Read QC and Preprocessing
One-Sentence Definition
Read QC and preprocessing evaluate and clean raw sequencing reads (quality, adapters, contamination, host DNA) before assembly or mapping.
Simple Explanation
Garbage in, garbage out. Check the reads before trusting any downstream genome or resistance call.
Detailed Scientific Explanation
Checks and fixes:
- Quality profiles — FastQC / MultiQC across a run
- Adapter and quality trimming — fastp, Trimmomatic
- Depth estimate — coverage = (reads × read length) / genome size; bacteria commonly target 30–100×
- Contamination / mixture — Kraken2 or ANI screens; mixed isolates break assemblies and typing
- Host removal — critical in Metagenomics from clinical specimens
- Duplicates / optical artifacts — depends on library prep
Red flags: bimodal GC, unexpected species fraction, low coverage in part of the genome, adapter carryover.
Mechanism
Sliding-window quality trimming, k-mer classification against reference databases, and mapping-based host depletion.
Clinical Importance
- A mixed culture sequenced as “one isolate” can produce a false resistance profile or a false outbreak link
Research Importance
- Reproducible QC thresholds are part of methods reporting
Diagnostic Relevance
- First gate of the WGS Bioinformatics Pipeline
AMR Relevance
- Low coverage regions can cause false-negative resistance gene calls
Related MOCs
Active Recall Questions
- How do you estimate coverage?
- Which QC failure most threatens outbreak typing?
- Why remove host reads in clinical metagenomics?
Connections
- Sequencing Data Formats · Genome Assembly · Culture and Isolation (purity upstream)