Aim: To compare pH and conductivity used in the determination of growth in reconstituted skim milk (RSM), to determine whether the presence of one or two plasmids in Lactococcus lactis had any influence on growth, and whether AbiS improved bacteriophages resistance of L. lactis.

Methods and Results: Conductivity and pH were used to determine growth in RSM. A small increase in the generation time was found with increasing number of plasmids, while their size was unimportant. The introduction of a plasmid-encoding AbiS did only enhance the level of phage resistance significant when other plasmids encoding either AbiS1 or the restriction modification system LlaBIII was present.

Conclusions: The earliest detection of growth was observed by measuring pH, rather than conductance. The plasmid-encoded AbiS system has a potential to be used as a phage resistance mechanisms in L. lactis during milk fermentations, especially when combined with other anti-phage mechanisms.

Significance and Impact of the Study: This study widened the knowledge about the influence of plasmid intruduction on the growth rate of L. lactis, which is important for the construction of new strains. The level of protection against 936 groups of phages was only significant when the mechanism was present togehter with the RM system LlaBIII.