Tectivirus can mediate indirect ecological adaptations in the B. cereus group

Gillis, Annika [UCL] Mahillon, Jacques [UCL]

Many of the phenotypical characteristics displayed by the members of the Bacillus cereus group are associated with Mobile Genetic Elements, including plasmids, insertion sequences, and particularly, phages. A wide array of experiments has shown that phages can drive rapid bacterial evolution in different species by imposing strong selection for phage-resistant bacteria and other bacterial life traits. However, the potential contribution of phages to the adaptation of the B. cereus group members into their different environmental niches still poorly understood. Tectiviridae, a phage family preying on the B. cereus group, includes tail-less temperate phages where the dsDNA is located within a lipid-containing membrane, covered by an icosahedral protein capsid. For instance, GIL01, GIL16 and Bam35 are tectiviruses infecting B. thuringiensis, while AP50 has been reported to infect B. anthracis. Since these tectiviruses are able to reside as linear plasmids into their host cells, it is important to understand the selective pressures experienced by the bacteria when facing these phages. Therefore, the aim of this work was to study the different bacterial resistance phenotypes shown against GIL01 and GIL16 tectiviruses. Several B. thuringiensis strains were subjected to a selection pressure after repeated propagation with GIL01 and GIL16 clear plaque (CP) mutants. These CP mutants elicited an elevated killing efficiency since they propagate exclusively lytically. Bacterial mutants, displaying complete resistance to the CP phages, were found at low frequency. They exhibited various colony morphotypes and showed distinct adaptation features, as illustrated by their biofilm formation and exopolysaccharide production, sporulation rate and long-term survival capacity. These observations indicate that phages can drive the ecological adaptation of the B. cereus group members. At the moment, candidate genes that could be involved in these life traits are being identified. These results will shed light on phages receptor(s) and on the bacterial adaptation strategies to virus life-styles.