Antibiotics
One-Sentence Definition
Antibiotics are antimicrobial agents that kill or inhibit bacteria by attacking structures or pathways that bacteria have and human cells do not.
Simple Explanation
Selective toxicity: hit something the bacterium needs and we don’t have.
Detailed Scientific Explanation
By target
| Target | Classes | Notes |
|---|---|---|
| Cell wall synthesis | β-lactams (penicillins, cephalosporins, carbapenems, monobactams), glycopeptides (vancomycin) | Bactericidal; need actively dividing cells |
| Protein synthesis (50S) | Macrolides, lincosamides, oxazolidinones, chloramphenicol | Mostly bacteriostatic |
| Protein synthesis (30S) | Aminoglycosides, tetracyclines | Aminoglycosides bactericidal, concentration-dependent |
| DNA replication | Fluoroquinolones (gyrase/topoisomerase IV) | Broad, but collateral damage and toxicity |
| RNA polymerase | Rifamycins | Rapid resistance if used alone |
| Folate pathway | Sulfonamides, trimethoprim | Synergistic in combination |
| Membrane | Polymyxins (colistin), daptomycin | Last-line; nephrotoxicity |
Pharmacodynamics
- Time-dependent (β-lactams): efficacy tracks time above MIC → frequent dosing or extended infusion
- Concentration-dependent (aminoglycosides, fluoroquinolones): peak/MIC ratio → once-daily high dose
- AUC/MIC (vancomycin, linezolid)
Spectrum and stewardship — narrow-spectrum when the organism is known; broad only while it is not. See Antimicrobial Stewardship.
Mechanism
Selective toxicity depends on targets absent or sufficiently different in humans: peptidoglycan, 70S ribosome, bacterial gyrase, bacterial folate synthesis.
Clinical Importance
- Choice depends on site (CSF/bone/urine penetration), host (renal/hepatic function, allergy, pregnancy), and local resistance patterns
Research Importance
- The pipeline is thin; see AI for Antibiotic Discovery and Protein Design for Antimicrobials
Diagnostic Relevance
- Susceptibility interpreted against breakpoints (EUCAST/CLSI) in Antimicrobial Susceptibility Testing
AMR Relevance
Every use selects for resistance — see Antimicrobial Resistance and Mechanisms of Antibiotic Resistance.
Related MOCs
Active Recall Questions
- What does “selective toxicity” mean and give two examples of exploited targets.
- Time-dependent vs concentration-dependent killing — how does each change dosing?
- Why is rifampicin rarely used as monotherapy?