Editing Quorum sensing (section)

====''Curvibacter'' sp.====
[[Curvibacter|''Curvibacter'' sp.]] is a gram-negative curved rod-formed bacterium which is the main colonizer of the epithelial cell surfaces of the early branching metazoan ''[[Hydra vulgaris]]''.<ref>{{Cite web|url=https://www.researchgate.net/publication/45278794|title=Curvibacter fontana sp. nov., a microaerobic bacteria isolated from well water|website=ResearchGate|language=en|access-date=2019-03-13}}</ref><ref name=":3">{{cite journal | vauthors = Pietschke C, Treitz C, Forêt S, Schultze A, Künzel S, Tholey A, Bosch TC, Fraune S | display-authors = 6 | title = Host modification of a bacterial quorum-sensing signal induces a phenotypic switch in bacterial symbionts | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 114 | issue = 40 | pages = E8488–E8497 | date = October 2017 | pmid = 28923926 | pmc = 5635886 | doi = 10.1073/pnas.1706879114 | bibcode = 2017PNAS..114E8488P | doi-access = free }}</ref> Sequencing the complete [[Genome#Prokaryotic genomes|genome]] uncovered a circular chromosome (4.37 Mb), a plasmid (16.5 kb), and two [[operon]]s coding each for an AHL (N-acyl-homoserine lactone) synthase (''curI1'' and ''curI2'') and an AHL receptor (''curR1'' and ''curR2'').<ref name=":3" /> Moreover, a study showed that these host associated ''Curvibacter'' bacteria produce a broad spectrum of AHL, explaining the presence of those operons.<ref name=":3" /> As mentioned before, AHL are the quorum sensing molecules of gram-negative bacteria, which means ''Curvibacter'' has a quorum sensing activity.

Even though their function in host-microbe interaction is largely unknown, ''Curvibacter'' quorum-sensing signals are relevant for host-microbe interactions.<ref name=":3" /> Indeed, due to the [[oxidoreductase]] activity of ''Hydra'', there is a modification of AHL signalling molecules (3-oxo-homoserine lactone into 3-hydroxy-homoserine lactone) which leads to a different host-microbe interaction. On one hand, a phenotypic switch of the colonizer ''Curvibacter'' takes place. The most likely explanation is that the binding of 3-oxo-HSL and 3-hydroxy-HSL causes different conformational changes in the AHL receptors ''curR1'' and ''curR2''. As a result, there is a different DNA-binding motif affinity and thereby different target genes are activated.<ref name=":3" /> On the other hand, this switch modifies its ability to colonize the epithelial cell surfaces of ''Hydra vulgaris''.<ref name=":3" /> Indeed, one explanation is that with a 3-oxo-HSL quorum-sensing signal, there is an up-regulation of flagellar assembly. Yet, [[flagellin]], the main protein component of flagella, can act as an immunomodulator and activate the innate immune response in ''Hydra''. Therefore, bacteria have less chance to evade the immune system and to colonize host tissues.<ref name=":3" /> Another explanation is that 3-hydroxy-HSL induces carbon metabolism and fatty acid degradation genes in ''Hydra''. This allows the bacterial metabolism to adjust itself to the host growth conditions, which is essential for the colonization of the ectodermal mucus layer of ''Hydrae''.<ref name=":3" />