Phage-resistant mutants of Lactobacillus delbrueckii may have functional properties that differ from those of parent strains.

Three commercial phage sensitive Lactobacillus delbrueckii strains (identified as Ab(1), YSD V and Ib(3)), and four spontaneous phage-resistant mutants isolated from them were tested for their capacity to activate the gut mucosal immune response in mice, as indicated by the numbers of IgA-producing cells. Random Amplified Polymorphic DNA (RAPD) analysis revealed a strong genetic homology between the sensitive strains and their respective derivatives. The phage-resistant mutants exhibited high levels of phage resistance, elevated stability of this phenotype and technological properties comparable to those of their respective parent strains. The tolerance to acidic conditions, bile salts and lysozyme was strain dependent and total cell viability losses as a result of exposure to all three stresses ranged from 2.0 to 3.7 log units. All the strains were highly resistant to a simulated gastric solution of pH 3, while significant additional losses in cell viability were observed when acid treated cells were exposed to bile salts and lysozyme. BALB/c mice received pure cultures of Lb. delbrueckii sensitive and phage-resistant strains for 2, 5 or 7 consecutive days. The ability of the parent strains to activate the small intestine immune response was preserved or enhanced in phage-resistant mutants. The maximal proliferation of IgA(+) cells was observed at day 5 or 7, depending on the strain. Mutants isolated in this study using natural selection strategies had improved phage resistance, adequate technological properties and satisfactory gut mucosal immunostimulation ability, and so would be good candidates for industrial applications in functional foods.