Characterization of closely related lactococcal starter strains which show differing patterns of bacteriophage sensitivity.

Aims: To characterize a group of closely related Lactococcus lactis subsp. lactis casein starter strains used commercially, which differ in their sensitivity to bacteriophages isolated from the same industrial environment.
Methods and Results: Nine strains of L. lactis, six of which had been used as starter cultures for lactic casein manufacture, were shown to be closely related by pulsed-field gel electrophoresis and total DNA profiles. Nineteen phages which propagated on one or more of these starter strains were isolated from industrial casein whey samples. The phages were all small isometric-headed and could be divided into five groups on the basis of host range on the nine strains. Most of the phages did not give a PCR product with primers designed to detect the two most common lactococcal small isometric phage species (936 and P335). The hosts could be divided into six groups depending on their phage sensitivity. Plasmids encoding genes for the cell envelope associated PI-type proteinase, lactose metabolism and specificity subunits of a type I restriction/modification system were identified.
Conclusions: This work demonstrates how isolates of the same starter strain may come to be regarded as separate cultures because of their different origins, and how these closely related strains may differ in some of their industrially relevant characteristics.
Significance and Impact of the Study: This situation may be very common among lactococci used as dairy starter cultures, and implies that the dairy industry worldwide depends on a small number of different strains.