Overview of Antimicrobial Drug Mechanisms

Antimicrobial drugs are classified based on their mechanisms of action against various pathogens, including bacteria, mycobacteria, viruses, fungi, helminths, and protozoa. Understanding these mechanisms is crucial for effective clinical application and combating antimicrobial resistance.

Antibacterial Drugs

Inhibition of Cell Wall Synthesis

These drugs target the synthesis of peptidoglycan, an essential component of bacterial cell walls, leading to cell lysis and death.

• Phosphonomycin: Inhibits the conversion of UDP-NAG to UDP-NAM by blocking pyruvyl transferase.

• Cycloserine: Inhibits enzymes that add alanines to peptide side-chains.

• Bacitracin: Blocks secretion of NAG and NAM from the cytoplasm by binding bactoprenol and preventing its dephosphorylation.

• Vancomycin: Binds to terminal D-ala-D-ala chains on NAM peptides, preventing transglycosylation and transpeptidation.

• Beta-lactams (Penicillins, Cephalosporins, Carbapenems, Monobactams): Inhibit the transpeptidation reaction that cross-links peptide side-chains of the peptidoglycan backbone.

Inhibition of Protein Synthesis

These drugs interfere with bacterial ribosomes, inhibiting translation and protein production.

• Aminoglycosides (Streptomycin, Gentamicin): Bind to the 30S subunit, causing misreading of mRNA.

• Tetracyclines: Bind to the 30S subunit at the tRNA docking site (A site).

• Chloramphenicol: Blocks the enzymatic site of the 50S subunit, preventing peptide bond formation.

• Lincosamides, Streptogramins, Macrolides: Bind to the 50S subunit, preventing ribosome movement along mRNA.

• Mupirocin: Selectively binds to bacterial tRNA for leucine, preventing isoleucine incorporation.

• Oxazolidinones (Linezolid): Block formation of the 70S initiation complex, preventing translation.

Disruption of Cytoplasmic Membrane

These agents compromise membrane integrity, leading to cell death.

• Gramicidin: Forms pores in the membrane, causing leakage of cellular contents.

• Polymyxin: Destroys cytoplasmic membranes, especially in Gram-negative bacteria; toxic to human kidneys.

Inhibition of Metabolic Pathways

These drugs interfere with essential bacterial metabolic processes, such as folic acid synthesis.

• Dapsone: Interferes with folic acid synthesis.

• Sulfonamides: PABA analogs that inhibit dihydrofolic acid production.

• Trimethoprim: Inhibits the enzyme converting dihydrofolic acid to tetrahydrofolic acid.

Inhibition of Nucleic Acid Synthesis

These drugs target enzymes involved in DNA replication and transcription.

• Fluoroquinolones (Ciprofloxacin): Inhibit DNA gyrase, preventing DNA replication.

• Nitroimidazoles (Metronidazole): Generate toxic free radicals under anaerobic conditions, damaging DNA.

Antimycobacterial Drugs

These drugs are specialized antibacterials used to treat mycobacterial infections, such as tuberculosis and leprosy.

Inhibition of Cell Wall Synthesis

• Ethambutol: Prevents formation of mycolic acid in the cell wall.

• Isoniazid: Blocks the gene for an enzyme that forms mycolic acid.

Disruption of Cytoplasmic Membrane

• Pyrazinamide: Disrupts membrane transport and prevents repair of damaged proteins.

Inhibition of Nucleic Acid Synthesis

• Clofazimine: Binds to DNA, preventing replication and transcription.

• Rifamycin: Binds to RNA polymerase, preventing transcription.

Antiviral Drugs

Antiviral drugs target unique aspects of viral replication, as viruses lack cell walls and cytoplasmic membranes.

Inhibition of Protein Synthesis

• Antisense nucleic acid drugs (Fomiversen): Bind to viral mRNA, blocking translation.

Inhibition of Metabolic Pathways

• Drugs that inhibit viral uncoating (Amantadine, Rimantadine): Neutralize acidic environments necessary for viral uncoating.

• Protease inhibitors: Block the active site of viral proteases, preventing maturation of viral proteins (important in HIV therapy).

Inhibition of Nucleic Acid Synthesis

• Nucleotide/nucleoside analogs (Acyclovir, Ribavirin, Adenosine arabinoside): Inhibit DNA/RNA synthesis by acting as false substrates.

• Reverse transcriptase inhibitors (Azidothymidine): Inhibit viral reverse transcriptase, used in HIV treatment.

Prevention of Virus Attachment

• Attachment antagonists (Arildone, Pleconaril): Block viral attachment to host cells.

• Neuraminidase inhibitors (Zanamivir): Prevent influenza virus from attaching to cells.

Antifungal Drugs

Antifungal drugs exploit differences between fungal and human cells, particularly in cell wall and membrane composition.

Inhibition of Cell Wall Synthesis

• Echinocandins: Inhibit synthesis of glucan, a key component of the fungal cell wall.

Inhibition/Disruption of Cell Membrane

• Polyenes (Amphotericin B, Nystatin): Bind ergosterol, forming pores in the membrane.

• Azoles (Fluconazole): Inhibit ergosterol synthesis.

• Allylamines (Terbinafine): Inhibit ergosterol synthesis.

Inhibition of Nucleic Acid Synthesis

• 5-Fluorocytosine: Converted by fungal enzymes to 5-fluorouracil, an analog of uracil that inhibits RNA function.

Antihelminthic Drugs

These drugs target parasitic worms by disrupting their metabolic pathways or cell membranes.

Inhibition of Metabolic Pathways

• Benzimidazole derivatives: Inhibit microtubule formation and glucose uptake.

Disruption of Cytoplasmic Membrane

• Praziquantel, Ivermectin: Increase cell membrane permeability, leading to paralysis and death of the parasite.

Antiprotozoan Drugs

Antiprotozoal agents target unique metabolic or nucleic acid synthesis pathways in protozoa.

Inhibition of Metabolic Pathways

• Atovaquone: Interrupts the electron transport chain.

• Benzimidazole: Inhibits microtubule formation and glucose uptake.

• Furazolidone: Blocks carbohydrate metabolism.

• Proguanil, Pyrimethamine: Block folic acid biosynthesis.

• Sulfonamides: PABA analogs that inhibit dihydrofolic acid production.

• Trimethoprim: Inhibits conversion of dihydrofolic acid to tetrahydrofolic acid.

Inhibition of Nucleic Acid Synthesis

• Nitroimidazoles: Generate toxic free radicals under anaerobic conditions, damaging DNA.

• Pentamidine: Binds to nucleic acids, inhibiting replication, transcription, and translation.

• Quinolones (Chloroquine): Mechanism is not fully understood.

Summary Table: Major Classes of Antimicrobial Drugs and Their Mechanisms