Lecture 22: Antimicrobial Drugs

  • Antibiotic: antimicrobial medications naturally produced by micro-organisms

    • commonly used as a synonym for antibacterial.

    • First antibiotic: penicillin

      • discovery of penicillin:

        • colones were smaller around penicillium mold

          • penicillium mold makes chemicals called penicillin

Antimicrobial Drug Terms

  • Antimicrobial action can either kill microbe or inhibit the growth of microbe:

    • “-cidal”: kills microbes

      • Bactericidal: Drugs that function by killing the targeted microbe.

    • “-static”: inhibits microbe growth

      • keeps microbe from growing long enough for immune system to kill it

        • Bacteriostatic: Drugs that inhibit the growth of the microbe. This allows the host's immune system enough time to clear the infection.

    • Examples of drugs that kill or inhibit growth of microbes:

      • antibacterial

      • anti-fungal

      • anti-protozoan

      • anti-helminthic

      • antiviral (inhibits replication)

Designing a Good Antimicrobial Drug

  • Selective Toxicity: hurts microbes more than the host

    • The principle of designing a drug that harms the microbe significantly more than the host

    • targets structures or processes unique to the microbe = drug target

  • Drug Target: the part of the microbe that is affected by the drug

    • most drugs bind to molecule or structure in microbe and breaking it

      • blocks microbe from functioning properly and ultimately leads to its death

Common Drug Targets

  • Drug Targets for Antibacterial Drugs:

    1. Cell Wall

    2. Cell Membrane

    3. Ribosomes

    4. Metabolic Pathways

    5. DNA/ RNA synthesis

Targeting the Cell Wall

  • Drugs that target the cell wall:

    • inhibit peptidoglycan synthesis

    • All drugs in this category are bactericidal

      • all Kill bacteria

    • Bacitracin: prevents transport of peptidoglycan subunits out of the cell wall

      • bactericidal drug

      • Target: cell wall

      • Mechanism:

        • during cell division, peptidoglycan subunits are exported out of the cell wall

          • Bacitracin blocks the peptidoglycan subunits from being exported

    • Glycopeptides: bind peptidoglycan subunits so they can’t be incorporated

      • bactericidal drug

      • Glycopeptide Antibiotic: Vancomycin

      • Target: cell wall

      • Mechanism:

        • after export, peptioglycans subunits are suppose to be bound to other existing peptidoglycan subunits to build cell wall

          • glycopeptide drug bind to peptioglycans subunits instead, preventing them from being incorporated into cell wall

    • Beta-lactams: bind and block penicillin binding protein (PBP) enzymes so it can’t build peptidoglycan

      • bactericidal drug

      • Glycopeptide Antibiotic: Penicillin

      • Target: cell wall

      • Mechanism:

        • PBP enzyme: builds peptidoglycan, takes peptidoglycan subunits and incorporates them into existing peptidoglycan cell wall

          • beta-lactam blocks PBP enzyme from building peptidoglycan cell wall

      • called beta-lactams because of their chemical structure, containing beta-lactam rings

Targeting the Cell Membrane

  • Drugs that target the cell membrane:

    • All drugs in this category are bactericidal

      • all Kill bacteria

    • Daptomycin: target Gram positive cell membrane and pokes holes in it

      • bactericidal drug

      • Target: Gram positive cell membrane

      • Mechanism:

        • pokes holes in cell membrane

          • causes cell to leak, leading to cell death

        • only works in gram positive cells because daptomysin cannot cross the outer membrane of gram negative cells

      • Memory Device: daptomycin has + sign in name so it targets gram positive, polymyxin does not so it targets gram negative

    • Polymyxin: target Gram negative outer membrane and pokes holes in it

      • bactericidal drug

      • Target: Gram negative outer membrane

      • Mechanism:

        • pokes holes in outer membrane

          • causes cell to leak, leading to cell death

        • only works in gram negative cells because only gram negative cells have an outer membrane

Targeting Nucleic Acid Synthesis

  • Drugs that target Nucleic Acid (RNA or DNA) Synthesis:

    • All drugs in this category are bactericidal

      • all Kill bacteria

    • Rifamycin: blocks RNA polymerase

      • bactericidal drug

      • Target: bacterial RNA synthesis

      • Mechanism:

        • during transcription, bacteria uses bacterial RNA polymerase to build RNA

          • Rifamycin blocks bacterial RNA synthesis during transcription

      • Memory device: Rifamycin and RNA both starts with R

    • Fluoroquinolone: blocks DNA gyrase

      • bactericidal drug

      • Target: DNA synthesis

      • Mechanism:

        • during replication, bacteria uses topoisomerase to relieve the torsional strain called DNA gyrase

          • Fluoroquinolone blocks DNA gyrase

      • Memory Device: "Fluoro"/"Flurry" as in spinning/gyrating.

    • Metronidazole: binds DNA and blocks it

      • bactericidal drug

      • Target: DNA synthesis

      • Mechanism:

        • Metronidazole binds DNA itself and blocks it

        • only active in anaerobic cells

      • Memory Device: no oxygen left, you have “met” your end

Targeting Protein Synthesis

  • Drugs that target Protein Synthesis:

    • Chloramphenicol: binds to the large ribosomal subunit to stop protein synthesis.

      • bacteriostatic drug

      • Target: large ribosome subunit

    • Lincosamides: binds to the large ribosomal subunit to stop protein synthesis.

      • bacteriostatic drug

      • Target: large ribosome subunit

    • Macrolides: binds to the large ribosomal subunit to stop protein synthesis.

      • bacteriostatic drug

      • Target: large ribosome subunit

    • Aminoglycosides: binds to the small ribosomal subunit to stop protein synthesis.

      • bactericidal drug

        • the only bactericidal in the group of drugs that target protein synthesis

          • memory device: A for Aminoglycoside is also for Assassin

      • Target: small ribosome subunit

    • Tetracyclines: binds to the small ribosomal subunit to stop protein synthesis.

      • bacteriostatic drug

      • Target: small ribosome subunit

    • Memory Device:

      • C.L.a.M. = target the large ribosome subunit

        • C: Chloramphenicol

        • L: Lincosamides

        • a: and

        • M: Macrolides

      • A. T. = target the small ribosome subunit

        • A: Aminoglycosides

        • T: Tetracyclines

Targeting Metabolic Pathways

  • Pathway for making Folic acid:

    • PABA: chemical precursor for building folic acid

      • enzyme converts PABA into Dihydrofolic acid

        • Dihydrofolic acid formed

          • enzyme converts Dihydrofolic acid into Tetrahydrofolic acid

            • Tetrahydrofolic acid formed

  • Drugs that target Folic Acid Synthesis (metabolic pathway):

    • All drugs in this category are bacteriostatic

    • but if sulfonamide and trimethoprim are given together, they are bactericidal

    • Sulfonamides: blocks enzyme from converting PABA into Dihydrofolic acid

      • individually, bacteriostatic drug

      • Target: folic acid synthesis (metabolic pathway)

      • Mechanism: Sulfonamides fits into active site of enzyme where PABA normally fits, so enzyme can’t find PABA and cannot catalyze the reaction

        • blocks the synthesis of folic acid

    • Trimethoprim: blocks enzyme from converting Dihydrofolic acid into Tetrahydrofolic acid

      • individually, bacteriostatic drug

      • Target: folic acid synthesis (metabolic pathway)

      • Mechanism: Trimethoprim fits into active site of enzyme where Dihydrofolic acid normally fits, so enzyme can’t find Dihydrofolic acid and cannot catalyze the reaction

        • blocks the synthesis of folic acid

  • Drugs that target Mycolic Acid Synthesis (metabolic pathway):

    • Isoniazid: weakens mycolic acid cell wall in mycobacterium species

      • bactericidal drug, but only when cells are dividing

        • problem for cells like tuberculosis because they don’t divide often

      • Target: mycolic acid synthesis in cell wall of mycobacterium (metabolic pathway)

      • Mechanism:

        • prevents bacteria from making more mycolic acid during cell division

Spectrum of Activity:

  • Broad Spectrum: target a wide range of bacteria, including gram positive or gram negative bacteria

  • Narrow Spectrum: target a limited range of bacteria, usually only gram positive or only gram negative bacteria

Anti-fungal Drugs

  • Three targets for Anti-fungal Drugs:

    1. ergosterol: steroid that is part of structure of fungi cell membrane

    2. beta glucans: part of fungi cell wall

    3. chitin: part of fungi cell wall

Anti-Viral Drugs
Herpes

  • Acyclovir: herpes drug; viral enzymes convert acyclovir into active form that blocks DNA synthesis

    • target: DNA synthesis (nucleic acid synthesis)

    • Mechanism:

      • only active inside of cell that are infected with herpes virus

        • enzyme within infected herpes virus cell converts acyclovir into active form

          • acyclovir then blocks DNA synthesis

HIV

  • HIV Drug targets:

    1. fusion / entry

      • fusion inhibitors: drugs that block membrane fusion, preventing virus from entering cell

    2. reverse transcriptase

      • reverse transcriptase inhibitors: keep viral RNA from being copied into DNA

    3. integrase

      • integrase inhibitors: prevents viral DNA from being integrated into host cell genome

    4. protease

      • protease inhibitors: block viral proteins from being made

SARS-CoV-2

  • SARS-CoV-2 Drug targets:

    1. entry

      • drug uses antibodies bind to receptors and prevent entry

      • drug binds to protease that would normally help virus with getting into cell

    2. protease

      • protease inhibitors: block viral proteins from being made

    3. replicase (viral RNA synthesis)

      • drugs are nucleoside analog that give viruses lots of mutation materials

        • virus is so mutated that it is not able to function

Route of Administration

  • Route of Administration include:

    • intravenous (IVIV )

    • intramuscular (IMIM)

    • oral

    • topical routes

Interaction of Drugs within the Body

  • Tissue Distribution: where drug goes in the body

    • some drugs are more effective different parts of the body

  • Metabolism: where in the body does the drug go to be metabolized

  • Excretion of Drug: The speed at which the body breaks down a drug

    • determines the dosage frequency (e.g., once daily vs. four times daily).

Drug Resistance

  • Susceptible: drug works to kill or inhibit growth of pathogen

    • refers specifically to the pathogen.

      • A pathogen is susceptible when a drug effectively kills it or inhibits its growth.

  • Resistant: drug DOES NOT work to kill or inhibit growth of a pathogen

    • also refers specifically to the pathogen.

    • A pathogen is resistant when it has developed a mechanism to avoid or neutralize the effects of a drug, making the medication useless against it.

  • It is never the patient who is "resistant" or "susceptible" to an antibiotic; it is the pathogen causing the infection.

Resistance Mechanisms

  • Pathogens utilize four primary strategies to resist the effects of antimicrobial drugs:

    1. Efflux: cell (pathogen) has drug pump that pumps drug out of cell

      • The pathogen possesses pumps located on the cell surface.

        • As the drug enters the cell, these pumps actively transport the drug molecules back out before they can reach their target.

    2. Blocking Drug Entry: prevent drug from entering cell (pathogen)

      • Pathogens prevent the drug from entering cell in the first place

        • example: gram negative bacteria naturally resistant to some drugs that cannot pass through cell membrane

    3. Enzyme Inactivates Drugs: enzyme within a cell (pathogen) destroys drug

      • The cell produces specific enzymes that chemically destroy or modify the drug.

        • This can involve cutting the drug molecule into pieces, causing a protein-based drug to unfold, or otherwise altering it to render it inactive.

      • Examples:

        • Beta-lactamase: An enzyme that specifically breaks bond in ring structure found in beta-lactam drugs (like penicillin)

          • type of drug resistance to drugs that target cell wall

        • Carbapenemase: An enzyme that breaks carbapenem drugs by breaking their structure

          • type of drug resistance to drugs that target cell wall

    4. Altering Drug Target Molecule: cell (pathogen) changes molecule so drug can’t bind to it anymore

      • Drugs typically work by binding to a specific molecule (the drug target) to block a process

        • The pathogen mutates the gene for that target molecule, but can still function normally

          • The drug can no longer bind to the altered target

            • drug is not effective anymore

      • Example: MecA gene (MRSA - Methicillin Resistant Staphylococcus Aureus):

        • bacteria have PBP gene that codes for PBP enzyme (builds peptidoglycan)

          • beta-lactam drugs bind and block PBP enzymes so it can’t build peptidoglycan cell wall

            • MRSA still have regular PBP gene, but also evolved an additional PBP gene, called MecA gene

              • MecA gene encodes a different type of PBP enzyme, called PBP 2a enzyme

                • enzyme builds peptidoglycan but beta-lactam drugs can no longer bind to it

                  • makes drug ineffective

How is Resistance Acquired?

  1. Random Mutation: Resistance begins with a random mutation in a single cell within a population of susceptible cells.

    • This mutation might create a new gene or alter an existing protein.

    • mutation is completely useless until drug comes into environment

  2. Selection Pressure: drug present in environment and kills all cells that are susceptible to the drug, allowing only single mutated cells to survive

    • When a drug is introduced, it kills all susceptible (S) cells

    • The resistant (R) cell survives.

  3. Vertical Gene Transfer: The resistance gene is passed from the surviving parent cell to its offspring during cell division.

    • The resistant (R) cell that survived replicates rapidly, since it has no competition for resources

  4. Horizontal Gene Transfer: Resistance genes can be shared between pre-existing cells and becomes resistant

    • occurs via:

      • Transformation: pre-existing cell picks up DNA from the environment.

        • can occur within different species

          • how drug resistance can jump species - another pathogen picks up resistance gene, making it now resistant too

      • Transduction: drug resistant DNA transferred via viruses.

        • occurs within the same species

      • Conjugation: Direct cell-to-cell transfer of drug resistant DNA

        • occurs within the same species

Spread of Resistance in Communities and Agriculture

  • Person-to-Person: Resistance spreads through the community from an initially infected individual with a drug resistant pathogen, to other patients or healthcare providers.

  • Agricultural Use: Antibiotics are given to livestock prophylactically (before they are sick) to increase growth yields

    • this constant exposure to antibiotics encourages development of antibiotic resistant bacteria

      • antibiotic bacteria can move to humans by:

        • Meat: antibiotic meat is improperly cooked or handled

        • Produce: Animal feces containing resistant bacteria are used as fertilizer for vegetable crops, which then enter the human food supply.

  • While the CDC regulates person-to-person spread, the FDA regulation of antibiotics in agriculture is poor and less stringent.

Antibiotic Stewardship

  • Healthcare Provider Responsibility:

    • Avoid mis-prescription and over prescription

    • Reserve "last resort" antibiotics for when truly need

    • Follow hospital guidelines to halt spread of disease

  • Patient Responsibility:

    • Follow instructions exactly: take the all antibiotics and do not skip doses.

    • understand that bacterial antibiotics are not effective for viral or fungal infections

  • Government and Public:

    • monitoring and regulation by CDC and FDA