Phage satellites induced by virulent phages are mobilized by natural competence leading to phage resistance in a new host.

A phage satellite (PS) typically resides within repeat regions (attL and attR sites) of a bacterial genome. Its genome ranges from 7 to 20-kb and includes genes encoding an integrase along with regulatory and DNA replication functions. However, it lacks genes associated with viral structural proteins. Streptococcus thermophilus (S.t.) is extensively used to produce yogurt and specialty cheeses. Intriguingly, the majority of S.t. strains harbor a PS while very few possess a complete prophage, suggesting that PSs may confer advantages to their hosts. In this study, we showed that PSs of S.t. can excise from the bacterial chromosome, at a very low rate, without any phage interaction. Furthermore, we found that they can also be induced by virulent phages. By leveraging CRISPR-Cas9, we selected S.t. cells devoid of any PS (delta-PS strain). Then, we mobilized a PS from one strain to a delta-PS strain, using only natural competence, bypassing the need for a helper phage. The resulting strain exhibited increased resistance to virulent phages. Through the isolation of phage mutants escaping the resistance phenotype, we pinpointed a specific phage protein responsible for the induction of a PS. Lastly, we demonstrated that a PS can be significantly induced by a virulent phage, which, in turn, greatly promotes its transfer and specific integration into new cells through natural competence. Our study introduces a novel natural approach to develop phage-resistant strains.