Tectivirus that infect the B. cereus group: diversity, host spectrum and beyond

The Tectiviridae are 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. In the case of tectivirus preying on Gram-positive bacteria, specifically on the Bacillus cereus group, GIL01, GIL16 and Bam35 are representatives. GIL01 and relatives are capable to reside as temperate phages that not integrate the host genome upon infection and remain as an autonomous linear plasmid in the cell. In order to study the occurrence of the prophage form of GIL01 and relatives, 587 strains of a B. cereus s.l. collection were examined. PCR and phage propagation experiments indicate that pGIL01-related molecules occurred in about 4.8% of the strains examined. In an effort to assess the host ranges of both GIL01 and GIL16, 23 strains of the B. cereus group which did not harbor pGIL01-related elements (3 B. cereus sensu stricto, 5 B. cereus sensu lato, 10 B. thuringiensis and 5 B. mycoides) were tested. The results showed that none of the strains tested were sensitive to GIL01 and GIL16, at least under the experimental conditions used. In order to carry out an effective infection process, viruses must recognize and bind to the host cell via a receptor molecule. Little is known about phage receptors in gram-positive bacteria, and much lesser for receptors of tectivirus. Therefore, the next step 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