Antibiotics and Bacterial Resistance - lect 18

Disrupting Cell Membranes

• Valinomycin:

  • Isolated from bacteria; bacteria use it to inhibit other bacteria.

  • Highly effective at chelating potassium ions (1000x more effective than for sodium).

  • Disrupts potassium concentration inside the cell, leading to cell death.

  • Too toxic for clinical use in humans.

• Polymyxin B:

  • Isolated from bacteria.

  • Contains unusual amino acids and a long fatty chain for anchoring into the cell membrane.

  • Causes leakage of small molecules, disrupting the ionic potential.

  • Tolerable by humans.

Impairing Protein Synthesis

• Target bacterial ribosomes, which differ from human ribosomes.

• Interfere with translation (protein synthesis).

• Bill Clemmons (Caltech): Studied the structure of ribosome subunits to identify antibiotic targets.

Targeting Ribosome Subunits: Aminoglycosides

• Based on carbohydrates with nitrogen replacing some hydroxyl groups.

• Effective against Gram-negative bacteria.

  • Around neutral pH, NH groups are protonated, aiding transport across the membrane of Gram-negative bacteria, which contain anionic lipids.

• Limitations:

  • Poor oral availability due to Lipinski's rule violations such as too many hydrogen bond donors and acceptors.

• Mechanism of Action:

  • Protonated amine groups form ionic pairs with phosphate groups on the ribosome.

  • Interferes with mRNA function.

Bacterial Resistance to Aminoglycosides

• Bacteria acetylate the amine groups using esterases or N-kinases.

  • Acetylation prevents protonation at neutral pH and hinders binding to the target site.

• Bacteria phosphorylate hydroxyl groups on the sugars.

  • Introduces negative charges, repelling the phosphate groups on the ribosome.

Tetracyclines

• Broad-spectrum antibiotics effective against Gram-positive and Gram-negative bacteria.

• Inhibit protein synthesis by preventing tRNA from binding to the ribosome.

• Structure:

  • Four fused rings.

  • Hydrophobic top half and hydrophilic edge.

  • Examples: chlorotetracycline, doxycycline, minocycline.

• The precise arrangement of hydrogen bonds on the hydrophilic edge is crucial for activity.

  • Forms specific hydrogen bonds with a metal ion in the tRNA binding site.

Bacterial Resistance to Tetracyclines

• Efflux Pumps: Bacteria expel tetracyclines using efflux pumps.

• Ribosome Mutation: Mutations in the ribosome prevent tetracycline binding.

Overcoming Resistance

• Adding additional hydrogen bonding recognition sites to tetracyclines (e.g., tigecycline).

Chloramphenicol

• Broad-spectrum antibiotic.

• Toxic to humans; used when the risk is justified by the severity of the infection.

• Undergoes phase II metabolism, conjugating to glucuronic acid to form chloramphenicol glucuronide.

• Binds to the ribosomal unit, interfering with mRNA transcription.

  • Involves pi-stacking and hydrogen bonding in the active site.

Bacterial Resistance to Chloramphenicol

• Bacteria acetylate a hydroxyl group necessary for chelating to potassium ions using simple esterases.

Gene Transfer

• Gene transfer contributes to antibiotic resistance.

Antibiotics and Cancer Research

• There were 16,000 hits three years ago compared to treating cancer.

• Lung cancer has a high mortality rate.

Disrupting Cell Membranes

• Valinomycin:- Isolated from bacteria; bacteria use it to inhibit other bacteria.

  • Highly effective at chelating potassium ions (1000x more effective than for sodium). This selectivity is due to valinomycin's structure, which features a hydrophobic exterior that allows it to dissolve in the cell membrane, and a hydrophilic interior that specifically binds potassium ions.

  • Disrupts potassium concentration inside the cell, leading to cell death. This disruption affects the cell's ability to maintain osmotic balance and conduct essential functions.

  • Too toxic for clinical use in humans. Its toxicity arises from its ability to disrupt ion gradients in human cells as well.

• Polymyxin B:- Isolated from bacteria.

  • Contains unusual amino acids and a long fatty chain for anchoring into the cell membrane. The fatty chain is crucial for its interaction with the lipid bilayer.

  • Causes leakage of small molecules, disrupting the ionic potential. By creating pores in the membrane, it leads to loss of essential metabolites and disruption of cellular processes.

  • Tolerable by humans; however, it can still cause nephrotoxicity and neurotoxicity, limiting its use.

Impairing Protein Synthesis

• Target bacterial ribosomes, which differ from human ribosomes. This difference allows for selective targeting of bacterial protein synthesis without significantly affecting human cells.

• Interfere with translation (protein synthesis). They bind to the ribosome and disrupt the process of mRNA translation into proteins.

• Bill Clemmons (Caltech): Studied the structure of ribosome subunits to identify antibiotic targets. His work has provided valuable insights into the mechanisms of antibiotic action and resistance.

Targeting Ribosome Subunits: Aminoglycosides

• Based on carbohydrates with nitrogen replacing some hydroxyl groups. The presence of nitrogen enhances their interaction with anionic components of bacterial cells.

• Effective against Gram-negative bacteria.- Around neutral pH, NH groups are protonated, aiding transport across the membrane of Gram-negative bacteria, which contain anionic lipids. The protonation is essential for their ability to cross the outer membrane of Gram-negative bacteria.

• Limitations:- Poor oral availability due to Lipinski's rule violations such as too many hydrogen bond donors and acceptors. This limits their clinical use to intravenous or intramuscular administration.

• Mechanism of Action:- Protonated amine groups form ionic pairs with phosphate groups on the ribosome.

  • Interferes with mRNA function, leading to misreading of the genetic code and premature termination of protein synthesis.

Bacterial Resistance to Aminoglycosides

• Bacteria acetylate the amine groups using esterases or N-kinases.- Acetylation prevents protonation at neutral pH and hinders binding to the target site. This enzymatic modification is a common mechanism of resistance.

• Bacteria phosphorylate hydroxyl groups on the sugars.- Introduces negative charges, repelling the phosphate groups on the ribosome. This phosphorylation disrupts the drug-target interaction.

Tetracyclines

• Broad-spectrum antibiotics effective against Gram-positive and Gram-negative bacteria. They inhibit a wide range of bacteria by targeting protein synthesis.

• Inhibit protein synthesis by preventing tRNA from binding to the ribosome. This action blocks the addition of amino acids to the growing peptide chain.

• Structure:- Four fused rings. This unique structure is essential for their mechanism of action.

  • Hydrophobic top half and hydrophilic edge. This amphipathic nature allows them to interact with both the ribosome and the cellular environment.

  • Examples: chlorotetracycline, doxycycline, minocycline. These different tetracyclines have variations in their chemical structures, affecting their pharmacokinetic properties.

• The precise arrangement of hydrogen bonds on the hydrophilic edge is crucial for activity.- Forms specific hydrogen bonds with a metal ion in the tRNA binding site. This interaction is critical for blocking tRNA binding and protein synthesis.

Bacterial Resistance to Tetracyclines

• Efflux Pumps: Bacteria expel tetracyclines using efflux pumps. These pumps actively transport the antibiotic out of the cell, reducing its intracellular concentration.

• Ribosome Mutation: Mutations in the ribosome prevent tetracycline binding. Mutations alter the structure of the ribosome, preventing the antibiotic from binding effectively.

Overcoming Resistance

• Adding additional hydrogen bonding recognition sites to tetracyclines (e.g., tigecycline). This modification enhances the drug's binding affinity to the ribosome, even in the presence of resistance mechanisms.

Chloramphenicol

• Broad-spectrum antibiotic. It is effective against a wide range of bacteria, but its use is limited due to toxicity.

• Toxic to humans; used when the risk is justified by the severity of the infection. Its toxicity includes bone marrow suppression and aplastic anemia.

• Undergoes phase II metabolism, conjugating to glucuronic acid to form chloramphenicol glucuronide. This conjugation facilitates its excretion from the body.

• Binds to the ribosomal unit, interfering with mRNA transcription.- Involves pi-stacking and hydrogen bonding in the active site. This interaction inhibits peptide bond formation.

Bacterial Resistance to Chloramphenicol

• Bacteria acetylate a hydroxyl group necessary for chelating to potassium ions using simple esterases. This acetylation prevents the drug from binding effectively to the ribosome.

Gene Transfer

• Gene transfer contributes to antibiotic resistance. Bacteria can acquire resistance genes through plasmids, transposons, and other mobile genetic elements.

Antibiotics and Cancer Research

• There were 16,000 hits three years ago compared to treating cancer. Some antibiotics have shown potential as anticancer agents.

• Lung cancer has a high mortality rate. Research is ongoing to explore the potential of antibiotics and other compounds in treating lung cancer.

Disrupting Cell Membranes

• Valinomycin:- Isolated from bacteria; bacteria use it to inhibit other bacteria.

  • Highly effective at chelating potassium ions (1000x more effective than for sodium). This selectivity is due to valinomycin's structure, which features a hydrophobic exterior that allows it to dissolve in the cell membrane, and a hydrophilic interior that specifically binds potassium ions.

  • Disrupts potassium concentration inside the cell, leading to cell death. This disruption affects the cell's ability to maintain osmotic balance and conduct essential functions.

  • Too toxic for clinical use in humans. Its toxicity arises from its ability to disrupt ion gradients in human cells as well.

• Polymyxin B:- Isolated from bacteria.

  • Contains unusual amino acids and a long fatty chain for anchoring into the cell membrane. The fatty chain is crucial for its interaction with the lipid bilayer.

  • Causes leakage of small molecules, disrupting the ionic potential. By creating pores in the membrane, it leads to loss of essential metabolites and disruption of cellular processes.

  • Tolerable by humans; however, it can still cause nephrotoxicity and neurotoxicity, limiting its use.

Impairing Protein Synthesis

• Target bacterial ribosomes, which differ from human ribosomes. This difference allows for selective targeting of bacterial protein synthesis without significantly affecting human cells.

• Interfere with translation (protein synthesis). They bind to the ribosome and disrupt the process of mRNA translation into proteins.

• Bill Clemmons (Caltech): Studied the structure of ribosome subunits to identify antibiotic targets. His work has provided valuable insights into the mechanisms of antibiotic action and resistance.

Targeting Ribosome Subunits: Aminoglycosides

• Based on carbohydrates with nitrogen replacing some hydroxyl groups. The presence of nitrogen enhances their interaction with anionic components of bacterial cells.

• Effective against Gram-negative bacteria.- Around neutral pH, NH groups are protonated, aiding transport across the membrane of Gram-negative bacteria, which contain anionic lipids. The protonation is essential for their ability to cross the outer membrane of Gram-negative bacteria.

• Limitations:- Poor oral availability due to Lipinski's rule violations such as too many hydrogen bond donors and acceptors. This limits their clinical use to intravenous or intramuscular administration.

• Mechanism of Action:- Protonated amine groups form ionic pairs with phosphate groups on the ribosome.

  • Interferes with mRNA function, leading to misreading of the genetic code and premature termination of protein synthesis.

Bacterial Resistance to Aminoglycosides

• Bacteria acetylate the amine groups using esterases or N-kinases.- Acetylation prevents protonation at neutral pH and hinders binding to the target site. This enzymatic modification is a common mechanism of resistance.

• Bacteria phosphorylate hydroxyl groups on the sugars.- Introduces negative charges, repelling the phosphate groups on the ribosome. This phosphorylation disrupts the drug-target interaction.

Tetracyclines

• Broad-spectrum antibiotics effective against Gram-positive and Gram-negative bacteria. They inhibit a wide range of bacteria by targeting protein synthesis.

• Inhibit protein synthesis by preventing tRNA from binding to the ribosome. This action blocks the addition of amino acids to the growing peptide chain.

• Structure:- Four fused rings. This unique structure is essential for their mechanism of action.

  • Hydrophobic top half and hydrophilic edge. This amphipathic nature allows them to interact with both the ribosome and the cellular environment.

  • Examples: chlorotetracycline, doxycycline, minocycline. These different tetracyclines have variations in their chemical structures, affecting their pharmacokinetic properties.

• The precise arrangement of hydrogen bonds on the hydrophilic edge is crucial for activity.- Forms specific hydrogen bonds with a metal ion in the tRNA binding site. This interaction is critical for blocking tRNA binding and protein synthesis.

Bacterial Resistance to Tetracyclines

• Efflux Pumps: Bacteria expel tetracyclines using efflux pumps. These pumps actively transport the antibiotic out of the cell, reducing its intracellular concentration.

• Ribosome Mutation: Mutations in the ribosome prevent tetracycline binding. Mutations alter the structure of the ribosome, preventing the antibiotic from binding effectively.

Overcoming Resistance

• Adding additional hydrogen bonding recognition sites to tetracyclines (e.g., tigecycline). This modification enhances the drug's binding affinity to the ribosome, even in the presence of resistance mechanisms.

Chloramphenicol

• Broad-spectrum antibiotic. It is effective against a wide range of bacteria, but its use is limited due to toxicity.

• Toxic to humans; used when the risk is justified by the severity of the infection. Its toxicity includes bone marrow suppression and aplastic anemia.

• Undergoes phase II metabolism, conjugating to glucuronic acid to form chloramphenicol glucuronide. This conjugation facilitates its excretion from the body.

• Binds to the ribosomal unit, interfering with mRNA transcription.- Involves pi-stacking and hydrogen bonding in the active site. This interaction inhibits peptide bond formation.

Bacterial Resistance to Chloramphenicol

• Bacteria acetylate a hydroxyl group necessary for chelating to potassium ions using simple esterases. This acetylation prevents the drug from binding effectively to the ribosome.

Gene Transfer

• Gene transfer contributes to antibiotic resistance. Bacteria can acquire resistance genes through plasmids, transposons, and other mobile genetic elements.

Antibiotics and Cancer Research

• There were 16,000 hits three years ago compared to treating cancer. Some antibiotics have shown potential as anticancer agents.

• Lung cancer has a high mortality rate. Research is ongoing to explore the potential of antibiotics and other compounds in treating lung cancer.