| Isolation and purification of lactic acid bacteria with antimicrobial activity by double agar plate diffusion method from mixed strains preserved at food fermentation Laboratory of Northwest A&F University,we get lactic acid bacteria S2. The fermentation products of strain S2 have good antibacterial activity to the common harmful bacteria and pathogenic bacteria in food. By means of morphological identification, physiological and biochemical identification, 16 S rDNA gene sequence identification, the lactic acid bacteria was identified as lactobacillus paracasei.After eliminating the interference of the lactic acid, acetic acid and hydrogen peroxide,we got conclusion that it is the bacteriocins produced by the lactic acid have antibacterial activity.By comparing the antibacterial effect with nisin, we got the antibacterial potency of Lb.paracasei-S2 bacteriocin crude extract was 3530IU/mL.The results showed bacteriocin of Lb.parcasei-S2 had obvious action in antibacterial upon Gram-negative bacteria and Gram-positive bacteria, and the antibacterial spectrum was wide. The micro-dilution broth method was used to determine the minimum inhibitory concentration(MIC) of the bacteriocin. To Escherichia coli, the MIC of bacteriocin were 110 IU/mLl;To Staphylococcus aureus of Gram-positive bacteria, the MIC of bacteriocin were 83IU/m L.Antibacterial substance of Lb. paracasei-S2 show antibacterial activity after 4h incubation and reach the maximum after 30 h. Lb. paracasei-S2 has strong ability to produce acid.The pH eventually stabilized at around 3.In this experiment, the effects of pH value, temperature, enzymes and ultraviolet on bacteriocin of Lb.parcasei-S2 antibacterial activity were nvestigated.The bacteriocin had a good endurance to temperature. When the Staphylococcus aureus and Escherichia coli were treated by bacteriocin at 121℃ for 20 min, the bacteriocin of Lb. paracasei-S2 keep good antibacterial effect;The results showed that antibacterial activity of the bacteriocin was between 2 and 6; while pH values was adjusted to above 6.0,antibacterial activity of the bacteriocin was greatly reduced. After treatmented by protease, the antibacterial activity disappeared while amylase and lipase had no effect. After UV irradiation treatment, the antibacterial effect did not significantly change.The results offered a theoretical base for the development and ultilization of bacteriocin as a kind of natural food preservative of antibacterial bacteriocin.As the experimental results sugges, the bacteriocin of Lactobacillus paracasei S2 has wider antimicrobial spectrum and certain stability in food processing.It is believed that the application of bacteriocin of Lactobacillus paracasei S2 as a kind of natural food preservative will be very broadly in the future. |
