Responses of Fermentation Characteristics and Microbial Communities to Vitamin B-12 Supplementation in In Vitro Ruminal Cultures

Vitamin B-12, an important cofactor involved in propionate formation, is synthesized exclusively by bacteria and archaebacteria. Humans need to intake vitamin B-12 through food, and dairy products are generally the best source of vitamin B-12. In the present study, the effects of vitamin B-12 supplementation in diets on in vitro ruminal fermentation characteristics and microbial communities were investigated to provide a reference for increasing the vitamin B-12 content in milk by dietary supplementation. A completely randomized design was carried out using the in vitro rumen culture technique, and 5 vitamin B-12 dose levels (0, 0.5, 1.0, 2.0, and 4 mg/g of dry matter) were used. The results showed that vitamin B-12 supplementation in diets decreased acetate: propionate ratio and butyrate concentration. The change in the acetate: propionate ratio can be attributed to the increased relative abundances of the Proteobacteria phylum and the Negativicutes class, both of which are involved in propionate metabolism. The decrease in butyrate concentration can likely be attributed to a reduction in relative abundance of species belonging to the Clostridia class, which are known as the predominant butyrate producers in the mammalian intestine. In addition, vitamin B-12 supplementation in diets reduced the CH4 production by altering the species composition of the archaeal community. In conclusion, dietary supplementation of vitamin B-12 resulted in rumen perturbation. In vivo studies should be conducted cautiously when evaluating the effects of vitamin B-12 supplementation on the synthesis and absorption of it, as well as its content in milk.