#FEMSmicroBlog: Growing cable bacteria with agar pillar


Cable bacteria are multicellular chains living in freshwater sediments. In here, these centimeter-long filaments align according to the redox gradient. This positioning allows them to separate redox half-reactions occurring at both ends and conduct electricity by transferring electrons from cell to cell along the chain. The study “Tracing long-distance electron transfer and cable bacteria in freshwater sediments by agar pillar gradient columns” in FEMS Microbiology Ecology reveals how to easily grow cable bacteria in the lab to study them. Corinna Sachs presents this new method for the #FEMSmicroBlog as well as the importance of cable bacteria. #FascinatingMicrobes


Changing the view on redox zonation of sediments

In most sediments, electron acceptors align in strict vertical zones according to their redox chemistry and thus separate the oxic from the sulfidic zone. When cable bacteria were first found within upper marine sediments about ten years ago, the perception of diffusion-based redox zonation was completely overthrown. It seemed that cable bacteria could circumvent this phenomenon.

By forming centimetre-long filaments, cable bacteria can uncouple redox half-reactions and transfer electrons from one end of the cable filament to the other end of the cable.

Cable bacteria form centimetre-long filaments and uncouple redox half reactions to transfer electrons from one end of the cable filament to the other end.

Electrons are gained in an oxidation reaction of sulphur and are transferred onto oxygen in a reduction reaction. This process set a new paradigm for the biogeochemistry of sediments and is known as long-distance electron transfer. In comparison to their own body lengths, electron transfer over a few centimeters is an immense achievement for microbes.


Agar pillar to grow cable bacteria

Via sulfur cycling, cable bacteria impact the cycling of other elements in sediments and in the ecosystem. The global distribution of cable bacteria, both in marine and freshwater habitats, highlights their influence also on the carbon cycle with implications for the world climate. This importance makes it crucial to study cable bacteria in depth.

However, due to their peculiar lifestyle, cable bacteria have proven difficult to cultivate. A quick and simple strategy – the “agar pillar” approach – has been introduced in the study “Tracing long-distance electron transfer and cable bacteria in freshwater sediments by agar pillar gradient columns” in FEMS Microbiology Ecology.

This experimental setup represents a (semi-)natural, but sediment-free experimental compartment. Within this sediment column, a central agar pillar is embedded and provides a compartment for targeted cable bacteria enrichment. In there, the bacteria are in equilibrium with the surrounding sediment.

Agar pillar method to study cable bacteria
Agar pillar method to study cable bacteria. From Sachs et al. (2022).

This approach is ideal for possible downstream applications requiring clean samples. At the same time, it is easy and cheap to implement and cable bacteria can be grown from environmental samples in only a few weeks.


Verifying electron transfer by microprofiling and microscopy

The study used fluorescence microscopy to show that cable bacteria filaments were indeed present within the agar pillar. Long-distance electron transfer was confirmed by tracing geochemical patterns over time and depth in the agar pillar and the surrounding sediment.

Fluorescence microscopy of cable bacteria
Fluorescence microscopy of cable bacteria. From Sachs et al. (2022).

By using state-of-the-art microprofiling sensors, the study discovered that an electric potential developed over time – a clear hint for the growth of electricity-producing cable bacteria. At the same time, the sulfidic zone shifted downwards thus suggesting sulfide consumption by cable bacteria.


Detection of new freshwater cable bacteria within the microbial community

By sequencing (nearly) the full 16S rRNA gene, a very precise phylogenetic analysis was possible. Notably, cable bacteria are affiliated within the Desulfobulbaceae family. The study found that the agar pillars for the surface-water sediments were preferentially colonized by cable bacteria related to Candidatus Electronema and discovered several potentially novel species within this genus.

But not only cable bacteria populated the agar pillar. Among the microbial community, as direct neighbors of cable bacteria, the study also found co-enriched fermenters. These findings hint at the possibility of fermenters using cable bacteria filaments as living electron conductors.

Bacterial fermenters are possibly using the cable bacteria filament to conduct electrons.

In summary, this study shed new light on the diversity and ecology of cable bacteria in freshwater systems. The agar pillar approach provides a new strategy to cultivate these fascinating microbes for (electro-)fueling research.


About the author of this blog

Corinna Sachs is a Bioinformatics Research Scientist, working in the biotech industry in Denmark. She holds two Master of Science degrees in marine environmental sciences and microbiology from the University of Oldenburg, Germany. During her PhD research in environmental microbiology (started at Helmholtz Zentrum München and continued at Bayreuth University, Germany), she investigated long-distance electron transfer in freshwater cable bacteria and their roles in the sediment ecosystem. During this time, she discovered her passion for the more data-driven and computational side of microbiology. Corinna’s current research focuses on systems biology and exploring microbial potential via bioinformatics approaches for more sustainable applications.

About this blog section

The section #FascinatingMicrobes for the #FEMSmicroBlog explains the science behind a paper and highlights the significance and broader context of a recent finding. One of the main goals is to share the fascinating spectrum of microbes across all fields of microbiology.

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