Symbiotic light, fantastic! : The Characterisation of Host and Symbiont Proteomes.

Within hours of hatching the Hawaiian bobtail squid, Euprymna scolopes, is colonised with the bioluminescent bacterium Vibrio fischeri in a specialised light organ. At night the epithelial crypts of the light organ contain the highest density of bacteria (up to 10^9cfu) which emit light that is used to avoid predation. However, a daily routine is established where the contents of the light organ, including 95% of the symbiont population, are vented at dawn when E.scolopes buries in the substrate. This cycle is completed where the remaining population of V.fischeri repopulate the crypts to similar levels as previously by nightfall. It is this relationship between colonisation, venting and growth in a host/symbiont system and the different genes that are expressed at each stage in both E.scolopes and V.fischeri that the authors have studied to determine which genes are required to establish a host/symbiont relationship.

The analysis of the host proteins expressed in the light organ showed the most abundant proteins are involved with the innate immune system, oxidative stress (superoxide dismutase) and other signalling pathways including NFκB (involved in DNA transcription under stress responses such as infection). Host proteins involved with iron sequestration and peptidoglycan recognition were also detected in the light organ and could explain the specificity for V.fischeri colonisation. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) intermediates were found in high quantities in the light organ which play a key role in the continuation of the squid/Vibrio symbiosis. V.fischeri must adapt to this environment to allow it to colonise the light organ which again allows for specificity of colonisers and allows some protection of the squid tissues.

Proteins expressed between V.fischeri found in the light organ of E.scolopes and V.fischeri in culture were compared and identified and indicates that the 25 most abundant V.fischeri proteins were proteins such as luciferase (key to fluorescence), alkyl hydroperoxide reductases, and a host of cold shock proteins. These analyses identify a range of different stress responses such as oxidative stress where the expression of catalases such as katA and peroxiredoxins may indicate a mechanism for V.fischeri to protect itself from ROS and RNS produced by the host tissues. Quorum sensing between bacteria is identified through the abundance of immunoglobulin like domains allowing the adhesion of bacteria to host surfaces which is vital for the original colonisation. Motility proteins are also abundant in the colonising V.fischeri community prior to the expulsion of the symbiont through venting with the regeneration of flagella a few hours before. V.fischeri also expresses several haem utilising genes such as HutA to combat iron sequestration by the host. Symbiont metabolism also changes where, at night whilst at high concentrations in the light organ, V.fischeri ferments chitin for energy however, after expulsion, the remaining bacteria anaerobically respire glycerol.

This study, by using advanced proteomics methods, allowed for more accurate identification of host and symbiont protein expression than previous studies. These results allow greater insight and greater options to further understanding through future research into host/symbiont relationships such as the mechanism of flagellar regeneration prior to venting and the mechanisms leading to the specificity for specific bacteria to form a symbiosis with a specific host.

A review of : Shleicher,T.R.; Nyholm,S.V.; 2011; Characterizing the Host and Symbiont Proteomes in the Association between the Bobtail Squid, Euprymna scolopes, and the Bacterium, Vibrio fischeri; PLoS ONE 6(10):e25649. doi: 10.1371/journal.pone.0025649