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Antibiotic Classes: Classification, Mechanism, and Clinical Use

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1. Cell Wall Synthesis Inhibitors

Beta-Lactams

Penicillins (Bactericidal)

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Cephalosporins (Bactericidal)

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Carbapenems (Bactericidal)

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Monobactams

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Glycopeptides (Bactericidal)

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Other Cell Wall Inhibitors

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2. DNA Synthesis Inhibitors

Fluoroquinolones (Bactericidal)

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Nitroimidazoles (Bactericidal)

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Trimethoprim-Sulfamethoxazole (TMP-SMX)

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Nitrofurantoin

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3. RNA Synthesis Inhibitors

Rifamycins (Bactericidal)

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4. Ribosome 50S Inhibitors (Bacteriostatic)

Macrolides

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Lincosamides

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Chloramphenicol

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Oxazolidinones

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5. Ribosome 30S Inhibitors

Aminoglycosides (Bactericidal)

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Tetracyclines (Bacteriostatic)

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Quick Reference Summary Table

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Antibiotic Mechanisms of Action

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🔴 TARGET 1: Cell Wall Synthesis

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Beta-Lactams (Penicillins, Cephalosporins, Carbapenems, Monobactams)

1. The beta-lactam ring mimics the D-Ala-D-Ala terminus of the peptidoglycan precursor (NAM-pentapeptide)
2. The drug binds irreversibly to Penicillin-Binding Proteins (PBPs) — enzymes (transpeptidases) that cross-link peptidoglycan strands
3. Transpeptidation is blocked — no new cross-links can form
4. The cell wall weakens as it continues to grow without repair
5. Osmotic pressure causes cell lysis and death

Bactericidal. Effective only against actively dividing bacteria (requires ongoing cell wall synthesis).

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Glycopeptides (Vancomycin, Teicoplanin)

1. Vancomycin is a large molecule - it cannot bind PBPs directly
2. Instead, it binds directly to the D-Ala-D-Ala terminus of the peptidoglycan precursor (NAM-pentapeptide) via hydrogen bonds
3. This sterically blocks both transglycosylation (chain elongation) AND transpeptidation (cross-linking)
4. Cell wall synthesis halts → cell lysis

Bactericidal. Only active against Gram-positive organisms (too large to cross the outer membrane of GN bacteria).

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Daptomycin (Lipopeptide)

1. In the presence of calcium, daptomycin inserts its lipid tail into the bacterial cell membrane (Gram-positive only)
2. Forms ion channels (pores) in the membrane
3. Rapid efflux of K⁺ ions → depolarization of membrane potential
4. Disruption of DNA, RNA, and protein synthesis simultaneously
5. Rapid cell death

Bactericidal. Fastest-killing antibiotic. Inactivated by pulmonary surfactant - NOT used for pneumonia.

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Bacitracin (Topical)

Topical use only due to nephrotoxicity systemically.

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🔵 TARGET 2: DNA Replication & Repair

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Fluoroquinolones (Ciprofloxacin, Levofloxacin, Moxifloxacin)

1. Enter the bacterial cell and bind to DNA gyrase (topoisomerase II) in Gram-negative bacteria and topoisomerase IV in Gram-positive bacteria
2. These enzymes normally relieve torsional stress ahead of the replication fork and decatenate daughter chromosomes after replication
3. Fluoroquinolones stabilize the enzyme-DNA cleavage complex — they trap the enzyme while it has cut the DNA
4. The double-strand DNA breaks accumulate and cannot be resealed
5. DNA fragmentation → cell death

Bactericidal. Concentration-dependent killing.

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Metronidazole (Nitroimidazole)

1. Metronidazole is a prodrug — it is selectively taken up by anaerobic and microaerophilic organisms
2. Intracellularly, low-redox-potential electron carriers (ferredoxin in anaerobes) donate electrons to the nitro group
3. This reduction generates highly reactive nitro radical intermediates
4. These radicals directly damage bacterial DNA — causing strand breaks and base modifications
5. DNA is degraded → rapid cell death

Bactericidal. Active ONLY against anaerobes and protozoa (requires anaerobic conditions for activation). Aerobic bacteria lack the reducing environment needed.

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Rifamycins (Rifampin, Rifaximin)

1. Rifampin enters the bacterial cell and binds to the beta subunit of bacterial RNA polymerase (encoded by rpoB gene)
2. Binding occurs near the active site of the enzyme, in the RNA/DNA channel
3. This blocks the initiation of RNA transcription — the enzyme can bind DNA and form the open complex but cannot begin synthesizing RNA
4. Without mRNA, protein synthesis stops
5. Cell death follows

Bactericidal. Never used as monotherapy (rapid resistance emerges via single rpoB point mutations). Always combined (e.g., with INH + PZA + EMB for TB).

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Trimethoprim-Sulfamethoxazole (TMP-SMX)

GTP / PABA
    ↓  ← [SMX blocks here — inhibits dihydropteroate synthase]
Dihydropteroic acid
    ↓
Dihydrofolic acid (DHF)
    ↓  ← [TMP blocks here — inhibits dihydrofolate reductase]
Tetrahydrofolic acid (THF)  ← active form needed for purine/pyrimidine synthesis
    ↓
DNA precursors (purines, thymidine)

• SMX (sulfonamide) is a structural analogue of PABA — competitively inhibits dihydropteroate synthase
• TMP competitively inhibits dihydrofolate reductase (DHFR) with 50,000-100,000x greater affinity for bacterial DHFR vs. human DHFR
• Humans obtain folate from diet - bacteria must synthesize it - so this is a selective target
• Sequential blockade produces a synergistic bactericidal effect (each drug alone is bacteriostatic)

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Nitrofurantoin

1. Taken up by bacteria and reduced by bacterial nitrofuran reductase into reactive intermediates
2. These intermediates damage ribosomal proteins, DNA, and cell respiration simultaneously
3. Multi-target damage makes resistance very rare

Concentrates in urine only — useful for lower UTI only, not systemic infections.

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🟢 TARGET 3: Protein Synthesis — 50S Ribosomal Subunit

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Macrolides (Erythromycin, Clarithromycin, Azithromycin)

1. Bind irreversibly to the 23S rRNA of the 50S ribosomal subunit
2. Block the peptide exit tunnel — the tunnel through which the growing polypeptide chain passes
3. Peptide chain cannot elongate beyond a few amino acids
4. Protein synthesis arrests → bacteriostatic (bactericidal at high concentrations)

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Clindamycin (Lincosamide)

1. Binds to the 50S ribosomal subunit at the same site as macrolides (23S rRNA, peptidyl transferase center)
2. Inhibits peptide bond formation (peptidyl transferase activity)
3. Prevents translocation of the ribosome along mRNA
4. Protein synthesis halts → bacteriostatic

Cross-resistance with macrolides common (both target the same ribosomal site - MLS resistance).

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Chloramphenicol

1. Binds reversibly to the 23S rRNA of the 50S subunit at the peptidyl transferase center
2. Inhibits peptidyl transferase — prevents formation of the peptide bond between the growing chain and the incoming amino acid
3. Protein synthesis stops → bacteriostatic (bactericidal for H. influenzae, N. meningitidis, S. pneumoniae)

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Linezolid (Oxazolidinone)

1. Binds to the 23S rRNA of the 50S subunit near the A site
2. Prevents formation of the 70S initiation complex — the 30S and 50S subunits cannot come together properly
3. Translation is blocked before it even begins — at the initiation stage
4. This unique mechanism means no cross-resistance with other protein synthesis inhibitors

Bacteriostatic against most organisms. Also a weak MAO inhibitor — risk of serotonin syndrome.

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🟡 TARGET 4: Protein Synthesis — 30S Ribosomal Subunit

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Aminoglycosides (Gentamicin, Tobramycin, Amikacin, Streptomycin)

1. Enter the bacterial cell via oxygen-dependent transport (this is why they are inactive against strict anaerobes)
2. Bind irreversibly to the 16S rRNA of the 30S subunit (specifically the A site — aminoacyl-tRNA decoding site)
3. Cause misreading of the genetic code — wrong amino acids are inserted into the protein
4. Misfolded/aberrant proteins are inserted into the cell membrane, increasing permeability
5. More drug enters → irreversible binding → accelerating cycle → rapid cell death

Bactericidal. Concentration-dependent killing (higher peak concentration = better killing). Require oxygen for uptake — inactive against anaerobes.

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Tetracyclines (Doxycycline, Tetracycline, Minocycline)

1. Enter bacteria via active transport (outer membrane porins in GN, then energy-dependent pump in cytoplasm)
2. Bind reversibly to the 16S rRNA of the 30S subunit at the A site
3. Block binding of aminoacyl-tRNA to the A site — the ribosome cannot "accept" the next amino acid
4. Peptide chain elongation stops
5. Protein synthesis is inhibited → bacteriostatic

Reversible binding = bacteriostatic (unlike aminoglycosides which bind irreversibly = bactericidal).

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Summary Diagram

BACTERIAL CELL
┌─────────────────────────────────────────────────────────┐
│                                                         │
│  CELL WALL                                              │
│  ├── Beta-lactams → Block transpeptidation (PBPs)       │
│  ├── Vancomycin → Block D-Ala-D-Ala binding             │
│  └── Daptomycin → Membrane depolarization               │
│                                                         │
│  DNA                                                    │
│  ├── Fluoroquinolones → Trap DNA gyrase / Topo IV       │
│  ├── Metronidazole → Free radical DNA strand breaks     │
│  └── Rifampin → Block RNA polymerase                    │
│                                                         │
│  FOLATE PATHWAY                                         │
│  └── TMP-SMX → Sequential blockade → No DNA precursors  │
│                                                         │
│  RIBOSOME (50S)                                         │
│  ├── Macrolides → Block peptide exit tunnel             │
│  ├── Clindamycin → Inhibit peptidyl transferase         │
│  ├── Chloramphenicol → Inhibit peptidyl transferase     │
│  └── Linezolid → Block 70S initiation complex           │
│                                                         │
│  RIBOSOME (30S)                                         │
│  ├── Aminoglycosides → Misreading of mRNA (irreversible)│
│  └── Tetracyclines → Block tRNA-A site binding          │
│                                                         │
└─────────────────────────────────────────────────────────┘

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Bactericidal vs. Bacteriostatic at a Glance

Clinical note: Bactericidal agents are preferred in immunocompromised patients, endocarditis, and meningitis - conditions where the host immune system cannot clear residual organisms.