Mechanisms of Antimicrobial Action: Inhibition of Cell Wall Synthesis

Mechanisms of Antimicrobial Action

Introduction

Antimicrobial agents are chemicals used to inhibit or kill microorganisms, including bacteria and fungi. Their effectiveness depends on their ability to target structures or processes unique to pathogens, minimizing harm to the host. Selective toxicity is a key principle, meaning the drug targets features present in microbes but absent in human cells.

• Selective toxicity: The ability of a drug to target pathogens without harming the host.

• Antimicrobial drugs: Chemicals that inhibit the growth of or kill microorganisms.

• Chemotherapy: The use of chemicals to treat disease, especially infections.

Antimicrobial drugs can be categorized by their mechanisms of action, which include inhibition of cell wall synthesis, protein synthesis, nucleic acid synthesis, metabolic pathways, and interference with cell membrane integrity or attachment.

Inhibition of Cell Wall Synthesis

Overview

Drugs that inhibit cell wall synthesis are selectively toxic to bacteria and fungi because animal cells lack cell walls. These drugs prevent the formation or cross-linking of essential cell wall components, leading to cell lysis due to osmotic pressure.

• Cell wall: A rigid structure that protects cells from osmotic pressure; present in bacteria and fungi, absent in animal cells.

• Peptidoglycan: The main structural component of bacterial cell walls, composed of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) subunits cross-linked by short peptides.

Inhibition of Synthesis of Bacterial Walls

Bacterial cell walls are primarily made of peptidoglycan. Drugs that inhibit peptidoglycan synthesis disrupt the integrity of the bacterial cell wall, causing cell death.

• Beta-lactams: A major class of antibiotics (e.g., penicillins, cephalosporins, carbapenems) that prevent cross-linking of NAM subunits by inhibiting transpeptidase enzymes.

• Bacitracin: Blocks the transport of peptidoglycan precursors across the cytoplasmic membrane.

• Vancomycin: A glycopeptide antibiotic that interferes with the formation of alanine-alanine bridges between NAM subunits.

• Cycloserine: Disrupts the formation of peptide cross-bridges in peptidoglycan.

• Ethambutol and Isoniazid: Inhibit the synthesis of mycolic acids in the cell walls of Mycobacterium species (agents of tuberculosis and leprosy).

These drugs are most effective against actively growing bacteria, as they prevent the synthesis of new cell wall material but do not affect existing peptidoglycan. Dormant cells are generally unaffected.

Table: Major Drugs Inhibiting Bacterial Cell Wall Synthesis

Drug/Class	Target/Mechanism	Example Organisms
Beta-lactams	Inhibit transpeptidase (cross-linking of NAM subunits)	Gram-positive and Gram-negative bacteria
Bacitracin	Blocks transport of peptidoglycan precursors	Gram-positive bacteria
Vancomycin	Disrupts alanine-alanine bridges in peptidoglycan	Gram-positive bacteria
Cycloserine	Disrupts peptide cross-bridge formation	Mycobacteria
Isoniazid, Ethambutol	Inhibit mycolic acid synthesis	Mycobacterium tuberculosis, Mycobacterium leprae

Inhibition of Synthesis of Fungal Walls

Fungal cell walls are composed of polysaccharides, including 1,3-β-glucan, which is not found in mammalian cells. Drugs that inhibit fungal cell wall synthesis are selectively toxic to fungi.

• Echinocandins: A class of antifungal drugs (e.g., caspofungin) that inhibit the enzyme responsible for synthesizing 1,3-β-glucan.

• Without glucan, fungal cells cannot maintain cell wall integrity, leading to osmotic rupture and cell death.

Table: Major Drugs Inhibiting Fungal Cell Wall Synthesis

Drug/Class	Target/Mechanism	Example Organisms
Echinocandins	Inhibit 1,3-β-glucan synthesis	Fungi (e.g., Candida, Aspergillus)

Key Equations and Concepts

• Peptidoglycan structure: Alternating NAG and NAM subunits cross-linked by peptides.

• Selective toxicity: $\text{Selective toxicity} = \frac{\text{Effect on pathogen}}{\text{Effect on host}}$

Examples and Applications

• Penicillin: Used to treat infections caused by Gram-positive bacteria by inhibiting cell wall synthesis.

• Caspofungin: Used to treat invasive fungal infections by inhibiting glucan synthesis in fungal cell walls.

Additional info: The notes infer the importance of cell wall synthesis inhibitors in clinical treatment, especially for diseases like tuberculosis and leprosy, which require long-term therapy due to slow bacterial growth rates.