1Hiroshima Univ. eng., 2Hiroshima Univ. ISSD
It is well known that the majority of environmental microorganisms resist from cultivation, especially in pure culture. Although this phenomenon is an essentially important issue in the field of microbiology, the reason or mechanism has not been clarified yet. Nitrospira is an environmentally important microbial type responsible for major nitrite oxidation in variety of environments but have few examples of success in isolation. On the other hand, our group has successfully isolated several Nitrospira strains by an application of newly developed method. In this study, using Nitrospira pure culture as a model of unculturable microorganisms, mechanisms that control the proliferation of unculturable microorganisms was investigated by focusing on microbial interactions and dormancy-awakening.
In the natural environment, Nitrospira co-exists with heterotrophic bacteria utilizing organic metabolites excreted from Nitrospira as substrates. We assumed that a part of those co-exisiting-microorgasnisms has a potential to promote growth of Nitrospira. We first isolated heterotrophic bacteria co-exists with Nitrospira, then selected microorganisms that possesses potential of promoting nitrite oxidation activity of Nitrospira. On the other hand, metabolic by-products excreted by Nitrospira itself inhibit its activity. From these observations, it is suggested that the growth of Nitrospira is promoted by heterotrophic bacteria through the removal of the growth inhibitor excreted from Nitrospira. Furthermore, it was found that Nitrospira goes dormant easily under certain condition, and awake under the certain condition, suggesting that the presence of unknown factors controlling dormancy and awakening. Two set of those results indicated that Nitrospira is hardly isolated by conventional batch culture, and this mechanism is likely to share in other microorganisms as well, as a common property.