Abstract

This study was aimed at assessing the different selective media and incubation conditions for achieving an appropriate method in the isolation and enumeration of Lactobacillusplantarum PTCC 1058, Lactobacillus delbrueckii ssp. bulgaricus and Streptococcusthermophilus in probiotic yogurt. The yogurt samples containing 1% of L. plantarum PTCC 1058 were prepared using YoFlex Express cultures (containing L. delbrueckii ssp.bulgaricus and S. thermophilus) and stored at 4°C. After 24 h, they were cultivated in seven different agar media including: de Man, Rogosa and Sharpe (MRS) agar, MRS-Bile agar, acidified MRS agar (pH = 5.2), yogurt-lactic-agar (YL), Elliker’s lactic-agar (EL), M17 medium and S. thermophilus (ST) agar as pour plate and surface plate culture, and were incubated in aerobic and anaerobic conditions at three different incubation temperatures: 15, 37 and 45°C, for 48 to 72 h. For more practical examination, L. plantarum PTCC 1058and yogurt bacteria were enumerated every week over 6 weeks. Also, the ability of L. plantarum PTCC 1058 and yogurt bacteria to grow in concentration of 0.5 to 5% of bile salts were investigated. The results indicated that both L. plantarum PTCC 1058 and L. bulgaricus could grow in low concentration (≤ 2%) of bile salts. So, MRS-Bile-agar medium was not suitable for differentiation of L. plantarum PTCC 1058 and yogurt bacteria. The best condition for differentiation of L. plantarum PTCC 1058 from yogurt bacteria was cultivation on MRS medium and incubation at 15°C for 72 h. YL and EL media and aerobic incubation at 37°C for 48 h were suitable for enumeration of L. bulgaricus, and surface plate culture in YL, M17 and ST media and aerobic incubation at 45°C for 48 h was useful for enumeration of S. thermophilus in yogurt.

Key words: Lactobacillus plantarum PTCC 1058, probiotic yogurt, differential culture, yogurt starter bacteria.