Antimicrobial Drugs: Mechanisms, Spectrum, and Resistance

Antimicrobial Drugs

Introduction to Antimicrobial Drugs

Antimicrobial drugs are agents that interfere with the growth of microbes within a host. They are essential tools in the treatment of infectious diseases and have revolutionized modern medicine.

• Antimicrobial drugs: Substances that inhibit or kill microorganisms, including bacteria, fungi, protozoa, and helminths.

• Paul Ehrlich: Introduced the concept of the "magic bullet"—a drug that targets pathogens without harming the host.

• Selective toxicity: The ability of a drug to kill or inhibit harmful microbes without damaging the host's cells.

• Salvarsan: The first effective antimicrobial drug, used to treat syphilis.

• Chemotherapy: The use of chemicals to treat disease, especially infections and cancer.

• Alexander Fleming: Discovered penicillin in 1928, produced by the mold Penicillium.

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

Problems of Chemotherapy for Different Infections

Challenges in Treating Microbial Infections

The effectiveness of chemotherapy varies depending on the type of pathogen. The main challenges are due to differences in cell structure and physiology between prokaryotes, eukaryotes, and viruses.

• Bacterial (Prokaryotic) Infections: Easier to target because their cellular structures differ significantly from human cells.

• Fungal, Protozoan, and Helminthic (Eukaryotic) Infections: More difficult to treat because these organisms are eukaryotes and share more similarities with human cells, increasing the risk of host toxicity.

• Viral Infections: Most challenging because viruses replicate inside host cells and use the host's machinery, making selective toxicity difficult.

Key Terms in Antimicrobial Therapy

Definitions

• Spectrum of Activity: The range of different microbes against which an antimicrobial agent acts.

• Narrow-spectrum antibiotic: Effective against a limited range of microorganisms (e.g., only Gram-positive bacteria).

• Broad-spectrum antibiotic: Effective against a wide variety of microorganisms, including both Gram-positive and Gram-negative bacteria.

• Superinfection: The growth of a pathogen that has developed resistance to an antibiotic, often resulting in the overgrowth of opportunistic pathogens.

Sources of Antibiotics

Representative Sources and Examples

Many antibiotics are derived from natural sources, particularly bacteria and fungi.

Source Organism	Antibiotic Produced
Bacillus subtilis	Bacitracin
Bacillus 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

Modes of Action of Antimicrobial Drugs

Overview of Mechanisms

Antimicrobial drugs target specific structures or functions in microbial cells. The main modes of action include:

• Inhibition of cell wall synthesis: Prevents bacteria from forming a functional cell wall (e.g., penicillins, cephalosporins, vancomycin).

• Inhibition of protein synthesis: Interferes with ribosomal function, blocking translation (e.g., chloramphenicol, tetracyclines, streptomycin).

• Inhibition of nucleic acid synthesis: Blocks DNA replication or RNA transcription (e.g., rifamycins, quinolones, fluoroquinolones).

• Injury to plasma membrane: Disrupts membrane integrity, causing cell death (e.g., polymyxin B, lipopeptides).

• Inhibition of essential metabolite synthesis: Blocks metabolic pathways (e.g., sulfonamides inhibit folic acid synthesis).

Inhibitors of Cell Wall Synthesis

Penicillins and Cephalosporins

• Penicillins: Contain a β-lactam ring; inhibit peptidoglycan synthesis in bacterial cell walls.

• Natural penicillins: Narrow spectrum, susceptible to penicillinase (β-lactamase).

• Semi-synthetic penicillins: Modified to resist penicillinase or broaden spectrum.

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

• Cephalosporins: Structurally similar to penicillins; classified by generations:

  • 1st generation: Narrow spectrum, Gram-positive

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

  • 3rd generation: Includes pseudomonads, injectable

  • 4th generation: Oral administration

• Vancomycin: Glycopeptide antibiotic, important as a last line of defense against MRSA.

• Isoniazid (INH): Inhibits mycolic acid synthesis, used for mycobacterial infections (e.g., tuberculosis).

Inhibitors of Protein Synthesis

Mechanisms and Examples

• Chloramphenicol, tetracyclines, streptomycin: Bind to bacterial ribosomes, blocking translation and protein production.

• These drugs exploit differences between prokaryotic (70S) and eukaryotic (80S) ribosomes for selective toxicity.

Inhibitors of Nucleic Acid Synthesis

Mechanisms and Examples

• Rifamycins: Inhibit mRNA synthesis; used for tuberculosis and leprosy.

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

Injury to the Plasma Membrane

Mechanisms and Examples

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

• Polymyxin B: Disrupts Gram-negative bacterial membranes; used topically in combination with bacitracin and neomycin.

Inhibitors of Essential Metabolite Synthesis

Mechanisms and Examples

• Sulfonamides (sulfa drugs): Inhibit folic acid synthesis by competing with PABA (para-aminobenzoic acid) for the active site of the enzyme.

• Broad spectrum of activity.

Laboratory Tests to Guide Chemotherapy

Methods for Determining Drug Effectiveness

• Disk diffusion (Kirby-Bauer) test: Measures the zone of inhibition around antibiotic disks on an agar plate.

• Broth dilution test: Determines the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of an antibiotic.

Effects of Drug Combinations

Synergism and Antagonism

• Synergism: The combined effect of two drugs is greater than the effect of either drug alone.

• Antagonism: The combined effect of two drugs is less than the effect of either drug alone.

Mechanisms of Microbial Resistance

How Microbes Resist Antimicrobial Drugs

• Enzymatic destruction or inactivation of the drug (e.g., β-lactamase production).

• Prevention of drug penetration to the target site.

• Alteration of the drug's target site.

• Rapid efflux (ejection) of the drug from the cell.

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 the prescribed regimen.

• Using someone else's leftover prescription.

Prevention Strategies

• Always finish the prescribed regimen.

• Never use leftover antibiotics.

• Avoid unnecessary prescriptions.

• Use specific antibiotics rather than broad-spectrum agents when possible.

• Correct choice and dosage are essential.

Future of Chemotherapeutic Agents

Emerging Strategies

• Development of antimicrobial peptides.

• Phage therapy (using bacteriophages to target bacteria).

• Advances in understanding microbial genetics to design targeted therapies.

Summary Table: Generations of Cephalosporins

Generation	Spectrum	Notes
1st	Narrow (Gram-positive)	Limited Gram-negative activity
2nd	Extended	Includes some Gram-negative
3rd	Broad	Includes pseudomonads; injectable
4th	Broadest	Oral administration

Additional info: For more detailed mechanisms and examples, refer to Tables 20.3 and 20.4 in your textbook.