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Applied and Environmental Microbiology, December 2005, p. 7838-7845, Vol. 71, No. 12
0099-2240/05/$08.00+0 doi:10.1128/AEM.71.12.7838-7845.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Marine Biotechnology Institute, Heita, Kamaishi, Iwate 026-0001,1 Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555,2 Central Research Institute of Oral Science, School of Dentistry, Asahi University, Hozumi, Mizuho, Gifu 501-0296, Japan3
Received 14 June 2005/ Accepted 11 August 2005
A thermophilic syntrophic bacterium, Pelotomaculum thermopropionicum strain SI, was grown in a monoculture or coculture with a hydrogenotrophic methanogen, Methanothermobacter thermautotrophicus strain
H. Microscopic observation revealed that cells of each organism were dispersed in a monoculture independent of the growth substrate. In a coculture, however, these organisms coaggregated to different degrees depending on the substrate; namely, a large fraction of the cells coaggregated when they were grown on propionate, but relatively few cells coaggregated when they were grown on ethanol or 1-propanol. Field emission-scanning electron microscopy revealed that flagellum-like filaments of SI cells played a role in making contact with
H cells. Microscopic observation of aggregates also showed that extracellular polymeric substance-like structures were present in intercellular spaces. In order to evaluate the importance of coaggregation for syntrophic propionate oxidation, allowable average distances between SI and
H cells for accomplishing efficient interspecies hydrogen transfer were calculated by using Fick's diffusion law. The allowable distance for syntrophic propionate oxidation was estimated to be approximately 2 µm, while the allowable distances for ethanol and propanol oxidation were 16 µm and 32 µm, respectively. Considering that the mean cell-to-cell distance in the randomly dispersed culture was approximately 30 µm (at a concentration in the mid-exponential growth phase of the coculture of 5 x 107 cells ml1), it is obvious that close physical contact of these organisms by coaggregation is indispensable for efficient syntrophic propionate oxidation.
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