Abstract
Recently, we demonstrated that fermentation conditions have a strong impact on subsequent survival of Lactococcus lactis strain MG1363 during heat and oxidative stress, two important parameters during spray drying. Moreover, employment of a transcriptome-phenotype matching approach revealed groups of genes associated with robustness towards heat and/or oxidative stress. To investigate if other strains have similar or distinct transcriptome signatures for robustness, we applied an identical transcriptome-robustness phenotype matching approach on the L. lactis strains IL1403, KF147 and SK11, which have previously been demonstrated to display highly diverse robustness phenotypes. These strains were subjected to an identical fermentation regime as was performed earlier for strain MG1363 and consisted of twelve conditions, varying in the level of salt and/or oxygen, as well as fermentation temperature and pH. In the exponential phase of growth, cells were harvested for transcriptome analysis and assessment of heat and oxidative stress survival phenotypes. The variation in fermentation conditions resulted in differences in heat and oxidative stress survival of up to five 10-log units. Effects of the fermentation conditions on stress survival of the L. lactis strains were typically strain-dependent, although the fermentation conditions had mainly similar effects on the growth characteristics of the different strains. By association of the transcriptomes and robustness phenotypes highly strain-specific transcriptome signatures for robustness towards heat and oxidative stress were identified, indicating that multiple mechanisms exist to increase robustness and, as a consequence, robustness of each strain requires individual optimization. However, a relatively small overlap in the transcriptome responses of the strains was also identified and this generic transcriptome signature included genes previously associated with stress (ctsR and lplL) and novel genes, including nanE and genes encoding transport proteins. The transcript levels of these genes can function as indicators of robustness and could aid in selection of fermentation parameters, potentially resulting in more optimal robustness during spray drying.
Highlights
Owing to their spoilage-preventing, texture-improving and flavor-enhancing properties, lactic acid bacteria have a long history of application in food fermentations [1, 2]
To compare the effect of fermentation conditions on the growth characteristics, L. lactis strains IL1403, KF147 and SK11 were grown under the twelve different conditions that were previously applied to strain MG1363 [11]
In this study we demonstrated that fermentation conditions have a large impact on heat and oxidative stress survival of L. lactis strains
Summary
Owing to their spoilage-preventing, texture-improving and flavor-enhancing properties, lactic acid bacteria have a long history of application in food fermentations [1, 2]. One of the most widely used lactic acid bacteria in the food industry is Lactococcus lactis, notably for the production of cheese and butter(milk) [2]. These milk fermentation processes are typically initiated with the addition of starter cultures containing high concentrations of one or multiple L. lactis strains. During the production of these starter cultures prior to application in the food industry, L. lactis strains encounter severe stresses, for example heat and oxidative stress during spray drying [3,4,5]. Viability of starter cultures is essential for an adequate contribution to the fermentation endproduct, justifying the industrial interest to better understand and improve robustness [1]
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