Abstract
Recruitment via settlement of pelagic larvae is critical for the persistence of benthic marine populations. For many benthic invertebrates, larval settlement occurs in response to surface microbial films. Larvae of the serpulid polychaete Hydroides elegans can be induced to settle by single bacterial species. Until now, only Pseudoalteromonas luteoviolacea had been subjected to detailed genetic and mechanistic studies. To determine if the complex structures, termed tailocins, derived from phage-tail gene assemblies and hypothesized to be the settlement cue in P. luteoviolacea were present in all inductive bacteria, genomic comparisons with inductive strains of Cellulophaga lytica, Bacillus aquimaris and Staphylococcus warneri were undertaken. They revealed that the gene assemblies for tailocins are lacking in these other bacteria. Negatively stained TEM images confirmed the absence of tailocins and revealed instead large numbers of extracellular vesicles in settlement-inductive fractions from all three bacteria. TEM imaging confirmed for C. lytica that the vesicles are budded from cell surfaces in a manner consistent with the production of outer membrane vesicles. Finding multiple bacteria settlement cues highlights the importance of further studies into the role of bacterial extracellular vesicles in eliciting settlement and metamorphosis of benthic marine larvae.
Highlights
The serpulid polychaete Hydroides elegans has emerged as a useful model for studying bacterial induction of settlement and metamorphosis[13,30]
Side-by-side preparations of bacterial products of C. lytica, B. aquimaris and S. warneri comparable to the same preparations from P. luteoviolacea were examined with transmission electron microscopy (TEM) to visually determine if tailocins were present, and, if so, were arranged in the complexes seen in P. luteoviolacea
This study further addresses the question, if tailocins are not transcribed in other inductive bacterial species, what are the mechanisms by which they induce larvae of the tubeworm Hydroides elegans to attach to surfaces and metamorphose?
Summary
The serpulid polychaete Hydroides elegans has emerged as a useful model for studying bacterial induction of settlement and metamorphosis[13,30]. Huang and colleagues[36] carried out random transposon mutagenesis on the bacterium and identified gene sequences that correlated with the inductive activity Analysis of those gene sequences implicated elements of phage-tail proteins in the bacterial genome. Among the original biofilm-bacterial species identified by Huang and Hadfield[33] that, in addition to P. luteoviolacea, induced larval settlement was another broadly distributed inductive marine bacterium, Cellulophaga (Cytophaga) lytica. Two biofilm bacterial strains have been found to induce settlement in larvae of H. elegans: Bacillus aquimaris and Staphylococcus warneri These two strains are Gram-positive, whereas both P. luteoviolacea and C. lytica are Gram-negative strains. This study further addresses the question, if tailocins are not transcribed in other inductive bacterial species, what are the mechanisms by which they induce larvae of the tubeworm Hydroides elegans to attach to surfaces and metamorphose?
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