Tectiviruses preying on the Bacillus cereus group : prevalence, diversity and phage-host interactions

Gillis, Annika [UCL]

With the increasing number of Bacillus cereus sensu lato genomes sequenced, it has become evident that the genetic pool of phages infecting this group of bacteria is large and diverse. However, the contribution of phages to the adaptation of the B. cereus group members to their environmental niches has been disregarded. Therefore, the present work focuses on a peculiar phage family, the Tectiviridae, for which it has been shown that it can establish a linear plasmidial prophage state when infecting members of this bacterial group. The first part of this thesis focused on discovering novel tectiviruses infecting the B. cereus group. The screening and propagation tests indicated that tectiviruses occurred in less than 3% of the bacterial isolates. Partial DNA sequencing of variable regions indicated that a greater diversity than previously observed exists within the family Tectiviridae. Analysis of the tectiviruses host range showed that no simple relationship can be established between the infection patterns of these phages and their diversity. An ecological approach to study the interactions taking place between tectiviruses and their Bacillus thuringiensis host was then addressed by assessing the impact of lysogeny and phage-resistance on some bacterial life traits. Whole-genome-sequencing analysis of B. thuringiensis phage-resistant mutants suggested that several enzymes involved in cell wall biosynthesis might be responsible for the resistance phenotype. Besides, evaluation of the impact of tectiviral lysogeny on different bacterial life traits provided evidence that tectiviruses may drive changes and ecological adaptations in the B. cereus group members. The last part of this work focused on developing an atomic force microscopy-based approach to gain insights into the nanoscale surface properties of B. thuringiensis swarmer-cells, since it was found that tectiviral lysogeny and phage-resistance alter, among others, bacterial swarming motility. Beyond a fundamental contribution towards understanding the genetic diversity and infection patterns of tectiviruses in the B. cereus group, this study sheds new light on phage receptor(s) and on the bacterial adaptation strategies to phage life-styles.