Antimicrobial Drugs: Mechanisms, Resistance, and Clinical Applications

Chapter 20: Antimicrobial Drugs

Introduction to Antimicrobial Drugs

Antimicrobial drugs are essential tools in the treatment of infectious diseases. This chapter explores the definitions, mechanisms, and clinical applications of these drugs, as well as the challenges posed by resistance.

Key Definitions

• Chemotherapy: The use of chemicals to treat disease, particularly the use of antimicrobial drugs to treat infections caused by microorganisms.

• Antibiotic: A substance produced by a microorganism that inhibits the growth of or kills other microorganisms.

• Antimicrobial drug: Any agent, natural or synthetic, that kills or inhibits the growth of microorganisms.

• Selective toxicity: The ability of a drug to target harmful microbes without damaging the host.

Challenges in Antifungal and Antiviral Therapy

• Antifungal drugs: Fungi are eukaryotic organisms, like human cells, making it difficult to find drugs that are selectively toxic to fungi without harming the host.

• Antiviral drugs: Viruses use host cellular machinery for replication, so drugs that inhibit viruses often also affect host cells, leading to toxicity.

Spectrum of Activity

• Broad-spectrum antibiotics: Effective against a wide range of bacteria, including both Gram-positive and Gram-negative species.

• Narrow-spectrum antibiotics: Target specific types of bacteria, minimizing disruption to normal microbiota.

Superinfections

• Definition: An infection occurring after or on top of an earlier infection, especially following broad-spectrum antibiotic therapy that disrupts normal flora.

• Examples: Clostridioides difficile colitis, oral or vaginal candidiasis.

Major Actions of Antimicrobial Drugs

• Inhibiting cell wall synthesis: Drugs like penicillins and cephalosporins prevent the formation of peptidoglycan, weakening bacterial cell walls and causing lysis.

• Inhibiting protein synthesis: Drugs such as tetracyclines and aminoglycosides target bacterial ribosomes, disrupting protein production.

• Injuring the plasma membrane: Polymyxins disrupt membrane integrity, leading to cell death.

• Inhibiting nucleic acid synthesis: Quinolones and rifamycins interfere with DNA or RNA synthesis.

• Inhibiting synthesis of essential metabolites: Sulfonamides block folic acid synthesis, a pathway not present in humans.

Penicillin: Structure, Mechanism, and Resistance

• Structure: Penicillins contain a β-lactam ring essential for their activity.

• Mechanism: Inhibit transpeptidase enzymes involved in peptidoglycan cross-linking, leading to cell wall weakness and lysis.

• Natural vs. Semi-synthetic: Natural penicillins (e.g., penicillin G) are produced by Penicillium species; semi-synthetic penicillins are chemically modified to broaden their spectrum or resist β-lactamases.

• Resistance: Bacteria may produce β-lactamase enzymes that hydrolyze the β-lactam ring, rendering penicillins ineffective.

Bacitracin

• Mechanism: Inhibits cell wall synthesis by interfering with the transport of peptidoglycan precursors across the cytoplasmic membrane.

• Use: Commonly used topically due to toxicity if administered systemically.

• Vancomycin – Inhibits cell wall synthesis

  • Derived from a species of Streptomyces

  • Narrow spectrum

  • Important in fight against MRSA

Protein Synthesis Inhibitors

• Target: Bacterial 70S ribosomes (distinct from eukaryotic 80S ribosomes).

• Streptomycin: Binds to the 30S subunit, causing misreading of mRNA.

• Tetracycline: Blocks attachment of tRNA to the ribosome, preventing protein elongation.

Antiviral Medications

• Difficulty: Viruses replicate inside host cells, making selective toxicity challenging.

• Actions:

  • Entry & fusion inhibitors: Block viral entry into host cells.

  • Uncoating, genome integration, and nucleic acid synthesis inhibitors: Prevent viral replication.

  • Nucleic acid analogs: Mimic nucleotides, causing premature chain termination.

  • Protease inhibitors: Block viral protein processing.

  • Exit inhibitors: Prevent release of new virions (e.g., oseltamivir/Tamiflu).

Interferons

• Definition: Host-produced proteins that interfere with viral replication.

• Function: Induce antiviral states in neighboring cells and activate immune responses.

HIV Treatment

• Antiretroviral drugs: Target various stages of the HIV life cycle (e.g., reverse transcriptase inhibitors, protease inhibitors).

• Multi-drug therapy: Combination of drugs (HAART) reduces resistance and improves efficacy.

Antimalarial Drugs

• Examples: Chloroquine, artemisinin, and quinine derivatives are used to treat malaria caused by Plasmodium species.

Laboratory Testing of Antimicrobial Susceptibility

Kirby-Bauer Test (Disk Diffusion Test)

• Procedure: Disks impregnated with antibiotics are placed on an agar plate inoculated with the test organism.

• Zone of inhibition: Clear area around the disk where bacterial growth is prevented; its diameter is measured to assess susceptibility.

• Interpretation:

  • Sensitive: Large zone; organism is inhibited by the drug.

  • Intermediate: Moderate zone; possible clinical efficacy.

  • Resistant: Small or no zone; organism is not inhibited.

Minimum Inhibitory Concentration (MIC) Test

• Definition: The lowest concentration of an antimicrobial that prevents visible growth of a microorganism.

• Interpretation: Used to determine the effective dosage for treatment.

Antimicrobial Resistance

• Innate resistance: Natural, inherent resistance of certain bacteria to specific drugs.

• Acquired resistance: Resistance developed through mutation or gene acquisition.

• Persister cells: Dormant variants of regular cells that are tolerant to antibiotics.

• Superbugs: Bacteria that are resistant to multiple antibiotics.

• Mechanisms of resistance:

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

  • Prevention of drug penetration (e.g., altered porins in Gram-negative bacteria).

  • Alteration of target site (e.g., modified ribosomal binding sites).

  • Rapid efflux (e.g., efflux pumps expel drugs from the cell).

• Contributing factors: Overuse and misuse of antibiotics accelerate the development of resistance.

Drug Interactions

• Synergism: The effect of two drugs together is greater than the sum of their individual effects.

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

Summary Table: Mechanisms of Action of Major Antimicrobial Drugs

Mechanism	Drug Examples	Target
Inhibit cell wall synthesis	Penicillins, Cephalosporins, Bacitracin	Peptidoglycan cross-linking
Inhibit protein synthesis	Streptomycin, Tetracycline	70S ribosome
Injure plasma membrane	Polymyxins	Phospholipid bilayer
Inhibit nucleic acid synthesis	Quinolones, Rifampin	DNA/RNA polymerases
Inhibit metabolite synthesis	Sulfonamides	Folic acid pathway

Key Equations

• Zone of Inhibition (Kirby-Bauer): Measured in millimeters (mm) as the diameter of the clear area around the antibiotic disk.

• MIC Determination: The lowest concentration (in μg/mL) at which no visible growth occurs.

Additional info: This summary includes expanded academic context and examples to ensure completeness and clarity for exam preparation.