Phaeobacter inhibens controls bacterial community assembly on a marine diatom.

Bacterial communities can have an important influence on the function of their eukaryotic hosts. However, how microbiomes are formed and the influence that specific bacteria have in shaping these communities is not well understood. Here, we used the marine diatom Thalassiosira rotula and the algal associated bacterium Phaeobacter inhibens as a model system to explore these questions. We exposed axenic (bacterial-free) T. rotula cultures to bacterial communities from natural seawater in the presence or absence of P. inhibens strain 2.10 or a variant strain (designated NCV12a1) that lacks antibacterial activity. We found that after 2 days the bacterial communities that assembled on the host were distinct from the free-living communities and comprised predominately of members of the Proteobacteria, Bacteroidetes and Cyanobacteria. In the presence of P. inhibens a higher abundance of Alphaproteobacteria, Flavobacteriia and Verrucomicrobia was detected. We also found only minor differences between the communities that established in the presence of either the wild type or the variant P. inhibens strain, suggesting that the antibacterial activity of P. inhibens is not the primary cause of its influence on bacterial community assembly. This study highlights the dynamic nature of algal microbiome development and the strong influence individual bacterial strains can have on this process.

Dual function of tropodithietic acid as antibiotic and signaling molecule in global gene regulation of the probiotic bacterium Phaeobacter inhibens.

Antibiotics are typically regarded as microbial weapons, but whereas their function at concentrations lethal for bacteria is often well characterized, the role of antibiotics at much lower concentrations as possibly found under natural conditions remains poorly understood. By using whole-transcriptome analyses and phenotypic screenings of the marine bacterium Phaeobacter inhibens we found that the broad-spectrum antibiotic tropodithietic acid (TDA) causes the same regulatory effects in quorum sensing (QS) as the common signaling molecule N-acyl-homoserine lactone (AHL) at concentrations 100-fold lower than the minimal inhibitory concentration against bacteria. Our results show that TDA has a significant impact on the expression of ~10% of the total genes of P. inhibens, in the same manner as the AHL. Furthermore, TDA needs the AHL associated LuxR-type transcriptional regulator, just as the AHL molecule. Low concentrations of antibiotics can obviously have a strong influence on the global gene expression of the bacterium that produces it and drastically change the metabolism and behaviour of the bacterium. For P. inhibens this includes motility, biofilm formation and antibiotic production, all important for settlement on new host-associated surfaces. Our results demonstrate that bacteria can produce antibiotics not only to antagonise other bacteria, but also to mediate QS like endogenous AHL molecules.

Genome sequence of Phaeobacter caeruleus type strain (DSM 24564(T)), a surface-associated member of the marine Roseobacter clade.

In 2009 Phaeobacter caeruleus was described as a novel species affiliated with the marine Roseobacter clade, which, in turn, belongs to the class Alphaproteobacteria. The genus Phaeobacter is well known for members that produce various secondary metabolites. Here we report of putative quorum sensing systems, based on the finding of six N-acyl-homoserine lactone synthetases, and show that the blue color of P. caeruleus is probably due to the production of the secondary metabolite indigoidine. Therefore, P. caeruleus might have inhibitory effects on other bacteria. In this study the genome of the type strain DSM 24564(T) was sequenced, annotated and characterized. The 5,344,419 bp long genome with its seven plasmids contains 5,227 protein-coding genes (3,904 with a predicted function) and 108 RNA genes.