Diversity in Robustness of Lactococcus lactis Strains during Heat Stress, Oxidative Stress, and Spray Drying Stress

Abstract

f In this study we tested 39 Lactococcus lactis strains isolated from diverse habitats for their robustness under heat and oxidative stress, demonstrating high diversity in survival (up to 4 log units). Strains with an L. lactis subsp. lactis phenotype generally displayed more-robust phenotypes than strains with an L. lactis subsp. cremoris phenotype, whereas the habitat from which the strains had been isolated did not appear to influence stress survival. Comparison of the stress survival phenotypes with already available comparative genomic data sets revealed that the absence or presence of specific genes, including genes encoding a GntR family transcriptional regulator, a manganese ABC transporter permease, a cellobiose phosphotransferase system (PTS) component, the FtsY protein, and hypothetical proteins, was associated with heat or oxidative stress survival. Finally, 14 selected strains also displayed diversity in survival after spray drying, ranging from 20% survival for the most robust strains, which appears acceptable for industrial application, to 0.1% survival for the least-tolerant strains. The high and low levels of survival upon spray drying correlated clearly with the combined robustness under heat and oxidative stress. These results demonstrate the relevance of screening culture collections for robustness under heat and oxidative stress on top of the typical screening for acidifying and flavor-forming properties. Based on their spoilage-preventing and flavor-enhancing characteristics, lactic acid bacteria (LAB) have been employed since ancient times in the fermentation of foods, e.g., fruits, vegetables, cereal grains, meat, and milk (1, 2). Nowadays, many of these processes have been industrialized, and fermentation is typically initiated by the addition of starter cultures, which contain high concentrations of one or multiple LAB strains (1, 2). As starter cultures require metabolic activity to contribute to the taste and texture of the fermentation end products, there has been an increasing industrial interest in studying robustness phenotypes during industrial production and processing (1), which involves preservation by either freezing or drying techniques (3-5). The major disadvantages of frozen starter cultures are the inconvenience and costs of transport and storage at low temperature, and, therefore, drying techniques are preferred (3-5). During spray drying, cultures are exposed to severe heat and oxidative stress (6, 7), typically resulting in lower survival rates of starter cultures than freeze-drying (3, 4). Therefore, freeze-drying is currently the most often applied industrial drying method (3-5). However, spray drying appears a more cost-effective and energy-efficient drying alternative for the preservation of starter cultures (3-5), providing strains that display high robustness under the stresses encountered in this process can be identified. This appears feasible, as studies on stress phenotypes typically result in highly strainspecific robustness phenotypes, e.g., for the gastro...

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