Controlling Microbial Growth in the Body: Antimicrobial Drugs and Their Mechanisms

Chapter 10: Controlling Microbial Growth in the Body

Introduction to Antimicrobial Drugs

Antimicrobial drugs are essential tools in microbiology and medicine, used to control and eliminate pathogenic microorganisms within the body. Understanding their types, mechanisms, and history is crucial for effective clinical application.

• Drug: Any chemical substance that affects physiology in any manner.

• Therapeutic agents: Drugs that act against diseases.

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

• Antibiotics: Antimicrobial agents produced naturally by organisms, typically bacteria or fungi.

History of Antibiotic Discovery

The development of antibiotics revolutionized the treatment of infectious diseases. Key milestones include:

• Early 1900s: Paul Ehrlich developed chemical compounds (e.g., arsenic-based drugs) that killed microbes, including Trypanosome parasites and Treponema pallidum (syphilis), though some were toxic to humans.

• 1929 (Fleming): Penicillin, produced by Penicillium fungus, was identified as the first antibiotic effective against bacteria. Large-scale production began in the 1940s.

• 1932 (Domagk): Sulfanilamide, the first commercially available antimicrobial, was effective against many bacterial infections.

• Mid-1900s: Discovery of soil microorganisms (e.g., Streptomyces, Bacillus) as sources of antibiotics.

Types of Antimicrobial Agents

Antimicrobial agents are classified based on their origin and method of production:

• Antibiotics: Naturally occurring antimicrobials produced by other organisms.

• Semi-synthetics: Chemically modified antibiotics that are more effective, longer-lasting, or easier to administer than natural antibiotics.

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

Example: Penicillium chrysogenum produces penicillin, which inhibits bacterial growth (as shown by a clear zone around the fungus in culture).

Major Sources and Examples of Antibiotics

Most antibiotics are derived from specific genera of microorganisms, especially soil bacteria and fungi.

Microorganism	Antimicrobial
Penicillium	Penicillin G
Streptomyces	Streptomycin, Erythromycin, Gentamicin, Mupirocin, Vancomycin, Rifampin, Bacitracin, Polymyxin
Bacillus	Bacitracin
Acremonium	Cephalosporins

Additional info: The greatest number of antibiotics are produced by Streptomyces, Bacillus, Penicillium, and Acremonium.

Selective Toxicity of Chemotherapeutic Agents

Effective antimicrobial drugs exhibit selective toxicity, meaning they target pathogens without harming the host.

• Bacterial drugs: Constitute the largest number of antimicrobial agents due to unique bacterial structures (e.g., cell wall, ribosomes).

• Drugs for eukaryotic infections: Fewer options exist for treating fungi, protozoa, and helminths due to similarities with human cells.

Key Terms in Antimicrobial Therapy

• Chemotherapy: Use of chemicals to treat disease.

• Selective toxicity: Ability of a drug to target pathogens with minimal harm to the host.

• Minimum inhibitory concentration (MIC): Lowest concentration of a drug that prevents visible growth of a microorganism.

• Molecular selectivity: Drug targets unique molecular structures in pathogens.