br Short Communication Lactococcus lactis is traditionally u

Short Communication Lactococcus lactis is traditionally used as a starter culture in the production of fermented dairy food products, such as cheese, sour cream, and buttermilk (Cavanagh et al., 2015). Two main pathways of lactose uptake and metabolism have been described for L. lactis. In the first pathway, during uptake, lactose is phosphorylated via the lactose phosphotransferase system to lactose-6-phpsphate, which is then hydrolyzed into galatose-6-phosphate and glucose by phospho-β-galactosidase. Galatose-6-phosphate is then fermented to lactic cgrp antagonist via the tagatose-6-phosphate (T6P) pathway. In the second pathway, lactose is internalized into the cell via a specific permease on the cell membrane and is then cleaved to galactose and glucose by β-galactosidase; galactose then enters the Leloir pathway (Solopova et al., 2012). Studies have suggested that the Leloir pathway of lactose metabolism normally occurs in the genome of lactic acid bacteria (LAB) whereas the T6P pathway is species specific in L. lactis (Wu et al., 2015). This finding indicates that the lactose metabolism of L. lactis strains may differ within species. β-Galactosidase and 6-phospho-β-galactosidase are the key enzymes of lactose metabolism in LAB (Lansky et al., 2014; Wu et al., 2015), and lactose metabolism has been linked to the formation of flavor compounds, such as diacetyl, acetoin, acetaldehyde, and acetic acid, via a pyruvate intermediate (Smit et al., 2005). However, few studies have investigated the variations in the activities of these enzymes and the use of lactose across L. lactis strains and their correlation with fermentation properties. In this study, we used the 16S rDNA approach to identify 16 strains of L. lactis isolated from various sources, including human feces, fermented vegetable, and dairy products (Supplemental Table S1; https://doi.org/10.3168/jds.2018-15973), such as fermented yak milk and dairy fan—a unique traditional cheese-like dairy product produced in the Yunnan province (Liu et al., 2009). To test the lactose usability of the strains, de Man, Rogosa, and Sharpe (MRS)-lactose broth with lactose as the sole carbon source was prepared by replacing glucose in the medium with lactose (Wu and Shah, 2017). The strains were cultivated in the MRS-lactose broth at 30°C under aerobic conditions, and their growth and acid production characteristics were compared including a type strain ATCC19435. Fermentation characteristics of 8 selected strains (ATCC19435, MA14, 15M2, 5G2, 6G5, DQHXNQ38–12, ZN8, and D-XJ4–12) that showed the highest growth and acid production were further studied in reconstituted skim milk. The cell-free supernatants of these 8 L. lactis strains after12 h of incubation in skim milk were analyzed for sugar metabolism, and the β-galactosidase and 6-phospho-β-galactosidase activities were measured as previously described (Wu and Shah, 2017). Gas chromatography-mass spectrometry analysis of the compounds produced in fermented milk was performed and principal component analysis was used to characterize the strains (Settachaimongkon et al., 2014; Zhao et al., 2016). Significant differences (P < 0.05) between the strains were determined by 1-way ANOVA using IBM SPSS Statistics version 21.0 (IBM Corp., Armonk, NY). The 15 L. lactis strains obtained from various habitats exhibited significant differences in growth and acid production characteristics in the MRS-lactose broth (Figure 1A and 1B) but showed similar growth rates and reached similar cell densities when grown in normal MRS broth (data not shown). Compared with the other strains, the strains MA14, 15M2, DQHXNQ38–12, 5G2, and 6G5 exhibited significantly higher growth rates with a rapid decrease in the pH value on culturing in skim milk for 2 to 8 h and reached the stationary phase after 8 h. All of these strains were isolated from dairy niches, such as fermented yak milk, sour cream, and Qula, a traditional cheese made from yak milk in Qinghai province (Duan et al., 2008). Our data support the theory that niche-specific adaptations that occur in environmental isolates play an important role in the transition of bacterial strains, which previously belonged to another niche, to the dairy niche; for example, the ancestor of L. lactis originally inhabited the plant niche (Siezen et al., 2011; Bachmann et al., 2012). The strains H30G-6, HeNa-21–8-GMM, HeNa-28–3-GMM, 15M9, DYNDL61M06, DQHXNQ39M56, DSCAB14M3, and ZN8 exhibited a slow but continuous increase in cell densities and a decrease in pH during incubation. The strains D-XJ4–12 and DYNDL19–5 exhibited a weaker growth and acid-producing ability than strains 5G2 and 15M2.