Antimicrobial Agents: Mechanisms, Spectrum, and Clinical Applications

Antimicrobial Agents: Overview

Definition and Sources

Antimicrobial agents are substances that kill or inhibit the growth of microorganisms, including bacteria and fungi. They can be naturally produced, semisynthetic, or fully synthetic.

• Antibiotics: Microbially produced antimicrobial agents, primarily by bacteria and fungi.

• Less than 1% of known antibiotics are clinically useful due to issues of efficacy and toxicity.

• Semisynthetic antibiotics: Artificially modified natural antibiotics to enhance efficacy, broaden spectrum, or reduce side effects. They retain a "natural core."

• Synthetic antibiotics: Entirely man-made in the laboratory.

Selective toxicity is a key property, meaning drugs kill or inhibit microbial cells without simultaneously damaging host tissues.

Sources of Antibiotics

• Bacteria: Streptomyces, Bacillus

• Fungi: Penicillium, Cephalosporium

• Zones of inhibition in culture indicate antibiotic production by colonies.

Major Antibiotic Producers and Their Products

Producer Genus	Type of Microbe	Drug Example
Penicillium	Mold	Penicillins, Griseofulvin
Cephalosporium	Mold	Cephalosporins
Micromonospora	Bacteria	Gentamicin
Bacillus	Bacteria	Bacitracin, Polymyxin B
Chromobacterium	Bacteria	Aztreonam
Streptomyces	Filamentous bacteria	Streptomycin, Erythromycin, Tetracycline, Vancomycin, Chloramphenicol, Amphotericin B

Spectrum of Activity

Definitions and Examples

The spectrum of an antimicrobial drug refers to the range of microorganisms it affects.

• Narrow-spectrum drugs: Effective on a small range of microbes.

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

Examples:

• Bacitracin: Effective for Gram-positive bacteria, not Gram-negative.

• Polymyxin: Effective for Gram-negative bacteria, not Gram-positive.

• Tetracycline: Effective for most Gram-positive and Gram-negative bacteria.

Mechanisms of Action of Antimicrobial Drugs

1. Drugs Affecting the Bacterial Cell Wall

These drugs inhibit the synthesis of peptidoglycan, an essential component of the bacterial cell wall, leading to cell lysis.

• Penicillins and cephalosporins: Block peptidoglycan synthesis.

• Penicillins that do not penetrate the outer membrane are less effective against Gram-negative bacteria.

• Broad-spectrum penicillins and cephalosporins can cross Gram-negative cell walls.

2. Drugs That Disrupt Cell Membrane Function

These drugs interact with membrane phospholipids, causing leakage of cellular contents, especially in Gram-negative bacteria.

• Polymyxins: Disrupt membrane integrity.

3. Drugs That Affect Nucleic Acid Synthesis

These drugs block the synthesis of nucleotides, inhibit DNA replication, or stop transcription.

• Quinolones: Synthetic drugs that inhibit DNA gyrase, preventing DNA replication.

• Example equation: $\text{Inhibition of DNA gyrase} \rightarrow \text{No DNA replication}$

4. Drugs That Block Protein Synthesis

These drugs interfere with ribosomal function, preventing protein synthesis.

• Aminoglycosides (e.g., streptomycin, gentamycin): Bind to the 30S ribosomal subunit.

• Tetracyclines: Inhibit the 30S ribosome, blocking tRNA attachment to the A site.

• Macrolides (e.g., erythromycin): Attach to the 50S ribosomal subunit.

5. Drugs That Affect Metabolic Pathways

These drugs inhibit key metabolic processes unique to bacteria.

• Sulfonamides (sulfa drugs): Block the synthesis of folic acid, which is essential for nucleic acid synthesis in bacteria.

• Only active in bacteria because humans do not synthesize their own folic acid.

• Example equation: $\text{Sulfonamide} + \text{Enzyme} \rightarrow \text{No folic acid synthesis}$

Antibacterial Drug Classes and Examples

Beta-lactam Antimicrobials

• Penicillins and cephalosporins: Interfere with cell wall synthesis.

• Penicillins G and V: Most important natural forms; drug of choice for Gram-positive cocci and some Gram-negative bacteria (e.g., meningococci, syphilis).

• Semisynthetic penicillins (ampicillin, amoxicillin): Broader spectrum, effective against Gram-negative infections.

• Penicillinase-resistant: Methicillin, nafcillin, cloxacillin.

• Primary problems: Allergies and resistant bacterial strains.

Fluoroquinolones

• Bind to DNA gyrase (Gram-negative) and topoisomerase IV (Gram-positive).

• Broad spectrum; treat urinary tract infections and other bacterial infections.

Aminoglycosides

• Produced by soil actinomycetes.

• Effective against aerobic Gram-negative rods and some Gram-positive bacteria.

• Streptomycin: Used for bubonic plague, tularemia, tuberculosis.

• Gentamicin: Less toxic, used for Gram-negative rods.

Tetracycline Antibiotics

• Broad spectrum; block protein synthesis by binding ribosomes.

Macrolides and Related Antibiotics

• Erythromycin: Lactone ring with sugars; attaches to 50S ribosomal subunit.

• Moderate spectrum, low toxicity.

• Used orally for Mycoplasma pneumonia, legionellosis, Chlamydia, pertussis, diphtheria, and as prophylaxis before intestinal surgery.

• Effective for penicillin-resistant gonococci, syphilis, acne.

• Newer semi-synthetic macrolides: Clarithromycin, Azithromycin.

Sulfonamides (Sulfa Drugs)

• Most important synthetic drugs that block folic acid synthesis in bacteria.

Antifungal Drugs

Challenges and Toxicity

Fungal cells are eukaryotic, making selective toxicity difficult. Drugs toxic to fungi may also harm human cells.

Antiviral Chemotherapeutic Agents

Drugs for Treating Influenza

• Selective toxicity is challenging due to viruses' obligate intracellular nature.

• Relenza and Tamiflu: Block neuraminidase in influenza A and B, preventing viral release.

Drugs for Treating HIV and AIDS

• Nucleoside reverse transcriptase inhibitors (NRTIs)

• Non-nucleoside reverse transcriptase inhibitors (NNRTIs)

• Integrase inhibitors

• Protease inhibitors

• Fusion inhibitors

Category	Effect on Virus	Example
Binding inhibitors	Insert into HIV DNA, prevent addition of new bases (competitive inhibition)	Maraviroc, Selzentry
Fusion inhibitors	Interact with reverse transcriptase, block viral entry	Enfuvirtide
Protease inhibitors	Block HIV protease, prevent viral assembly	Atazanavir, Darunavir, Saquinavir
Pharmacokinetic enhancers	Increase effectiveness of other anti-HIV drugs	Cobicistat (Tybost)

Interferons (IFN)

• Human-based glycoproteins produced by fibroblasts and leukocytes.

• Therapeutic benefits:

  • Reduce healing time and complications of infections

  • Prevent or reduce symptoms of cold and papillomavirus

  • Slow progress of certain cancers, leukemias, and lymphomas

  • Treat Kaposi's sarcoma in AIDS patients

COVID-19 Treatment

• Severe COVID-19 is primarily due to inflammation, not direct viral damage.

• Dexamethasone, budesonide, fluvoxamine: Reduce inflammation.

• Convalescent antibodies from recovered patients show positive effects.

• Remdesivir: Shortens hospital stay.

• Molnupiravir (nucleoside analog) and Paxlovid (protease inhibitors): Show exceptional activity.

Drug Resistance

Mechanisms and Spread

Microorganisms can develop resistance to drugs through genetic changes and gene transfer.

• Spontaneous mutations in chromosomal genes.

• Acquisition of new genes via transfer from other species (plasmids, transposons).

• Intermicrobial transfer of resistance (R) factors.

Factors Increasing Antibiotic Resistance

• Sharing antibiotics with others.

• Stopping antibiotics before completing the prescribed regimen.

• Incorrect diagnosis and prescription by physicians.

• Use of antibiotics in livestock herds.

Summary Table: Mechanisms of Antimicrobial Action

Mechanism	Drug Class	Example
Cell wall synthesis inhibition	Beta-lactams	Penicillins, Cephalosporins
Cell membrane disruption	Polymyxins	Polymyxin B
Protein synthesis inhibition	Aminoglycosides, Tetracyclines, Macrolides	Streptomycin, Tetracycline, Erythromycin
Nucleic acid synthesis inhibition	Quinolones	Ciprofloxacin
Metabolic pathway inhibition	Sulfonamides	Sulfamethoxazole

Additional info: Some context and drug examples were inferred and expanded for clarity and completeness.