Study On Improving The Resistance Of Lactococcus Lactis And The Production Of Bacteriocin-like Substance

Lactic acid bacteria?LAB?,studied in many researches,has probiotic effect as the food safety microbiology.But in the actual process of industrial production,it will often face non-suitable living environment,and these adverse environments limite the application of LAB as probiotics.Lactococcus lactis is the model strain of LAB,and the research results of resistance study of non-suitable environment will help to improve the anti-stress propertiy of other LAB strains.The paper studied the physiological mechanism of increased environmental resistance of L.lactis NZ9000,a derived strain of L.lactis MG1363,by treatment of TG enzyme.Secondly,the effect of adding TG in the culture medium of L.lactis NFL,and the nutritional components and stage fermentation strategy on the bacteriocin production of L.lactis NFL were also studied.The main results were as follows:1.L.lactis NZ9000 strain can only tolerate 0.04%bile salt stress?the lowest survival rate was 0.003%?.It was worth noting that after treatment by TG,the survival rate of strain NZ9000 was increased by 60.3 times.Previous TEM observation showed that the cell wall of strain NZ9000,treated by 9 U/mL TG,was 1.92 times thicker than the strain without TG treatment.We supposed that it may be the result of the increased cross-linking level between short peptide substrances in cell walls.2.Additionally,strain NZ9000 treated with TG exhibited significantly increased resistances on multiple environmental stresses.For example,relative to the control?non-treatment by TG?,the survival rate of the experimental group?treatment by TG?treated with gastric acid stress for 2 h increased by nearly 36.0 times,treated with 500?g/mL erythromycin for 2 h increased by 5.04 times,treated with 800?g/mL cefazolin increased by 5.33 times.After 15 days of low temperature treatment at-20?,the survival rate of experimental group increased by nearly 17.2 times.And the addition of TG enzyme significantly increased the environmental stress resistance of Lactobacillus plantarum 415,but it had no significant effect on L.lactis NFL.3.In order to obtain higher bacteriocin production,the culture medium and fermentation methods were optimized,the results were shows that the best growth medium was still the original MRS medium.In the second stage of fermentation process,the most suitable carbon source was lactose,the organic nitrogen source was beef paste,and the inorganic salt was CuSO₄-5H₂O.The suitable condition of the first stage of fermentation was eration,addition of hemin and constant pH 6.5.However,in the second stage of fermentation,non-ventilation and natural pH were more favorable for the production of BLS.Finally,with the optimization of bacteriocin producing medium and culture conditions,the biomass of strain NFL was increased by 12.9%and the yield of BLS was increased by nearly 2.04 times.