Studies On Biological Properties And Protoplasts Formation In Streptococcus Thermophilus

Streptococcus thermophilus is a familiar lactobacillus in latic acid fermentation . To further understand the biological properties of streptococcus thermophilus in yoghurt and develop it, Streptococcus Thermophilus was isolated and identified from yoghurt. Effects of gastrointestinal factors on the survival of Streptococcus Thermophilus were studied. Protoplasts formation and regeneration in Streptococcus Thermophilus were also studied, and the studied of the technique could provide a corre-sponding methodology to genetic engineer of Streptococcus Thermophilus. This thesis includes three sections.In the first section, the isolation and identification of Streptococcus Thermophilus were studied. In this study, Streptococcus Thermophilus was isolated by the use of ERIE agar culture medium. The suspicious colonies were treated with Gram staining and Gram-positive spherical bacteria were indentified. In comparision with standard specie, the same morphological and strainning character and biochemical reaction were revealed. Results indicated that Streptococcus Thermophilus from IRIE agar culture medium was spherical, which could grow at 45℃and could not grow at 10℃, 6.5%Nacl, pH6.5.In the second section, Effects of gastrointestinal factors on the survival of Streptococcus Thermophilus, and the heat tolerance of it were studied. The survival rate of Streptococcus Thermophilus is evaluated. Results showed that Streptococcus Thermophilus was in logarithmic growing time after in was incubated for 4h. Streptococcus Thermophilus had the abilities of growth reproduction. Thermophilus Streptococcus could not survive while incubating at pH2 for 0.5h. The survival rate of Streptococcus Thermophilus was 27.00% while incubating at pH3 for 0.5h. This study showed that Streptococcus Thermophilus had a low acid to tolerance, but the heat tolerance of it was high. Heating at 50℃for 15min showed no effect on survival of Streptococcus Thermophilus. The survival rate of Streptococcus Thermophilus was 1.29% while heating at 80℃for 15min.In the three section, Protoplasts formation and regeneration in Streptococcus Thermophilus were studied. The production of Streptococcus Thermophilus of protoplasts was perfomed lytic enzymes containing 1 u g/ml 5 u g/ml, 10 u g/ml Mutanolysin respectively. Effects of Mutanolysin concentration and enzyme digestion time on the production rate of protoplast were studied. We also investigated regeneration of protoplasts of Streptococcus Thermophilus in different culture environments and effects of culture time on protoplasts regeneration. Results in the study indicated that protoplasts formation in Streptococcus Thermophilus have been successfully performed by using 10 u g/ml Mutanolysin for 10min. Moreover, the rate of regeneration of protoplasts was hign when it was cultured in O₂ by double-layer regeneration medium. Long culture time was harm to protoplasts regeneration. We could culture protoplasts for 1-2d at 0℃-4℃.