Antimicrobial Therapy: Principles, Drug Types, and Microbial Sources

Principles of Antimicrobial Therapy

Definition and Goals

Antimicrobial chemotherapy is the use of drugs to destroy infectious agents without harming the host's cells. The ideal antimicrobial agent selectively targets the pathogen while minimizing toxicity to the host.

• Antimicrobial drugs can be produced naturally by microorganisms or synthesized in the laboratory (synthetic).

• Selective toxicity is the ability of a drug to kill or inhibit microbial cells without simultaneously damaging host tissues.

Types of Antimicrobial Agents

Antibiotics

Antibiotics are microbially produced antimicrobial agents, typically derived from bacteria and fungi. They are the foundation of antimicrobial chemotherapy.

• Less than 1% of known antibiotics are clinically useful due to issues such as toxicity or poor absorption.

• Antibiotics can be modified to enhance efficacy, resulting in semisynthetic antibiotics.

Semisynthetic and Synthetic Antibiotics

• Semisynthetic antibiotics: Artificially modified natural antibiotics to enhance their efficacy. They retain a natural core structure.

• Modifications can:

  • Broaden or alter the spectrum of activity

  • Improve efficacy

  • Reduce side effects

• Synthetic antibiotics: Entirely man-made in the laboratory, not derived from natural sources.

Spectrum of Activity

Broad vs. Narrow Spectrum

The spectrum of an antimicrobial drug refers to the range of microbes it affects.

• Broad-spectrum drugs: Effective against a wide variety of microbial types, including both Gram-positive and Gram-negative bacteria (e.g., tetracycline).

• Narrow-spectrum drugs: Effective against a limited group of microbes, such as only Gram-positive bacteria (e.g., bacitracin).

• Medium spectrum: Drugs effective against some Gram-positive and Gram-negative bacteria.

Sources of Antimicrobial Drugs

Microbial Producers

Many antibiotics are produced by bacteria and fungi. The most important producer genera include:

• Bacteria: Streptomyces, Bacillus

• Fungi: Penicillium, Cephalosporium

Zones of inhibition on agar plates indicate colonies that produce antibiotics.

Table: Selected Microbial Sources of Antibiotics

Producer Genus	Type of Microbe	Drugs Formed
Penicillium	Mold	Penicillins, Griseofulvin
Cephalosporium	Mold	Cephalosporins
Micromonospora	Bacteria	Gentamicin
Bacillus	Bacteria	Bacitracin, Polymyxin B
Chromobacterium	Bacteria	Aztreonam
Streptomyces	Filamentous bacteria	Streptomycin, Erythromycin, Tetracycline, Vancomycin, Chloramphenicol, Amphotericin B

Key Terms and Concepts

• Antimicrobial chemotherapy: The use of drugs to treat infections caused by microorganisms.

• Antibiotic: A substance produced by a microbe that inhibits or kills other microbes.

• Spectrum of activity: The range of microorganisms affected by a particular drug.

• Selective toxicity: The ability of a drug to target pathogens without harming the host.

• Semisynthetic antibiotic: A naturally produced antibiotic that has been chemically modified.

• Synthetic antibiotic: An antibiotic produced entirely by chemical synthesis.

Examples and Applications

• Penicillins are effective against Gram-positive bacteria and are produced by Penicillium molds.

• Tetracycline is a broad-spectrum antibiotic produced by Streptomyces species.

• Bacitracin is a narrow-spectrum antibiotic effective mainly against Gram-positive bacteria, produced by Bacillus.

Additional info: The notes above are expanded and clarified for academic completeness, including definitions and examples for key terms and concepts relevant to college-level microbiology.