🦠 Understanding Antibiotic Susceptibility
**Antibiotic susceptibility** must be carefully evaluated **before selecting an antibiotic** based on culture reports.
**Two key factors to consider:**
- ⚠️ **Intrinsic bacterial resistance to certain antibiotics.**
- 🧪 **In vivo (real-world) vs. In vitro (lab-based) effectiveness.**
🚫 Intrinsic Resistance - Antibiotics That Should NOT Be Used
Some bacteria are **naturally resistant** to specific antibiotics, making them **ineffective choices**, even if lab reports suggest sensitivity.
**Examples:**
- ❌ **Fosfomycin** has **no role in treating Acinetobacter infections**.
- ❌ **Tigecycline** is **ineffective against Pseudomonas aeruginosa**.
⚠️ In Vivo vs. In Vitro Susceptibility
**Antibiotic effectiveness should be based on real-world (in vivo) conditions, not just laboratory (in vitro) susceptibility.**
**Examples of antibiotics that appear effective in lab tests but fail in clinical use:**
- ❌ **Daptomycin cannot be used for pneumonia** – it is inactivated by lung surfactant.
- ❌ **Tigecycline cannot be used for bacteremia** – it has **poor serum concentrations**.
🔬 Key Learning
**Just because an antibiotic appears effective in lab tests does NOT mean it works in real infections.**
Always consider:
- 🛡️ **Intrinsic bacterial resistance.**
- ⚠️ **Pharmacokinetics and site of action in vivo.**
- 📊 **Proper antibiotic selection based on clinical efficacy.**
📢 Key Takeaways
- ✔️ **Intrinsic bacterial resistance must be considered before choosing an antibiotic.**
- ✔️ **Daptomycin & Tigecycline have specific clinical limitations.**
- ✔️ **Antibiotic selection should be based on in vivo effectiveness, not just lab reports.**
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