Comparison of the Expression Levels of Exopolysaccharide Genes in Streptococcus thermophilus LBB.T54 Grown in Semi-Defined Media and Milk

Exopolysaccharide (EPS) production by Streptococcus thermophilus strains is exploited by dairy companies to obtain fermented milk with improved texture. In previous studies of S. thermophilus LBB.T54, however, EPS yields in M17 were negligible compared to milk. It remained unclear whether differences in the composition of the medium simply stimulated the growth and viable cell counts or also changed the expression level of the eps cluster. The aim of the study was to test the potential change of the expression levels of selected genes from the eps cluster in S. thermophilus LBB.T54 when grown in M17, compared to milk. Strain LBB.T54 was cultured in LM17 (M17 with lactose) or milk at 37oC. qPCR was performed with primer pairs targeting the epsD, epsE and rpoA genes. The rpoA gene was used to normalize the results. qPCR measurements clearly indicated a 2-4 fold higher expression levels of both epsD and epsE in strain S. thermophilus LBB.T54 when grown in milk compared to LM17. Growth in other semi-defined media (M17 with glucose, Elliker broth and modified Trypticase soy broth with glucose) resulted in similar expression levels of epsD and epsE as that in LM17. EPS yields in milk reached 33 mg/l while in LM17 they did not exceed 5 mg/l. In the process of starter culture preparation and from technological point of view, the application of LM17 medium or another semi-defined medium seems to be counter-productive compared to milk-based media when EPS yields are essential for products obtained with LBB.T54.