Method for screening Lactobacillus casei fermentation agent

A method for screening a Lactobacillus casei fermentation agent, comprising the following steps:

• (1) inoculating Lactobacillus casei into a liquid culture medium and culturing to obtain an inoculated Lactobacillus casei liquid culture medium;
• (2) extracting a total DNA from the Lactobacillus casei, amplifying a tuf gene and a lac cluster gene, using the tuf gene as a control, and using an equation Nrelatives=(1+E)−ΔCT to determine a plasmid copy number of the lac cluster gene, wherein the tuf gene is an elongation factor Tu of a single copy gene on a L. casei genome;
• Nrelatives is a plasmid copy number;
• E is an amplification efficiency, and E=10−1/K, K is a standard curve slope;
• −ΔCT is a difference value between a number of a first cycle and a number of a second cycle, wherein a fluorescent signal of an amplified product of the lac cluster gene reaches a set threshold through the first cycle, a fluorescent signal of an amplified product of the tuf gene reaches the set threshold through the second cycle;
• the tuf gene is a gene of an elongation factor Tu of a single copy gene on a Lactobacillus casei genome, and a GenBank number of the tuf gene is AJ418937.2;
• and
• the lac cluster gene is a lacG gene;
• wherein
• the step of using the equation Nrelatives=(1+E)−ΔCT to determine a plasmid copy number of the lacG gene in the step (2) comprises:
• using tuf2-F/R:
• tuf2-F:
• SEQ ID NO:
• 1,
• tuf2-R:
• SEQ ID NO:
• 2;
• and lacG-F/R:
• lacG-F:
• SEQ IS NO:
• SEQ ID NO:
• 3,
• lacG-R:
• SEQ ID NO:
• 4 as primers, detecting a CT value through a quantitative polymerase chain reaction (qPCR), and calculating expression levels of the tuf gene and lacG gene;
• and calculating the plasmid copy number through the ΔCT;
• (3) preparing a crude enzyme liquid of β-fructosidase, and determining an enzymatic activity of the β-fructosidase of the Lactobacillus casei;
• and
• (4) performing a screening on the Lactobacillus casei fermentation agent according to the plasmid copy number of the lacG gene and the enzymatic activity of the β-fructosidase.