Prophage Origin of a Virulent Phage Appearing on Fermentations of Lactobacillus casei S-1

For protection from the abnormal fermentation ofLactobacillus casei S-1 caused by contamination of a virulent phage, φFSV, the origin of this phage was studied. Morphologies, viral structural proteins, and DNA structures of three independent isolates of φFSV were compared with those of φFSW, which is lysogenized in strain S-1. The results showed (i) that the morphology of φFSV phages is indistinguishable from that of φFSW and (ii) that all viral structural components found in φFSW are present in the particles of φFSV's. In addition, restriction endonuclease analyses of viral DNA showed that theHin dIII-digested fragments of φFSW DNA, the sum of which covered at least 94.7% of this phage genome, were conserved in the φFSV DNA digests. Results of Southern filter hybridization of the S-1 and prophage-cured cell (C239) DNAs with φFSV DNA as a probe revealed that C239 had lost most of the φFSV DNA sequence, whereas S-1 had about one copy of the φFSV DNA sequence. These results indicate that virulent phage φFSV is derived from the lysogenized phage φFSW. Therefore, the appearance of φFSV can be eliminated by using the prophage-cured derivative of S-1.