Antibiotics classification
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antibiotic classification mechanism of action cell wall protein synthesis
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Antibiotic Classification
Antibiotics are classified by chemical structure, mechanism of action, spectrum of activity, and target site. Below is a comprehensive overview.
1. By Mechanism of Action
🔬 Cell Wall Synthesis Inhibitors
β-Lactam Antibiotics
The largest and most widely used class. They contain a β-lactam ring and kill bacteria by binding penicillin-binding proteins (PBPs), blocking transpeptidase and inhibiting cross-linking of peptidoglycan in the bacterial cell wall.
| Sub-class | Ring structure | Key drugs |
|---|---|---|
| Penicillins | β-lactam + thiazolidine (5-membered) | Penicillin G/V, ampicillin, amoxicillin, nafcillin, oxacillin, piperacillin |
| Cephalosporins | β-lactam + dihydrothiazine (6-membered) | Cefazolin (1st), cefuroxime (2nd), ceftriaxone (3rd), cefepime (4th), ceftaroline (5th) |
| Carbapenems | Modified thiazolidine ring | Imipenem, meropenem, ertapenem, doripenem |
| Monobactams | Single β-lactam ring only (no second ring) | Aztreonam (Gram-negatives only; safe in penicillin allergy) |
β-Lactamase inhibitors (clavulanic acid, sulbactam, tazobactam, avibactam, vaborbactam, relebactam) are combined with β-lactams to overcome bacterial β-lactamase resistance. — Fishman's Pulmonary Diseases and Disorders
Penicillin sub-groups:
- Natural: Pen G, Pen V
- Aminopenicillins (extended spectrum): ampicillin, amoxicillin
- Antistaphylococcal (penicillinase-resistant): nafcillin, oxacillin, dicloxacillin
- Antipseudomonal: piperacillin, ticarcillin
- β-Lactam/β-lactamase inhibitor combinations: amoxicillin-clavulanate, ampicillin-sulbactam, piperacillin-tazobactam
Cephalosporin generations:
| Generation | Coverage | Examples |
|---|---|---|
| 1st | Gram+ (MSSA), limited Gram− | Cefazolin, cephalexin |
| 2nd | Broader Gram−, some anaerobes | Cefuroxime, cefoxitin, cefaclor |
| 3rd | Broad Gram−, CNS penetration | Ceftriaxone, cefotaxime, ceftazidime |
| 4th | Gram+/Gram−, antipseudomonal | Cefepime |
| 5th | MRSA coverage | Ceftaroline |
Glycopeptides
- Mechanism: Interfere with insertion of glycan subunits into the cell wall (bind D-Ala-D-Ala terminus of peptidoglycan precursors)
- Examples: Vancomycin, teicoplanin, dalbavancin, oritavancin
- Coverage: Gram-positive organisms, including MRSA; vancomycin is the drug of choice for MRSA
Lipopeptides
- Examples: Daptomycin
- Mechanism: Disrupts the bacterial cell membrane (distinct from cell wall synthesis per se)
- Coverage: Gram-positives including MRSA and VRE
Other cell wall agents
- Bacitracin: Inhibits cell wall synthesis by forming a complex with C55-phenol pyrophosphate; topical use
- Fosfomycin: Inhibits MurA (first step of peptidoglycan synthesis)
🔬 Protein Synthesis Inhibitors
| Class | Ribosomal Target | Key Drugs | Notes |
|---|---|---|---|
| Aminoglycosides | 30S subunit | Gentamicin, tobramycin, amikacin, streptomycin | Irreversible binding; bactericidal; ototoxicity & nephrotoxicity |
| Tetracyclines | 30S subunit (blocks aminoacyl-tRNA) | Tetracycline, doxycycline, minocycline, tigecycline | Bacteriostatic; broad spectrum; avoid in pregnancy/children |
| Macrolides | 50S subunit | Erythromycin, azithromycin, clarithromycin | Inhibit translocation of peptidyl-tRNA; cover atypicals |
| Clindamycin | 50S subunit | Clindamycin | Same binding site as macrolides; covers anaerobes & Gram+ |
| Chloramphenicol | 50S subunit | Chloramphenicol | Inhibits peptidyl transferase; risk of aplastic anemia |
| Lincosamides | 50S subunit | Clindamycin, lincomycin | Same as above |
| Oxazolidinones | 50S subunit (also inhibits 30S-50S initiation complex) | Linezolid, tedizolid | Bacteriostatic; covers MRSA/VRE |
| Streptogramins | 50S subunit | Quinupristin-dalfopristin | Combination bactericidal |
— Miller's Review of Orthopaedics, 9th ed.
🔬 Nucleic Acid Synthesis Inhibitors
| Class | Target | Key Drugs | Spectrum |
|---|---|---|---|
| Fluoroquinolones | DNA gyrase (topoisomerase II) & topoisomerase IV | Ciprofloxacin, levofloxacin, moxifloxacin, ofloxacin | Broad; Gram−, some Gram+, atypicals |
| Rifamycins | RNA polymerase (β-subunit) | Rifampin, rifabutin, rifaximin | TB, MRSA combination, C. difficile |
| Metronidazole | DNA strand breakage (after reduction) | Metronidazole | Anaerobes, protozoa |
| Nitrofurantoin | Multiple DNA targets | Nitrofurantoin | UTI only (limited tissue distribution) |
🔬 Folate Synthesis Inhibitors
| Class | Target | Key Drugs |
|---|---|---|
| Sulfonamides | Dihydropteroate synthase (DHPS) — blocks dihydropteroic acid formation | Sulfamethoxazole, sulfadiazine |
| Diaminopyrimidines | Dihydrofolate reductase (DHFR) | Trimethoprim |
| Combination | Sequential blockade of folate pathway | Trimethoprim-sulfamethoxazole (TMP-SMX) — bactericidal synergy |
TMP-SMX has bactericidal activity against S. pneumoniae, H. influenzae, and M. catarrhalis, but does not cover atypical pathogens. — Fishman's Pulmonary Diseases and Disorders
🔬 Cell Membrane Disruptors
| Class | Key Drugs | Notes |
|---|---|---|
| Polymyxins | Polymyxin B, colistin (polymyxin E) | Cyclic polypeptides; last-resort for MDR Gram-negative (e.g., Acinetobacter, Pseudomonas) |
| Lipopeptides | Daptomycin | Gram-positive; NOT used for pneumonia (inactivated by surfactant) |
2. Summary Table by Class
| Class | Mechanism | Examples | Spectrum |
|---|---|---|---|
| Penicillins | Cell wall (PBP binding) | Amoxicillin, piperacillin | Variable Gram+/Gram− |
| Cephalosporins (1st–5th gen) | Cell wall (PBP binding) | Cefazolin → Ceftaroline | Progressively broader |
| Carbapenems | Cell wall (PBP binding) | Imipenem, meropenem | Very broad (incl. ESBL) |
| Monobactams | Cell wall (PBP binding) | Aztreonam | Gram− only |
| Glycopeptides | Cell wall (D-Ala-D-Ala) | Vancomycin, teicoplanin | Gram+ only |
| Aminoglycosides | 30S ribosome | Gentamicin, amikacin | Gram−, synergy for Gram+ |
| Tetracyclines | 30S ribosome | Doxycycline, tigecycline | Broad, atypicals |
| Macrolides | 50S ribosome | Azithromycin, erythromycin | Gram+, atypicals |
| Clindamycin | 50S ribosome | Clindamycin | Gram+, anaerobes |
| Oxazolidinones | 50S ribosome | Linezolid | Gram+, MRSA/VRE |
| Chloramphenicol | 50S ribosome | Chloramphenicol | Broad |
| Fluoroquinolones | DNA gyrase/Topo IV | Ciprofloxacin, levofloxacin | Broad |
| Rifamycins | RNA polymerase | Rifampin | TB, Gram+ |
| Sulfonamides | DHPS (folate synthesis) | Sulfamethoxazole | Broad |
| Trimethoprim | DHFR (folate synthesis) | TMP-SMX | Broad |
| Metronidazole | DNA damage | Metronidazole | Anaerobes, protozoa |
| Polymyxins | Cell membrane | Colistin, Polymyxin B | MDR Gram− |
| Lipopeptides | Cell membrane | Daptomycin | Gram+ |
3. By Spectrum of Activity
| Spectrum | Classes/Examples |
|---|---|
| Narrow Gram+ | Penicillin G/V, cefazolin, vancomycin, daptomycin |
| Narrow Gram− | Aztreonam, colistin |
| Broad spectrum | Carbapenems, piperacillin-tazobactam, fluoroquinolones, TMP-SMX, tigecycline |
| Atypical organisms | Macrolides, doxycycline, fluoroquinolones |
| MRSA | Vancomycin, linezolid, daptomycin, ceftaroline, TMP-SMX |
| Anaerobes | Metronidazole, clindamycin, carbapenems, cefoxitin |
| Mycobacteria | Rifampin, isoniazid, ethambutol, pyrazinamide |
| Fungi | Amphotericin B, azoles, echinocandins (not true antibiotics but antimicrobials) |
4. Bactericidal vs. Bacteriostatic
| Bactericidal | Bacteriostatic |
|---|---|
| β-Lactams, glycopeptides, aminoglycosides, fluoroquinolones, metronidazole, daptomycin, rifampin | Tetracyclines, macrolides, clindamycin, linezolid, chloramphenicol, sulfonamides, trimethoprim |
Note: "bactericidal" and "bacteriostatic" are concentration- and organism-dependent distinctions; some agents can be bactericidal at high concentrations or against certain organisms.
Sources:
- Miller's Review of Orthopaedics, 9th Edition — Table 1.31, Mechanisms of Action
- Fishman's Pulmonary Diseases and Disorders, 2-Volume Set — β-Lactam Antibiotics; Trimethoprim-Sulfamethoxazole sections
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