Amoxicillin as First-Line Treatment for UTI Bacteria
Amoxicillin is NO LONGER recommended as first-line empiric therapy for urinary tract infections due to high resistance rates (median 75% of E. coli isolates resistant), but it may be used for culture-directed treatment of susceptible isolates only. 1
Bacteria Susceptible to Amoxicillin (Culture-Directed Only)
When urine culture and susceptibility testing confirm susceptibility, amoxicillin can treat:
β-lactamase-Negative Organisms Only:
- Escherichia coli (only if susceptibility confirmed) 2
- Proteus mirabilis (only β-lactamase-negative strains) 2
- Enterococcus faecalis (only β-lactamase-negative strains) 2, 1
Critical Resistance Context
The WHO Expert Committee removed amoxicillin from recommended first-line options in 2021 after Global Antimicrobial Resistance Surveillance System (GLASS) data from 22 countries showed 75% median resistance (range 45-100%) among E. coli urinary isolates. 1
Why Amoxicillin Fails Empirically:
- Nearly 1 in 3 bacterial strains causing cystitis or pyelonephritis demonstrate amoxicillin resistance 3
- Resistance rates have made empiric use inappropriate in most clinical settings 1
- Treatment failures occur when amoxicillin is used against resistant organisms 4
Current First-Line Recommendations Instead
For lower urinary tract infections, use these first-line agents instead: 1
- Nitrofurantoin (5-day course)
- Trimethoprim-sulfamethoxazole (if local resistance <20%)
- Fosfomycin (single 3g dose)
- Amoxicillin-clavulanate (NOT plain amoxicillin)
When Amoxicillin May Be Appropriate
Culture-Directed Treatment Only:
Use amoxicillin ONLY after culture confirms susceptibility (β-lactamase-negative organisms). 2
Specific Scenario - VRE UTI:
- High-dose ampicillin (18-30g IV daily) or amoxicillin (500mg every 8 hours) may overcome ampicillin-resistant VRE in urinary tract infections due to high urinary drug concentrations 1
- Clinical and microbiological eradication rates of 88.1% and 86% respectively reported for ampicillin-resistant VRE UTI treated with ampicillin 1
Critical Pitfalls to Avoid
Never use amoxicillin empirically for UTI - resistance rates are too high for acceptable clinical outcomes 1, 3
Always obtain culture and susceptibility before considering amoxicillin - empiric use leads to treatment failures 1, 4
Distinguish between amoxicillin and amoxicillin-clavulanate - the addition of clavulanate maintains high E. coli susceptibility rates, while plain amoxicillin does not 1, 5
Do not use for β-lactamase-producing organisms - these are inherently resistant regardless of reported MIC 2