DNA extract characterization process for microbial detection methods development and validation

Quantitative polymerase chain reaction (qPCR) assays used in pathogen detection require rigorous methods development including characterizing DNA extraction products. A DNA extract characterization process is demonstrated using DNA extracted from five different cells types (two Gram-negatives: Escherichia coli, and Burkholderia thailandensis, spores and vegetative cells from the Gram-positive Bacillus cereus, and yeast Saccharomyces cerevisiae) with six different methods.

DNA extract quantity (concentration and extraction efficiency) and quality (purity and intactness) varied by cell type and extraction method enabling the demonstration of different DNA characterization methods. DNA purity was measured using UV spectroscopy, where the A260/A280 and A260/A230 ratios are indicators of different contaminants. Reproducibility of UV spectroscopy measurements decreased for DNA concentrations less than 17.5 ng/μL. Forty-seven extracts had concentrations greater than 17.5 ng/μL, 25 had A260/A280 above 2.0, and 28 had A260/A230 ratios below 1.8 indicating RNA and polysaccharide contamination respectively. Based on a qPCR inhibition assay the contaminants did not inhibit PCR. Extract intactness was evaluated using microfluidic gel electrophoresis. Thirty-five samples had concentrations above the limit of quantification (LOQ, roughly 11 ng/ μL), 93.5% of the DNA was larger than 1kb and 1% was smaller than 300 bp. Extract concentrations ranged from 1502.2 ng/μL to below the LOQ when UV spectroscopy, fluorometry, and qPCR were used. LOQ for UV spectroscopic and fluorometric measurements were 3.5 ng/μL and 0.25 ng/μL respectively. The qPCR LOQ varied by cell type (5.72 × 10-3 ng/μL for E. coli, 2.66 × 10-3 ng/μL, for B. cereus, 3.78 × 10-3 ng/μL for B. thailandensis, and 7.67 × 10-4 ng/μL for S. cerevisiae). A number of samples were below the UV spectroscopy (n = 27), flurometry (n = 15), and qPCR (n = 3) LOQ.

The presented DNA extract characterization process provides measures of DNA quantity and quality applicable to microbial detection methods development and validation studies. Evaluating DNA quality and quantity results in a better understanding of process LOD and contributing factors to suboptimal assay performance. The samples used demonstrated the use of different DNA characterization methods presented but did not encompass the full range of DNA extract characteristics.