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J. Bacteriol. doi:10.1128/JB.00027-08
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Ultrastructural characterization of the prokaryotic symbiosis in "Chlorochromatium aggregatum"

Department Biology I, Electron Microscopy, Ludwig-Maximilians-Universität München, Menzingerstr. 67, D-80638 München, Germany; Department Biology I, Section Microbiology, Ludwig-Maximilians-Universität München, Maria-Ward-Str. 1a, D-80638 München, Germany

^* To whom correspondence should be addressed. Email: wanner{at}lrz.uni-muenchen.de.

	Abstract

The phototrophic consortium "Chlorochromatium aggregatum" currently represents the most highly developed interspecific association of bacteria and consists of green sulfur bacteria, so-called epibionts, surrounding a central, motile, chemotrophic bacterium. In order to identify subcellular structures characteristic for this symbiosis, consortia were studied by a combination of high resolution analytical SEM, TEM, 3D reconstruction and image analysis. Epibionts are interconnected, to a lower extent also with the central bacterium, by electron-dense hair-like filaments. In addition, numerous periplasmic tubules extend from the outer membrane of the central bacterium and are in direct contact to the outer membrane of the epibionts. In each epibiont cell, the attachment site to the central bacterium is characterized by absence of chlorosomes and an additional 17 nm-thick layer (epibiont contact layer, ECL) attached to the inner side of the cytoplasmic membrane. The ECL is only occasionally observed in pure cultures of the epibiont, where it occurs in about 10-20% of the free-living cells. A striking feature of the central bacterium is the presence of one or two hexagonally packed flat crystals (central bacterium crystal, CBC) per cell. The CBC reaches 1 µm in length, is 35 nm thick and consists of a bilayers of subunits with a spacing of 9 nm. A detailed model for consortia is presented summarizing the conclusions on i) cohesion of the cells, ii) common periplasmic space between central bacterium and epibiont, iii) ECL as symbiosis-specific structure, and iv) formation of the interior paracrystalline structures, central bacterium membrane layer (CML) and CBC.