Development of lytic Lactococcus lactis bacteriophages in a Cheddar cheese plant
Introduction
In cheese plants lytic lactococcal bacteriophages have good opportunities for development of new phages because aseptic conditions cannot be established for the fermentation. The phages can be transferred from one fermentation to the next via whey, aerosols, dust etc., the vats are large and the cheese milk cannot be heat treated sufficiently to destroy the phages without decreasing the quality of the cheese. Therefore phages constitute a major problem in the industrial production of cheese, especially for Cheddar cheese (Whitehead and Cox, 1935, Jarvis, 1987).
Several lactococcal phages have been isolated from whey and characterised with regard to host range, burst size, morphology and DNA/DNA homology (for a review see Josephsen and Neve, 1998). The taxonomic studies have demonstrated that the lactococcal phages can be divided into 10 (originally 12) distinct species with no significant DNA homology between the species, but with high homology within the species (Jarvis et al., 1991, Jarvis, 1995). Until recently the majority of the lytic phages belonged to the small isometric-headed 936 species, however, lately phages belonging to the P335 species have also appeared (Moineau et al., 1992, Josephsen et al., 1994). Prolate-headed phages belonging to the c2 species have also been isolated in several dairy plants (Jarvis et al., 1991).
Usually single or multiple strain starter cultures are used for industrial production of cheese, especially Cheddar cheese. It is difficult to make them phage resistant enough for long term industrial use. Mixed undefined starter cultures contain an unknown number of strains and are therefore generally believed to be able to resist bacteriophage infection better than single and multiple starters. Moreover, mixed-undefined strain starters are considered to be more difficult to control in large scale production and the range of different starter cultures is limited, especially for non-citrate fermenting starters.
Few studies have been published concerning the development of lytic phages in a dairy plant over a long period of time. In investigations with the mixed Cheddar starter TK5 (Nielsen et al., 1987) we have followed the development of lytic phages during several years in the Danish cheese factory at Øster Tørslev, where the TK5 starter has been used for more than 12 years, during the first 11 years as the sole starter. We have earlier reported on these investigations based on the first five years and on investigations on 62 bacterial isolates from the TK5 starter (Josephsen and Nielsen, 1988, Josephsen et al., 1994). During the whole period of 12 years whey samples from the cheese factory have been collected and the activity of the phages of the whey against the TK5 starter culture itself and against the 62 isolates have been determined (E. Waagner Nielsen, personal communication).
Here we report on three phages, jw30, jw31 and jw32, which were isolated from whey samples from Øster Tørslev Cheese Factory, characterised and compared with the previously isolated phages from the same cheese factory (Josephsen et al., 1994).
Section snippets
Bacteria and bacteriophages
Sixty-two Lactococcus lactis strains had been isolated from the mixed Cheddar starter TK5 (Josephsen and Nielsen, 1988). Phages jw30, jw31 and jw32 were isolated from factory whey samples collected in 1989, 1993 and 1994, on the TK5 isolates W22, W53 and W34. The three phages were always propagated on these isolates. The small isometric-headed type phages, P008 and P335 (Braun et al., 1989), propagated on the host strain, L. lactis subsp. lactis var. diacetylactis F7/2, were included in this
Discussion
The mixed undefined starter TK5 was used in a Danish cheese plant for production of Cheddar cheese for more than 11 years before any inhibition of the acid production due to bacteriophages was experienced despite the presence of phages in the whey. The phages gradually became able to attack an increasing number of isolates from the starter culture. Three “new” phages were isolated from whey samples collected from the second half of the production period and they were compared with “old” phages
Conclusion
From 1982 to 1993 the mixed undefined starter TK5 was used for production of Cheddar cheese without any inhibition of the acid production due to bacteriophages, however, there was always phages present in the whey. The phages gradually became able to attack an increasing number of isolates from the starter culture. Comparison of phages, always small isometric-headed, isolated at different times from the whey showed that three “new” phages had broader host ranges and larger burst sizes than the
Acknowledgements
The technical skill of Bashir Aideh, Bo S. Hansen, Michael S. Agnild, Susanne G. Jensen and Jonna Mathiesen is highly appreciated. This work was supported by The Royal Veterinary and Agricultural University.
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Postal address: Bundesanstalt für Milchforschung, Kiel, Germany.