MOLECULAR ECOLOGY OF STREPTOCOCCUS-THERMOPHILUS BACTERIOPHAGE INFECTIONS IN A CHEESE FACTORY

A mozzarella cheese factory using an undefined, milk-derived Streptaco ccus thermophilus starter system was monitored longitudinally for 2 ye ars to determine whether the diversity of the resident bacteriophage p opulation arose from environmental sources or from genetic changes in the resident phage in the factory. The two hypotheses led to different predictions about the genetic diversity of the phages. With respect t o host range, 12 distinct phage types were observed. With two exceptio ns, phages belonging to different lytic groups showed clearly distinct restriction patterns slid multiple isolates of phages showing the sam e host range exhibited identical or highly related restriction pattern s, Sequencing studies in a conserved region of the phage genome reveal ed no point mutations in multiple isolates of the same phage type, whi le up to 12% nucleotide sequence diversity was observed between the di fferent phage types. This diversity is as large as that between the mo st different sequences from phages in our collection. These observatio ns make unlikely a model that postulates a single phage invasion event and diversification of the phage during its residence in the factory, In the second stage of our factory study, a defined starter system wa s introduced that could not propagate the resident factory phage popul ation, Within a week, three new phage types were observed in the facto ry while the resident phage population was decreased but not eliminate d. Raw milk was the most likely source of these new phages, as phages with identical host ranges and restriction patterns were isolated from raw milk delivered to the factory during the intervention trial. Appa rently, all of the genetic diversity observed in the S. thermophilus p hages isolated during our survey was already created in their natural environment. A better understanding of the raw-milk ecology of S. ther mophilus phages is thus essential for successful practical phage contr ol.