Antimicrobial Drugs: History, Sources, Mechanisms, and Resistance

Antimicrobial Drugs: History, Sources, Mechanisms, and Resistance

Introduction

This study guide summarizes the key concepts from Chapter 10 of Microbiology with Diseases by Taxonomy, focusing on the history, sources, mechanisms of action, clinical considerations, and resistance related to antimicrobial drugs. Understanding these principles is essential for microbiology students preparing for exams or clinical applications.

History of Antimicrobial Agents

Development and Discovery

• Chemotherapeutic agents: Chemicals that affect physiology in any manner, especially those used to treat diseases.

• Antimicrobial agents (antimicrobials): Drugs that treat infections caused by microorganisms.

• Paul Ehrlich: Introduced the concept of "magic bullets"—chemicals (e.g., arsenic compounds) that selectively kill microbes without harming the host.

• Alexander Fleming: Discovered penicillin, the first true antibiotic, released from the fungus Penicillium.

• Gerhard Domagk: Discovered sulfanilamide, the first widely used synthetic antimicrobial.

• Selman Waksman: Discovered antibiotics produced naturally by organisms, such as Streptomyces species.

Types of Antimicrobial Agents

• Antibiotics: Antimicrobial agents produced naturally by microorganisms.

• Synthetics: Antimicrobials that are completely synthesized in a laboratory.

• Semi-synthetics: Chemically altered antibiotics that are more effective, longer lasting, or easier to administer than naturally occurring ones.

Example: Antibiotic Effect of Penicillium chrysogenum

• Zone of inhibition: Area around the fungus where bacterial growth is prevented, demonstrating the antimicrobial effect.

Sources of Common Antibiotics and Semisynthetics

Many antibiotics are derived from bacteria and fungi. The following table summarizes key sources and their associated antimicrobials:

Microorganism	Antimicrobial
Penicillium chrysogenum	Penicillin G
Penicillium griseofulvum	Griseofulvin
Acremonium spp.	Cephalosporin
Amycolatopsis orientalis	Vancomycin
Amycolatopsis rifamycinica	Rifampin
Bacillus licheniformis	Bacitracin
Bacillus polymyxa	Polymyxin
Micromonospora purpurea	Gentamicin
Pseudomonas fluorescens	Mupirocin
Saccharopolyspora erythraea	Erythromycin
Streptomyces griseus	Streptomycin
Streptomyces fradiae	Neomycin
Streptomyces aureofaciens	Tetracycline
Streptomyces venezuelae	Chloramphenicol
Streptomyces nodosus	Amphotericin B
Streptomyces avermitilis	Ivermectin
Streptomyces noursei	Nystatin

Mechanisms of Antimicrobial Action

Selective Toxicity

• Successful chemotherapy requires selective toxicity: the drug must harm the pathogen without harming the host.

• Bacterial drugs are the most diverse; drugs for eukaryotic pathogens (fungi, protozoa) are more limited due to similarities with human cells.

Inhibition of Cell Wall Synthesis

• Most common agents prevent cross-linkage of N-acetylmuramic acid (NAM) subunits in peptidoglycan.

• Beta-lactams (e.g., penicillins, cephalosporins) inhibit enzymes that form cross-links, weakening the cell wall and causing lysis.

• Vancomycin and bacitracin inhibit cell wall synthesis by different mechanisms.

• Isoniazid and ethambutol disrupt mycolic acid formation in mycobacterial species (e.g., Mycobacterium tuberculosis).

• Effective only for growing cells; no effect on existing peptidoglycan.

• Fungal cell wall inhibitors (e.g., echinocandins) target glucan synthesis.

Inhibition of Protein Synthesis

• Prokaryotic ribosomes are 70S (30S + 50S), while eukaryotic ribosomes are 80S (40S + 60S).

• Drugs can selectively target bacterial translation (e.g., aminoglycosides, tetracyclines).

• Mupirocin inhibits isoleucyl-tRNA synthetase, blocking isoleucine incorporation in Gram-positive bacteria.

Disruption of Cytoplasmic Membranes

• Some drugs form channels in membranes, compromising integrity (e.g., polymyxin for Gram-negative bacteria).

• Amphotericin B and nystatin bind to ergosterol in fungal membranes, causing leakage.

• Humans are less affected due to cholesterol instead of ergosterol, but toxicity is possible.

Inhibition of Metabolic Pathways

• Antimetabolic agents exploit differences between pathogen and host metabolism.

• Sulfonamides and trimethoprim inhibit folic acid synthesis, essential for nucleotide production.

• Other agents block viral uncoating (e.g., amantadine) or protozoal electron transport (e.g., atovaquone).

Inhibition of Nucleic Acid Synthesis

• Some drugs block DNA replication or RNA transcription (e.g., quinolones inhibit DNA gyrase).

• Nucleotide/nucleoside analogs distort nucleic acid structure, preventing replication (used against viruses and cancer cells).

• Reverse transcriptase inhibitors target viral enzymes (e.g., HIV), with minimal effect on human cells.

Prevention of Virus Attachment, Entry, or Uncoating

• Attachment antagonists block viral proteins or host receptors, preventing infection (e.g., pleconaril).

• Uncoating inhibitors (e.g., arildone) prevent viral genome release.

Clinical Considerations in Prescribing Antimicrobial Drugs

Ideal Antimicrobial Agent

• Readily available, inexpensive, chemically stable, easily administered, nontoxic, nonallergenic, and selectively toxic against a wide range of pathogens.

Spectrum of Action

• Narrow-spectrum: Effective against a limited group of organisms.

• Broad-spectrum: Effective against many organisms, but may disrupt normal flora and cause secondary infections.

Effectiveness Testing

• Diffusion susceptibility (Kirby-Bauer) test: Measures zone of inhibition around antibiotic disks.

• Minimum inhibitory concentration (MIC) test: Determines the lowest concentration that inhibits growth.

• Minimum bactericidal concentration (MBC) test: Determines the lowest concentration that kills bacteria.

Routes of Administration

• Topical: Applied to external infections.

• Oral: Taken by mouth; convenient but variable absorption.

• Intramuscular (IM): Injected into muscle; allows moderate absorption.

• Intravenous (IV): Delivered directly into bloodstream; rapid and high levels.

Toxicity and Side Effects

• Some drugs are toxic to kidneys, liver, or nerves; caution needed for pregnant women.

• Therapeutic index (TI): Ratio of tolerated dose to effective dose; higher TI indicates greater safety.

• Allergic reactions (e.g., anaphylactic shock) are rare but serious.

• Disruption of normal microbiota can lead to secondary infections, especially in hospitalized patients.

Resistance to Antimicrobial Drugs

Development of Resistance

• Some pathogens are naturally resistant; others acquire resistance via mutations or horizontal gene transfer (e.g., R plasmids).

Mechanisms of Resistance

• Enzyme production that destroys or deactivates the drug (e.g., beta-lactamase).

• Prevention of drug entry into the cell.

• Alteration of drug target so binding is less effective.

• Alteration of metabolic pathways.

• Efflux pumps that expel the drug from the cell.

• Biofilm formation reduces drug penetration.

• Production of proteins that protect target enzymes (e.g., MfpA in Mycobacterium tuberculosis).

Multiple and Cross Resistance

• Pathogens may acquire resistance to multiple drugs, especially in healthcare settings.

• Multiple-drug-resistant (MDR) pathogens: Resistant to at least three antimicrobial agents.

• Cross resistance: Resistance to drugs with similar structures or mechanisms.

Preventing Resistance

• Maintain high drug concentrations to eliminate pathogens.

• Use drug combinations (synergism enhances effect; antagonism reduces efficacy).

• Use antimicrobials only when necessary.

• Develop new drugs and modify existing ones (second- and third-generation drugs).

• Research new antibiotics, bacteriocins, and drugs targeting unique microbial proteins.

Example: Beta-Lactamase Action

• Beta-lactamase (penicillinase): Enzyme that hydrolyzes the beta-lactam ring of penicillins, rendering them inactive.

Example: Synergism

• Combining two drugs (e.g., amoxicillin and clavulanic acid) can enhance antimicrobial activity beyond either drug alone.