Growth in milk: a challenging task for strains of the species Lactobacillus delbrueckii.

The ability of lactic acid bacteria (LAB) to grow in milk is a consequence of the presence and expression of genes responsible for the degradation of milk sugars and casein. The genes coding for the utilization of lactose and galactose (the main sugars of milk) are organized into the lac and gal operons, respectively. The genetic equipment involved in the proteolytic activity of LAB in milk is complex and still not well characterized. The aim of this research was to investigate some genetic determinants implicated in the catabolism of lactose, galactose and casein in the four known subspecies of L. delbrueckii, i.e. the subspecies delbrueckii, bulgaricus, lactis, and indicus. In particular some aspects were examined: the expression of the lacZ gene, encoding the β-galactosidase enzyme; the sequence of lacR gene and the presence of galT gene, encoding the galactosyl-1-phosphate uridyl transferase. In addition, the presence of the gene prtB, encoding a cell-wall anchored protease, was evaluated and associated with the ability to grow in milk. Differences in the distribution and/or regulation of these genes among the four subspecies of L. delbrueckii, were underlined and related to their adaptation to specific ecological niches.