A bacterial aphrodisiac?

Bacteria are everywhere. Well, pretty much everywhere. On that door handle, your bed, the air to the bottom of the ocean and the trillion inside your own bodies! Their pure diversity provides an exceedingly large study source to improve our biochemical and physiological understanding of cellular processes, but especially the effect bacteria can play on other organisms. Now, a recent study published in Cell has found that sexual reproduction can be regulated by the bacterium Vibrio fischeri (V.fischeri) (1).

A serendipitous discovery

Now, just to be clear, V.fischeri was found to regulate sexual reproduction in a choanoflagellate, S.rosetta, not in humans. Choanoflagellates are single-celled organisms that are the closest living relatives of animals, that survive by eating bacteria for breakfast. The presence of V.fischeri induces the cells to swarm and aggregate in loose gatherings of between 2-50 cells and with it a switch from asexuality to sexuality.

This loose aggregate formation is different from the developmental formation of multicellular rosettes, known to be triggered by specific lipids produced by the bacterium Algoriphagus machipongonensis (2).

Extracellular regulator of sex (EroS)

A series of experiments were conducted to find the biochemical source secreted by V.fischeri that acts as a bacterial aphrodisiac. The principal technique used was bioactivity guided fractionation of the V.fischeri culture to select and test components responsible for this phenomenon. The big hint came when a pre-protease treatment prevented the previously seen swarming activity. In conjunction with further fractionation and mass spectrometry the uncharacterised protein was found to be 90 kDa.

The protein was suitably named the extracellular regulator of sex – EroS for short. The eroS gene was heterologously expressed in in E.coli, the protein purified and was found to recapitulate the phenotype induced by V.fischeri.

Sequence alignment and mutagenesis showed eroS to be a chondroitin lyase, a type of GAG lyase. Since chondroitin disaccharides (products of a chondroitin lyase) were incapable of inducing the swarming alone, it is instead hypothesised that the action of the lyase is to cause a structural modification of S.rosetta that promotes its sexual behaviour.

A natural phenomenon?

Although the experiments show that V.fischeri is capable of inducing this behaviour is doesn’t necessarily mean that it will in natural conditions. However, as only 4×102 V.fischeri cells per ml are required, a concentration comparable to oligotrophic oceans, this proves that this ‘interkingdom interaction’ is indeed likely.

Since it was recently found that we have an even larger microbiome than previously thought*, understanding the functions and consequences that bacteria can have will aid a fuller understanding of ourselves and how the two kingdoms interact and link to diseases like autism and type II diabetes.

*Another 54 species have been discovered by Harvard scientists (3)

Further Reading

(1)  A.Woznica. Mating in the Closest Living Relatives of Animals Is Induced by a Bacterial Chondroitinase Cell. 2017 Sep 7;170(6):1175-1183

(2)  Alegado RA, Brown LW, Cao S, et al. A bacterial sulfonolipid triggers multicellular development in the closest living relatives of animals. Greenberg P, ed. eLife. 2012;1:e00013. doi:10.7554/eLife.00013.

(3) https://motherboard.vice.com/en_us/article/kz7bjm/scientists-sequence-the-dna-of-thousands-of-microbial-communities-living-in-healthy-people



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