Survival of a Lactococcus lactis strain varies with its carbohydrate preference under in vitro conditions simulated gastrointestinal tract

Lactococcus lactis G50 is a candidate probiotic bacterium with immunomodulatory activity. We evaluated the suitability of strain G50 as orally administered probiotics on the basis of its resistance to simulated gastrointestinal (GI) stress, including the presence of lysozyme, low pH, and bile, carbohydrate preference, and bacterial cell surface composition in vitro. This strain survived GI stresses but its resistance to lysozyme was affected by the type of available carbohydrate in the growth medium; it was unaffected with lactose, xylose and galactose as the carbon source but was significantly lower for fructose, sucrose and glucose. The resistance of strain G50 to low pH was unaffected by carbon source. Resistance to bile was determined by two methods; growth and survival study and varied with carbon source. The growth of strain G50 with 0.3% bile was lowest in lactose-containing broth, higher in broth containing xylose or galactose, and highest in broth containing sucrose, glucose, or fructose. In contrast, the survival of cells after 3 h incubation with 0.3% bile was highest for lactose. The hydrophobicity of bacterial cells, which can be related to epithelial adhesion in certain cases, was also highest for lactose. The fatty acid composition of cells grown on lactose differed from that of cells grown on other carbon sources. These results suggest that survival of strain G50 in the GI tract depends on the kinds of carbohydrates available. Carbohydrate preferences were observed for other strains of lactic acid bacteria under conditions of GI stress, and this preference varied with the strain and the type of GI stress. Careful consideration should be given to the selection of carbohydrates for in vitro testing of the survival of lactic acid bacteria in the GI tract.