Antimicrobial Drugs: Mechanisms, Classes, and Clinical Applications

Antimicrobial Drugs: Overview

Introduction to Antimicrobial Drugs

Antimicrobial drugs are agents used to treat infections by inhibiting or killing pathogenic microorganisms. The ideal antimicrobial drug exhibits selective toxicity, meaning it targets harmful microbes without damaging the host's cells.

• Chemotherapy: Treatment of disease with chemicals taken into the body.

• Antimicrobial drugs: Interfere with the growth of microbes within a host.

• Antibiotic: A substance produced by a microbe that, in small amounts, inhibits another microbe.

• Synthetic drugs: Chemically synthesized in the laboratory (e.g., sulfa drugs).

• Antibiotics: Naturally produced by microorganisms such as Streptomyces, Bacillus, Penicillium, and Cephalosporium.

Historical Perspective

• Paul Ehrlich: Developed the first chemotherapeutic agent, salvarsan, for syphilis.

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

• Historical use of molds in wound treatment predates modern antibiotics.

Criteria for Antimicrobial Drugs

Essential Properties

Antimicrobial drugs should ideally possess the following characteristics, though no drug is perfect:

• Selective toxicity (toxic to microbes, not host)

• Low or no hypersensitivity reactions

• Solubility in body fluids

• Low potential for resistance development

Spectrum of Antimicrobial Activity

Broad vs. Narrow Spectrum

The spectrum of activity refers to the range of microorganisms affected by a drug:

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

• Narrow-spectrum: Effective against a limited group (e.g., Penicillin G targets mainly Gram-positive bacteria).

• Superinfection: Overgrowth of non-susceptible organisms during antibiotic therapy.

Mechanisms of Action of Antimicrobial Drugs

Bactericidal vs. Bacteriostatic

• Bactericidal: Kill microbes directly.

• Bacteriostatic: Inhibit microbial growth, allowing host defenses to eliminate pathogens.

Major Mechanisms

• Inhibition of cell wall synthesis

• Inhibition of protein synthesis

• Injury to plasma membrane

• Inhibition of nucleic acid synthesis

• Inhibition of synthesis of essential metabolites

1. Inhibition of Cell Wall Synthesis

Drugs such as penicillins and cephalosporins interfere with the synthesis of peptidoglycan, weakening the bacterial cell wall and causing cell lysis. These drugs are most effective against actively growing bacteria and have selective toxicity because eukaryotic cells lack peptidoglycan.

2. Inhibition of Protein Synthesis

Many antibiotics target bacterial ribosomes, which differ structurally from eukaryotic ribosomes. This allows selective inhibition of microbial protein synthesis.

• Chloramphenicol: Binds 50S subunit, inhibits peptide bond formation.

• Erythromycin: Binds 50S subunit, prevents translocation.

• Tetracyclines: Interfere with tRNA attachment to the 30S subunit.

• Streptomycin: Changes shape of 30S subunit, causing mRNA misreading.

3. Injury to the Plasma Membrane

Some drugs disrupt the structure or function of microbial membranes, increasing permeability and causing cell death. Examples include polymyxin B (bacteria) and amphotericin B (fungi).

4. Inhibition of Nucleic Acid Synthesis

These drugs interfere with DNA replication or transcription. Because microbial and human nucleic acids are similar, these drugs often have limited selectivity and more side effects.

• Quinolones/Fluoroquinolones: Inhibit DNA gyrase (e.g., ciprofloxacin).

• Rifampin: Inhibits RNA synthesis; used for tuberculosis.

5. Inhibition of Synthesis of Essential Metabolites

Some drugs act as competitive inhibitors of key microbial enzymes. Sulfonamides (sulfa drugs) inhibit folic acid synthesis by mimicking para-aminobenzoic acid (PABA), a precursor in the pathway.

Major Classes of Antimicrobial Drugs

Inhibitors of Cell Wall Synthesis

• Penicillins: Natural (Penicillin G, V), semisynthetic (oxacillin, ampicillin), extended-spectrum.

• Cephalosporins: Multiple generations with increasing spectrum.

• Polypeptide antibiotics: Bacitracin (topical), vancomycin (last line for MRSA).

• Antimycobacterial antibiotics: Isoniazid (inhibits mycolic acid synthesis), ethambutol (inhibits incorporation of mycolic acid).

Inhibitors of Protein Synthesis

• Chloramphenicol: Broad spectrum, binds 50S subunit.

• Aminoglycosides: Streptomycin, neomycin, gentamicin; change shape of 30S subunit.

• Tetracyclines: Broad spectrum, interfere with tRNA attachment.

• Macrolides: Erythromycin, azithromycin; bind 50S subunit, prevent translocation.

• Glycylcyclines: Bind 30S subunit, inhibit translation (effective against MRSA).

Inhibitors of Nucleic Acid Synthesis

• Rifamycins: Inhibit RNA synthesis (anti-tuberculosis).

• Quinolones/Fluoroquinolones: Inhibit DNA gyrase (e.g., ciprofloxacin).

Inhibitors of Essential Metabolite Synthesis

• Sulfonamides: Inhibit folic acid synthesis (broad spectrum).

Injury to the Plasma Membrane

• Polymyxin B: Topical, used for Gram-negative bacteria.

• Lipopeptides: Cause structural changes in the membrane (used for MRSA).

Antifungal, Antiviral, Antiprotozoan, and Antihelminthic Drugs

Antifungal Drugs

• Polyenes (Amphotericin B): Bind to ergosterol, disrupt fungal membranes; used for systemic mycoses.

• Azoles: Inhibit ergosterol synthesis (e.g., miconazole, triazole).

• Allylamines: Used for azole-resistant infections (e.g., terbinafine).

• Griseofulvin: Inhibits microtubule formation; treats superficial dermatophytes.

Antiviral Drugs

• Amantadine: Inhibits viral uncoating (influenza).

• Acyclovir, Zidovudine: Inhibit nucleic acid synthesis (herpes, HIV).

• Protease and Integrase inhibitors: Used for HIV therapy.

• Interferons: Prevent spread of viruses to new cells.

Antiprotozoan and Antihelminthic Drugs

• Chloroquine: Inhibits DNA synthesis (malaria).

• Metronidazole: Interferes with anaerobic metabolism (Trichomonas, Giardia).

• Niclosamide: Inhibits ATP production (tapeworms).

• Praziquantel: Alters membrane permeability (flatworms).

• Mebendazole, Albendazole: Interfere with nutrient absorption (intestinal roundworms).

• Ivermectin: Causes paralysis of helminths (mainly in livestock).

Tests to Guide Chemotherapy

Determining Susceptibility

• MIC (Minimal Inhibitory Concentration): Lowest concentration of drug that prevents visible growth.

• MBC (Minimal Bactericidal Concentration): Lowest concentration that kills the organism.

• Antibiogram: Summary of antimicrobial susceptibilities of local bacterial isolates.

Disk Diffusion (Kirby-Bauer) Method

This standardized test measures the effectiveness of antibiotics against specific bacteria. Disks containing antibiotics are placed on an agar plate inoculated with the test organism. The zone of inhibition is measured to determine susceptibility.

• Results are interpreted as Sensitive (S), Resistant (R), or Intermediate (I) based on standardized charts.

• Mueller-Hinton agar is the recommended medium.

• Standardized inoculum size and incubation conditions are critical for reproducibility.

Antimicrobial Drug Resistance

Mechanisms of Resistance

• Enzymatic breakdown of drug (e.g., penicillinase).

• Prevention of drug penetration to target site.

• Alteration of drug target site (e.g., ribosomal changes).

• Efflux of drug from the cell.

Resistance genes are often carried on plasmids or transposons and can be transferred between bacteria by conjugation.

Factors Promoting Resistance

• Misuse of antibiotics (e.g., incomplete courses, use for viral infections, use in animal feed).

• Use of outdated or leftover antibiotics.

Drug Combinations and Side Effects

Effects of Drug Combinations

• Synergism: Combined effect is greater than the sum of individual effects (e.g., penicillin + clavulanic acid).

• Antagonism: Combined effect is less than individual effects (e.g., penicillin + tetracycline).

Side Effects of Antibiotics

• Opportunistic infections (e.g., oral candidiasis).

• Adverse effects such as discoloration of teeth (e.g., tetracycline).

Summary Table: Microbial Sources of Antibiotics

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

Additional info: This guide covers the essential concepts, mechanisms, and clinical considerations for antimicrobial drugs, as outlined in a standard microbiology curriculum.