Inactive "Ghost" Cells Do Not Affect Motility and Long-Range Electron Transport in Filamentous Cable Bacteria

Cable bacteria are multicellular filamentous microorganisms that perform electrogenic sulphur oxidation over centimetre-long distances. These filaments contain so-called "ghost cells", which display a highly reduced cytoplasmic content and a lack of metabolic activity. However, the origin and abundance of these ghost cells are not well understood, raising questions about their formation and potential impact on the functioning of the entire filaments. Here, we quantified the abundance of ghost cells in cable bacteria via a targeted propidium iodide staining technique and investigated their morphology and possible origin. Microscopy revealed that ghost cells are present in filaments under in situ conditions, and hence, they are not an artefact from filament sampling. Interestingly, filaments containing ghost cells retained gliding motility, as well as the capacity for long-distance electron transport, thus suggesting that the functionality of the filament as a whole remains largely unaffected by the presence of these ghost cells. Noteworthy is the higher frequency of ghost cells near the ends of filaments, and within filament fragments retrieved from oxic environments. Our findings provide new insights into the adaptive strategies of filamentous bacteria, highlighting their ability to maintain functionality at the organism level despite the fact that some individual cells are no longer metabolically active.