V.fischeri and E.scolopes: It takes two to tango!

The Bobtail Squid (Euprymna scolopes) maintains a population of bioluminescent bacteria (Vibrio fischeri) within a bi-lobed (light) organ, which is used to affect an anti-predatory behaviour called counter illumination. V.fischeri are not transferred from parent to larvae, as when they hatch the bobtail squid harbours none of these symbionts, but uses the light organ to attract them from its surroundings. The Bobtail squid goes through a cycle of expelling its symbionts in the morning and re-establishing a well populated colony during the day. The light organ is specifically attractive to V.fischeri. This paper reviews research into the symbiosis between these two organisms.

There are three stages of symbiotic colonisation in the case of the bobtail squid: Initiation, in which the symbionts enter the light organ and begin to divide, accommodation, in which high cell density is achieved and persistence, which is the continued re-growth of symbiont colonies after they have been expelled.

All of the aspects of the colonisation process are strongly linked to motility in the symbionts through flagella. This paper outlines the effect that some genetic mutations in this trait, all of which reduce the chances of the symbiont forming a symbiosis with the bobtail squid. Research has also shown that genes associated with the expression of flagella in V.fischeri could also have some control over other aspects of the bacteria, which could go some way to explaining why symbiosis was not properly re-established when the mutation was genetically complimented.

Luminescence is an important part of symbiotic persistence. The paper also goes on to explain in a similar fashion that studies on mutations in the genes controlling this function cause problems with initiation as well as persistence. It is suggested again that problems with these genes lead to problems with the overall biology of the symbionts and could explain why symbiosis does not easily occur.

Studies have shown that morphological changes occur in the squid’s light organ after initial colonisation by the symbionts. This most importantly includes a narrowing of part of the entry duct called the antechamber, into which bacteria enter the organ. It has been suggested that this helps to maintain an optimal bacteria colony size, as it seems to act as a physical barrier between the light organ and the surroundings. This aspect of the symbiosis is curious, as this type of morphological change is usually associated with harmful bacteria. It may be possible that the symbiotic relationship was originally brought about by pathogenic bacteria. Research in other animal groups have also found that bacteria considered to be “toxic” can actually cause beneficial tissue development, suggesting that the action of the bacteria is highly context dependent. The paper also goes on to suggest that the chemical pathways taken by pathogenic and symbiotic bacteria are very similar, though lead to very different ends.

The collation of this evidence seems to raise as many questions as it answers with regard to the symbiosis between V.fischeri and E.scolopes. In particular, more investigation into the similarities and differences between these symbionts and pathogens is required. Further study will also need to include genetic diversification in the bacteria among many other things, so that we can start to understand the crucial roles that beneficial microbes play on animal health and development.

A review of Visick K.L. and Ruby E.G. (2006) Vibrio fischeri and its host: it takes two to tango, Current opinion in microbiology 9 632-638