Euprymna scolopes: an additional association

The Euprymna scolopes is known for its fascinating association with Vibrio fischerii, which provides the squid with counter illumination whilst hunting at night. What is less known is that another bacterial association is found in this squid. Bacteria in genus Roseobacter appear to be associated with the accessory nidamental glad (ANG) in female E. scolopes. The ANG is an integral part of the female reproductive system, and Rosebacter are thought to be transmitted vertically; integrated in the egg of the baby squid. But what it does there is largely unknown. An analysis of the genomes of Rosobacter sp. isolated from the ANG suggest that these bacteria may communicate with their host through quorum sensing, as well as provides evidence of a specialised type 6 secretory system and genes for siderophores synthesis.

The genome sequences of 59 Rosebacter isolated from five mature females of E. scolopes were large, reflecting the metabolic diversity of these bacteria. Little genome reduction has occurred in these bacteria, even though they are tightly associated with a host. Contrasting to previous results, all of the isolates had genes for nitrate reductase and denitrifying enzymes, showing they can survive anaerobically.

The Type 4 Secretion System often found in Roseobacter was not found in these bacteria, but instead all isolates contained genes for the Type 6 Secretion System (see my last blog). The author hypothesised that this may be used for protection of the egg masses, as the T6SS is a particularly nasty cellular weaponry. However, they also mention is could be used as a form of communication between the host and bacteria. As the bacteria are located in the ANG epithelium and hemocytes, the T6SS could be used in the transfer of effector molecules to the host or symbiont. Subsequently altering biological activity, such as gene expression, enzyme activity or cell signalling.

Two quorum sensing LuxI homologs, similar to the ssaIR and ssbIR in Ruegeria sp, were detected in the bacteria. Only two isolates had both pairs, the remaining contained only the ssbIR. In Ruegeria sp. the ssaIR and ssbIR are part of an interconnected quorum-sensing network, which regulates biofilm formation, whilst simultaneously controlling swimming motility. The function the ssbIR alone is therefore unknown, yet due to the frequency of its occurrence, the author hypothesises it must have an integral function. One hypothesis was based on the raiIR in Rhizobium etli, which controls growth and nitrogen fixation. The author suggests that the luxIR could be used to regulate the growth of other the bacteria on the ANG.

Siderophores genes were found in all the bacteria, which is not common for Roseobacter. When the bacteria were grown in iron-limited conditions, an increase in siderophore synthesis occurred. Iron is often limiting in the marine environment, therefore having these siderophore may function as a selective advantage for these bacteria in competition with other bacteria without siderophores.

This paper provides the first insight into the association between E. scolopes and Rosebacter, presenting some interesting specialisations in these Rosebacter. The function of these bacteria on the ANG are still unknown, but are thought to contribute to the protection and development of the young cephalopod.  In my opinion this paper could have had more detailed explanations of the quorum-sensing systems it was referring to as I found it a bit unclear, however the majority of the paper was very interesting.

Reference:

Collins, A. J., Fullmer, M. S., Gogarten, J. P., & Nyholm, S. V. (2015). Comparative genomics of Roseobacter clade bacteria isolated from the accessory nidamental gland of Euprymna scolopes. Frontiers in microbiology, 6.