Bacteria Most Likely to be Resistant to Amoxicillin
The bacteria most likely to be resistant to amoxicillin include Escherichia coli (with approximately 75% resistance), Enterobacter species, Citrobacter species, Serratia species, Klebsiella species, and Moraxella catarrhalis (nearly 100% resistance). 1, 2, 3
Major Resistant Bacteria and Mechanisms
Enterobacteriaceae with Intrinsic Resistance
- Enterobacter species, Citrobacter species, and Serratia species possess chromosomal inducible AmpC β-lactamases that confer intrinsic resistance to amoxicillin 4, 3
- These bacteria can easily express AmpC enzymes at high levels through mutation, leading to resistance to many β-lactam antibiotics 4, 5
- Klebsiella pneumoniae produces SHV-1 β-lactamases and K. oxytoca produces chromosomal K1 β-lactamase, causing resistance to amoxicillin 3
Common Respiratory Pathogens
- Moraxella catarrhalis is nearly 100% β-lactamase positive, making it highly resistant to amoxicillin 2, 6
- Between 10% and 42% of Haemophilus influenzae strains are β-lactamase positive and resistant to amoxicillin 2, 6
- Streptococcus pneumoniae has variable resistance rates (10-15% nationally, but up to 50-60% in some areas) due to altered penicillin-binding proteins 2, 6
Escherichia coli
- Global data shows that approximately 75% (range 45-100%) of E. coli urinary isolates are resistant to amoxicillin 1
- This high resistance rate has led to the removal of amoxicillin from WHO recommendations for empiric treatment of urinary tract infections 1
- E. coli can acquire resistance through plasmid-mediated β-lactamases or occasionally through hyperexpression of chromosomal AmpC enzymes 4, 7
Resistance Mechanisms
β-lactamase Production
- The primary mechanism of amoxicillin resistance is through β-lactamases that cleave the β-lactam ring, rendering the antibiotic inactive 8, 9
- Common β-lactamases include:
Other Resistance Mechanisms
- Altered penicillin-binding proteins (PBPs) with reduced affinity for β-lactams (common in S. pneumoniae and MRSA) 4, 2
- Reduced influx through porin loss or enhanced efflux pump activation 5
- Increased plasmid copy numbers or gene amplification 7
Clinical Implications
Treatment Considerations
- Amoxicillin-clavulanate remains effective against many amoxicillin-resistant strains that produce certain β-lactamases 1
- For suspected AmpC-producing Enterobacterales, carbapenems or cefepime may be more appropriate 11
- High-dose amoxicillin (80-90 mg/kg/day) may overcome intermediate resistance in S. pneumoniae 2, 6
- Local resistance patterns should guide empiric therapy decisions 2
Risk Factors for Resistant Organisms
- Recent antibiotic use (within previous 30 days) 2, 6
- Young age (less than 2 years in pediatric populations) 2
- Attendance at childcare facilities 2
- Healthcare-associated infections (particularly for AmpC producers) 11
Geographic Considerations
- Resistance patterns vary considerably by region and community 2, 6
- In low and lower-middle-income countries, resistance to amoxicillin among Gram-negative bacteria causing neonatal sepsis is particularly high 12
- The emergence of ESBL-producing organisms is becoming a significant problem in outpatient settings worldwide 10