By combining different sample preparation methods (chemical fixation, resin-embedding, and cryo-fixation) and imaging techniques (scanning electron microscopy, transmission electron microscopy and tomography, focused ion beam scanning electron microscopy, and atomic force microscopy) we systematically imaged the structure of intact cable bacteria filaments with varying diameters. In addition, we investigated the periplasmic fiber sheath that remains after the cytoplasm and membranes were removed by chemical extraction. Based on these investigations, we present a quantitative structural model of a cable bacterium.
Cable bacteria build their cell envelope by a parallel concatenation of ridge compartments that have a standard size. Larger diameter filaments simply incorporate more parallel ridge compartments. Each ridge compartment contains a ~50 nm diameter fiber in the periplasmic space. These fibers are continuous across cell-to-cell junctions, which display a conspicuous cartwheel structure that is likely made by invaginations of the outer cell membrane around the periplasmic fibers. The continuity of the periplasmic fibers across cells makes them a prime candidate for the sought-after electron conducting structure in cable bacteria.
The Cell Envelope Structure of Cable Bacteria. Cornelissen R, Bøggild A, Thiruvallur Eachambadi R, Koning RI, Kremer A, Hidalgo-Martinez S, Zetsche E-M, Damgaard LR, Bonné R, Drijkoningen J, Geelhoed JS, Boesen T, Boschker HTS, Valcke R, Nielsen LP, D'Haen J, Manca JV and Meysman FJR (2018) The Cell Envelope Structure of Cable Bacteria. Front. Microbiol. 9:3044. Front. Microbiol., 20 December 2018