Meeting Abstract
Bacteria and archaea have profusely colonized the earth for more than 3.5 billion years, forming dense microbial biofilms on virtually all marine surfaces. The first evolving animals, arising around 700 million years ago, adapted to the microbial world in many ways. One major animal adaptation is the use of biofilm bacteria or their products as signals for recruitment by larvae from at least seven marine invertebrate phyla. We have investigated the settlement biology of one such species, the circum-tropical serpulid polychaete Hydroides elegans. In-depth studies on larvae of H. elegans have revealed that its metamorphically competent larvae settle selectively in response to specific biofilm-dwelling bacterial species. The strongly inductive bacterium Pseudoalteromonas luteoviolacea produces complex clusters of bacteriocins, multi-protein structures evolutionarily derived from phage-tail elements, which induce metamorphosis of H. elegans. However, at least three other bacterial species isolated from biofilms and found to induce settlement of H. elegans lack the genes for bacteriocins. Among these, Cellulophaga lytica produces abundant outer membrane vesicles (OMVs) in culture, and cell-free suspensions of OMVs induce metamorphosis of H. elegans. To determine whether it is compounds in the bacterial membrane that surrounds both intact cells and OMVs or OMV contents that induce settlement, we isolated major membrane lipopolysaccharides from C. lytica and purchased bacterial peptidoglycan and found them not to be inductive when tested separately or in combination. This suggests it is “cargo” in the OMVs that is the inductive element. Knowledge of the precise inductive elements will allow us to explore the mechanisms by which larvae respond with the profound developmental events of settlement and metamorphosis.
