Deciphering GIL01 receptors in the Bacillus cereus group

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

The Tectiviridae includes tail-less bacteriophages with a double-layer capsid where the dsDNA is located within a lipid-containing membrane covered by a rigid icosahedral protein. This family is subdivided in two groups according to the host they may infect. The well known PRD1 infects Gram-negative enterobacteria such as Escherichia coli and Salmonella enterica while GIL01, GIL16 and Bam35 are representatives of tectivirus preying on Gram-positive bacteria, specifically the Bacillus cereus group. In addition to this difference, both groups display distinct propagation styles: the ones infecting Gram-negative bacteria lyse the host cell at completion of the infectious cycle whereas GIL01 and relatives are capable to reside as temperate phages adopting a prophage form, designated pGIL01, an autonomous linear plasmid. In order to carry out an effective infection process, viruses must recognize and bind to the host cell. The initial recognition is typically mediated by a molecule or molecular assemblies (receptors) exposed on the surface of a susceptible cell. Little is known about phage receptors in Gram-positive bacteria, and much lesser for receptors of tectivirus. Therefore, the aim of this work is to identify the receptor(s) for GIL01 family. GIL01 and relatives produce turbid plaques characteristic of lysogenic phages. After repeated propagation different spontaneous clear plaque (CP) variables were found in lysates. These CP mutants elicited a high killing efficiency since they propagate exclusively lytically. Interestingly, CP bacterial survivors, designed GIL01-relativesCPr, appeared at very low frequency. These resistant bacteria exhibited diverse morphologies and colony phenotypes. Indeed, the GIL01-relativesCPr showed different sensitivity to wild type GIL01-relatives phages, suggesting that a component of the cell wall could act as a receptor or mediate the interaction bacteria-phage. Intriguingly is the fact that the CP variants displayed different capacities to infect the resistant bacteria. At the moment, candidate genes in B. cereus that could be involved in the resistance shown by bacteria are being indentified. Coupled with the identification of tectivirus receptors, the host spectra of GIL01-relatives and the distribution of tectivirus among in the B. cereus group are being studied, information that will be valuable for diversity studies and receptor(s) identification.