N. NINOVA-NIKOLOVA and Z. URSHEV
LB BULGARICUM PLC – R & D Center, BG – 1202 Sofia, Bulgaria
Ninova-Nikolova, N. and Z. Urshev, 2013. Fast acidifying urease-deficient Streptococcus thermophilus isolate shows spontaneous deletion of its complete urease operon. Bulg. J. Agric. Sci., Supplement 2, 19: 112–116
Classical starters for fermented milk contain Streptococcus thermophilus strains, which are capable of degrading urea present in milk. The ammonium released from urea slows down the decrease of pH during fermentation. This problem may be overcome with the selection of urease-deficient (UD) variants of S. thermophilus. Spontaneous UD-mutants Ft3uD3 and Yt3uD3-1 were previously isolated from the population of S. thermophilus Ft3 and Yt3, respectively, which originate from industrial yoghurt starters. In this study PCR amplification of the genes present in the urease operon and comparison of the macrorestriction profiles with PFGE of the UD-derivatives and the parent strains was performed to explain the loss of urease activity. No amplification of the genes from the urease cluster was observed for the Yt3uD3-1 UD-derivative of Yt3, indicating that the whole urease operon was absent in this mutant strain. The comparison of the SmaI macroresriction profiles of Yt3uD3-1 and its parent strain indicated a 19 kb deletion in the UD-culture. Therefore we concluded that in the UD-mutant Yt3uD3-1 the whole urease operon was deleted. On the contrary, all genes from the urease cluster amplified in Ft3 and its UD-mutant Ft3uD3 with identical size of each amplicon. Also, no differences in the macrorestriction profile of Ft3 and its mutant Ft3uD3 were observed. Consequently the loss of urease activity in Ft3uD3 is most likely result of point mutation within the urease operon. As the stability of the UD-derivatives during serial passage is a key issue in the application of the obtained mutants, we assumed that deletion mutant Yt3uD3-1 is a better candidate as it cannot revert to its active urease phenotype. Nevertheless, both S. thermophilus UD-derivatives Ft3uD3 and Yt3uD3-1 were stable for at least 150 passages. These results confirm the reliability of S. thermophilus UD-strains for industrial application.