MICRO 4/8 EXAM 3

Antibiotics Targeting Cell Walls

• Many antibiotics target bacterial cell walls to inhibit cell wall synthesis.

  • Beta-Lactam Ring: Most antibiotics in this category possess a beta-lactam ring, which is an essential component that disrupts cell wall formation. This structure prevents the polymerization of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which are crucial for building the bacterial cell wall.
    • Examples of Antibiotics with Beta-Lactam Ring:
      • Penicillins
      • Cephalosporins

• Antibiotics for Mycobacterium: For certain bacteria such as Mycobacterium tuberculosis and Mycobacterium leprae, specialized antibiotics are required due to their unique cell wall structure that includes mycolic acid.

  • Treatment often involves peptide antibiotics specific to these types of bacteria.

Antibiotics Targeting Protein Synthesis

• Antibiotics that interfere with protein synthesis primarily target the bacterial ribosome, which consists of a small subunit and a large subunit.
  • The ribosome contains various sites where tRNA and mRNA interact during protein production.
  • Key antibiotics that affect protein synthesis include:
    • Macrolides:
      • Example: Erythromycin
      • Mechanism: Blocks the peptidyl (P) site of the ribosome, preventing peptide bond formation.
    • Tetracyclines:
      • Example: Doxycycline
      • Mechanism: Blocks the acceptor (A) site, inhibiting tRNA binding.
    • Aminoglycosides:
      • Example: Gentamicin
      • Mechanism: Prevents the peptide chain from exiting the ribosome by constricting the exit channel.

Antibiotics Targeting Nucleic Acids

• Two distinct groups of antibiotics target nucleic acids, impacting either bacterial DNA or RNA:
  • Quinolones:
    • Examples: Ciprofloxacin (Cipro), Levofloxacin
    • Mechanism: Inhibit bacterial topoisomerase, an enzyme vital for DNA replication, making them effective against severe infections.
  • RNA Polymerase Inhibitors:
    • Example: Erythromycin (also targets protein synthesis but has RNA polymerase inhibiting properties)
    • Selectivity Mechanism: While it can affect human RNA polymerase, bacterial RNA polymerase is significantly more susceptible to this antibiotic, allowing for effective targeting at acceptable dosage levels.

Antibiotics Targeting Plasma Membranes

• Antibiotics targeting the plasma membrane are generally not selectively toxic:
  • Polymyxin:
    • Mechanism: Disrupts the integrity of the plasma membrane but is used topically because it can also affect human cells.
    • Use Case: Commonly found in topical ointments (e.g., Neosporin).

Antibiotics Targeting Enzymes in Bacteria

• Sulfa Drugs:
  • Mechanism: Inhibit bacterial enzymes responsible for the synthesis of folic acid, an essential molecule for nucleic acid production.
  • Distinction: Humans cannot synthesize this vitamin and must obtain it through diet, meaning targeting this pathway in bacteria can be done without affecting human health.
  • Importance in Prenatal Care: Pregnant women are often advised to take folic acid supplements due to its vital role in DNA replication during fetal development, with insufficient levels leading to potential pregnancy complications.

Antivirals and Their Mechanisms

• Nucleoside and Nucleotide Analogs:
  • Function: Mimic natural nucleotides to incorporate into viral nucleic acid chains, disrupting replication.
• Assembly Inhibitors:
  • Example: Tamiflu
  • Function: Interferes with the assembly of viral components, impacting the viral lifecycle.
• Inhibitors of Uncoating and Fusion:
  • Prevent viral entry into cells or subsequent steps necessary for replication.
• HIV Treatment:
  • Combination therapies involving multiple inhibitors (nucleoside analogs, assembly inhibitors) are commonly used to prevent viral mutation and resistance.

Antifungal Drugs and Their Challenges

• Selectivity Issues with Fungi:
  • Fungi are eukaryotic organisms, making it challenging to find selectively toxic targets since they share cellular structures with human cells.
• Targets in Fungi:
  • Ergosterol: A unique component of the fungal cell membrane, targeted by antifungal drugs.
  • Chitin Cell Wall: Another target since humans do not have cell walls, allowing for targeted therapies.
  • Nucleic Acid Synthesis: Some antifungal drugs may also target nucleic acid synthesis, but they tend to have higher toxicity.
• Common Over-the-Counter Antifungal Treatments:
  • Typically target ergosterol in fungal membranes (e.g., treatments for ringworm or athlete’s foot).

Antiparasitic Drugs

• Antimalarial Drugs: Malaria treatment faces challenges due to emerging resistance in parasite populations, but there is ongoing research for more effective treatments.
• Anthelmintic Drugs:
  • Targeting parasitic worms, with some drugs being highly effective while others still face challenges in providing effective treatment options.

Antibiotic Resistance

• Timeline of Resistance Development:
  • Resistance has been observed shortly after the introduction of antibiotics due to natural selection. For example:
    • Sulfa drugs observed resistance within 10 years of their introduction.
    • Penicillin observed notable resistance just 3 years after its introduction in 1943.
• Mechanisms of Resistance Development:
  • Rapid bacterial reproduction rates increase mutation opportunities, resulting in higher chances for resistance traits to emerge.
    • Example: If resistance mutation odds are 1 in 40 million, with 40 million bacteria, chances of a resistant individual emerging is substantial.
• Contributing Factors to Resistance:
  • Inappropriate Clinical Use:
    • Prescribing antibiotics for non-bacterial infections (e.g., 90% of respiratory infections are viral).
    • Doctors becoming more cautious about antibiotic prescriptions, focusing on bacterial confirmation before prescribing.
  • Patient Compliance Issues:
    • Patients not completing antibiotic courses can leave behind more resilient bacteria, thus promoting resistance evolution.
• Misconceptions about Resistance:
  • Humans do not become resistant to antibiotics; the resistance develops in bacteria. Future antibiotic effectiveness depends on the sensitivity of the bacterial strain rather than the individual’s prior antibiotic use.

Conclusion

• Understanding antibiotic targeting mechanisms and the implications of antibiotic resistance is critical to developing successful treatment strategies and promoting responsible usage. The concepts outlined here highlight the importance of considering both the pharmacological aspects and the ecological dynamics that contribute to bacterial resistance.