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

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