Mutualistic relationships that involve close cell–cell interactions are most studied between bacterial and eukaryotic interactions, for example between pathogens and eukaryotic hosts (plant or animal). However symbiotic relationships between bacteria themselves have only been studied more recently. They can be found amongst archaea and bacteria in microbial mats where nutrient exchange and waste removal roles are crucial, in anaerobic methane-oxidizing communities of marine environments, or even in human digestive systems.

Microbial mats contain many bacterial species living in close interactions. 

Although these syntrophic cocultures appear to be more widespread than commonly expected, very little is known about the physical interaction that are formed in the cell–cell interactions and the specific chemistry involved to establish a mutually beneficial relationship.

The clue is in the genomes…

By sequencing the genomes of two different species of a symbiotic mixture, and comparing the unique features of these genomes to other free living species, we were able to identify specific, unique interactions that can be formed within this syntrophy. The study showed several types of pili (hair-like appendages found on the surface of many bacteria) and so-called close-adhesion proteins present in one of the partners, while unique e-pili (electron-transporting pili) are formed by the other partner. The details of this are described in a publication by Kyndt et. al., just released this month in Microorganisms: https://www.mdpi.com/2076-2607/8/12/1939

One interesting technical aspect of the study was that the sequencing of both species in the symbiosis was performed simultaneously (without separating the species). However, we were able to separate and assemble the genomes by a process called metagenomic binning. This was made possible by our in-house Illumina sequencing and the publicly available PATRIC bioinformatics platform.

One cell’s trash is another cell’s treasure.

You may wonder what the benefit is for each of these cells to form this symbiosis. In the studied model, the green sulfur bacteria (Prosthecochloris) grow by photosynthesis. This is similar to the photosynthesis you may know from plants and algae, however it occurs anaerobic (without oxygen) and uses hydrogen sulfide (H₂S) instead of H₂O, and deposits elemental sulfur globules (S⁰) outside the cells, as a waste product. The partner bacteria in the mixture (Desulfuromonas) loves this, and uses this sulfur as an electron acceptor, converting it to sulfide. This sulfide in turn is used by the green sulfur bacteria to performs its photosynthesis, thereby completing the cycle.

Overview of interactions between Prosthecochloris ethyilca (green) and Desulfuromonas acetoxidans (brown/red). Image created in BioRender.com

Close interactions are preferred.

Thanks to the recent study, we now have a model by which so-called ‘tight adhesion’ (Tad) pili and large agglutination proteins from the green sulfur Prosthecochloris are key elements in the formation of the syntrophic complex. Once close cell–cell interactions are formed, the closed sulfur cycle can be established by electron transfer through specialized e-pili and several cytochromes produced by the Desulfuromonas component.

In addition to the sulfur cycling, another benefit of this relationship is that the colorless bacterium has flagella. This gives the entire complex the advantage of mobility and allows it to swim towards light and nutrients.

It is interesting that both the Tad pili and the adhesion proteins are best known from studies of bacterial virulence factors. However, based on their presence in other nonpathogenic species, they seem to be more widespread amongst bacteria and are likely involved in many environmentally important symbiotic interactions between bacteria.