Differentiation of Streptococcus thermophilus strains in commercial Direct Vat Set yoghurt starter

Based on inhibition of acid production, 100 individual colonies of Streptococcus thermophilus isolated from commercial Direct Vat Set yoghurt starter DVS-M were typed into 2 groups of sensitive and insensitive to phage φ001, which was isolated from the lysis culture of the turbidity test. The ratio between the sensitive and insensitive isolates in DVS-M was calculated at 3:2. Whole-cell protein profile (WCPP) and randomly amplified polymorphic DNA (RAPD) analysis showed that both WCPP and RAPD patterns between sensitive and insensitive isolates were distinct, whereas isolates with the same sensitivity or insensitivity shared common WCPP and RAPD patterns. It suggested that all the sensitive isolates belong to one S. thermophilus strain and all the insensitive isolates belong to another one. This study revealed that DVS-M yoghurt starter consists of 2 S. thermophilus strains with different phage sensitivity.