The stressing life of Lactobacillus delbrueckii subsp. bulgaricus in soy milk.

Lactobacillus delbrueckii subsp. bulgaricus is a beneficial lactic acid bacterium and constitutes one of the most used, and thus consumed, dairy starters, worldwide. This homofermentative bacterium was the first lactobacillus described and is involved in the fermentation of yogurt and of diverse other fermented products, including cheeses. It has a long history of safe use, as well as documented probiotic lato sensu effects, including alleviation of lactose intolerance. Plant-based fermented products presently experience a considerable development, as a result of evolution of consumers' habits, in a general context of food transition. This requires research and development, and thus scientific knowledge, to allow such transition, including the development of fermented soy milks. These last indeed offer an alternative source of live and active bacteria. The yogurt starters L. delbrueckii subsp. bulgaricus, together with Streptococcus thermophilus, have been implemented to generate yogurt-type fermented soy milks worldwide. While the adaptation of these starters to the dairy environment has been extensively studied, little is known about L. delbrueckii adaptation to the soy environment. We therefore investigated its adaptation to soy milk and compared it to cow's milk. Surprisingly, it did not grow in soy milk, neither alone, nor in co-culture with S. thermophilus. Acidification of soy milk was however faster in the presence of both species. In order to deepen such adaptation, we then compared L. delbrueckii growth and survival in soy milk ultrafiltrate (SUF, the aqueous phase of soy milk) and compared it to cow's milk ultrafiltrate (MUF, the aqueous phase of cow milk). This comparison revealed major differences in terms of cell morphology and proteome composition. Lactobacilli appeared deformed and segmented in soy. Major differences in both the surface and the cellular proteome indicated upregulation of stress proteins, yet downregulation of cell cycle and division machinery. Altogether, these results suggest that soy milk may be a stressing environment for the yogurt starter L. delbrueckii subsp. bulgaricus.

Dairy Propionibacteria: Versatile Probiotics.

Dairy propionibacteria are used as cheese ripening starters, as biopreservative and as beneficial additives, in the food industry. The main species, Propionibacterium freudenreichii, is known as GRAS (Generally Recognized As Safe, USA, FDA). In addition to another dairy species, Propionibacterium acidipropionici, they are included in QPS (Qualified Presumption of Safety) list. Additional to their well-known technological application, dairy propionibacteria increasingly attract attention for their promising probiotic properties. The purpose of this review is to summarize the probiotic characteristics of dairy propionibacteria reported by the updated literature. Indeed, they meet the selection criteria for probiotic bacteria, such as the ability to endure digestive stressing conditions and to adhere to intestinal epithelial cells. This is a prerequisite to bacterial persistence within the gut. The reported beneficial effects are ranked according to property's type: microbiota modulation, immunomodulation, and cancer modulation. The proposed molecular mechanisms are discussed. Dairy propionibacteria are described as producers of nutraceuticals and beneficial metabolites that are responsible for their versatile probiotic attributes include short chain fatty acids (SCFAs), conjugated fatty acids, surface proteins, and 1,4-dihydroxy-2-naphtoic acid (DHNA). These metabolites possess beneficial properties and their production depends on the strain and on the growth medium. The choice of the fermented food matrix may thus determine the probiotic properties of the ingested product. This review approaches dairy propionibacteria, with an interest in both technological abilities and probiotic attributes.