Genetic Variation of Lactobacillus delbrueckii subsp. lactis Bacteriophages Isolated from Cheese Processing Plants in Finland.

The genomes of four Lactobacillus delbrueckii subsp. lactis bacteriophages were characterized by restriction endonuclease mapping, Southern hybridization, and heteroduplex analysis. The phages were isolated from different cheese processing plants in Finland between 1950 and 1972. All four phages had a small isometric head and a long noncontractile tail. Two different types of genome (double-stranded DNA) organization existed among the different phages, the pac type and the cos type, corresponding to alternative types of phage DNA packaging. Three phages belonged to the pac type, and a fourth was a cos-type phage. The pac-type phages were genetically closely related. In the genomes of the pac-type phages, three putative insertion/deletions (0.7 to 0.8 kb, 1.0 kb, and 1.5 kb) and one other region (0.9 kb) containing clustered base substitutions were discovered and localized. At the phenotype level, three main differences were observed among the pac-type phages. These concerned two minor structural proteins and the efficiency of phage DNA packaging. The genomes of the pac-type phages showed only weak homology with that of the cos-type phage. Phage-related DNA, probably a defective prophage, was located in the chromosome of the host strain sensitive to the cos-type phage. This DNA exhibited homology under stringent conditions to the pac-type phages.