MOLECULAR ECOLOGY AND EVOLUTION OF STREPTOCOCCUS-THERMOPHILUS BACTERIOPHAGES - A REVIEW

Bacteriophages attacking Streptococcus thermophilus, a lactic acid bac terium used in milk fermentation, are a threat to the dairy industry. These small isometric-headed phages possess double-stranded DNA genome s of 31 to 45 kb. Yoghurt-derived phages exhibit a limited degree of v ariability, as defined by restriction pattern and host range, while a large diversity of phage types have been isolated from cheese factorie s. Despite this diversity all S. thermophilus phages, virulent and tem perate, belong to a single DNA homology group. Several mechanisms appe ar to create genetic variability in this phage group. Site-specific de letions, one type possibly mediated by a viral recombinase/integrase, which transformed a temperate into a virulent phage, were observed. Re combination as a result of superinfection of a lysogenic host has been reported. Comparative DNA sequencing identified up to 10% sequence di versity due to point mutations. Genome sequencing of the prototype tem perate phage phi Sfi21 revealed many predicted proteins which showed h omology with phages from Lactococcus lactis suggesting horizontal gene transfer. Homology with phages from evolutionary unrelated bacteria l ike E. coli (e.g. lambdoid phage 434 and P1) and Mycobacterium phi L5 was also found. Due to their industrial importance, the existence of l arge phage collections, and the whole phage genome sequencing projects which are currently underway, the S. thermophilus phages may present an interesting experimental system to study bacteriophage evolution.