A comparison of hard and soft direct methods for DNA extraction from soil

Nucleic acid extraction is the first step in molecular biology studies of soil bacterial communities. The most common used soil DNA extraction method is the direct, hard extraction Mobio method, which uses bead beating to lyse bacteria. In this study we compared the Mobio method with a soft, enzymatic lysis extraction method. Next generation sequencing (Illumina and Pyrosequencing) of amplicons generated from four 16S primer pairs and DNA from 12 soils and 3 composts was used to compare the two extraction methods. Four bacterial orders, the delta proteobacterial Desulfuromonadales and gamma proteobacterial Pseudomonadales, Enterobacteriales, and Alteromonadales were more common in amplicons from soft extracted DNA, sometimes by two orders of magnitude. These groups can be a significant fraction of the bacterial population. For example the Pseudomonadales made up to 16 % and Enterobacteriales 10% of amplicons from Soft extracted DNA. The JG30-KF-CM45 order was under extracted by the enzymatic lysis extraction method. Results differed more by primer choice than extraction method and the phylogenetic resolution of differences between extraction methods changed with primer choice. Given how often Mobio extraction is used, these proteobacterial orders are probably under-represented in the studies of soil bacteria that use nucleic acid methods. Further improvements in soil DNA extraction are needed. Amplicons sequencing studies should use a range of different primers to confirm the phylogenetic resolution of their results. ImportanceSeveral large scale studies of soil bacteria that compare thousands of soil samples across continents have used the Mobio method for DNA extraction. Large scale studies like these are increasing with the recent establishment of the Global Soil Biodiversity Observation Network (Soil BON), which also uses the Mobio method. The results of this work will be used to make policy decisions about how to manage the soil and may be a guide for bioprospectors. As the Mobio method is so widely used, it is important to know its limitations. Studies that use the Mobio method underestimate the fraction of several proteobacterial groups. Most notably the Enterobacteria and Pseudomonas can be under extracted by 10-100 fold. The degree of under extraction varies with different soils.