Abstract
BackgroundLactococcus lactis is the main component of the mesophilic starters used in cheese manufacture. The success of milk fermentation relies on the viability and metabolic activity of the starter bacteria. Therefore, robust strains able to withstand the harsh conditions encountered during cheese manufacture and starter production are demanded. In this work, we have applied adaptive evolution under cell envelope stress imposed by the cell wall active bacteriocin Lcn972 to evolve strains with more robust phenotypes.ResultsConsecutive exposure of the starter strain L. lactis IPLA947 to Lcn972 yielded a stable mutant, L. lactis R5, with enhanced survival when challenged with hydrogen peroxide. L. lactis R5 exhibited faster growth rates in aerobic fermentations in broth and was able to acidify milk to a lower pH in aerated milk cultures. The improved behavior of L. lactis R5 in the presence of oxygen did not translate into a better performance in the presence of heme (i.e. respiration metabolism) or into higher survival during storage at cold temperatures or after freeze-drying compared to the wild type L. lactis IPLA947. L. lactis R5 retained the same milk acidification rate and no changes in the consumption of lactose and production of organic acids were noticed. However, the profile of volatile compounds revealed a significant increase in 3-hydroxy-2-butanone (acetoin) in curds manufactured with L. lactis R5.ConclusionsBased on our results, L. lactis R5 can be proposed as a suitable dairy starter with improved survival under oxidative stress and enhanced metabolic traits. The results support the notion that adaptive evolution under cell envelope stress might be useful to generate strain diversity within industrial L. lactis strains.
Highlights
Lactococcus lactis is the main component of the mesophilic starters used in cheese manufacture
We have explored exposure of a L. lactis dairy starter strain to the cell wall active bacteriocin Lcn972 as a means of obtaining evolved strains with robust phenotypes
Adaptive evolution under cell envelope stress (CES) was conducted by a two-step process consisting of an adaptation step, which involved exposing the cultures to increasing amounts of Lcn972, and a subsequent stabilization step, in which bacterial cultures were grown for 100 generations in the absence of Lcn972
Summary
Lactococcus lactis is the main component of the mesophilic starters used in cheese manufacture. The success of milk fermentation relies on the viability and metabolic activity of the starter bacteria. Robust strains able to withstand the harsh conditions encountered during cheese manufacture and starter production are demanded. Lactic acid bacteria (LAB) are the main components of dairy starters and are used to produce a wide variety of fermented products such as cheese, butter, fermented milk, yogurt, etc., which are highly appreciated by consumers. Among LAB, Lactococcus lactis is the main component of the mesophilic starters used in cheese manufacture. Considering that the success of milk fermentation relies on the viability and metabolic activity of L. lactis, continuous efforts have been made to understand the physiology behind robust phenotypes of industrially relevant microorganisms (reviewed by [27]). Legal constraints and the negative opinion of consumers towards genetically modified organisms (GMOs) pose a barrier for further marketing and create demand for alternative strategies [12, 20]
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