TLDR; Spirochetes will occasionally recruit other microorganisms in order to use them as living "life support systems". Interestingly, similar partnerships seem to have evolved multiple times in lineages of bacteria with vastly different lifestyles.
In the early 1920's, Professor Boris Perfil’ev had visited a resort in Russia and had made the most peculiar discovery. While sampling sediments within the many therapeutic springs on the resort, he discovered white mats of what had appeared to be sulfur oxidizing bacteria. However, he was surprised to find that most of the mat's structure was made up of a branching network of filamentous bacteria and was interspersed with tiny vibrioid (comma-shaped) cells which quickly traveled through the water with the aid of their flagella. Initially, he thought they were multiple life-stages of the same polymorphic bacterium and gave the organism the name Thiodendron latens, which roughly translates to "sedentary sulfur tree".
Since then, the mats were later found to be made up of multiple closely interdependent species of bacteria. The filamentous cells belonged to a few species of spirochete, whereas the vibrioid cells were sulfate and thiosulfate reducers belonging to the genera Desulfovibrio, Desulfobacter and Dethiosulfovibrio. These "Thiodendron Consortia" have been found within a host of different environments, ranging from hot springs to sulfidic marine habitats, such as tidal flats. However, it was only in 2002 that the true function of these communities was discovered.
While most spirochetes are strictly fermenters, those within the Thiodendron consortia are able to bounce between fermentation and aerobic respiration by way of a shortened electron transport chain. However, while aerobic respiration produces a lot more energy than the fermentation of sugars, it can also lead to a significant build up of hydrogen peroxide within their cytoplasm. Since the spirochetes cannot detoxify it on their own, this often results in death.
In order to avoid this fate, they fuel the growth of "sulfidogens" (bacteria that produce sulfide) by providing them with waste products of fermentation, such as pyruvate (It's also possible that the sulfidogens receive nitrogenous byproducts from their hosts when they die and undergo lysis. In a sense, their own remains might serve as a nutritious fertilizer).
This allows the sulfidogens to respire and produce hydrogen sulfide, which enters the spirochete's cytoplasm and functions as a sort of "cleansing" antioxidant. The end result of this process are a number of beautiful, refractive sulfur granules that accumulate within the spirochetes as they grow.
(Above) The white microbial mat shown above is the Thiodendron consortium and can be seen growing alongside what appears to be decaying kelp. (Lower Left) A collection of spirochetes that form up the structure of the Thiodendron. (Lower Right) A close-up of a sulfidogen in the process of cell division.
Side Note : While this form of mutualism might seem bizarre, this process has evolved multiple times, giving rise to other relationships in which prokaryotes depend upon one another for the detoxification of waste. For example, the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 experiences higher mortality rates in the presence of the molecular oxygen it produces through photosynthesis. As a result, it grows more readily in the presence of the oxygen-respiring heterotroph Meiothermus ruber due to the protection it offers.
Citations :
https://www.sciencedirect.com/science/article/abs/pii/S0303264720301957
https://sci-hub.st/10.1007/s11021-005-0001-3
https://www.researchgate.net/publication/41509760_Spirochaeta_perfilievii_sp_nov_oxygen-tolerant_sulfide_oxidizing_sulfur_and_thiosulfate-reducing_spirochete_isolated_from_a_saline_spring
https://www.researchgate.net/publication/335938166_RNA-based_qPCR_as_a_tool_to_quantify_and_to_characterize_dual-species_biofilms
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