Antimicrobial Drugs: Mechanisms, Actions, and Resistance

Antimicrobial Drugs: Introduction

Overview

Antimicrobial drugs are essential tools in the treatment and prevention of infectious diseases. They target specific structures or processes in microbes, minimizing harm to the host organism. Understanding their mechanisms and resistance is crucial for effective clinical use.

• Chemotherapy: Use of drugs to treat disease, especially infections.

• Antimicrobial drugs: Agents that interfere with the growth of microbes within a host, often working in conjunction with the immune system.

• Selective toxicity: The ability of a drug to target structures present in bacteria but absent in the host, resulting in minimal host damage.

• Antibiotics: Substances produced by microbes that, in small amounts, inhibit the growth of other microbes.

Classification of Antibiotics

Spectrum of Action

Antibiotics are classified based on their spectrum of activity and clinical application.

• Broad-band Antibiotics: Active against a diverse group of bacteria (e.g., tetracycline, carbapenems).

• Narrow-band Antibiotics: Active against specific pathogens (e.g., ethambutol for Mycobacteria).

• Reserve Antibiotics: Used for specific applications, such as infections with methicillin-resistant Staphylococcus aureus (MRSA).

Antibiotics – Mode of Action

Types of Effects

Antibiotics can exert different effects on bacterial cells, which are important for clinical decision-making.

• Bacteriostatic: Inhibit growth but do not kill bacteria; effect is reversible and only present while the antibiotic is administered.

• Bactericidal: Kill bacteria; effect is non-reversible. Includes direct and accessory bactericidal antibiotics.

• Bacteriolytic: Cause lysis and death of bacteria, resulting in a decrease in both total and viable cell counts.

Comparison Table: Antibiotic Effects

Type	Total Cell Count	Viable Cell Count
Bacteriostatic	Constant	Constant
Bactericidal	Constant	Decreases
Bacteriolytic	Decreases	Decreases

Measuring Antibiotic Activity

Minimal Inhibitory Concentration (MIC)

The MIC is the lowest concentration of an antibiotic that prevents visible growth of a microorganism.

• Determined by observing the zone of inhibition around a disk impregnated with antibiotic.

• Used to guide effective dosing in clinical settings.

Minimal Bactericidal Concentration (MBC)

The MBC is the lowest concentration of an antibiotic that kills the microorganism.

• Identified by transferring cultures from MIC tests to antibiotic-free media; if no growth occurs, MBC is established.

Sites of Action of Antibiotics

Major Targets

Antibiotics act on several key bacterial processes and structures:

• Cell wall synthesis

• Protein biosynthesis

• Transcription

• Replication

• Membrane integrity

• Metabolism

Inhibitors of Cell Wall Synthesis

Major Drug Classes

Cell wall synthesis inhibitors are among the most effective antibiotics, especially against Gram-positive bacteria.

• Cycloserine

• Fosfomycin

• Peptide antibiotics: Bacitracin, Vancomycin

• β-Lactam antibiotics: Penicillins (natural and semisynthetic), Cephalosporins

• Antimycobacterium antibiotics: Isoniazid (INH), Ethambutol

Peptidoglycan and Cell Wall Structure in Bacteria

Peptidoglycan Composition

Peptidoglycan is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria.

• Repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM)

• Peptide chains attached to NAM residues

• Crosslinking of peptide chains provides strength and rigidity

Crosslinking by Transpeptidation

Transpeptidation is the process by which peptide chains are crosslinked, a key step targeted by β-lactam antibiotics.

Peptidoglycan Biosynthesis

Stages of Synthesis

Peptidoglycan biosynthesis involves several steps:

1. Cytoplasmic stage: Synthesis of amino sugar and peptide precursors

2. Membrane stage: Transport of precursors across the membrane via bactoprenol

3. Cell wall stage: Polymerization and crosslinking by transpeptidase enzymes

Specific Antibiotics and Their Mechanisms

Cycloserine

Cycloserine is an alanine analog that inhibits the racemization of L-alanine to D-alanine and the formation of D-alanyl-D-alanine dipeptides, preventing crosslinking in peptidoglycan synthesis.

• No racemization of L-alanine to D-alanine

• No crosslinking by transpeptidation

Fosfomycin

Fosfomycin is an epoxide antibiotic that inactivates the enzyme UDP-N-acetylglucosamine-3-enolpyruvyltransferase (MurA), blocking the first committed step in peptidoglycan biosynthesis.

• Acts as a phosphoenolpyruvate (PEP) analog

• Alkylates active site cysteine residue

• Used for urinary tract infections

Peptide Antibiotics

Peptide antibiotics such as bacitracin and vancomycin interfere with cell wall synthesis in different ways.

• Bacitracin: Polypeptide antibiotic, primarily active against Gram-positive bacteria. Produced by Bacillus species. Interferes with synthesis of linear sugar strands of peptidoglycan and inhibits dephosphorylation of undecaprenyl pyrophosphate, blocking precursor transport.

• Vancomycin: Glycopeptide antibiotic that inhibits cell wall synthesis by binding to D-Ala-D-Ala termini of peptidoglycan precursors, preventing crosslinking.

Summary Table: Mechanisms of Cell Wall Synthesis Inhibitors

Antibiotic	Target	Mechanism
Cycloserine	D-Ala synthesis	Inhibits racemization and dipeptide formation
Fosfomycin	MurA enzyme	Blocks first step in peptidoglycan synthesis
Bacitracin	Undecaprenyl pyrophosphate	Blocks precursor transport
Vancomycin	D-Ala-D-Ala termini	Prevents crosslinking

Key Terms and Concepts

• Antimicrobial resistance: The ability of microbes to withstand the effects of drugs that once could successfully treat them.

• Transpeptidase: Enzyme responsible for crosslinking peptidoglycan strands.

• Bactoprenol: Lipid carrier molecule that transports peptidoglycan precursors across the cell membrane.

Example Application

Understanding the mechanisms of action and resistance of antimicrobial drugs is essential for selecting appropriate therapies and combating antibiotic resistance in clinical microbiology.