Role of fumarate reductase on the fermentation properties of Lactobacillus delbrueckii ssp. bulgaricus.

Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus are symbiotic starters widely used in yogurt fermentation. They exchange metabolites to meet their nutritional demands during fermentation, promoting mutual growth. Although S. thermophilus produces fumaric acid, and the addition of fumaric acid has been shown to promote the growth of L. bulgaricus monoculture, whether fumaric acid produced by S. thermophilus is used by L. bulgaricus during coculture remains unclear. Furthermore, the importance of fumaric acid metabolism in the growth of L. bulgaricus is yet to be elucidated. Therefore, in this study, we investigated the importance of fumaric acid metabolism in L. bulgaricus monocultures and coculture with S. thermophilus. We deleted the fumarate reductase gene (frd), responsible for the metabolism of fumaric acid to succinic acid, in L. bulgaricus strains 2038 and NCIMB 701373. Both Δfrd strains exhibited longer fermentation times than their parent strains, and fumaric acid was metabolized to malic acid rather than succinic acid. Coculture of Δfrd strains with S. thermophilus 1131 also resulted in a longer fermentation time, and the accumulation of malic acid was observed. These results indicated that fumaric acid produced by S. thermophilus is utilized by L. bulgaricus as a symbiotic substance during yogurt fermentation and that the metabolism of fumaric acid to succinic acid by FRD is a key factor determining the fermentation ability of L. bulgaricus.

Cooperative action of gut-microbiota-accessible carbohydrates improves host metabolic function.

Microbiota-accessible carbohydrates (MACs) exert health-promoting effects, but how each MAC impacts gut microbiota and regulates host physiology remains unclear. Here, we show that l-arabinose and sucrose cooperatively act on gut microbiota and exert anti-obesogenic effects. Specifically, l-arabinose, a monosaccharide that is poorly absorbed in the gut and inhibits intestinal sucrase, suppresses diet-induced obesity in mice in the presence of sucrose. Additionally, the suppressive effect of l-arabinose on adiposity is abrogated in mice lacking the short-chain fatty acid (SCFA) receptors GPR43 and GPR41. Mechanistically, l-arabinose increases the relative abundance of acetate and propionate producers (e.g., Bacteroides), while sucrose enhances SCFA production. Furthermore, l-arabinose and sucrose activate the glycolytic and pentose phosphate pathways of Bacteroides, respectively, indicating that they synergistically promote acetate production through distinct pathways. These findings suggest that each MAC has a unique property and thus may serve as a precision gut-microbiota modulator to promote host homeostasis.

Lactobacillus delbrueckii subsp. bulgaricus 2038 and Streptococcus thermophilus 1131 Induce the Expression of the REG3 Family in the Small Intestine of Mice via the Stimulation of Dendritic Cells and Type 3 Innate Lymphoid Cells.

Accumulating evidence clarifies that intestinal barrier function, for example, by the mucus layer, antimicrobial peptides, immune systems, and epithelial tight junctions, plays crucial roles in maintaining our health. We reported previously that yogurt fermented with Lactobacillus delbrueckii subsp. bulgaricus 2038 and Streptococcus thermophilus 1131 induced the gene expression of the regenerating family member 3 (REG3) family, which encodes antimicrobial peptides in the small intestine, although it was unclear how the yogurt activated the intestinal cells related to it. Here, we evaluated the cytokine production from the intestinal immune cells stimulated by these strains in vitro and in vivo to elucidate the mechanism for the induction of the REG3 family by the yogurt. The results showed that stimulation by both strains induced interleukin (IL)-23 production from bone marrow-derived dendritic cells (DCs) and IL-22 production from small intestinal lamina propria (LP) cells. In addition, oral administration of these strains to mice increased IL-23p19+ LPDCs and IL-22+ type 3 innate lymphoid cells and induced the expression of Reg3g in small intestinal tissue. Moreover, we showed that the activities for the induction of IL-23 by DCs were strain dependent on L. bulgaricus and S. thermophilus and that S. thermophilus 1131, which is the predominant species in the yogurt, exhibited relatively higher activity compared to the other strains of S. thermophilus. Our findings suggested that these yogurt starter strains, L. bulgaricus 2038 and S. thermophilus 1131, have the potential to maintain and improve intestinal barrier function by stimulating immune cells in the LP.

Conjugative plasmid from Lactobacillus gasseri LA39 that carries genes for production of and immunity to the circular bacteriocin gassericin A.

Gassericin A is a circular bacteriocin produced by Lactobacillus gasseri strain LA39. We found a 33,333-bp plasmid, designated pLgLA39, in this strain. pLgLA39 contained 44 open reading frames, including seven genes related to gassericin A production/immunity (gaa), as well as genes for replication, plasmid maintenance, and conjugative transfer. pLgLA39 was transferred from LA39 to the type strain of L. gasseri (JCM 1131) by filter mating. The transconjugant exhibited >30-fold-higher more resistance to gassericin A and produced antibacterial activity. Lactobacillus reuteri LA6, the producer of reutericin 6, was proved to harbor a plasmid indistinguishable from pLgLA39 and carrying seven genes 100% identical to gaa. This suggests that pLgLA39 might have been transferred naturally between L. gasseri LA39 and L. reuteri LA6. The seven gaa genes of pLgLA39 were cloned into a plasmid vector to construct pGAA. JCM 1131(T) transformed with pGAA expressed antibacterial activity and resistance to gassericin A. pGAA was segregationally more stable than a pGAA derivative plasmid from which gaaA was deleted and even was more stable than the vector. This suggests the occurrence of postsegregational host killing by the gaa genes. pLgLA39 carried a pemIK homolog, and segregational stabilization of a plasmid by the pLgLA39-type pemIK genes was also confirmed. Thus, pLgLA39 was proved to carry the genes for at least two plasmid maintenance mechanisms, i.e., gaa and pemIK. Plasmids containing a repA gene similar to pLgLA39 repA were distributed in several L. gasseri strains.