PhD Defence Laura Louise Lindqvist

Supervisor
Professor Lone Gram, DTU Bioengineering
Co-supervisors
Assistant professor Shengda Zhang, DTU Bioengineering
Senior scientist Eva Sonnenschein, DTU Bioengineering (now Swansea University)
Examiners
Associate Professor Mogens Kilstrup, DTU Bioengineering
Professor Mathias Middelboe, University of Copenhagen
Professor Alison Buchan, University of Tennessee
Chair
Professor Lars Jelsbak, DTU Bioengineering

Microbial secondary metabolites have for decades been exploited for their biotechnological potential, e.g. as antibiotics, antitumor compounds, and food additives. They are, as opposed to primary metabolites, not involved in growth and proliferation. For a long time, these compounds where therefore disregarded as dispensable for the cell. However, this is far from the truth, as secondary metabolites play important roles in interactions with the environment and other microorganisms by acting as antibiotics, nutrient scavengers, and signals, to name a few. However, our understanding of the ecological role of secondary metabolites is still limited.
The purpose of this PhD project was to explore the ecological implications of secondary metabolite production, using the antibiotic tropodithietic acid (TDA), produced by bacteria of the marine
Roseobacter group, as a case study. TDA is a multifunctional secondary metabolite which is both a potent antibiotic, an iron chelator, and as a quorum sensing (QS) signal. QS is a process where gene expression is regulated in response to cell density. TDA therefore has the potential to perform a number of ecological roles. We showed that abolishing TDA production affects motility, cell morphology, and three putative horizontal gene transfer (HGT) systems in the producing strain, here Phaeobacter piscinae. We propose that these changes reflect a role for TDA as a global coordinator of colonization and adaption to novel niches, thus significantly impacting the lifecycle of P. piscinae.
One of the affected HGT systems was a so-called gene transfer agent (GTA), a virus-like particle found in many Rhodobacteraceae. GTAs, as opposed to classical bacteriophages, randomly package pieces of host DNA and transfer it to other bacteria in a manner combining aspects of transduction and natural
transformation. GTAs can therefore facilitate HGT in a bacterial population, which may in turn contribute to genome plasticity and the spread of antibiotic resistance. In our work, we found that TDA likely interacts with a QS regulator, PgaR, to repress the expression of GafA, a direct activator of GTA release. These results add to the growing understanding of the regulatory network governing GTA activation.
Lastly, pinpointing triggers of secondary metabolism may provide a clue to what their ecological role is. tracking secondary metabolite production is a daunting task, as the concentrations found in Nature are often below chemical detection limits. As an alternative to chemical detection, we developed two genetic reporter-fusions, where a fluorescent protein is produced upon activation of the biosynthetic machinery for TDA production. These reporter-fusions permit high-throughput studies of TDA production.
Ultimately, the work carried out in this work contributes to the growing understanding of the versatile roles secondary metabolites play. From an ecological perspective, this provides novel insights into how microorganisms interact with their surrounding environment. Additionally, a greater understanding of secondary metabolism may facilitate the discovery of novel applications for this diverse group of compounds.