Impact of DNA sequencing and analysis methods on 16S rRNA gene bacterial community analysis in dairy products

Abstract

DNA sequencing and analysis methods were compared for 16S rRNA V4 PCR amplicon and gDNA mock communities encompassing nine bacterial species commonly found in milk and dairy products. The communities were examined using Illumina MiSeq and Ion Torrent PGM DNA sequencing methods followed by the QIIME 1 (UCLUST) and Divisive Amplicon Denoising Algorithm 2 (DADA2) data analysis pipelines including taxonomic comparisons to the Greengenes and Ribosomal Database Project (RDP) databases. Examination of the PCR amplicon mock community with these methods resulted in Operation Taxonomy Units (OTUs) and Amplicon Sequence Variants (ASVs) that ranged from a low of 13 to high of 118 and were dependent on the DNA sequencing method and read assembly step. The elevated numbers of OTUs and ASVs included assignments to spurious taxa as well as sequence variants of the nine species included in the mock community. Comparisons between the gDNA and PCR amplicon mock communities showed that combining gDNA from the different strains prior to PCR resulted in up to 8.9-fold greater numbers of spurious OTUs and ASVs. However, the DNA sequencing method and initial data assembly steps conferred the largest effects on predictions of bacterial diversity, independent of the mock community type (PCR amplicon or gDNA; Bray-Curtis R2 = 0.88 and weighted Unifrac, R2 = 0.32). Overall, DNA sequencing performed with the Ion Torrent PGM and analyzed with DADA2 and the Greengenes database resulted in the most accurate predictions of the mock community phylogeny, taxonomy, and diversity.\n\nImportanceValidated methods are urgently needed to improve DNA-sequence based assessments of complex bacterial communities. In this study, we used 16S rRNA PCR amplicon and gDNA mock community standards, consisting of nine, dairy-associated bacterial species, to evaluate the most commonly applied 16S rRNA marker gene DNA sequencing and analysis platforms used in evaluating dairy and other bacterial habitats. Our results show that bacterial metataxonomic assessments are largely dependent on the DNA sequencing platform and read curation method used. DADA2 improved sequence annotation compared with QIIME 1, and when combined with the Ion Torrent PGM DNA sequencing platform and the Greengenes database for taxonomic assignment, the most accurate representation of the dairy mock community standards was reached. This approach will be useful for validating sample collection and DNA extraction methods and ultimately investigating bacterial population dynamics in milk and dairy-associated environments.