DESIGN OF A PHAGE-INSENSITIVE LACTOCOCCAL DAIRY STARTER VIA SEQUENTIAL TRANSFER OF NATURALLY-OCCURRING CONJUGATIVE PLASMIDS

The plasmid-free Lactococcus lactis subsp. cremoris MG1614 is highly p hage sensitive and lacks lactose fermenting ability (Lac) and primary casein degrading ability (Prt). Food grade gene transfer systems were used to sequentially superimpose different phage defense systems on th is background, resulting in a gradual increase in resistance to bacter iophage in the derivatives. pLP712, encoding Lac and Prt, was then tra nsferred to one of these hosts, into which plasmids encoding adsorptio n inhibition, restriction modification, and abortive infection had alr eady been introduced. This resulted in a phage-resistant strain which was successfully used as a single-strain starter for cheddar cheese ma nufacture under industrial conditions.