Antifungal resistance mechanisms

Resistance Mechanisms - Fungi Fight Back

• Target Modification: The most common mechanism.
  • Azoles: Point mutations in the ERG11 gene alter the target enzyme, 14-α-demethylase, reducing drug binding.
  • Echinocandins: Mutations in FKS1/FKS2 genes change the structure of β-(1,3)-D-glucan synthase.
  • Polyenes: Alterations in ERG genes reduce ergosterol in the cell membrane, decreasing Amphotericin B binding sites.
• Efflux Pumps: Overexpression of genes for ABC or MFS transporters actively pumps azoles out of the cell.
• Reduced Drug Access/Metabolism:
  • Flucytosine: Impaired uptake (cytosine permease mutation) or decreased conversion to its active form (cytosine deaminase mutation).

⭐ Cross-resistance is a major clinical issue; a mutation conferring resistance to one azole often applies to others in the class.

Target Alterations - Changing the Locks

Fungi evolve by modifying the drug's molecular target, akin to changing a lock. This prevents the drug "key" from fitting, rendering it ineffective. This is a primary resistance mechanism for several major antifungal classes, driven by specific genetic mutations.

Drug Transport - Pump It or Block It

• Efflux Pumps (Azoles): A primary mechanism of azole resistance.

  • Transporter proteins actively expel drugs, preventing accumulation at the target enzyme, Erg11p.
  • Key superfamilies: ATP-binding cassette (ABC) and Major Facilitator Superfamily (MFS).
  • Caused by overexpression of genes like CDR1, CDR2 (ABC) and MDR1 (MFS).
  • 📌 Mnemonic: ABC/MFS pumps Actively Boot Chemicals/Molecules From Site.

• Decreased Drug Uptake (Flucytosine):

  • Resistance to flucytosine ($5-FC$) often involves impaired drug entry.
  • Mutations in the cytosine permease enzyme reduce $5-FC$ transport into the cell.
  • This limits conversion to active toxic metabolites like 5-fluorouracil.

⭐ Upregulation of efflux pumps is a key reason Candida auris exhibits high-level, multidrug resistance, particularly to azoles.

Biofilm Formation - Slime Shield Stronghold

• Fungi, especially Candida species, form biofilms on surfaces like catheters and implants.
• This creates a physical barrier composed of an extracellular matrix (ECM), rich in polysaccharides like β-glucan.
  • The dense ECM physically traps antifungal agents, preventing them from reaching fungal cells.
  • It also creates a protected, low-nutrient, and low-oxygen environment.
• Cells within the biofilm's deeper layers are often metabolically quiescent (persister cells), reducing the efficacy of drugs targeting active processes.

⭐ Biofilms significantly upregulate efflux pump expression, actively expelling any antifungal that penetrates the matrix, contributing to multi-drug resistance.

High-Yield Points - ⚡ Biggest Takeaways

• Azole resistance most often involves mutations in the ERG11 gene or the upregulation of drug efflux pumps.
• Polyene (Amphotericin B) resistance is linked to altered cell membrane ergosterol composition, which reduces drug binding.
• Echinocandin resistance typically results from mutations in the FKS1 gene, modifying the target enzyme β-(1,3)-D-glucan synthase.
• Flucytosine resistance occurs via decreased activity of cytosine permease or cytosine deaminase.
• Biofilm formation acts as a physical barrier, limiting drug penetration.