Antimicrobial Drugs: Mechanisms, Spectrum, Resistance, and Clinical Applications

History and Principles of Chemotherapy

Origins of Chemotherapy

Chemotherapy in microbiology refers to the use of chemicals to treat infectious diseases. The concept of selective toxicity, introduced by Paul Ehrlich, is fundamental: drugs should target pathogens without harming the host.

• Selective Toxicity: The ability of a drug to target microbial cells without affecting host cells.

• Magic Bullet: Ehrlich's idea of a compound that specifically targets pathogens.

• Antibiotic: A substance produced by a microbe that inhibits the growth of another microbe.

• Antimicrobial Drugs: Synthetic substances that interfere with microbial growth.

Discovery of Penicillin

Alexander Fleming discovered penicillin in 1928, marking the beginning of the antibiotic era. The first clinical trials of penicillin occurred in 1940.

• Penicillin: The first widely used antibiotic, effective against many bacterial infections.

Sources and Spectrum of Antibiotics

Representative Sources of Antibiotics

Antibiotics are derived from various microorganisms, including bacteria, actinomycetes, and fungi.

• Gram-positive rods: Bacillus subtilis (Bacitracin), Paenibacillus polymyxa (Polymyxin)

• Actinomycetes: Streptomyces species produce many antibiotics (e.g., Streptomyces nodosus produces Amphotericin B).

• Fungi: Penicillium species produce penicillin.

Microorganism	Antibiotic
Bacillus subtilis	Bacitracin
Paenibacillus polymyxa	Polymyxin
Streptomyces nodosus	Amphotericin B
Streptomyces venezuelae	Chloramphenicol
Streptomyces aureofaciens	Chlortetracycline and tetracycline
Streptomyces erythraeus	Erythromycin
Streptomyces fradiae	Neomycin
Streptomyces griseus	Streptomycin
Micromonospora purpurea	Gentamicin
Cephalosporium spp.	Cephalothin
Penicillium griseofulvum	Griseofulvin
Penicillium chrysogenum	Penicillin

Spectrum of Antimicrobial Activity

The spectrum of activity describes the range of organisms an antimicrobial drug can affect.

• Broad-spectrum antibiotics: Effective against a wide variety of microbes.

• Narrow-spectrum antibiotics: Target specific groups of bacteria.

Drug	Prokaryotes	Eukaryotes
Penicillin G	Gram-positive bacteria
Streptomycin	Gram-negative bacteria, Mycobacteria
Tetracycline	Gram-negative, Gram-positive, Chlamydias, Rickettsias	Fungi, Protozoa
Metronidazole		Protozoa
Acyclovir		Viruses

Mechanisms of Action of Antimicrobial Drugs

Major Mechanisms

Antimicrobial drugs act by targeting essential microbial processes.

• Inhibition of cell wall synthesis: Penicillins, cephalosporins, bacitracin, vancomycin

• Inhibition of protein synthesis: Chloramphenicol, erythromycin, tetracyclines, streptomycin

• Inhibition of nucleic acid replication and transcription: Quinolones, rifampin

• Injury to plasma membrane: Polymyxin B

• Inhibition of essential metabolite synthesis: Sulfanilamide, trimethoprim

Antibacterial Antibiotics: Inhibitors of Cell Wall Synthesis

Penicillins

Penicillins contain a β-lactam ring and prevent cross-linking of peptidoglycans, interfering with cell wall construction, especially in gram-positive bacteria.

• Natural penicillins: Penicillin G (injection), Penicillin V (oral)

• Semisynthetic penicillins: Modified to resist penicillinase or broaden spectrum (e.g., oxacillin, ampicillin)

• Penicillinase-resistant penicillins: Methicillin, oxacillin

• Extended-spectrum penicillins: Effective against gram-negatives and gram-positives

• Penicillins plus β-lactamase inhibitors: Combine with clavulanic acid to inhibit penicillinase

Cephalosporins

Cephalosporins are structurally similar to penicillins but have a different β-lactam ring. They are grouped by generation and are effective against a broad range of bacteria.

Polypeptide Antibiotics

• Bacitracin: Topical application; effective against gram-positives

• Vancomycin: Glycopeptide; last line against MRSA

• Teixobactin: New class; effective against gram-positives

Antibacterial Antibiotics: Inhibitors of Essential Metabolites

Sulfonamides

Sulfonamides inhibit folic acid synthesis, which is necessary for nucleic acid and protein synthesis. They competitively bind to the enzyme that normally binds PABA, a folic acid precursor.

• Drug synergism: Combination of trimethoprim and sulfamethoxazole (TMP-SMZ) increases efficacy.

Antifungal and Antiviral Drugs

Antifungal Drugs

Antifungal drugs target unique features of fungal cells.

• Affect fungal sterols: Interrupt ergosterol synthesis, increasing membrane permeability

• Affect fungal cell walls: Target β-glucan

• Inhibit nucleic acids: Flucytosine interferes with RNA synthesis

• Block microtubule assembly: Inhibit cell division

Antiviral Drugs

Antiviral drugs block various stages of viral infection.

• Entry and fusion inhibitors: Block virus binding or fusion with host cell

• Uncoating, genome integration, and nucleic acid synthesis inhibitors: Prevent viral uncoating or integration; nucleoside analogs inhibit RNA/DNA synthesis

• Assembly and exit inhibitors: Protease and neuraminidase inhibitors

• Interferons: Proteins that inhibit viral replication

Tests to Guide Chemotherapy

Disk-Diffusion Method (Kirby-Bauer Test)

The disk-diffusion method uses paper disks with antibiotics placed on agar containing the test organism. The zone of inhibition indicates sensitivity.

E Test

The E test determines the minimal inhibitory concentration (MIC), the lowest concentration preventing bacterial growth.

Broth Dilution Tests

Broth dilution tests determine MIC and minimal bactericidal concentration (MBC) by observing bacterial growth in wells with varying drug concentrations.

Resistance to Antimicrobial Drugs

Mechanisms of Resistance

Bacteria can develop resistance through various mechanisms.

• Persister cells: Survive antibiotic exposure due to genetic traits

• Superbugs: Resistant to multiple antibiotics

• Horizontal gene transfer: Resistance genes spread via plasmids or transposons

Mechanisms of Resistance

• Blocking entry: Prevent antibiotic from entering cell

• Inactivation by enzymes: Enzymes degrade or modify antibiotic

• Alteration of target molecule: Target changes so antibiotic cannot bind

• Efflux of antibiotic: Pumps remove antibiotic from cell

Antibiotic Misuse and Its Consequences

Misuse of Antibiotics

Misuse of antibiotics accelerates resistance. Common misuse includes:

• Using outdated or weakened antibiotics

• Using antibiotics for viral infections

• Using antibiotics in animal feed

• Failing to complete prescribed regimen

• Using someone else’s prescription

Effects of Drug Combinations

Synergism and Antagonism

• Synergism: Combined effect of two drugs is greater than either alone

• Antagonism: Combined effect is less than either alone

Future of Chemotherapeutic Agents

Emerging Strategies

• Targeting virulence factors: Disrupting mechanisms that allow pathogens to cause disease

• Sequestering iron: Limiting iron availability to pathogens

• Antimicrobial peptides: Produced by various organisms, including bacteria (bacteriocins)

• Phage therapy: Using bacteriophages to target bacterial infections

Additional info: Academic context was added to clarify mechanisms, sources, and clinical applications of antimicrobial drugs, as well as to explain resistance and future directions.