Antimicrobial Drugs

Introduction to Antimicrobial Drugs

Antimicrobial drugs are agents that interfere with the growth and survival of microbes within a host. Their development revolutionized the treatment of infectious diseases, allowing for targeted therapy against bacteria, fungi, protozoa, and viruses. The concept of selective toxicity—destroying harmful microbes without damaging host cells—is central to antimicrobial chemotherapy.

• Paul Ehrlich introduced the idea of the "magic bullet" and coined the term chemotherapy for the use of drugs to treat disease.

• Salvarsan was one of the first chemotherapeutic agents.

• In 1928, Alexander Fleming discovered penicillin, produced by Penicillium.

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

Problems with Chemotherapy

Challenges in Treating Different Microbial Groups

Chemotherapy is more effective against prokaryotes than eukaryotes or viruses due to differences in cell structure and replication mechanisms.

• Prokaryotes (bacteria) have unique targets (e.g., cell wall, ribosomes) not found in host cells.

• Eukaryotes (fungi, protozoa, helminths) resemble host cells, increasing the risk of host toxicity.

• Viruses replicate inside host cells, using host machinery, making selective targeting difficult.

Example: Antiviral drugs must target viral-specific processes without harming host cell functions.

Sources of Antibiotics

Representative Microbial Producers

Antibiotics are derived from various microorganisms, including bacteria and fungi. The following table summarizes key sources:

Spectrum of Antimicrobial Activity

Narrow vs. Broad Spectrum

The spectrum of activity describes the range of microbes an antimicrobial drug affects.

• Narrow-spectrum antibiotics: Target specific groups (e.g., only Gram-positive bacteria).

• Broad-spectrum antibiotics: Affect a wide range of organisms (e.g., both Gram-positive and Gram-negative bacteria).

Example: Penicillin is narrow-spectrum; tetracycline is broad-spectrum.

Modes of Action of Antimicrobial Drugs

Five Major Mechanisms

Antimicrobial drugs act by targeting essential microbial processes:

1. Inhibition of cell wall synthesis (e.g., penicillins, cephalosporins, vancomycin)

2. Inhibition of protein synthesis (e.g., chloramphenicol, erythromycin, tetracyclines, streptomycin)

3. Inhibition of nucleic acid replication and transcription (e.g., quinolones, rifampin)

4. Injury to plasma membrane (e.g., polymyxin B, lipopeptides)

5. Inhibition of essential metabolite synthesis (e.g., sulfonamides, trimethoprim)

Superinfection

Definition and Clinical Importance

A superinfection occurs when a target pathogen develops resistance to an antibiotic, allowing it to proliferate. These infections can be opportunistic and difficult to treat.

Inhibitors of Cell Wall Synthesis

Penicillins

Penicillins inhibit peptidoglycan synthesis, weakening bacterial cell walls and causing lysis.

• Natural penicillins: Penicillin G (injection), Penicillin V (oral); narrow spectrum, susceptible to penicillinases.

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

• Extended-spectrum penicillins: Effective against many Gram-negative bacteria.

Structure: All penicillins share a β-lactam ring, which is essential for activity.

Cephalosporins

Cephalosporins are structurally related to penicillins and also inhibit cell wall synthesis.

• 1st generation: Narrow spectrum, Gram-positive

• 2nd generation: Extended spectrum, includes Gram-negative

• 3rd generation: Includes pseudomonads, injected

• 4th generation: Oral

Polypeptide Antibiotics

• Vancomycin: Glycopeptide from Streptomyces; last line against MRSA.

Antimycobacterial Antibiotics

• Isoniazid (INH): Inhibits mycolic acid synthesis, essential for mycobacterial cell walls.

Inhibitors of Protein Synthesis

Mechanisms and Examples

These drugs target bacterial ribosomes (70S), which differ from eukaryotic ribosomes (80S), allowing selective toxicity.

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

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

• Tetracyclines: Interfere with attachment of tRNA to ribosome.

Inhibitors of Nucleic Acid Synthesis

Rifamycins and Quinolones

• Rifamycin: Inhibits mRNA synthesis; used for tuberculosis and leprosy.

• Quinolones/Fluoroquinolones: Inhibit DNA gyrase, blocking DNA replication; used for urinary tract infections and pneumonia.

Injury to the Plasma Membrane

Lipopeptides and Polymyxin B

• Lipopeptides: Cause structural changes in membranes, effective against Gram-positive bacteria.

• Polymyxin B: Disrupts Gram-negative bacterial membranes; used topically, often combined with bacitracin and neomycin.

Competitive Inhibitors

Sulfonamides (Sulfa Drugs)

Sulfonamides inhibit folic acid synthesis by competing with PABA (para-aminobenzoic acid) for the active site of the enzyme.

• Broad spectrum

• Structurally similar to PABA

Tests to Guide Chemotherapy

Diffusion and Dilution Methods

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

• E-test: Determines minimal inhibitory concentration (MIC).

• Broth dilution test: Determines minimal bactericidal concentration (MBC).

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 drug alone.

Example: Amoxicillin-clavulanic acid shows synergistic inhibition compared to aztreonam.

Bacterial Resistance to Antibiotics

Mechanisms of Resistance

Bacteria can develop resistance through several mechanisms:

1. Blocking entry: Modifying porins to prevent antibiotic entry.

2. Inactivation by enzymes: Producing enzymes (e.g., β-lactamase) that degrade antibiotics.

3. Alteration of target molecule: Changing antibiotic target sites to reduce binding.

4. Efflux of antibiotic: Pumping antibiotics out of the cell via membrane proteins.

Antibiotic Misuse and Prevention of Resistance

Factors Contributing to Resistance

• Using outdated or weakened antibiotics

• Using antibiotics for viral infections (e.g., common cold)

• Using antibiotics in animal feed

• Failing to complete prescribed regimens

• Using leftover prescriptions

Prevention Strategies

• Always finish the prescribed regimen

• Never use leftover antibiotics

• Avoid unnecessary prescriptions

• Use specific antibiotics rather than broad-spectrum agents

• Correct choice and dosage

Future of Chemotherapeutic Agents

Emerging Strategies

• Antimicrobial peptides: Small proteins with broad-spectrum activity.

• Phage therapy: Use of bacteriophages to target specific bacteria.

• Advances in microbial genetics for targeted drug development.

Key Terms and Definitions

• Spectrum of activity: Range of organisms affected by a drug.

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

• Superinfection: Infection by resistant organisms following antibiotic treatment.

Summary Tables

Representative Sources of Antibiotics

Spectrum of Activity of Antibiotics

Key Equations

• Minimal Inhibitory Concentration (MIC): The lowest concentration of an antimicrobial that will inhibit the visible growth of a microorganism after overnight incubation.

• Minimal Bactericidal Concentration (MBC): The lowest concentration of an antibacterial agent required to kill a particular bacterium.

Additional info: Some content was expanded for clarity and completeness, including definitions, examples, and mechanisms based on standard microbiology textbooks.