GENETIC DIVERSITY OF BACTERIOPHAGES HIGHLY SPECIFIC FOR STREPTOCOCCUS THERMOPHILUS STRAIN LBB.A

Eight virulent bacteriophages phi A1, phi A2, phi A5, phi A6, phi DmA, phi SfA, phi PtA and phi A7, that showed high specificity for Streptococcus thermophilus strain LBB.A, the streptococcal component of an industrial yogurt starter were isolated. The comparison of the HindIII and EcoRV restriction Patterns of the phage DNA revealed limited relatedness between the isolates, clustering together only phi A5 with phi A6 and indicating similarity between phi SfA, phi PtA and phi A7. All eight bacteriophages were classified as cos-type phages. An internal region within a conserved 2.2 khp EcoRI DNA fragment typical for the majority of S. thermophilus bacteriophages was present in six of the phage isolates and was absent in phi DmA and phi A1. PCR amplification of the VR2 region of the antireceptor gene yielded an 800 hp product for seven of the phages and a 700 hp product for phi A2. The genome size of the eight phage isolates was in the range of 33.4-37.3 kbp. Of a total of 62 other S. thermophilus cultures phi A1, phi DmA and phi PtA were able to infect only two strains with reduced effectiveness of plaquing. Despite the broad genetic diversity of phages of S. thermophilus strain LBB.A, a two-step selection procedure, involving successive challenge of the bacterial culture with phi A1 and phi A7, was sufficient to obtain mutants resistant to all eight phages.