Abstract
Cable bacteria are multicellular filamentous bacteria within the Desulfobulbaceae that couple the oxidation of sulfide to the reduction of oxygen over centimeter distances via long distance electron transport (LDET). So far, none of the freshwater or marine cable bacteria species have been isolated into pure culture. Here we describe a method for establishing a stable single-strain cable bacterium culture in partially sterilized sediment. By repeated transfers of a single cable bacterium filament from freshwater pond sediment into autoclaved sediment, we obtained strain GS, identified by its 16S rRNA gene as a member of Ca. Electronema. This strain was further propagated by transferring sediment clumps, and has now been stable within its semi-natural microbial community for several years. Its metagenome-assembled genome was 93% complete, had a size of 2.76 Mbp, and a DNA G + C content of 52%. Average Nucleotide Identity (ANI) and Average Amino Acid Identity (AAI) suggest the affiliation of strain GS to Ca. Electronema as a novel species. Cell size, number of outer ridges, and detection of LDET in the GS culture are likewise consistent with Ca. Electronema. Based on these combined features, we therefore describe strain GS as a new cable bacterium species of the candidate genus Electronema, for which we propose the name Candidatus Electronema aureum sp.nov. Although not a pure culture, this stable single-strain culture will be useful for physiological and omics-based studies; similar approaches with single-cell or single-filament transfers into natural medium may also aid the characterization of other difficult-to-culture microbes.
Highlights
The vast majority of microbial organisms has yet to be cultured; it has been estimated that only 1 % of microbes can be cultured with standard isolation techniques [35]
We present a simple two-step process to establish a single strain cable bacterium culture: First, we remove a single cable bacterium filament from an environmental sample and second, we reintroduce the same filament into autoclaved sediment from its source environment
Cable bacteria developed in the top centimeter of the sediment within a month, and were ready to be used as a source for single filament transfers
Summary
The vast majority of microbial organisms has yet to be cultured; it has been estimated that only 1 % of microbes can be cultured with standard isolation techniques [35]. Cable bacteria developed in the top centimeter of the sediment within a month, and were ready to be used as a source for single filament transfers. The single cable bacterium filament was swiftly dragged by glass hook through the corridor and into the clump of sediment from the autoclaved core (Fig. 1c).
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