In this study, we hypothesized that bacterial exopolysaccharides (EPS) may serve as energy substrates for K-selected bacteria in soils, and we would be able to identify and culture previously uncultured species using various EPS as growth substrates.
The exopolysaccharides gellan, indican_2, and cellulose were produced by the bacteria Sphingomonas elodea (ATCC 31461), Beijerinckia indica (ATCC 9039) and Gluconacetobacter xylinus (ATCC 53524) respectively. Two experiments were performed using these exopolysaccharides (EPS) as growth substrates. The first experiment involved detecting EPS-degrading bacteria by using a stable isotope probing technique. 13C labeled EPS were used as growth substrates for Big Hill Spring (BHS) and Paint Pots mound (PP) soil samples. EPS-degrading bacteria were identified using DNA density ultracentrifugation coupled with 454-pyrosequencing of the 16S rRNA genes recovered from 13C-labelled DNA fractions. Several uncultured Xanthomonadaceae sp. were enriched on cellulose (produced by G. xylinus) during incubation from BHS soil and several previously uncultured Planctomycetes were highly enriched on indican_2 (produced by B. indica) incubation from PP soil. The second experiment was to isolate novel bacteria by using gellan or indican_2 as sole carbon sources. Based on cultivation results, we found several previously uncultured bacteria could be identified and isolated from our gellan (produced by S. elodea) and indican_2 plates. For instance, we successfully isolated previous uncultured Acidobacteria by using gellan and indican_2 as our carbon source.
All in all, in this study, we found some previously uncultured bacteria belonging to the groups of Acidobacteria, Verrucomicrobia, Planctomycetes, Chloroﬂexi, Actinobacteria, Cyanobacteria, Proteobacteria, Bacteroidetes, Armatimonadetes and Candidate division OD1, which are readily enriched or cultured using these EPS substrates. These poorly understood groups, especially some members of Planctomycetes, showed preference for our EPS indican_2. The research results indicate that using novel EPS as carbon sources might be a new way to improve current cultivation techniques.