1Genetics Strains Research Center, NIG, 2JSPS Superlative Postdoctoral Research Fellow (SPD), 3Technical Department, TechnoSuruga Laboratory Co. Ltd, Transdisciplinary Research Integration Center, NIG, Graduate School of Biosphere Science, Hiroshima Univ.
Filtration with a 0.2-µm filter is a common method for removing microbes from samples used in scientific research and many other applications. However, several studies have shown that novel lineages with diverse functional potential can nonetheless remain in filtered environmental samples. Here, we report two types of bacteria cultivated from samples filtered through 0.2-µm filters. Environmental samples were three-fold filtered with 0.2-µm filter units. Each filtrate was added to different types of growth medium and incubated; nearly 60 isolates were selected after cultivation. One actinobacterial isolate displayed ultramicro-sized cells (< 0.1 µm3) even under eutrophic culture conditions, and formed an independent cluster with undescribed species and environmental clones on the phylogenetic tree. The other proteobacterial isolate showed a large filamentous morphology, although some of its cells exhibited a curved rod or slender spiral form. This cellular polymorphism may lead to its filterability through 0.2-µm filters. In addition, phylogenetic analysis revealed that this isolate was a novel class-level representative in the phylum Proteobacteria. Thus, our results indicate that unique cultivable microbes are present in filterable fractions. Furthermore, we will present results of genomic and phenotypic analyses of these isolates, which are currently underway.