Antimicrobial Drugs: Mechanisms, Applications, and Resistance

Chapter 20: Antimicrobial Drugs

Introduction to Antimicrobial Drugs

Antimicrobial drugs are essential tools in the treatment of infectious diseases. This chapter explores their definitions, mechanisms of action, and the challenges associated with their use, including resistance.

Key Definitions

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

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

• Antimicrobial drug: A synthetic or natural substance that kills or inhibits the growth of microorganisms.

• Selective toxicity: The ability of a drug to target microbial cells without damaging host cells.

Challenges in Antifungal and Antiviral Therapy

• Antifungal drugs: Fungi are eukaryotic, 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 must target viral processes without affecting host cells, which is challenging.

Spectrum of Antimicrobial Activity

• Broad-spectrum antibiotics: Effective against a wide range of bacteria (both Gram-positive and Gram-negative). Example: tetracycline.

• Narrow-spectrum antibiotics: Effective against specific families of bacteria. Example: penicillin G (mainly Gram-positive bacteria).

Superinfections

• Superinfection: An infection occurring after or on top of an earlier infection, especially following broad-spectrum antibiotic use, which disrupts normal microbiota.

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

Major Actions of Antimicrobial Drugs

• Inhibiting cell wall synthesis: Drugs like penicillins and cephalosporins prevent peptidoglycan cross-linking, weakening bacterial cell walls.

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

• Injuring the plasma membrane: Polymyxins disrupt membrane integrity, causing cell leakage.

• Inhibiting nucleic acid synthesis: Drugs like quinolones and rifampin 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: Contains a β-lactam ring essential for its activity.

• Mechanism: Inhibits transpeptidase enzymes, preventing cross-linking of peptidoglycan in bacterial cell walls, leading to cell lysis.

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

• Bacterial resistance: Common mechanisms include production of β-lactamase enzymes that hydrolyze the β-lactam ring.

Bacitracin

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

• Use: Mainly topical due to toxicity.

Inhibition of Protein Synthesis

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

• 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 targeting difficult.

• Actions:

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

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

  • Nucleic acid analogs: Mimic nucleotides, causing premature chain termination (e.g., acyclovir).

  • Protease inhibitors: Block viral protein processing (e.g., HIV therapy).

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

Interferons

• Definition: Host-produced proteins that inhibit viral replication and modulate the immune response.

• Function: Induce production of antiviral proteins in neighboring cells.

HIV Treatment

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

• Multi-drug therapy: Combines drugs to reduce resistance and improve efficacy (Highly Active Antiretroviral Therapy, HAART).

Antimalarial Drugs

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

Laboratory Testing of Antimicrobial Drugs

Kirby-Bauer Disk Diffusion Test

• Purpose: Determines the susceptibility of bacteria to antibiotics.

• Procedure: Antibiotic-impregnated disks are placed on an agar plate inoculated with the test organism. After incubation, zones of inhibition are measured.

• Zone of inhibition: The clear area around a disk where bacterial growth is prevented; its size indicates susceptibility.

• Interpretation:

  • Sensitive: Large zone; bacteria are inhibited by the drug.

  • Intermediate: Moderate zone; partial inhibition.

  • Resistant: Small or no zone; bacteria are not inhibited.

Minimum Inhibitory Concentration (MIC) Test

• Purpose: Determines the lowest concentration of an antimicrobial that inhibits visible growth of a microorganism.

• Interpretation: The MIC is the lowest drug concentration with no visible growth in broth or agar dilution tests.

Antimicrobial Resistance

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

• Acquired resistance: Resistance developed through mutation or acquisition of resistance genes.

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

• Superbugs: Bacteria that are resistant to multiple antibiotics (e.g., MRSA).

• Mechanisms of resistance:

  • Enzymatic destruction or inactivation: e.g., β-lactamase production.

  • Prevention of penetration: Altered porins in Gram-negative bacteria.

  • Alteration of target site: Mutation in ribosomal proteins or enzymes.

  • Rapid efflux: Pumping out of drugs by efflux pumps.

• Contributing factors: Overuse and misuse of antibiotics in medicine and agriculture accelerate resistance development.

Drug Interactions

• Synergism: The effect of two drugs together is greater than the sum of their individual effects (e.g., penicillin and streptomycin).

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

Summary Table: Major Actions of Antimicrobial Drugs

Action	Example Drugs	Target
Inhibit cell wall synthesis	Penicillins, cephalosporins, bacitracin	Peptidoglycan cross-linking
Inhibit protein synthesis	Streptomycin, tetracycline, erythromycin	70S ribosome
Injure plasma membrane	Polymyxin B	Cell membrane integrity
Inhibit nucleic acid synthesis	Quinolones, rifampin	DNA/RNA synthesis
Inhibit essential metabolite synthesis	Sulfonamides	Folic acid pathway

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