Selection and characterization of probiotic culture of Streptococcus thermophilus from dahi.

摘要

For the isolation of probiotic cultures of Streptococcus thermophilus from dahi, 120 samples were collected from the southern regions of Punjab, Pakistan. 11 isolates were obtained, and 6 were scrutinized for antibacterial activities against foodborne pathogens. The carbohydrate fermentation profile of these 6 strains was determined by the API50 CHL system. Additionally, these strains were amplified for their 16S rRNA regions to confirm their genotypic relationship. Furthermore, phenotypic characteristics among these strains were established by S-layer protein analysis of their cell walls by SDS-PAGE and by plasmid profiling. The outer cell wall layers of these strains were found to have 6-14 different sizes of protein bands of 27, 34, 37, 40, 45 and 60 kDa. Similarly, except for S02FT, all strains had a single prominent plasmid of 23 kbp, whereas S02FT had an additional plasmid of 9 kbp. On the basis of this unique feature and a wide spectrum antibacterial spectrum against pathogenic bacteria, S. thermophilus S02FT was further characterized. This culture showed an optimum antibacterial activity of 800 AU/ml at pH 5.0-5.5 and a temp. of 30-37 degrees C. It grew well in in vitro acidic conditions and tolerated up to 2% bile salts. It was resistant to nalidixic acid, ciprofloxacin, gentamicin and sulfamethoxazol, but showed intermediate behaviour in the presence of vancomycin and erythromycin.