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Journal Club:”Direct Exchange of Electrons Within Aggregates of an Evolved Syntrophic Coculture of Anaerobic Bacteria” - OR: How Bacteria Hook up to Share Energy

Another curious observation made the science rounds the past week: wired, electric bacteria. Reading this article reminded me of a review article on dissimilatory bacteria I read before, and one of the most interesting talks I ever attended in my life titled "Eavesdropping on Bacterial Conversations".

What did they do?

Figure 1. Conceptual diagram showing electric symbiotic
relationship between G. metallieducens and G. sulfurreducens.

Summers, who is Microbiologist working in the Lovley lab at the University of Massachusetts, was studying Fe(III) reducing bacteria in the soil. They wondered what would happen when Fe(III) reducing bacteria would deplete Fe(III) available in the soil. In order to study this question, the research group co-cultured two strains of geobacter bacteria: Geobacter metallireducens and Geobacter sulfurreducens. The research team thought that combining the former bacteria that can oxidize ethanol in order to obtain energy, but normally must pass obtained electrons onto Fe(III) which was not present in the solution, with the latter strain which cannot metabolize, but can reduce fumerate to succinate while consuming hydrogen released by the previous bacterial species, would lead to the formation of an electrical symbiosis. The effect where one bacteria lives off the products of another bacteria is called syntrophy, and the authors wondered how this might happen.

They hypothesized that together these two bacteria could metabolize ethanol completely. In order to select for those bacteria that were capable of doing so, nine co-cultures were grown together and sequentially transferred when each co-culture was able to metabolize at least 70% of the supplied ethanol.

After a few months, they observed red bacterial aggregates forming. The role of interspecies hydrogen transfer has been debated in the field. The group wanted to study importance of this function within these aggregates and therefore knocked out a gene required to make hydrogenase. To their surprise, knocking out hydrogenase function did not have any effects on ethanol metabolism efficiency, but it sped up the formation of the aggregates (21 days instead of 7 months). This observation suggested that alternative mechanisms for electron transfer must other than interspecies hydrogen transport.

Upon sequencing the genomes of the co-cultured bacteria, a single mutation in G. sulfurreducens encoding an enhancer binding protein gene was found. Expressing this mutation in wild type strains also reduced aggregate formation time down to 21 days suggesting that this mutation is sufficient to promote aggregate formation.

The mutation lead to increased expression of various targets amongst which is a pili-associated cytochrome c-type protein which normally promotes electron transfer to Fe(III) oxides. Knocking out the gene inhibited growth of the bacteria even after more than nine months. Thus, this observation strongly suggests that G. metallireducens is passing its electrons directly to G. sulfurreducens without a hydrogen intermediate, giving us a concrete example for a sort of electric symbiosis.

Why is this significant?

According to Ken Nealson, who was asked about this research by The New Scientist, there are potential far reaching implications of this kind of symbiosis for disease treatment and diagnosis. Professor Nealson also wondered if human cells in our body are wired like these bacteria. On a personal note, I also see potential applications for the environmental and/or sustainable biotechnology field.

In our next journal club we will look at an article analyzing the microbial community inside cow stomachs.

As usual, I encourage everyone to read the original article which can be obtained from here:

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