Development of lytic Lactococcus lactis bacteriophages in a Cheddar cheese plant

The mixed TK5 starter culture was used in a Danish factory as the only starter for production of Cheddar cheese for more than 11 years before the factory experienced serious bacteriophage attacks with inhibition of the acid production in the curd. The cheese whey contained some phages from the beginning, and gradually new phages appeared able to infect an increasing number of isolates. Three bacteriophages jw30, jw31 and jw32 were isolated from the factory whey collected in 1989–94 and compared with lytic bacteriophages isolated in the period 1982–86 (Josephsen et al., 1994) DNA hybridisation showed that the type phage P008 had high homology to the new phages jw30, jw31 and jw32 as had the phages isolated in 1982–84. The new phages had broader host ranges and higher burst sizes than the previously isolated phages, showing that the lytic phages had become more virulent with time.