基于蔗糖代谢途径分析保加利亚乳杆菌的后酸化性能

弱后酸化能力是乳酸菌作为发酵剂使用时的重要特性。为研究保加利亚乳杆菌蔗糖代谢途径对发酵乳后酸化起到的作用，该研究比较了五株保加利亚乳杆菌的后酸化性能，分析了保加利亚乳杆菌的生长情况、糖代谢和产酸能力，探究了蔗糖代谢产酸相关基因的差异表达及关键酶活性，并研究了外源添加蔗糖代谢关键酶前后保加利亚乳杆菌的生长情况。结果表明，Lb. 1后酸化能力最弱，在以蔗糖为碳源的培养基中生长缓慢，KLDS 1.0207后酸化能力最强。发酵24 h后，二者蔗糖转化率分别为5.85%和85.39%，乳酸含量分别为1.13 g/L和11.81 g/L。在含双糖（乳糖:蔗糖=1:1.5）的MRS培养基中生长时，KLDS 1.0207蔗糖代谢途径中基因sacA、pgi、gap、pgk、ldh表达量极显著高于Lb. 1（P<0.01），KLDS 1.0207蔗糖酶活性显著高于Lb. 1（P<0.05），达到1.31 U/mg。在Lb. 1的蔗糖培养基中补充蔗糖酶后，OD600 nm增至原来的5倍。因此，蔗糖酶活性对保加利亚乳杆菌代谢蔗糖至关重要，编码蔗糖酶的sacA基因表达下调显著减弱蔗糖酶活性，菌株后酸化能力明显下降。

Analysis of Post-acidification Property of Lactobacillus bulgaricus Based on Sucrose Metabolism Pathway

Weak post-acidification ability is an important characteristic of lactic acid bacteria when being used as a starter culture. In order to study the role of Lactobacillus bulgaricus sucrose metabolic pathway on post-acidification of fermented milk, in this study, the post acidification performances of five Lactobacillus bulgaricus strains was compared, the growth status, sugar metabolism and acid production capacity of Lactobacillus bulgaricus were analyzed, the differential expression of genes related to sucrose metabolism and acid production and the activities of key enzymes were investigated, and the growth of Lactobacillus bulgaricus before and after the exogenous addition of key enzymes for sucrose metabolism, were studied. The results showed that Lb. 1 had the weakest post-acidification ability and grew slowly in the medium with sucrose as the carbon source. KLDS 1.0207 had the strongest post-acidification ability. After fermentation for 24 h, the sucrose conversion rates were 5.85% and 85.39%, respectively, and the lactic acid contents were 1.13 g/L and 11.81 g/L, respectively, for these two strains. The expression levels of sacA, pgi, gap, pgk and ldh in the sucrose metabolism pathway of KLDS 1.0207 were extremely significantly higher than those for Lb.1 when MRS containing disaccharide (lactose:sucrose=1:1.5) was used as the growth medium (P<0.01), with the activity of sucrose enzyme of KLDS 1.0207 being significantly higher (reaching 1.31 U/mg) than that of Lb. 1 (P<0.05). The OD600 nm increased by 5 times after sucrase was supplemented into the sucrose medium of Lb. 1. Therefore, the activity of sucrase is crucial for the sucrose metabolism in Lactobacillus bulgaricus. The downregulation of the expression of sacA gene encoding sucrase significantly weakened the activity of sucrase, and the post-acidification ability of the strain decreased significantly.