Close Relationship of Virulent Bacteriophages of Streptococcus salivarius subsp. thermophilus at Both the Protein and the DNA Level

Summary: Comparisons of 23 virulent phages of Streptococcus salivarius subsp. thermophilus were made based on morphology, host-range, structural protein analysis. DNA restriction patterns and DNA homology. All the phages had isometric heads (diameters 55·58 nm) and striated tails (lengths 221·275 nm). The genome sizes ranged from 37 to 43 kb. The electrophoretic profiles of the structural proteins were related, with at least two major bands of about 23 and 29 kDa in common for all the phages except three, φ80, φ96 and φ99. These latter phages, which had a particular protein composition (main proteins of 29 and 40 kDa) and high DNA homology between each other, showed only a low DNA homology with the other phages. Extensive DNA homologies were demonstrated by Southern hybridization for all the other phages. All these results suggest a close evolutionary relationship between the phages.