PCR-SSCP comparison of 16S rDNA sequence diversity in soil DNA obtained using different isolation and purification methods

Abstract

This study compared different methods of direct DNA extraction and purification from a silt loam soil and investigated the relationship between DNA quantity and sequence diversity. Five extraction methods and four purification techniques were investigated. Quantities of DNA extracted were between 3.4+/-0.55 and 54.3+/-8.18 &mgr;g g(-1) (dry wt) of soil with OD(260)/OD(230) purity ratios between 0.80 and 1.15. Analysis of sequence diversity in all extracts was conducted using PCR-single strand conformation polymorphism (SSCP). Profiles generated using universal 16S rDNA primers (Com1/Com2) were found to be identical when used to amplify 16S rDNA extracted directly from soil. The genus Pseudomonas was targeted in order to reduce profile complexity, which was apparent when using universal 16S rDNA primers, and which hindered direct comparison of sequence diversity. A Pseudomonas culture library and non-cultured Pseudomonas 16S rDNA genes were used to provide a background count of Pseudomonas operational taxonomic units present in the soil. Cloning and sequencing of amplicons generated using a Pseudomonas-specific (Ps-for) and a universal 16S rDNA (Com2) primer, coupled with nested amplification (Com1/Com2 amplification from Ps-for/Ps-rev amplicons), used in conjunction with SSCP, revealed that environmental contaminants co-extracted with DNA, such as humic acid, significantly reduced primer specificity. SSCP was sensitive enough to reveal template bias in different primer sets. PCR-restriction fragment length-SSCP of Pseudomonas 16S rDNA amplified from soil-extracted DNA revealed distinct differences in sequence representation between extraction methods and showed that greater DNA yield is not synonymous with higher sequence diversity. We, therefore, suggest that DNA extractions from soil should be evaluated not only in terms of quantity and purity, but also in terms of the sequence diversity present. SSCP proved to be a valuable tool for the assessment of the methodologies commonly used in PCR-mediated microbial ecology studies.