Antimicrobial Drugs
Antimicrobial
Chemotherapy
Definition: Chemotherapy refers to the treatment of disease with drugs.
Historical Context:
Modern chemotherapy began with Paul Ehrlich.
Introduced the concept of a “Magic Bullet” aimed to selectively find and destroy pathogens without harming the host.
Paul Ehrlich's Contributions
Early Work:
Described that organisms take up different dyes (gram staining) and hypothesized similar methods could be applied to control microorganisms.
Employed trial and error methodology in his research.
Significant Discovery:
After many failures, succeeded on the 606th try, discovering Salvarsan, an arsenic derivative effective against syphilis.
Ehrlich’s Principles of Chemotherapy
Affinity Requirement:
There must be an affinity between some part of the drug and a receptor on the pathogen.
Toxic Potential:
The drug should have toxic potential that ensures the pathogen is destroyed upon binding.
Differentiable Metabolic Processes:
The metabolic process impacted by the drug must differ significantly from the corresponding processes in the host organism.
Types of Antimicrobial Agents
Chemotherapeutic Drugs
Definition:
Chemical compounds synthesized in the laboratory that exhibit antimicrobial effects.
Example:
Sulfa Drugs: Synthetic antimicrobials that interfere with bacterial growth.
Antibiotics
Definition:
Natural compounds with antimicrobial effects, produced partially by living organisms.
Purpose:
Generally aimed to inhibit the growth of other organisms.
Example:
Penicillin.
Historical Discoveries in Antibiotics
Alexander Fleming:
Discovered Penicillin in 1928, marking the first natural antibiotic.
Selman Waksman:
Worked with Actinomycetes leading to the discovery of antibiotics like actinomycin, streptomycin, and neomycin; these continue to produce more than half of our natural antibiotics.
Dorothy Hodgkin:
Used x-ray crystallography to determine the structure of penicillin, advancing the creation of semi-synthetic antibiotics.
Sources of Common Antibiotics (Table 10.1)
Fungi:
Penicillium chrysogenum → Penicillin G
Penicillium griseofulvum → Griseofulvin
Acremonium spp. → Cephalothin
Bacteria:
Amycolatopsis orientalis → Rifampin
Bacillus licheniformis → Bacitracin
Streptomyces griseus → Streptomycin
Examples of various antibiotics including Gentamicin, Tetracycline, etc.
Fundamentals of Antimicrobial Chemotherapy
Bacteriostatic vs. Bactericidal:
Bacteriostatic: Prevents bacterial growth (static).
Bactericidal: Kills bacteria (cidal).
Treatment efficacy depends on the profile of the infection and the specific bacteria involved.
Spectrum of Antimicrobial Activity
Narrow Spectrum Antimicrobials: Target specific subsets of pathogens.
Broad Spectrum Antimicrobials: Affect various types of bacteria but may also target normal flora, risking superinfection.
Dosage and Administration
Factors:
Amount and administration route of the medicine are critical for effectiveness.
Drugs exhibit a half-life, the rate at which half of the agent is eliminated from the body.
Multiple doses may be required to maintain effective drug levels in the body.
Routes of administration can affect drug effectiveness (e.g., gastrointestinal absorption).
Drug Interactions
Some antimicrobials are used in combination to enhance effectiveness.
Interactions may occur whereby two static drugs may contribute to a cidal effect.
Other drugs may inhibit each other's effectiveness or absorption, leading to treatment failure.
Criteria for Effective Antimicrobial Drugs
Selective Toxicity: Toxic to the invader and not the host.
Minimal Hypersensitivity: Should not produce hypersensitivity reactions in most hosts.
Solubility in Body Fluids: Should remain soluble.
Rate of Metabolism and Excretion: Should not be rapidly metabolized.
Long Shelf Life: Stability in storage.
Preservation of Normal Flora: Should not eliminate beneficial microbial flora.
Resistance Developments: Should not easily lead to resistance.
Broad Spectrum of Action: Effective against various pathogens.
Cost-Effectiveness: Should be affordable and easy to manufacture.
Few Exhibiting All Properties: Few antimicrobial agents meet all criteria effectively.
Selective Toxicity Explained
This feature is crucial, as drugs must effectively target pathogens while sparing host cells.
A comprehensive understanding of microbial pathogens, their structures, and metabolic pathways helps develop selective drugs that act specifically against bacteria.
Modes of Action of Antimicrobial Drugs
Various mechanisms include:
Inhibition of cell wall synthesis.
Inhibition of protein synthesis.
Inhibition of nucleic acid synthesis (DNA/RNA).
Inhibition of plasma membrane function.
Inhibition of enzyme activity.
Inhibition of the spindle apparatus.
Inhibition of the electron transport chain.
Specific Modes of Action
Inhibition of Pathogen's Attachment or Entry into Host Cell
Examples: Arildone, Pleconaril, Enfuvirtide.
Inhibition of Cell Wall Synthesis
Penicillins: Interfere with the cross-linking of peptidoglycan layers leading to cell lysis.
Bacitracin: Blocks attachment of peptidoglycan polysaccharides; toxic at high levels.
Inhibition of Nucleic Acid Synthesis
Rifampin: Blocks bacterial RNA polymerase, crucial for transcription; effective against Mycobacterium tuberculosis.
Quinolones: Inhibit DNA gyrase, preventing replication.
Inhibition of Protein Synthesis
Aminoglycosides: Interfere with the 30S ribosomal subunit, causing errors in protein synthesis.
Tetracyclines: Inhibit attachment of tRNA to 30S subunit.
Inhibition of Plasma Membrane Function
Polymyxin B: Binds phosphate in bacterial membranes causing cell lysis.
Amphotericin B: Binds to sterols in fungal membranes, leading to cell rupture but has toxicity effects on kidneys.
Development of Drug Resistance
Mechanisms of Resistance
Drug resistance develops as bacteria evolve, integrating mechanisms such as:
Changing drug entry pathways (altering membrane structure).
Developing metabolic pathways to inactivate drugs (e.g., via β-lactamase).
Altering drug targets (mutating protein binding sites).
Strategies to Minimize Resistance
Complete the full course of antibiotic therapy.
Use combination therapies for enhanced efficacy (synergistic effect).
Develop new drugs strategically against emerging resistance patterns.
Common Drug-Resistant Bacteria
Methicillin-Resistant Staphylococcus aureus (MRSA): Semisynthetic penicillin resistant to β-lactamases, causing severe infections, particularly in hospitals.
Vancomycin-Resistant Enterococci (VRE): Resistance to glycopeptide antibiotics, presenting dangerous treatment challenges in clinical settings.
Carbapenem-Resistant Enterobacteriaceae (CRE): Bacteria developing resistance through mobile genetic elements, complicating treatment.
Multidrug-Resistant Mycobacterium tuberculosis (MDR-TB): Highly resistant, with multiple treatment options limited by resistance profiles.
Tests for Microbial Susceptibility
Agar Diffusion Method (Kirby-Bauer Test)
Involves inoculating media with the microorganism and placing antibiotic disks to measure zone of inhibition.
Use Dilution Method
A series of tubes with varying concentrations of the antibiotic is utilized. Turbidity indicates effectiveness.
Minimum Inhibitory Concentration (MIC)
The lowest concentration preventing microbial growth.
Minimum Bactericidal Concentration (MBC)
The lowest concentration that results in bacterial death.
Factors Affecting Susceptibility Testing
The diffusion rate of the antimicrobial agent.
Type of media used.
Incubation conditions including temperature and duration.
Initial concentration of the drug.
Each section provides detailed insights into antimicrobial principles, historical context, and mechanisms, which collectively contribute to a comprehensive understanding of the topic.