Squid symbiont causes light organ maturation

Symbiotic relationships between Vibrio species and invertebrates have been observed for many years. In some cases the benefits from such a relationship can mean the difference between death and survival. The symbiosis between Vibrio fischeri and the bobtail squid Euprymna scolopes is an example of this, as V. fischeri inhabit a light organ within E. scolopes and emits bioluminescence which can cause counter-illumination, and thus hides its host from potential predators. E. scolopes hatch aposymbiotically and acquire V. fischeri from the seawater that surrounds them using mucus that is shed from the ciliated epithelium of the recently hatched E. scolopes, and thus the symbiotic relationship is begun. 

In a study by Aschtgen et al (2016), the effect of V. fischeri colonization on the light organ maturation was investigated. This maturation can be detected early by looking at the increased number of haemocytes (macrophage-like blood cells), which are associated with the increased colonization in E. scolopes. Outer membrane vesicles (OMV) produced by V. fischeri were hypothesized to deliver signals that triggered the maturation of the light organ in E. scolopes, and regulated haemocyte trafficking in surface epithelia. The data also led to the hypothesis that haemocyte trafficking was only fully induced after the V. fischeri individuals reached the deep crypts of the light organ.

To test this haemocyte trafficking, seawater containing juvenile E. scolopes was inoculated to contain 100mg of protein ml^-1 of OMV inoculated from Gram-negative bacteria. The results showed that they all had similar amounts of haemocyte trafficking, which led Aschtgen et al to conclude that only a high number of V. fischeri in the deep crypts can deliver OMV at the levels needed in order for them to carry out their signaling on these haemocytes. It also showed that the species of V. fischeri would be the only species to be able to colonize the crypts within the light organs of E. scolopes. Further testing with a higher dose of OMV showed that haemocyte appearance occurred much earlier than in the previous test (At 3 h compared to 18-24 h). This led Aschtgen at al to say that inoculated cells must divide 10-12 times before these deep crypts are populated enough to start producing OMV.

This study provided evidence to show that OMV from V. fischeri induced light organs to mature, and that only this species can colonize areas in the light organs of E. scolopes. It even showed the effects of delayed expulsion of these symbionts to lead to an increase of trafficking. This last finding seemed to go beyond the scope of the study, leading me to say that this study was successful in finding answers. It will be interesting to see if there are any further developments in the near future with regards to the OMV production in these symbionts, and to compare these findings with other symbiotic Vibrio species.

Reviewed paper: Aschtgen, Marie-Stephanie., Wetzel, Keith., Goldman, William., McFall-Ngai, Margaret., and Ruby, Edward. (2016). Vibrio fischeri-derived outer membrane vesicles trigger host development. Cellular Microbiology. 18(4): 488-499. http://onlinelibrary.wiley.com/wol1/doi/10.1111/cmi.12525/full