Chemotherapy
Antibiotics kill or slow bacteria by attacking cell walls, protein factories, or DNA machinery. They do nothing against viruses, fungi, or parasites—precision choice and full courses are essential to cure patients and limit resistance.
🔬 Major Classes & Quick Facts
| Class | Key Examples | Main Target | Route | Typical Use |
|---|---|---|---|---|
| β-Lactams | Amoxicillin, Ceftriaxone, Meropenem | Cell-wall synthesis | PO / IV / IM | Pneumonia, UTI, skin |
| Macrolides | Azithromycin, Clarithromycin | 50S ribosome | PO / IV | Atypical pneumonia, pertussis |
| Tetracyclines | Doxycycline, Minocycline | 30S ribosome | PO / IV | Acne, rickettsia, cholera |
| Aminoglycosides | Gentamicin, Amikacin | 30S ribosome (bactericidal) | IV / IM | Sepsis, gram-negative rods |
| Fluoroquinolones | Ciprofloxacin, Levofloxacin | DNA gyrase | PO / IV | UTI, pyelonephritis, GI |
| Sulfonamides + TMP | Cotrimoxazole (TMP-SMX) | Folate pathway | PO / IV | Pneumocystis pneumonia, UTI |
| Glycopeptides | Vancomycin, Teicoplanin | Cell wall (different site) | IV (PO for C. diff) | MRSA, severe gram-positive |
| Oxazolidinones | Linezolid, Tedizolid | 50S ribosome | PO / IV | VRE, MRSA pneumonia |
| Lincosamides | Clindamycin | 50S ribosome | PO / IV | Anaerobic, bone/joint, toxin suppression |
| Nitroimidazoles | Metronidazole, Tinidazole | DNA breakage | PO / IV | Anaerobes, C. diff, protozoa |
🧠 Key Concepts
1. Spectrum
- Narrow: Penicillin G (mostly streptococci)
- Broad: Piperacillin-tazobactam, ceftriaxone, ciprofloxacin
2. Resistance Mechanisms (MEMORIZE)
- Enzymes (β-lactamase)
- Mutation of target (DNA gyrase, PBP)
- Permeability drop (porin loss)
- Pumps (efflux)
Memory hook: “EM-PP” — Enzymes, Mutation, Permeability, Pumps.
3. Prescribing Principles
- Confirm bacterial infection when possible
- Culture before starting if safe
- Use local antibiograms
- Complete the full course (prevents regrowth of tougher bugs)
⚠️ Class-Specific Toxicities
β-Lactams
- Rash, C. diff colitis
- Seizures at high dose (penicillins, imipenem)
Aminoglycosides
- Nephrotoxicity (↑Cr)
- Ototoxicity (vertigo, hearing loss)
- Monitor peak & trough
Fluoroquinolones
- Tendon rupture (esp. Achilles)
- QT prolongation
- Photosensitivity
Vancomycin
- Red-man syndrome (rapid infusion)
- Nephrotoxicity with high trough
- Monitor trough 15-20 mg/L for severe infection
Linezolid
- Thrombocytopenia (platelets weekly)
- Serotonin syndrome with SSRIs
Metronidazole
- Disulfiram-like reaction with alcohol
- Peripheral neuropathy with long courses
🎯 High-Yield Bullets
- Penicillin = streptococci; add clavulanate to beat β-lactamase
- Ceftriaxone = first-line gonorrhea, meningitis, typhoid
- Azithromycin = atypical pneumonia, pertussis, single-dose for chlamydia
- Doxycycline = rickettsia, Lyme, cholera, anthrax, malaria prophylaxis
- Ciprofloxacin = UTI, gastroenteritis; NOT gonorrhea (resistance)
- TMP-SMX = Pneumocystis, Stenotrophomonas, some MRSA
- Vancomycin = MRSA; oral = C. diff colitis
- Linezolid = oral option for VRE/MRSA pneumonia
- Metronidazole = anaerobes below diaphragm + protozoa
📖 Abbreviations
| Abbrev | Meaning | Abbrev | Meaning |
|---|---|---|---|
| MRSA | Methicillin-Resistant Staph. aureus | VRE | Vancomycin-Resistant Enterococcus |
| TMP-SMX | Trimethoprim-Sulfamethoxazole | UTI | Urinary-Tract Infection |
| PO | By mouth | IV | Intravenous |
| Cr | Serum creatinine | QTc | Corrected QT interval |
🧭 Summary
Match the drug class to the bug, site, and patient factors. Know the mechanism → spectrum → toxicity → resistance chain and you’ll prescribe confidently—and safely.
Antibiotics are precision weapons—choose the right caliber, aim at the correct target, and stop firing when the battle is won.