Fig. 1: The conductive fiber sheath in cable bacteria is composed of a layer of protein on top of an acidic polysaccharide layer.

A STEM-HAADF imaging demonstrates that the fiber sheath is composed of parallel fibers imposed on a basal sheath. One 2D image (left panel) and two 3D tomographic reconstructions are shown. Independent replicas (N = 2) showed similar results. B AFM-IR spectra of fiber sheaths at cell areas and cell junctions (OPO laser, spectra are background corrected and averaged, cell area N = 14, junctions N = 11, a.u. is arbitrary units). C Fiber sheath AFM-IR mapping of the signal (a.u.) from the 1643 cm⁻¹ Amide I protein band (QCL laser, arbitrary units; see Supplementary Fig. 7 for corresponding AFM height and deflection images. Independent replicas (N = 2) showed similar results.). Representative ToF-SIMS depth profiles of fiber sheaths obtained in positive (D) and negative mode (E). A selection of fragments from different compound classes is shown (general organic carbon fragments: C₂H₂⁺ and C₂H⁻, protein derived fragments: C₂H₆N⁺ and CNO⁻, carbohydrate derived fragments: CHO⁺ and C₂H₃O₂⁻ and sulfur and transition metals). See Supplementary Fig. 1 and Supplementary Note 1 for further information. Counts of individual fragments were scaled to improve clarity as indicated in the figure legends. The counts from Ni₃S₃⁻ are the sum of all ⁵⁸Ni and ⁶⁰Ni isotopologues. Arrows denote the middle of the fiber sheath as calibrated by in situ AFM (59 ± 6 nm, see Supplementary Fig. 2).