Even though their function in host-microbe interaction is largely unknown, "Curvibacter" quorum-sensing signals are relevant for host-microbe interactions. 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. On the other hand, this switch modifies its ability to colonize the epithelial cell surfaces of "Hydra vulgaris". 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. 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".