Antibiotic Selection Based on Culture Susceptibilities
Switch to amoxicillin/clavulanate or trimethoprim/sulfamethoxazole for oral therapy, as both are susceptible and appropriate first-line alternatives to cephalexin for most urinary and skin/soft tissue infections. 1
Key Interpretation of Culture Results
The culture shows the organism is susceptible to nearly all tested antibiotics except cefazolin, which is marked "NR" (not reported) with an MIC ≤1. 2 This NR designation is critical—cefazolin susceptibility testing serves as a surrogate for oral first-generation cephalosporins like cephalexin (Keflex), and when cefazolin shows resistance or indeterminate results, cephalexin should not be relied upon. 2
Recommended Antibiotic Switches
For Urinary Tract Infections:
- Trimethoprim/sulfamethoxazole (≤20 MIC): Highly effective for UTIs with excellent urinary penetration; use if local resistance rates are <20% 1
- Nitrofurantoin (≤16 MIC): Excellent choice for uncomplicated lower UTIs with high urinary concentrations 1
- Amoxicillin/clavulanate (≤2 MIC): Appropriate alternative with good urinary penetration 1
Avoid fluoroquinolones (ciprofloxacin, levofloxacin) as first-line despite susceptibility, as they should be reserved for more serious infections to minimize resistance development and collateral damage. 1, 3
For Skin and Soft Tissue Infections:
- Amoxicillin/clavulanate: First-line choice for mild to moderate infections 1
- Trimethoprim/sulfamethoxazole: Appropriate alternative for purulent infections 1
- Ceftriaxone or cefepime: If parenteral therapy needed for moderate-severe infections 1
Clinical Decision Algorithm
Step 1: Identify infection type (UTI vs skin/soft tissue vs other)
Step 2: Assess severity:
- Mild/uncomplicated → oral therapy preferred
- Moderate/severe → consider parenteral options initially
Step 3: Select narrowest spectrum agent from susceptible options:
- UTI (uncomplicated): Nitrofurantoin > TMP/SMX > amoxicillin/clavulanate 1
- UTI (complicated/pyelonephritis): Ceftriaxone IV initially, then oral step-down 1
- Skin/soft tissue (mild): Amoxicillin/clavulanate > TMP/SMX 1
- Skin/soft tissue (moderate-severe): Ceftriaxone or ampicillin/sulbactam IV 1
Important Caveats
The cefazolin NR result is a red flag: Modern CLSI guidelines use cefazolin as a surrogate marker for oral first-generation cephalosporins, and this indeterminate result suggests cephalexin may not achieve adequate pharmacodynamic targets. 2
Avoid carbapenem overuse: Despite susceptibility to imipenem and meropenem, these should be reserved for confirmed multidrug-resistant organisms or severe sepsis to preserve their efficacy. 1
Consider infection source control: For complicated infections, antibiotic optimization alone may be insufficient without drainage, debridement, or removal of foreign bodies. 1