A new phage on the 'Mozzarella' block: Bacteriophage 5093 shares a low level of homology with other Streptococcus thermophilus phages

Streptococcus thermophilus bacteriophage 5093 is a virulent phage that infects the industrial Mozzarella starter CSK939. The genome of phage 5093 is 37,184 base pairs (bps) containing 50 open reading frames (orfs). Genetic analysis revealed that the genome of phage 5093 is highly mosaic when compared with other sequenced S. thermophilus phages. This is particularly apparent in the late gene cluster with regions displaying high homology to prophage sequences of non-dairy streptococci and limited homology to either pac or cos-type S. thermophilus phages. In addition, a definitive antireceptor gene was not observed - suggesting that phage 5093 may have developed a different system for host recognition. Interestingly, the phage does contain a methylase domain that probably evolved as a phage counter-defence mechanism. These findings suggest that phage 5093 may represent a third group of S. thermophilus phage and provide the link between phages that infect S. thermophilus and its non-dairy ancestors.