Tectivirus preying on the Bacillus cereus group: diversity, phage-host interactions and beyond

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

Tectiviridae comprises tail-less phages with a double-layer capsid where the ~15kb linear dsDNA is located within a lipid-containing membrane covered by a rigid icosahedral protein capsid. This family contains two groups: the lytic PRD1 phage infects Gram-negative bacteria, while GIL01, GIL16 and AP50 are temperate ones preying on Gram-positive bacteria, specifically on the Bacillus cereus sensu lato (s.l.) group. Although the biology of PRD1 is well studied, little is known about the interactions taking place between tectiviruses and their respective Gram-positive host. Therefore, this work aimed at characterizing the interactions between tectivirus and their B. cereus s.l. host cells. The occurrence of pGIL01-like phages was addressed among 700 B. cereus s.l. strains. PCR and propagation tests revealed that pGIL01-like molecules occurred in less than 5% of the examined strains. Despite this limited distribution, some new GIL01-like variants were found, and preliminary DNA sequencing indicated that a greater diversity exist within the Tectiviridae. In an effort to assess the host range of GIL01, GIL16 and their new cousins, 100 strains of B. cereus s.l. devoid of pGIL01-like elements were tested. The results showed that less than 30% of the bacteria were sensitive to the tectiviruses tested. Interestingly, some strains were susceptible to both GIL01 and GIL16, whereas others could be infected only by one representative. These results strongly suggest that more than one molecule can act as tectivirus receptor in the bacterial surface. For studying adsorption properties and identification of tectivirus receptors, spontaneous as well as transposon-induced bacterial mutant libraries were obtained and mutants displaying either complete or specific phage resistances were obtained. Bacterial mutants displaying complete resistance to tectivirus were found at low frequency. They exhibited various colony morphotypes and showed distinct adaptation features, as illustrated by their biofilm formation, sporulation rate and long-term survival capacity. These results will shed new light on bacteriophages receptor(s) and on the bacterial adaptation strategies to virus life-styles.