A Strategy for Rotation of Different Bacteriophage Defenses in a Lactococcal Single-Strain Starter Culture System †

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A new strategy for starter culture rotations was developed for a series of phage-resistant clones genetically derived from a single strain of Lactococcus lactis subsp. lactis. Phage-resistant derivatives carrying different defense systems were constructed via conjugation with various plasmids encoding abortive infection (Abi/Hsp) and/or restriction and modification (R/M) systems of different specificity. The plasmids included pTR2030 (Hsp+ R+/M+), pTN20 (Abi+ R+/M+), pTRK11 (R+/M+), and pTRK68 (R+/M+). Selected phage-resistant transconjugants or transformants were evaluated in different rotation sequences through cycles of the Heap-Lawrence starter culture activity test in milk contaminated with phage and whey from the previous cycle. When used in consecutive sequence, derivative strains carrying the R/M systems encoded by pTN20, pTRK11, and pTRK68 retarded phage development when the initial levels of phage contamination were below 102 PFU/ml but not when levels were increased to 103 PFU/ml. Use of a derivative bearing pTR2030 (Hsp+ R+/M+) at the beginning of the rotation prevented phage development, even when the initial levels of phage contamination were high (106 PFU/ml). Alternating the type and specificity of R/M and Abi defenses through the rotation prevented phage proliferation and in some cases eliminated contaminating phages. A model rotation sequence for the phage defense rotation strategy was developed and performed successfully over nine cycles of the Heap-Lawrence starter culture activity test in the presence of high-titer commercial phage composites. This phage defense rotation strategy is designed to protect a highly specialized Lactococcus strain from phage attack during continuous and extended use in the dairy industry.

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