Geographical diversity of Streptococcus thermophilus phages in Chinese yoghurt plants

The aim of this study was to investigate the diversity of phages from Chinese yoghurt plants. Restriction analysis of six isolated Streptococcus thermophilus (St) phages showed that Φ101, Φ102 and Φ108 with different origins but with the same host strain St1 were different while phages Φ901, Φ903 and Φ907 with host strain St9 from the same origin were homologous. DNA packaging mechanism analysis proved the isolated phages to be pac-type except Φ108. The sensitive strain St1 could be infected by two different packaging mechanisms. Probing and sequencing of the variable region (VR2) fragment revealed that VR2 in aggressive phages (Φ101, Φ102 and Φ108) infecting host strain St1 was deleted. Phage Φ901 has an 845 bp amplicon and the similarity coefficient of the sequence was lower than 96% compared with other reported phages, which shows phage Φ901 did not belong to any known grouping.