 6
2. If Enterococcus faecalis isolated - Amoxicillin remains excellent choice 2
3. If Streptococcus agalactiae - Amoxicillin preferred; use 5-7 days for uncomplicated UTI 3

For Complicated UTIs with Systemic Symptoms:

Use amoxicillin plus aminoglycoside combination for empiric treatment of complicated UTI requiring hospitalization 1. This provides broader gram-negative coverage while awaiting cultures.

Common Pitfalls to Avoid

• Never use amoxicillin empirically for suspected E. coli UTI - 75% global resistance rate makes treatment failure likely 1
• Do not assume amoxicillin-clavulanic acid and amoxicillin are interchangeable - The clavulanic acid component is critical for overcoming resistance 4
• For VRE UTIs, use high-dose amoxicillin (500 mg every 8 hours) to achieve adequate urinary concentrations that overcome resistance 1
• Avoid amoxicillin in patients with recent antibiotic exposure - Prior β-lactam use increases likelihood of resistant organisms 5