Mechanisms of Antimicrobial Resistance

Mechanisms of Antimicrobial Resistance: AMR Fundamentals - Resistance Rising

AMR: Microbe's ability to withstand antimicrobial drugs, previously effective for treatment.

• Intrinsic Resistance: Natural, inherent to the organism; predictable.
  • E.g., Gram-negative bacteria resistant to Penicillin G (due to outer membrane impermeability).
• Acquired Resistance: Microbe obtains the ability to resist a drug to which it was previously susceptible.
  • E.g., MRSA (Methicillin-Resistant Staphylococcus aureus) due to acquisition of the mecA gene.
• Genetic Basis (Acquired):
  • Mutations: Spontaneous changes in bacterial DNA.
  • Horizontal Gene Transfer (HGT): Transfer of resistance genes between bacteria. 📌 Can Transfer Too (Conjugation, Transduction, Transformation).
    • Conjugation: Gene transfer via direct cell-to-cell contact (e.g., plasmids).
    • Transduction: Gene transfer via bacteriophages (viruses infecting bacteria).
    • Transformation: Uptake of naked DNA from the environment.
  • Mobile Genetic Elements (MGEs): DNA segments that can move within or between genomes (e.g., plasmids, transposons, integrons).

⭐ Vancomycin resistance in Staphylococcus aureus (VRSA) often arises from acquiring the vanA gene cluster from vancomycin-resistant enterococci (VRE) via conjugation on a plasmid. This is a critical example of HGT leading to high-level resistance against a last-resort antibiotic.

Mechanisms of Antimicrobial Resistance: Target Tampering - The Altered Lock

Bacteria alter drug target sites, reducing binding. "Altered Lock, Key Won't Fit."

• PBP Modification:
  • MRSA: mecA gene → PBP2a (low β-lactam affinity).
  • S. pneumoniae: Altered PBPs (penicillin resistance).
• Ribosomal Alteration:
  • Macrolides: Methylation of 23S rRNA (erm genes). (📌 ERM = Erythromycin Ribosomal Methylase)
  • Aminoglycosides: 16S rRNA mutations.
  • Linezolid: 23S rRNA (domain V) mutations.
• DNA Gyrase/Topoisomerase IV Alteration:
  • Fluoroquinolones: Mutations in gyrA, gyrB (DNA gyrase) or parC, parE (topoisomerase IV).
• Cell Wall Precursor Alteration:
  • Vancomycin: $D-Ala-D-Ala \rightarrow D-Ala-D-Lac$ (vanA gene cluster) or $D-Ala-D-Ser$ (vanB, vanC genes).

⭐ The mecA gene product, PBP2a, confers resistance to virtually all β-lactam antibiotics.

Mechanisms of Antimicrobial Resistance: Drug Defense - Destroy & Discard

Bacteria neutralize antibiotics by: enzymatic destruction or active removal.

• Enzymatic Inactivation/Modification (Destroy)

  • β-Lactamases: Hydrolyze β-lactam ring. Major classes:

    Class	Examples	Key Feature
    A	ESBLs, KPC	Serine; clavulanate-inhibited (ESBLs)
    B	NDM-1, VIM	Metallo ($Zn^{2+}$); broad spectrum
    C	AmpC	Serine; cephalosporinase; inducible
    D	OXA	Serine; hydrolyzes oxacillin/carbapenems

  • Aminoglycoside-Modifying Enzymes:
    • Acetyltransferases (AAC)
    • Phosphotransferases (APH)
    • Nucleotidyltransferases (ANT)
  • Chloramphenicol Acetyltransferase (CAT): Acetylates chloramphenicol.

• Decreased Drug Accumulation (Discard via Efflux Pumps)

  • Actively pump out antibiotics.
  • Tetracycline-specific: Tet(A), Tet(K) pumps.
  • Multi-Drug Resistance (MDR) Pumps:
    • ABC (ATP-Binding Cassette) superfamily
    • MFS (Major Facilitator Superfamily)
    • RND (Resistance-Nodulation-Division) family (e.g., AcrAB-TolC in E. coli)
    • 📌 Mnemonic for MDR pumps: "All My Resistance": ABC, MFS, RND.

⭐ Carbapenemases like NDM-1 confer resistance to nearly all β-lactams, including carbapenems, and are often plasmid-mediated.

Mechanisms of Antimicrobial Resistance: Entry Denied & Bypassed - The Great Escape

• 1. Decreased Permeability/Uptake (Entry Blocked)

  • Gram-negatives: Porin loss/mutation in outer membrane ↓ drug entry (e.g., β-lactams, FQs, aminoglycosides).
    • E. coli: OmpF, OmpC mutations.
    • P. aeruginosa: OprD loss (↓ carbapenem, esp. imipenem, entry).
  • 

• 2. Alternative Metabolic Pathways (Target Bypassed)

  • Bacteria develop new pathways, rendering drug ineffective.
  • Examples:
    • Sulfonamides: Altered dihydropteroate synthase (DHPS) or ↑ PABA.
    • Trimethoprim: Altered dihydrofolate reductase (DHFR).

⭐ Loss of OprD porin in Pseudomonas aeruginosa is a key mechanism of imipenem resistance. Meropenem may be less affected due to alternative entry routes.

High‑Yield Points - ⚡ Biggest Takeaways

• Enzymatic degradation: β-lactamases (e.g., ESBLs, KPCs) inactivate penicillins & cephalosporins.
• Target modification: Altered PBPs (MRSA), ribosomal methylation (macrolides, clindamycin), gyrase mutations (quinolones).
• Reduced permeability: Porin channel loss limits entry of drugs like carbapenems.
• Efflux pumps: Multi-drug resistance (MDR) pumps actively expel antibiotics (e.g., tetracyclines, fluoroquinolones).
• Target bypass/protection: VanA/VanB genes in VRE alter peptidoglycan precursor, reducing vancomycin binding.
• Biofilm formation provides a protective niche and facilitates horizontal gene transfer, increasing resistance spread.