The use of cadmium resistance on the phage-resistance plasmid pNP40 facilitates selection for its horizontal transfer to industrial dairy starter lactococci

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Abstract
Aims: To facilitate the horizontal transfer and selection of phage‐resistance plasmids in industrial lactococci. Methods and Results: Cadmium‐resistance properties similar to those previously identified in Lactococcus were linked to the well‐known phage‐resistance plasmid pNP40. This finding was exploited to facilitate delivery of the plasmid to an industrial cheese starter Lactococcus lactis DPC4268. Additionally, 25 different cadmium‐sensitive cheese starter lactococci were also identified as potential recipients for the phage‐resistance plasmid pNP40, and also the plasmids pAH90/pAH82 which also encode cadmium resistance. All three plasmids were successfully conjugated to strain DPC4268. Cheddar cheese was manufactured in industry with the pNP40 phage‐resistant transconjugant. Significance and Impact of the Study: Food‐grade enhancement of phage resistance in industrial starter strains has been made simpler by the use of this selection, especially since the majority of potential recipient starter strains analysed were cadmium sensitive.