Antimicrobial Drugs: Principles, Sources, and Selective Toxicity

Antimicrobial Drugs

Interactions Between Drug and Microbe

Antimicrobial drugs are designed to target and eliminate microbial pathogens while minimizing harm to the host. The concept of selective toxicity is central to antimicrobial therapy, aiming for drugs that kill or inhibit microbes without damaging host tissues.

• Selective toxicity: The ability of a drug to target microbial cells specifically, sparing host cells.

• As pathogens become more similar to host cells (e.g., eukaryotic microbes), achieving complete selective toxicity becomes more difficult, increasing the risk of side effects.

• Antimicrobial drugs should ideally kill or inhibit microbes without causing harm to the patient.

Example: Antibiotics targeting bacterial cell walls (which humans lack) are highly selectively toxic, while drugs for fungal infections (eukaryotes) may have more side effects.

Principles of Antimicrobial Therapy

The main goal of antimicrobial therapy is to administer a drug that destroys the infective agent without harming the host's cells. Drugs can be produced naturally (antibiotics) or synthetically (antimicrobial agents).

• Antibiotics: Substances produced by microorganisms that inhibit or kill other microbes.

• Synthetic antimicrobials: Chemically manufactured drugs with antimicrobial activity.

Example: Penicillin is a naturally produced antibiotic, while sulfonamides are synthetic antimicrobials.

Representative Sources of Antibiotics

Major Microbial Producers of Antibiotics

Antibiotics are derived from various microorganisms, each producing drugs with specific targets and spectra of activity. The following table summarizes key sources and their representative antibiotics.

Microorganism	Antibiotic
GRAM-POSITIVE RODS Bacillus subtilis Paenibacillus polymyxa	Bacitracin Polymyxin
ACTINOMYCETES Streptomyces nodosus Streptomyces venezuelae Streptomyces aureofaciens Saccharopolyspora erythraea Streptomyces fradiae Streptomyces griseus Micromonospora purpurea	Amphotericin B Chloramphenicol Chlortetracycline and tetracycline Erythromycin Neomycin Streptomycin Gentamicin
FUNGI Cephalosporium spp. Penicillium griseofulvum Penicillium chrysogenum	Cephalothin Griseofulvin Penicillin

Additional info: Actinomycetes, especially Streptomyces species, are prolific producers of antibiotics used in clinical medicine. Fungi such as Penicillium and Cephalosporium are sources of penicillins and cephalosporins, respectively.

Key Terms and Concepts

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

• Spectrum of activity: The range of microbes against which an antibiotic is effective (narrow-spectrum vs. broad-spectrum).

• Selective toxicity: The property of a drug to harm the pathogen without harming the host.

Applications and Examples

• Bacitracin: Used topically for skin infections; produced by Bacillus subtilis.

• Penicillin: First widely used antibiotic; effective mainly against Gram-positive bacteria; produced by Penicillium chrysogenum.

• Streptomycin: Used to treat tuberculosis; produced by Streptomyces griseus.