Classification of antibiotic and which bacteria they act
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I now have rich source material from Goodman & Gilman's and Campbell-Walsh Urology. Let me compile a comprehensive answer.
Classification of Antibiotics & Their Bacterial Spectrum
Antibiotics can be classified along several dimensions: chemical/structural class, mechanism of action, spectrum of activity, and bactericidal vs bacteriostatic effect.
I. Bactericidal vs Bacteriostatic
| Bactericidal | Bacteriostatic |
|---|---|
| β-Lactams | Tetracyclines |
| Aminoglycosides | Macrolides (azithromycin, erythromycin) |
| Fluoroquinolones | Sulfonamides |
| Vancomycin | Trimethoprim |
| Chloramphenicol | |
| Clindamycin |
Nitrofurantoin is generally bacteriostatic but can be bactericidal at high doses.
— Campbell-Walsh Urology, Table 55.4
II. Classification by Mechanism of Action
1. Cell Wall Synthesis Inhibitors
A. β-Lactams
All share the β-lactam ring and inhibit penicillin-binding proteins (PBPs), blocking peptidoglycan cross-linking.
Resistance: β-lactamase production, altered PBPs (e.g., MRSA), reduced porin penetration.
| Subclass | Examples | Gram (+) Coverage | Gram (−) Coverage |
|---|---|---|---|
| Aminopenicillins | Amoxicillin, Ampicillin | Streptococcus, Enterococci | Proteus mirabilis |
| + β-lactamase inhibitor | Amoxicillin-clavulanate, Ampicillin-sulbactam | Streptococcus, Enterococci, Staph (not MRSA) | P. mirabilis, Klebsiella, H. influenzae |
| Antistaphylococcal penicillins | Nafcillin, Oxacillin, Dicloxacillin | Streptococcus, Staphylococcus (not MRSA) | None |
| Antipseudomonal penicillins | Piperacillin-tazobactam | Streptococcus, Enterococci | Most GNRs including Pseudomonas aeruginosa |
| 1st-gen cephalosporins | Cefazolin, Cefalexin | Streptococcus, Staph (not MRSA) | E. coli, P. mirabilis, Klebsiella |
| 2nd-gen cephalosporins | Cefuroxime, Cefaclor (Group 1) | Streptococcus, Staph (not MRSA) | E. coli, P. mirabilis, H. influenzae, Klebsiella |
| Cefoxitin, Cefotetan (Group 2) | Streptococcus | E. coli, Proteus spp., H. influenzae, anaerobes | |
| 3rd-gen cephalosporins | Ceftriaxone | Streptococcus, Staph (not MRSA) | Most GNRs, excluding P. aeruginosa |
| Ceftazidime | Streptococcus | Most GNRs including P. aeruginosa | |
| 4th-gen cephalosporins | Cefepime | Streptococcus, Staph (not MRSA) | Broad GNRs including P. aeruginosa |
| 5th-gen cephalosporins | Ceftaroline | Streptococcus, MRSA | Broad GNRs (not Pseudomonas) |
| Monobactam | Aztreonam | None | Most GNRs including P. aeruginosa |
| Carbapenems | Imipenem, Meropenem, Ertapenem | Streptococcus, Staph (not MRSA) | Broadest — most GNRs including Pseudomonas (not Ertapenem) |
| Siderophore cephalosporin | Cefiderocol | Limited | Targets resistant GNRs via iron-chelation transport into outer membrane |
B. Glycopeptides
- Vancomycin — inhibits cell wall synthesis at a different point than β-lactams (binds D-Ala-D-Ala peptide terminus)
- Spectrum: Gram-positive only — S. aureus (including MRSA), S. epidermidis, Streptococcus, Enterococcus (VRE = resistant)
- Teicoplanin — similar spectrum to vancomycin
C. Fosfomycin
- Inhibits the first step of peptidoglycan synthesis (MurA enzyme)
- Spectrum: E. coli, Enterococcus faecalis (used for uncomplicated UTI)
2. Protein Synthesis Inhibitors
A. 30S Ribosomal Subunit Inhibitors
| Drug Class | Examples | Spectrum | Notes |
|---|---|---|---|
| Aminoglycosides | Gentamicin, Tobramycin, Amikacin, Streptomycin | Aerobic GNRs (E. coli, Pseudomonas, Klebsiella); synergistic on Gram (+) with β-lactams/vancomycin | Bactericidal; concentration-dependent; nephrotoxic/ototoxic |
| Tetracyclines | Tetracycline, Doxycycline, Minocycline | Broad spectrum — Gram (+), Gram (−), atypicals (Chlamydia, Mycoplasma, Rickettsia, Brucella) | Bacteriostatic |
| Glycylcyclines | Tigecycline | Very broad — MRSA, VRE, most GNRs, anaerobes (not Pseudomonas) | Overcomes classic tetracycline resistance |
B. 50S Ribosomal Subunit Inhibitors
| Drug Class | Examples | Spectrum | Notes |
|---|---|---|---|
| Macrolides | Erythromycin, Azithromycin, Clarithromycin | Gram (+) strep/staph, atypicals (Legionella, Mycoplasma, Chlamydia) | Bacteriostatic |
| Lincosamides | Clindamycin | S. aureus (not MRSA), Streptococcus, anaerobes | Bacteriostatic; no Gram (−) coverage |
| Chloramphenicol | Chloramphenicol | Broad — Gram (+), Gram (−), anaerobes, Rickettsia | Bacteriostatic; bone marrow toxicity |
| Oxazolidinones | Linezolid | Gram (+) only — MRSA, VRE, Streptococcus | Bacteriostatic |
| Streptogramins | Quinupristin-dalfopristin | Gram (+) — S. aureus, Streptococcus, VRE (E. faecium only) | Bactericidal in combination |
3. DNA/RNA Synthesis Inhibitors
A. Fluoroquinolones
- Mechanism: inhibit DNA gyrase (topoisomerase II) and topoisomerase IV → disrupt DNA replication
- Resistance: mutation in gyrase binding site, efflux pumps, porin changes
| Generation | Examples | Spectrum |
|---|---|---|
| 1st | Nalidixic acid | Gram (−) (urinary only) |
| 2nd | Ciprofloxacin, Ofloxacin | Broad GNRs including P. aeruginosa; limited Gram (+) |
| 3rd | Levofloxacin | Broad GNRs + enhanced Gram (+) (S. pneumoniae) |
| 4th | Moxifloxacin | Broad + anaerobes; not Pseudomonas |
B. Rifamycins
- Rifampicin (Rifampin) — inhibits bacterial DNA-dependent RNA polymerase
- Spectrum: Mycobacterium tuberculosis, S. aureus (used in combination); also N. meningitidis prophylaxis
C. Nitroimidazoles
- Metronidazole — forms toxic radical intermediates after anaerobic reduction
- Spectrum: Strict anaerobes (Bacteroides fragilis, Clostridium difficile) and protozoa (Giardia, Entamoeba, Trichomonas)
D. Nitrofurantoin
- Inhibits multiple bacterial enzyme systems (urinary concentrations required)
- Spectrum: E. coli, Staphylococcus saprophyticus, Enterococcus — UTI only
4. Folate Synthesis Inhibitors
| Drug | Target | Spectrum |
|---|---|---|
| Sulfonamides (sulfamethoxazole) | Dihydropteroate synthase (blocks PABA incorporation) | Gram (+), Gram (−) (Nocardia, Klebsiella granulomatis, H. ducreyi); also some fungi/parasites |
| Trimethoprim | Dihydrofolate reductase | Gram (+), Gram (−) — works synergistically with sulfonamides |
| TMP-SMX (co-trimoxazole) | Sequential blockade of folate pathway | E. coli, Staph, Streptococcus, H. influenzae, Pneumocystis jirovecii, Nocardia, Toxoplasma |
— Goodman & Gilman's Pharmacological Basis of Therapeutics
5. Cell Membrane Disruptors
| Drug | Mechanism | Spectrum |
|---|---|---|
| Polymyxins (colistin, polymyxin B) | Disrupt outer membrane of GNRs | Resistant GNRs — Acinetobacter, Pseudomonas, CRE; last-resort agents |
| Daptomycin | Depolarizes Gram (+) cell membrane | S. aureus (MRSA), Streptococcus, VRE |
III. Summary Spectrum Table
| Antibiotic Class | Gram (+) | Gram (−) | Anaerobes | Atypicals | Mycobacteria |
|---|---|---|---|---|---|
| Penicillin G/V | ✓✓ | Limited | Some | — | — |
| Aminopenicillins | ✓ | Limited | — | — | — |
| Antipseudomonal PCN | ✓ | ✓✓ (+ Pseudomonas) | ✓ | — | — |
| 1st-gen cephalosporins | ✓ | Limited | — | — | — |
| 3rd-gen cephalosporins | ✓ | ✓✓ | — | — | — |
| Carbapenems | ✓ | ✓✓ | ✓✓ | — | — |
| Vancomycin | ✓✓ (MRSA, VRE) | — | — | — | — |
| Aminoglycosides | Synergy only | ✓✓ | — | — | Streptomycin |
| Tetracyclines | ✓ | ✓ | Some | ✓✓ | — |
| Macrolides | ✓ | Limited | — | ✓✓ | Some |
| Fluoroquinolones | ✓ (3rd/4th) | ✓✓ | 4th gen | — | Some |
| Metronidazole | — | — | ✓✓ | — | — |
| TMP-SMX | ✓ | ✓ | — | Some | — |
| Rifampicin | ✓ | — | — | — | ✓✓ |
| Polymyxins | — | ✓✓ (resistant) | — | — | — |
| Linezolid | ✓✓ (MRSA, VRE) | — | — | — | Some |
Sources:
- Goodman & Gilman's The Pharmacological Basis of Therapeutics — Chapter: Antimicrobial Chemotherapy: Classes and Actions
- Campbell-Walsh Urology — Tables 55.4–55.6 (Bacteriostatic vs Bactericidal; Mechanism of Action; Antimicrobial Coverage)
- Katzung's Basic and Clinical Pharmacology, 16th Edition — Aminoglycosides & Spectinomycin chapter
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Antibiotic Classification
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Here's what's inside across 4 pages:
| Section | Content |
|---|---|
| I | Bactericidal vs Bacteriostatic — side-by-side colour table (blue/teal) |
| II | β-Lactams full subclass table (penicillins → carbapenems) + Glycopeptides |
| III | 30S inhibitors (aminoglycosides, tetracyclines) + 50S inhibitors (macrolides, linezolid, clindamycin, etc.) |
| IV | DNA/RNA inhibitors — fluoroquinolones, rifamycins, metronidazole, nitrofurantoin |
| V | Folate inhibitors — sulfonamides, TMP, TMP-SMX |
| VI | Cell membrane disruptors — polymyxins, daptomycin |
| VII | Master spectrum table (18 drug classes × 5 coverage categories, colour-coded ✓✓/✓/—) |
| VIII | Anti-mycobacterial agents — INH, rifampicin, PZA, EMB, streptomycin, dapsone |
Each section uses a distinct colour scheme (navy, teal, purple, orange, green, red) with alternating row shading for easy reading.
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