Aeromonas hydrophila Antibiotic Resistance Pattern
Aeromonas hydrophila demonstrates intrinsic resistance to penicillins (ampicillin, amoxicillin) and first-generation cephalosporins, while maintaining high susceptibility (>90%) to fluoroquinolones, third-generation cephalosporins, carbapenems, and aminoglycosides. 1, 2, 3
Intrinsic Resistance (Expected Resistance)
- All Aeromonas hydrophila strains are naturally resistant to ampicillin, amoxicillin, and first-generation cephalosporins due to chromosomally encoded beta-lactamases 3, 4
- Amoxicillin-clavulanate shows only 45% effectiveness against A. hydrophila, making it unreliable for empiric therapy 3
- Novobiocin resistance is common (>40% of isolates) and should not be used 5
High Susceptibility Antibiotics (>90% Susceptible)
- Fluoroquinolones (ciprofloxacin, ofloxacin, levofloxacin) demonstrate excellent activity with >90% susceptibility 1, 2, 3
- Third-generation cephalosporins (ceftazidime, cefotaxime, cefoperazone) show >90% susceptibility 2, 3, 4
- Fourth-generation cephalosporins (cefepime) maintain >90% susceptibility 2
- Carbapenems (imipenem, moxalactam) demonstrate >90% susceptibility 2, 4
- Aminoglycosides (amikacin) show >90% susceptibility, though gentamicin and kanamycin have slightly lower rates 2, 5, 4
- Aztreonam demonstrates high activity against all A. hydrophila strains 2, 3
- Chloramphenicol shows excellent activity (>90% susceptible) 3, 4
- Tetracycline maintains >90% susceptibility in most studies 3, 4
Moderate Susceptibility (Variable Resistance)
- Trimethoprim-sulfamethoxazole shows geographic variation: highly effective in some regions but increasing resistance noted in Taiwan 2, 3
- Piperacillin and mezlocillin demonstrate good activity but are less reliable than other beta-lactams 3
- Ticarcillin shows variable effectiveness depending on geographic region 3
Emerging Resistance Patterns
- Imipenem resistance has emerged at 13.7% in recent Brazilian isolates, representing a concerning trend for this previously highly effective agent 6
- Cefotaxime resistance reached 13.7% in recent studies, indicating declining third-generation cephalosporin effectiveness in some regions 6
- Ceftazidime resistance at 11.8% suggests geographic variation in third-generation cephalosporin susceptibility 6
- Ciprofloxacin resistance remains low at 3.9% but represents an increase from historical near-universal susceptibility 6
- Multidrug resistance (resistance to ≥3 antibiotic classes) occurs in 3.6% of isolates 6
Geographic Considerations
Taiwan isolates demonstrate higher resistance rates to tetracycline, trimethoprim-sulfamethoxazole, extended-spectrum cephalosporins, and aminoglycosides compared to U.S. and Australian strains 2
Brazilian isolates show concerning resistance patterns including 46.4% overall antibiotic resistance and 23.5% nalidixic acid resistance 6
Clinical Implications for Empiric Therapy
For severe A. hydrophila infections, the IDSA recommends doxycycline plus either ciprofloxacin or ceftriaxone as first-line combination therapy to prevent resistance development 7
Avoid empiric use of:
- Any penicillin without beta-lactamase inhibitor 3, 4
- First-generation cephalosporins 3, 4
- Amoxicillin-clavulanate (only 45% effective) 3
- Novobiocin, spiramycin, erythromycin (>40% resistance) 5
Preferred empiric agents based on resistance patterns: