Antibiotic resistance mechanisms

Enzymatic Destruction - The Drug Demolishers

• Bacteria produce enzymes that chemically modify and inactivate antibiotics, rendering them ineffective.
• Primary example: β-lactamases, which hydrolyze the amide bond in the β-lactam ring of penicillins and cephalosporins.
• Common in Staphylococcus aureus, Haemophilus influenzae, E. coli, and Klebsiella pneumoniae (which can produce Extended-Spectrum β-Lactamases, ESBLs).
• Other enzymes include aminoglycoside-modifying enzymes (e.g., acetyltransferases, phosphotransferases) that alter aminoglycoside structure.

⭐ Clavulanate, Sulbactam, and Tazobactam are β-lactamase inhibitors co-formulated with penicillins to overcome this resistance mechanism. 📌 CAST (Clavulanate, Avibactam, Sulbactam, Tazobactam).

Target Modification - If You Can't Beat 'Em, Change

• Bacteria alter the drug's target site, ↓ antibiotic binding & efficacy. 📌 "Change the lock so the key doesn't fit."

• Cell Wall Synthesis:

  • Beta-Lactams (e.g., Methicillin): Alteration of Penicillin-Binding Proteins (PBPs). The mecA gene in MRSA encodes PBP2a, which has low affinity for beta-lactams.
  • Vancomycin: Modification of peptidoglycan precursor.

    ⭐ VRE (Vancomycin-Resistant Enterococci) changes D-Ala-D-Ala to D-Ala-D-Lac, preventing vancomycin binding.

• Protein Synthesis:

  • Macrolides/Clindamycin: Methylation of 23S rRNA binding site (erm gene) prevents drug binding.

• Nucleic Acid Synthesis:

  • Fluoroquinolones: Mutations in DNA gyrase & topoisomerase IV.

Reduced Permeability & Efflux - The Fortress Defense

• Reduced Permeability: Bacteria limit antibiotic entry, primarily in Gram-negatives.

  • Mechanism: Downregulation or mutation of outer membrane porin channels.
  • Less influx means the antibiotic can't reach its intracellular target.
  • Classic Example: Pseudomonas aeruginosa resistance to carbapenems.

• Efflux Pumps: Actively transport antibiotics out of the cell.

  • ATP-dependent pumps that recognize and expel drugs.
  • Confers resistance to tetracyclines, macrolides, and fluoroquinolones.
  • 📌 Pump Medications To Flight (Pseudomonas, Macrolides, Tetracyclines, Fluoroquinolones).

⭐ Pseudomonas aeruginosa exhibits high intrinsic resistance due to its very restrictive outer membrane porins and a wide array of constitutively expressed and inducible efflux pumps (e.g., MexAB-OprM).

Genetic Mechanisms - Sharing the Cheat Codes

• Transformation: Bacteria uptake and incorporate naked DNA from the environment. A key mechanism for Streptococcus pneumoniae.
• Conjugation: Direct transfer of genetic material (plasmids, transposons) between bacteria via a sex pilus.

  ⭐ Plasmids, especially F-plasmids in Gram-negatives, are key drivers of multi-drug resistance transfer via conjugation.

• Transduction: A bacteriophage (virus) transfers genetic material from one bacterium to another.
• Transposons: "Jumping genes" that move resistance genes between plasmids and chromosomes.

High‑Yield Points - ⚡ Biggest Takeaways

• Enzymatic inactivation: β-lactamases are a classic example, breaking down penicillins and cephalosporins, leading to resistance.
• Target site modification: MRSA alters its penicillin-binding proteins (PBPs); fluoroquinolone resistance often involves mutated DNA gyrase.
• Active efflux pumps: These systems actively remove antibiotics like tetracyclines and macrolides from the bacterial cell.
• Decreased permeability: Gram-negative bacteria can mutate porin channels to prevent antibiotic entry into the cell.
• Metabolic pathway alteration: Bacteria may develop alternative pathways to bypass drugs like sulfonamides.