Sanger Sequencers for Fungal Identification: Introduction, Application, and Keynotes

Introduction

Sanger sequencing, developed by Frederick Sanger in 1977, is a chain-termination DNA sequencing method that remains the gold standard for molecular-level fungal identification.
It is widely used to determine the exact nucleotide sequence of specific fungal genes (e.g., ITS, D1/D2, LSU, SSU, β-tubulin, calmodulin) to accurately identify fungal species — especially those difficult to differentiate by morphology or culture alone.

Sanger sequencers operate based on capillary electrophoresis (CE) detection of fluorescently labeled dideoxynucleotides (ddNTPs), producing high-quality, single-read DNA sequences up to ~1000 bp.

Applications in Fungal Identification

1. Species-Level Identification

• Targeted sequencing of the ITS region (Internal Transcribed Spacer) — the universal DNA barcode for fungi.
• Confirms the identity of Candida, Aspergillus, Penicillium, Cryptococcus, and many environmental or opportunistic fungi.
• Used for clinical mycology, taxonomy, and phylogenetic studies.

2. Confirmation of MALDI-TOF or PCR Results

• Serves as a confirmatory test when phenotypic or MALDI-TOF identification is inconclusive.
• Resolves cryptic species (e.g., Aspergillus fumigatus complex, Candida parapsilosis complex).

3. Antifungal Resistance Gene Analysis

• Sequencing of ERG11, CYP51A, FKS, and CDR1/CDR2 genes to detect mutations responsible for azole or echinocandin resistance.

4. Phylogenetic and Epidemiological Studies

• Enables strain typing and evolutionary analysis of clinical and environmental isolates.
• Helps trace sources of hospital-acquired fungal outbreaks.

5. Reference Sequence Generation

• Produces high-quality reference sequences for database submission (GenBank, UNITE, MycoBank).
• Supports the development of fungal DNA barcode libraries for diagnostic use.

6. Quality Control and Teaching

• Used in academic, diagnostic, and research laboratories for training in molecular mycology and sequencing.

Popular Sanger Sequencers

1. ABI 3130/3130xl Genetic Analyzer
2. ABI 3500/3500xl Genetic Analyzer
3. ABI 3730/3730xl DNA Analyzer
4. SeqStudio Genetic Analyzer (Thermo Fisher Scientific)
5. CEQ 8000 (Beckman Coulter)

All these use capillary electrophoresis and fluorescent dye terminator chemistry for high-resolution sequencing.

Workflow Overview

1. DNA Extraction →
2. PCR Amplification (e.g., ITS1-ITS4 primers) →
3. PCR Product Purification →
4. Cycle Sequencing Reaction (ddNTPs) →
5. Capillary Electrophoresis (CE) →
6. Chromatogram Analysis (e.g., using Sequencing Analysis or BioEdit) →
7. BLAST Comparison (NCBI/UNITE databases)

Keynotes

1. Sanger sequencing remains the benchmark method for fungal DNA-based identification, offering >99% accuracy.
2. Particularly valuable for rare, slow-growing, or morphologically atypical fungi.
3. ITS region is the universal barcode; additional loci like β-tubulin, calmodulin, or LSU may be used for complex groups.
4. Produces a single read per locus, limiting throughput but ensuring high fidelity.
5. Cost-effective for small-scale testing; NGS is preferred for large sample sets.
6. Requires pure culture DNA and high-quality chromatograms for reliable identification.

Further Readings

1. https://www.cd-genomics.com/microbioseq/resource-rapid-bacterial-fungal-pathogen-identification-using-sanger-sequencing.html
2. https://www.abcam.com/en-us/knowledge-center/dna-and-rna/sanger-sequencing
3. https://www.cd-genomics.com/microbioseq/resource-its-sequencing-fungal-diversity-application.html
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC10643650/https://medicallabnotes.com/fungal-identification-by-pcr-sequencing-sanger-introduction-principle-clinical-significance-and-keynotes/amp/
5. https://geneticeducation.co.in/advantages-and-limitations-of-sanger-sequencing/