English

Abstract

Streptococcus thermophilus is used primarily as starter cultures to counter the harmful bacteria grown in cheese and yogurt making/preservation processes. These bacteria produce some exogenous toxins called bacteriocins having the antimicrobial activities against both Gram positive and Gram negative bacteria. In our study S. thermophilus growth was obtained at pH 5.5 and temperature 40°C. Bacteriocin activities were checked after their treatment with different enzymes, organic solvents, sodium chloride (NaCl) and detergents as well as their heat stability and effect of pH was studied. Bacteriocin activity was found heat stable at 100°C for 30 min and was found stable in the 3-10 pH range but lost the activity after the treatment with proteinase-K and protease enzymes. Activity was lost in treatment with lipase and amylase which shows the presence of lipo-glycolated peptide. Bacteriocin activity was lost on the presence of Dithiothreitol (DTT) and b-mercaptoethanol which showed the presence of disulphide bond present in bacteriocin and essential for its activity. Urea and ethylene diamine tetraacetic acid (EDTA) also affected the bacteriocin activity but found stable to survive in the presence of 6% NaCl. Antibacterial assay showed the strong growth inhibition of test bacteria. Bactericidal activity was further purified to homogeneity by ammonium sulphate precipitation and different chromatographic techniques. Molecular weight was calculated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as 2.7 kDa.   Key words: Streptococcus thermophilus, bacteriocin, antimicrobial peptide, yogurt culture bacteriocin, disc diffusion test.