Construction of a Bacteriophage-Resistant Derivative of Lactococcus lactis subsp. lactis 425A by Using the Conjugal Plasmid pNP40

Lactococcus lactis subsp. lactis 425A is an atypical strain which excretes a high concentration of α-acetolactate when grown in milk. The conjugative lactococcal plasmid pNP40, which encodes phage and nisin resistance, was introduced to strain 425A by conjugation, using resistance to phage and nisin as a selection. No phage-nisin resistance mutants were encountered. Transconjugants display complete resistance at both 21 and 39°C to those phage previously identified as lytic for 425A. Transconjugants lose their resistance characteristics when spontaneously cured of pNP40. The commercially important property of 425A—production of high levels of α-acetolactic acid—is unaffected by the presence of pNP40.