PhD defence: Jesper Juel Mauritzen

Due to special circumstances, the SCIENCE PhD school has granted permission to go through with the defense as originally planned, except that there will be only one opponent present at the oral defense (Lars Jelsbak).

Title: Regulation of virulence properties in the fish pathogen Vibrio anguillarum
Subtitle: The role of prophages and quorum sensing

Supervisor: Mathias Middelboe, Professor

Assessment committee:

• Niels-Ulrik Frigaard, Associate Professor (Chair), Department of Biology, University of Copenhagen, Denmark
• Martha R. J. Clokie, Professor, Department of Genetics and Genome Biology, University of Leicester, United Kingdom
• Lars Jelsbak, Professor MSO, Department of Biotechnology and Biomedicine, Technical University of Denmark, Denmark

Abstract:
Vibrio anguillarum is a marine fish pathogenic bacterium that causes vibriosis in many fish species leading to large economic losses in the aquaculture industry. Yet, the factors governing the pathogenicity in this bacterium are largely unknown. Many bacterial pathogens can communicate to coordinate their virulence programs through the process quorum sensing (QS) that relies on the production, release, and detection of signal molecules. As cells divide, produce and release QS signals, the accumulation of signals becomes a proxy of the cell density, and, at high signal concentrations, bacteria activate QS regulation in response to cell density. However, our insight in the genes controlled by QS in V. anguillarum is scarce. Here, I uncover the QS regulation in two V. anguillarum isolates and reveal a plethora of genes under QS control, including central processes such as metabolism and virulence.

Remarkably, the isolate-specific differences in both the activation and the extent of QS regulation were reflected in their virulence in a fish infection model, adding to our understanding of the mechanisms underlying strain-specific differences in virulence. Besides the role of QS in bacterial infection of fish, QS also plays a role in virus infection of the bacterium. Viruses that infect bacteria (phages) can form latent infections, where phage genomes reside dormant in host cells as part of their chromosome (prophages) until induced to multiply and kill their host cells. QS represses this switch from latent prophage to cell lysis and this work delve into the molecular mechanisms of how QS control prophage induction. Specifically, the master regulator of QS in V. anguillarum, VanT, directly binds the phage genome and represses a phage-encoded regulator that plays a part in prophage induction. This highlights a refined use of QS by the host cells to control prophage induction that is clearly distinct from a simple evolutionary arms race, and suggests that QS repression of prophage induction is important for phage-host coexistence.

Moreover, this thesis describes inoviruses in V. anguillarum and reveals a new family of non-tailed NO16-like phages, that are present as prophages inside the genomes of at least 30 species of Vibrio and widely distributed across geographical regions. Characterization of the genetic diversity and infection dynamics, of the previously overlooked group of phages revealed that both host cell density and phage-host ratios are important factors that determine the development of phage infections. The pervasiveness of these prophages with small genomes in V. anguillarum chromosomes indicate efficient dispersal and underlines that phage-host interactions may often be mutualistic rather than parasitic and drive microbial evolution. In support of this, prophage-containing isolates were more virulent toward a fish model compared to prophage-free isolates, underlining that phage infections can modify virulence of V. anguillarum. Altogether this thesis provides novel insight into the role of QS and prophages for the virulence of the important fish pathogen V. anguillarum and sheds light on the mechanisms of how QS control prophage induction.