Field preservation and DNA extraction methods for intestinal microbial diversity analysis in earthworms

We assessed the effect of DNA extraction and sample preservation methods on the DNA yield and quality obtained from earthworm (Aporrectodea caliginosa Savigny) gut samples and on the results obtained by bacterial and fungal automated ribosomal intergenic spacer analysis (ARISA) of DNA extracts. Methods based on a hexadecyltrimethylammonium bromide dithiotreitol (CTAB-DTT) extraction buffer yielded more favourable results than those based on a sodium dodecyl sulphate (SDS) buffer. For both of these buffers, incorporation of a bead-beating during the lysis step increased the ARISA-derived bacterial ribotype numbers and diversity estimates, as determined for gut wall samples (P < 0.01). Although spectrophotometric analysis indicated that DNA extracted by the CTAB-DTT and SDS-based methods were of comparable quality (P ≥ 0.05), the former method yielded > 1.5 times more DNA from both gut contents and gut walls of earthworms than the latter method (both incorporating the bead beating step) (P < 0.01). ARISA analysis detected more reproducible ribotypes and more microbial diversity in DNA extracted by the CTAB-DTT- as compared to the SDS-based method (P < 0.01). Significant difference between bacterial communities of gut contents and gut walls were detected within DNA extracted by the CTAB-DTT (but not by the SDS-based) method (Global R = 0.76, P < 0.001, analysis of similarity). Using the CTAB-DTT-based method, we showed that earthworm preservation in ethanol yielded higher quality DNA from gut contents than preservation in either chloroform or liquid N, as determined by spectrophotometry, PCR inhibition analysis and bacterial and fungal ARISA (P < 0.05). Bacterial or fungal communities in the gut contents of fresh and ethanol-preserved earthworms were more similar and were significantly different from those of earthworms preserved in chloroform or liquid N (Global R = 0.79 and 0.83 for bacteria and fungi, respectively; P < 0.001, analysis of similarity). We propose that ethanol preservation and the CTAB-DTT-based DNA extraction method described herein are also suitable for the analysis of gut-associated microbiota in other soil and sediment feeding invertebrates.