Comparison of droplet digital PCR and quantitative real-time PCR for examining population dynamics of bacteria in soil

The newly developed droplet digital PCR (DD-PCR) has shown promise as a DNA quantification technology in medical diagnostic fields. This study evaluated the applicability of DD-PCR as a quantitative tool for soil DNA using quantitative real-time PCR (qRT-PCR) as a reference technology. Cupriavidus sp. MBT14 and Sphingopyxis sp. MD2 were used, and a primer/TaqMan probe set was designed for each (CupMBT and SphMD2, respectively). Standard curve analyses on tenfold dilution series showed that both qRT-PCR and DD-PCR exhibited excellent linearity (R²=1.00) and PCR efficiency (≥92 %) across their detectable ranges. However, DD-PCR showed a tenfold greater sensitivity than qRT-PCR. MBT14 and MD2 were added to non-sterile soil at 0∼5×10 ⁸ and 0∼5×10⁷ cells per gram of soil, respectively (n=5). This bacterial load test indicated that DD-PCR was more sensitive and discriminating than qRT-PCR. For instance, DD-PCR showed a gradual DNA increase from 14 to 141,160 MBT14 rDNA copies μL DNA extract ^-1 as the bacterial load increased, while qRT-PCR could quantify the DNA (6,432 copies μL DNA^-1) at ≥5×10⁵ MBT14 per gram of soil. When temporal DNA changes were monitored for 3 weeks in the amended soils, the two technologies exhibited nearly identical changes over time. Linearity tests (y=a·x) revealed excellent quantitative agreement between the two technologies (a=0.98, R²=0.97 in the CupMBT set and a=0.90, R²=0.94 in the SphMD2 set). These results suggest that DD-PCR is a promising tool to examine temporal dynamics of microorganisms in complex environments.