Antimicrobial Drugs

Introduction to Antimicrobial Drugs

Antimicrobial drugs are agents that interfere with the growth of microbes within a host organism. These drugs are essential in the treatment of infectious diseases caused by bacteria, fungi, protozoa, and helminths. The development and use of antimicrobial drugs have revolutionized modern medicine.

• Antimicrobial drugs: Compounds that inhibit or kill microorganisms without causing significant harm to the host.

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

• Chemotherapy: The use of chemicals to treat disease, a term coined by Paul Ehrlich.

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

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

Discovery of Antibiotics

The discovery of antibiotics marked a significant milestone in medical history. In 1928, Alexander Fleming discovered penicillin, the first true antibiotic, produced by the mold Penicillium.

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

• Penicillin: The first antibiotic discovered, effective primarily against Gram-positive bacteria.

Problems of Chemotherapy for Different Infections

Challenges in Treating Various Pathogens

The effectiveness of chemotherapy varies depending on the type of pathogen. The main challenges arise from the biological differences between prokaryotes, eukaryotes, and viruses.

• Bacterial (prokaryotic) infections: Easier to target due to significant differences from human cells.

• Fungal, protozoan, and helminthic (eukaryotic) infections: More difficult to treat because these organisms are more similar to human cells, increasing the risk of host toxicity.

• Viral infections: Most challenging, as viruses replicate inside host cells and use host cellular machinery, making selective targeting difficult.

Key Terms

Definitions

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

• Narrow-spectrum antibiotic: Effective against a limited group 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 organisms.

Sources of Antibiotics

Representative Sources and Examples

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

Modes of Action of Antimicrobial Drugs

Overview of Mechanisms

Antimicrobial drugs act by targeting 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: Targets bacterial ribosomes, disrupting protein production (e.g., chloramphenicol, tetracyclines, streptomycin).

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

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

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

Examples of Drug Classes and Their Actions

• Penicillins: Inhibit cell wall synthesis; effective mainly against Gram-positive bacteria.

• Cephalosporins: Similar to penicillins but with broader spectrum; classified into generations based on activity.

• Vancomycin: Glycopeptide antibiotic; last line of defense against resistant Staphylococcus aureus (MRSA).

• Isoniazid (INH): Inhibits mycolic acid synthesis; used against mycobacteria (e.g., tuberculosis).

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

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

• Polymyxin B: Disrupts plasma membrane; effective against Gram-negative bacteria; used topically.

• Sulfonamides: Competitive inhibitors of folic acid synthesis; broad spectrum.

Antibiotic Spectrum and Superinfection

Spectrum of Activity

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

• Broad-spectrum antibiotics: Affect a wide range of bacteria, increasing the risk of disrupting normal flora and causing superinfections.

Superinfection

• Occurs when a resistant pathogen proliferates after normal flora are eliminated by broad-spectrum antibiotics.

• Can lead to opportunistic infections.

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.

MIC: The lowest concentration of an antimicrobial that inhibits visible growth of a microorganism. MBC: The lowest concentration of an antimicrobial that kills 99.9% of the initial bacterial population.

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 Become Resistant

• Enzymatic destruction or inactivation of the drug (e.g., beta-lactamases).

• 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 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 of Antibiotic Resistance

• 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.