An ancient, species-specific tagatose-6-phosphate pathway in Lactobacillus casei group for galactose reduction in cultured dairy foods

Abstract

Residual lactose and accumulated galactose in cultured dairy foods lead to several medical and industrial concerns such as lactose intolerance, galactosemia, and browning of pizza during baking. Our previous comparative KEGG study on galactose metabolism pathways in sequenced LAB strains has uncovered the presence of tagatose-6-phosphate (T6P) pathway in all of the completely sequenced Lactobacillus casei group strains including Lb. casei, Lb. paracasei and Lb. rhamnosus. In this study, we have demonstrated that T6P pathway, but not Leloir pathway, in Lb. casei group is more efficient for lactose and galactose catabolism than Leloir pathway in selected strains of Lb. acidophilus group, Streptococcus thermophilus and Lb. bulgaricus cultured in galactose- or lactose-based MRS media. However, the activity of β-galactosidase, which is a key enzyme in Leloir pathway in Lb. casei group strains, was not detected. In the milk, Lb. casei group strains catabolize less lactose than Str. thermophilus, Lb. bulgaricus and Lb. acidophilus group, but very limited galactose was accumulated in milk. Moreover, co-cultivation of Lb. casei group with Str. thermophilus or Lb. bulgaricus generated less galactose or lactose in the cultured milk. In addition, comparison of lac-gal gene cluster in sequenced Lb. casei group strains has shown the presence of an unknown PTS (PTSUnk) in this region. The EIIC of PTSUnk has conserved protein domain found in EIIC of PTSGat (galactose-specific PTS) and its gene expression was also highly up-regulated (> 200 fold changes) in the presence of galactose suggesting its possible role for galactose phosphorylation. In addition, it was found that lac-gal clusters in the genomes of Lb. casei group strains are not associated with any HGT events suggesting that this cluster may be an ancient pathway. This study demonstrates the use of Lb. casei group strains as functional dairy starter for lactose or galactose depletion in milk. Further characterization of PTSUnk in Lb. casei group using genetic manipulation has been carried out.