Antimicrobial Drugs: Mechanisms, Clinical Considerations, and Resistance

Controlling Microbial Growth in the Body: Antimicrobial Drugs

The History of Antimicrobial Agents

The development of antimicrobial agents revolutionized the treatment of infectious diseases. These agents include chemicals that affect physiology, chemotherapeutic agents (drugs acting against diseases), and antimicrobial agents (drugs treating infections).

• Paul Ehrlich: Introduced the concept of “magic bullets” and developed arsenic compounds targeting microbes.

• Alexander Fleming: Discovered Penicillin released from Penicillium.

• Gerhard Domagk: Discovered sulfanilamide, the first widely used antimicrobial.

• Selman Waksman: Coined the term “antibiotics” for naturally produced antimicrobial agents.

• Semi-synthetics: Chemically altered antibiotics for improved efficacy.

• Synthetics: Completely synthesized antimicrobials in laboratories.

Example: The antibiotic effect of Penicillium chrysogenum on Staphylococcus aureus demonstrates the principle of microbial antagonism.

Mechanisms of Antimicrobial Action

Antimicrobial drugs act through several mechanisms, targeting specific features of pathogens to achieve selective toxicity. The largest group targets bacteria, with fewer drugs available for eukaryotic pathogens and viruses.

• Inhibition of cell wall synthesis

• Inhibition of protein synthesis

• Disruption of cytoplasmic membranes

• Inhibition of metabolic pathways

• Inhibition of nucleic acid synthesis

• Prevention of virus attachment

Summary Diagram of Drug Targets

Inhibition of Cell Wall Synthesis

Drugs targeting cell wall synthesis are most effective against bacteria, especially during growth. The bacterial cell wall is composed of peptidoglycan, with NAG and NAM subunits cross-linked by peptide bridges.

• Beta-lactams: Bind to enzymes that cross-link NAM subunits, weakening the cell wall and causing lysis.

• Semi-synthetic beta-lactams: More stable, better absorbed, and active against a broader range of bacteria.

• Vancomycin and cycloserine: Interfere with bridges linking NAM subunits in Gram-positives.

• Bacitracin: Blocks secretion of NAG and NAM from cytoplasm.

• Isoniazid and ethambutol: Disrupt mycolic acid formation in mycobacteria.

Example: Beta-lactams are only effective against growing cells, as they prevent new peptidoglycan formation but do not affect existing layers.

Inhibition of Protein Synthesis

Protein synthesis inhibitors exploit differences between prokaryotic (70S) and eukaryotic (80S) ribosomes. However, mitochondria in eukaryotes contain 70S ribosomes, which can lead to toxicity.

• Aminoglycosides: Cause misreading of mRNA.

• Tetracyclines: Block docking site of tRNA.

• Chloramphenicol: Binds to 50S subunit, inhibiting peptide bond formation.

• Macrolides and lincosamides: Block 50S subunit, preventing movement through ribosome.

• Oxazolidinones: Block initiation of translation.

Disruption of Cytoplasmic Membranes

Some drugs disrupt membrane integrity by forming channels or inhibiting sterol synthesis.

• Amphotericin B: Attaches to ergosterol in fungal membranes, forming pores.

• Azoles and allyamines: Inhibit ergosterol synthesis.

• Polymyxin: Disrupts Gram-negative bacterial membranes; toxic to kidneys.

Inhibition of Metabolic Pathways

Antimetabolic agents are effective when pathogen and host metabolic processes differ.

• Quinolones: Interfere with malaria parasite metabolism.

• Heavy metals: Inactivate enzymes.

• Sulfonamides: Block folic acid synthesis by acting as competitive inhibitors of PABA.

• Antiviral agents: Target unique aspects of viral metabolism (e.g., uncoating, protease inhibitors).

Inhibition of Nucleic Acid Synthesis

Drugs in this category block DNA replication or mRNA transcription, often affecting both prokaryotic and eukaryotic cells. Nucleotide analogs distort nucleic acid shapes, preventing replication, transcription, or translation.

• Quinolones and fluoroquinolones: Act against prokaryotic DNA gyrase.

• Reverse transcriptase inhibitors: Target HIV replication; humans lack reverse transcriptase.

Prevention of Virus Attachment

Attachment antagonists block viral attachment or receptor proteins, representing a new area of drug development.

Clinical Considerations in Prescribing Antimicrobial Drugs

Ideal Antimicrobial Agent

• Readily available

• Inexpensive

• Chemically stable

• Easily administered

• Nontoxic and nonallergenic

• Selectively toxic against a wide range of pathogens

Spectrum of Action

The spectrum of action refers to the range of pathogens a drug affects. Narrow-spectrum drugs target few organisms, while broad-spectrum drugs target many. Broad-spectrum drugs may lead to secondary or superinfections by reducing normal flora.

Efficacy Testing

• Diffusion susceptibility test: Measures zone of inhibition.

• Minimum inhibitory concentration (MIC) test: Determines lowest concentration preventing growth.

• Minimum bactericidal concentration (MBC) test: Determines lowest concentration killing bacteria.

Routes of Administration

• Topical: For external infections.

• Oral: Self-administered, no needles.

• Intramuscular: Injection into muscle.

• Intravenous: Directly into bloodstream.

Safety and Side Effects

• Toxicity: May affect kidneys, liver, or nerves; special consideration for pregnant women.

• Allergies: Rare but potentially life-threatening (anaphylactic shock).

• Disruption of normal microbiota: May cause secondary infections or superinfections, especially in hospitalized patients.

Resistance to Antimicrobial Drugs

Development of Resistance

Resistance can be natural or acquired through mutations or acquisition of R-plasmids via transformation, transduction, or conjugation.

Mechanisms of Resistance

• Production of enzymes that destroy or deactivate drugs (e.g., β-lactamase).

• Slow or prevent drug entry into the cell.

• Alteration of drug targets.

• Alteration of metabolic pathways.

• Pumping drugs out of the cell (efflux pumps).

• Binding proteins that prevent drug action (e.g., MfpA protein in Mycobacterium tuberculosis).

Multiple Resistance and Cross Resistance

• Pathogens may acquire resistance to multiple drugs, especially in hospital settings.

• Cross resistance occurs when resistance to one drug confers resistance to similar drugs.

Retarding Resistance

• Maintain high drug concentration in patients to kill sensitive cells and inhibit others.

• Use drugs in combination (synergism vs. antagonism).

• Use antimicrobials only when necessary.

• Develop new drug variations and search for new antibiotics.

• Design drugs complementary to microbial protein shapes.

Summary Table: Mechanisms of Antimicrobial Action

Mechanism	Example Drugs	Target Pathogen
Inhibition of Cell Wall Synthesis	Penicillins, Cephalosporins, Vancomycin	Bacteria
Inhibition of Protein Synthesis	Aminoglycosides, Tetracyclines	Bacteria
Disruption of Cytoplasmic Membrane	Polymyxin, Amphotericin B	Bacteria, Fungi
Inhibition of Metabolic Pathways	Sulfonamides, Trimethoprim	Bacteria, Protozoa
Inhibition of Nucleic Acid Synthesis	Quinolones, Nucleotide analogs	Bacteria, Viruses
Prevention of Virus Attachment	Arildone, Pleconaril	Viruses

Additional info: This summary expands on brief points with academic context, definitions, and examples to provide a comprehensive, exam-ready study guide for antimicrobial drugs and resistance mechanisms.