Antibiotics are chemical substances produced by microorganisms or synthesized chemically that inhibit the growth of or kill bacteria. They are a cornerstone of modern medicine, used to treat bacterial infections by targeting specific bacterial structures or functions without harming the host.

Mechanisms of Action of Antibiotics

Antibiotics act by interfering with vital bacterial processes. The main mechanisms include:

1. Inhibition of Cell Wall Synthesis

The bacterial cell wall, composed mainly of peptidoglycan, is essential for maintaining cell shape and integrity. Antibiotics targeting this process prevent bacteria from forming a functional cell wall, leading to cell lysis due to osmotic pressure.

• Beta-lactams (penicillins, cephalosporins, carbapenems): Bind to penicillin-binding proteins (PBPs), inhibiting peptidoglycan cross-linking.
• Glycopeptides (e.g., vancomycin): Bind to D-alanyl-D-alanine residues, blocking incorporation into the cell wall.
• Bacitracin: Interferes with peptidoglycan subunit transport across the membrane.

2. Disruption of Cytoplasmic Membrane

Some antibiotics disrupt the bacterial cell membrane's selective permeability, causing leakage and cell death.

• Polymyxins (polymyxin B, colistin): Bind to membrane lipids in Gram-negative bacteria, disrupting integrity.

3. Inhibition of Protein Synthesis

Bacteria synthesize proteins using 70S ribosomes composed of 30S and 50S subunits, distinct from eukaryotic ribosomes, allowing selective targeting.

• 30S subunit inhibitors:
  • Aminoglycosides (e.g., gentamicin): Bind to 16S rRNA, causing mRNA misreading (bactericidal).
  • Tetracyclines: Block tRNA attachment, hindering elongation (bacteriostatic).
• 50S subunit inhibitors:
  • Macrolides (e.g., erythromycin), lincosamides, chloramphenicol, oxazolidinones (e.g., linezolid): Inhibit elongation or ribosome complex formation (bacteriostatic).

4. Inhibition of Nucleic Acid Synthesis

Antibiotics can target bacterial DNA replication or RNA transcription.

• Fluoroquinolones: Inhibit DNA gyrase and topoisomerase IV.
• Rifamycins: Bind to RNA polymerase, blocking RNA synthesis.

5. Antimetabolite Activity

Some antibiotics inhibit essential bacterial metabolic pathways.

• Sulfonamides and trimethoprim: Inhibit folic acid synthesis.
• Isoniazid: Targets mycolic acid synthesis in mycobacteria.

Classification Based on Bactericidal vs. Bacteriostatic Action

Bactericidal antibiotics kill bacteria directly (e.g., beta-lactams, aminoglycosides, fluoroquinolones).

Bacteriostatic antibiotics inhibit bacterial growth, allowing immune clearance (e.g., tetracyclines, macrolides).