A process for isolation of a strain of Lactobacillus having the ability of being established on human intestinal mucosa in vivo and being able to remain therein after oral administration for at least 10 days after the completion of the administration. By the process the new strains L. plantarum 299 (DSM 6595) and L. casei ssp. rhamnosus 271 (DSM 6594) have been isolated, which are useful for the prophylaxis or treatment of bacterial infections, especially in the form of a fermented nutrient composition.