Efficacy of four conjugal lactococcal phage resistance plasmids against phage in commercial Lactococcus lactis subsp cremoris cheese starter strains

The efficacy of four lactococcal phage resistance plasmids (pNP40, pMU1311, pD160 and pKP100) against phage was assessed after their conjugal transfer to four commercial Lactococcus lactis subsp. cremoris cheese starter strai ns and to the plasmid-free strain L. Inc ris subsp. cremoris MG1363. In MG1 363, only pNP40 conferred resistance to prolate phages c2 and 643. Highest levels of resistance to small isometric phages in MG1363 occurred when pNP4 0 was stacked together with pMU1311 or pD160. In the four starter strains, the plasmids conferred varying levels of resistance to small isometric phag es. Growth and acidification rates in milk of most transconjugants derived from the starter strains decreased, but this was not always due to loss of plasmid-encoded cell wall proteinase (lactocepin) activity. Only one transc onjugant grew during repeated subculture in milk with addition of factory w heys containing phages. This and the presence of bacteriocins encoded on pM U1311 and pD160 limited application of the plasmids to protect L. lactis su bsp. cremoris starters against phages in industry. However, some of the pla smids could be useful in extending the industry life of starters where fast acid production is not required or where bacteriocin production is accepta ble. (C) 2001 Elsevier Science Ltd. All rights reserved.