BackChapter 10: Controlling Microbial Growth in the Body – Antimicrobial Drugs
Study Guide - Smart Notes
Tailored notes based on your materials, expanded with key definitions, examples, and context.
Development of Antimicrobial Agents
Key Historical Figures
The discovery and development of antimicrobial agents revolutionized the treatment of infectious diseases. Several scientists made foundational contributions:
Paul Ehrlich (1854–1915): Introduced the concept of chemotherapy and selective toxicity. Developed "magic bullets"—chemicals that specifically target pathogens without harming the host. His arsenic compound was the first effective treatment for syphilis.
Alexander Fleming (1881–1955): Discovered penicillin from the mold Penicillium, the first widely used antibiotic, which dramatically reduced deaths from bacterial infections.
Gerhard Domagk (1895–1964): Discovered sulfanilamide, the first practical synthetic antimicrobial agent effective against bacterial infections.
Selman Waksman: Coined the term antibiotics and demonstrated that microorganisms could produce antimicrobial compounds.
Scientist | Major Contribution |
|---|---|
Paul Ehrlich | Chemotherapy, selective toxicity, "magic bullets" |
Alexander Fleming | Discovered penicillin from Penicillium mold |
Gerhard Domagk | Discovered sulfanilamide |
Selman Waksman | Coined the term "antibiotics" |
Principle of Selective Toxicity
Definition and Importance
Selective toxicity is the ability of an antimicrobial agent to harm the pathogen without causing significant damage to the host. This principle is essential for safe and effective therapy.
Targets structures or pathways unique to pathogens (e.g., cell walls, 70S ribosomes, DNA gyrase, ergosterol, reverse transcriptase, unique metabolic pathways).
Ensures high toxicity to the pathogen and low toxicity to the patient.
Mechanisms of Antimicrobial Action
Six Major Mechanisms
Inhibition of Cell Wall Synthesis: Prevents construction or repair of microbial cell walls, leading to cell lysis due to osmotic pressure. Mainly affects bacteria.
Inhibition of Protein Synthesis: Targets 70S ribosomes in prokaryotes, halting protein production and cell growth.
Disruption of Cytoplasmic Membrane: Damages membrane integrity, causing leakage of cellular contents and cell death. Especially effective against fungi (ergosterol target).
Inhibition of Metabolic Pathways: Blocks biochemical pathways unique to microbes (e.g., folic acid synthesis).
Inhibition of Nucleic Acid Synthesis: Prevents DNA replication and RNA transcription, stopping cell division and viral replication.
Prevention of Attachment to Host Cells: Blocks viral recognition, attachment, or entry, preventing infection initiation.
Drugs Affecting Cell Walls
Mechanisms and Examples
Beta-lactam antibiotics (e.g., penicillins, cephalosporins): Inhibit cross-linking of NAM subunits in peptidoglycan, weakening the cell wall.
Vancomycin and Cycloserine: Interfere with bridges between NAM subunits (especially in Gram-positive bacteria).
Bacitracin: Blocks transport of NAG and NAM from the cytoplasm to the cell wall.
Isoniazid and Ethambutol: Inhibit mycolic acid synthesis, targeting Mycobacterium tuberculosis.
Drug | Action |
|---|---|
Beta-lactams | Prevent cross-linking of NAM |
Vancomycin | Blocks bridges between NAM units |
Cycloserine | Interferes with bridge formation |
Bacitracin | Blocks transport of NAG/NAM |
Isoniazid | Inhibits mycolic acid synthesis |
Ethambutol | Inhibits mycolic acid synthesis |
Drugs Affecting Protein Synthesis
Mechanisms and Examples
Aminoglycosides: Bind 30S subunit, causing misreading of mRNA.
Tetracyclines: Block the A site on the 30S subunit, preventing tRNA binding.
Chloramphenicol: Inhibits peptide bond formation at the 50S subunit.
Macrolides, Lincosamides, Streptogramins: Prevent ribosomal movement along mRNA (50S subunit).
Antisense Nucleic Acids: Bind mRNA, blocking ribosome attachment.
Oxazolidinones: Block initiation of translation; used for resistant Gram-positive bacteria.
Drug/Class | Ribosomal Target | Mechanism |
|---|---|---|
Aminoglycosides | 30S | Distort ribosome; misread codons |
Tetracyclines | 30S | Block A site |
Chloramphenicol | 50S | Block peptide bond formation |
Macrolides/Lincosamides/Streptogramins | 50S | Freeze ribosomal movement |
Antisense nucleic acids | mRNA | Prevent ribosome attachment |
Oxazolidinones | Initiation complex | Block initiation of translation |
Drugs Affecting Cytoplasmic Membranes
Mechanisms and Examples
Polyenes (e.g., nystatin, amphotericin B): Bind ergosterol, forming pores in fungal membranes, causing leakage and cell lysis.
Azoles and Allylamines: Inhibit ergosterol synthesis, destabilizing fungal membranes.
Polymyxin: Disrupts Gram-negative bacterial membranes; reserved for severe infections due to toxicity.
Selective toxicity: Human cells lack ergosterol, so these drugs primarily affect fungi.
Drugs Affecting Metabolic Pathways
Mechanisms and Examples
Electron Transport Inhibitors: Used against protozoa and fungi; block ATP production.
Heavy Metals: Inactivate metabolic enzymes, halting cell metabolism.
Viral Activation Blockers: Prevent viruses from becoming metabolically active.
Metabolic Antagonists (e.g., sulfanilamide): Inhibit folic acid synthesis, selectively toxic to bacteria.
Drugs Affecting Nucleic Acid Synthesis
Mechanisms and Examples
Quinolones and Fluoroquinolones: Inhibit prokaryotic DNA gyrase, blocking DNA replication.
Nucleotide/Nucleoside Analogs: Mimic DNA/RNA building blocks, causing chain termination or faulty nucleic acids; used mainly against viruses and cancer cells.
Reverse Transcriptase Inhibitors: Block viral reverse transcriptase, preventing RNA viruses from synthesizing DNA (e.g., HIV therapy).
Drug Type | Target/Action | Main Result |
|---|---|---|
Quinolones/Fluoroquinolones | Prokaryotic DNA gyrase | Prevent bacterial DNA replication |
Nucleotide/Nucleoside Analogs | DNA or RNA structure | Distort nucleic acids, halt replication/transcription/translation |
Reverse Transcriptase Inhibitors | Viral reverse transcriptase | Prevent viral DNA formation from RNA |
Drugs Affecting Viral Attachment and Entry
Mechanisms and Examples
Attachment Antagonists (e.g., pleconaril): Block viral proteins or host receptors, preventing viral attachment and entry.
Uncoating Inhibitors (e.g., arildone): Prevent removal of viral capsid, blocking access to viral nucleic acid.
Sequence of viral infection: Attachment → Entry → Uncoating → Synthesis. These drugs act before synthesis begins.
Characteristics of an Ideal Antimicrobial Drug
Readily available
Inexpensive
Chemically stable
Easily administered
Nontoxic and nonallergenic
Selectively toxic against a wide range of pathogens
Spectrum of Antimicrobial Activity
Narrow-Spectrum vs. Broad-Spectrum Drugs
Narrow-Spectrum | Broad-Spectrum |
|---|---|
Effective against few organisms | Effective against many organisms |
More specific target | Less specific overall range |
Less disruption of normal microbiota | Greater disruption of normal microbiota |
Lower risk of superinfection | Higher risk of superinfection |
Microbial antagonism: Normal microbiota compete with pathogens; broad-spectrum drugs may disrupt this balance, leading to secondary infections.
Routes of Administration
Route | Main Advantage | Main Limitation |
|---|---|---|
Topical | Direct treatment of external infection | Limited penetration |
Oral | Convenient, self-administered | Variable absorption |
Intramuscular | Bypasses digestive tract | Painful, slower than IV |
Intravenous | Immediate delivery to bloodstream | Greater risk, requires professional administration |
Side Effects of Antimicrobial Therapy
Three Main Categories
Toxicity: Direct damage to host tissues (e.g., kidneys, liver, nerves).
Allergies: Immune response to the drug (e.g., rash, anaphylactic shock).
Disruption of Normal Microbiota: Loss of beneficial microbes, leading to secondary infections.
Side Effect | Cause | Possible Result |
|---|---|---|
Toxicity | Direct damage to host tissues | Kidney, liver, or nerve damage |
Allergy | Immune response to drug | Rash or anaphylactic shock |
Microbiota disruption | Death of normal flora | Secondary infection or superinfection |
Therapeutic Index and Therapeutic Window
Therapeutic Index (TI): A numerical measure of drug safety, calculated as:
High TI = safer drug; low TI = requires careful monitoring.
Therapeutic Window: The range of drug concentrations that are effective but not toxic.
Therapeutic Index | Therapeutic Window |
|---|---|
Numerical safety margin | Safe and effective concentration range |
Compares toxic and therapeutic doses | Describes concentrations in the body |
Development of Antimicrobial Resistance
How Resistance Arises
Microbial populations may contain naturally resistant cells.
Antimicrobial use kills sensitive cells, allowing resistant cells to survive and multiply.
Resistance can arise through chromosomal mutations or acquisition of resistance genes (e.g., R plasmids).
R Plasmids and Resistance
Definition and Importance
R plasmids (R factors): Extrachromosomal DNA carrying resistance genes.
Can encode drug-destroying enzymes, altered targets, efflux pumps, and more.
Spread rapidly between bacteria via horizontal gene transfer.
Mechanisms of Microbial Resistance
Drug-destroying enzymes: Inactivate or degrade the drug (e.g., beta-lactamases).
Reduced drug entry: Alter membrane structure or charge to prevent drug entry.
Altered target: Modify drug target to reduce binding.
Altered metabolism: Use alternative pathways or enzymes.
Compensating enzyme: Produce enzymes that bypass the drug's effect.
Efflux pumps: Actively expel drugs from the cell.
Biofilm growth: Biofilm matrix protects cells and limits drug penetration.
Resistance Mechanism | How it Protects the Microorganism |
|---|---|
Drug-destroying enzymes | Inactivate or break down the drug |
Reduced drug entry | Prevent enough drug from entering |
Altered target | Prevent effective drug binding |
Altered metabolism | Bypass the blocked pathway |
Compensating enzyme | Overcome the drug’s effect |
Efflux pumps | Pump drug out of the cell |
Biofilm growth | Protect cells and limit drug penetration |
Spread of Resistance Genes
Vertical transmission: Parent cell to daughter cells via binary fission.
Horizontal gene transfer: Between existing bacteria via transformation (uptake of free DNA), transduction (bacteriophage-mediated), or conjugation (direct cell-to-cell contact, often via R plasmids).
Cross Resistance vs. Multiple Resistance
Cross Resistance | Multiple Resistance |
|---|---|
One mechanism protects against similar drugs | Several mechanisms confer resistance to different drug types |
Usually involves structurally related drugs | Involves unrelated antimicrobial types |
Resistance to one drug causes resistance to another | Pathogen resists three or more drug types |
Strategies to Retard Resistance Development
Maintain high drug concentration for sufficient time: Ensures all pathogens are inhibited or killed.
Finish the entire prescription: Prevents survival of partially resistant cells.
Use combination therapy: Reduces the chance that a single cell will survive treatment (synergism).
Limit antimicrobial use: Use only when necessary to reduce selective pressure for resistance.
Semisynthetic drugs: Chemically modified derivatives of existing drugs designed to overcome resistance and improve efficacy.
Additional info: This guide expands on the PowerPoint outline by providing definitions, mechanisms, and clinical context for each drug class and resistance mechanism. It also clarifies the importance of selective toxicity, the rationale for combination therapy, and the public health implications of antimicrobial resistance.
