<i>Lactobacillus casei</i> Low-Temperature, Dairy-Associated Proteome Promotes Persistence in the Mammalian Digestive Tract

We found that incubation of probiotic <i>Lactobacillus casei</i> BL23 in milk at 4 °C prior to ingestion increased its survival in the mammalian digestive tract. To investigate the specific molecular adaptations of <i>L. casei</i> to milk, we used tandem mass spectrometry to compare proteins produced by <i>L. casei</i> BL23 at 4 °C in milk to those in exponential and stationary phase cells in laboratory culture medium at either 37 or 4 °C. These comparisons revealed a core of expressed <i>L. casei</i> proteins as well as proteins produced in either a growth-phase or temperature-specific manner. In total, 205 <i>L. casei</i> proteins were uniquely expressed or detected in higher abundance specifically as a result of incubation in milk and included an over-representation of proteins for cell surface modification, fatty acid metabolism, amino acid transport and metabolism, and inorganic ion transport. Genes for DltD (d-alanine transfer protein), FabH (3-oxoacyl-ACP synthase), RecA (recombinase A), and Sod (superoxide dismutase) were targeted for inactivation. The competitive fitness of the mutants was altered in the mouse intestine compared with wild-type cells. These results show that the food matrix can have a profound influence on dietary (probiotic) bacteria and their functional significance in the mammalian gut.