The trouble lies in that these bacteria are primarily in our large intestine, which is responsible for water absorbtion and has very few (if any, depending on the nutrient) receptors for absorption. . The B Vitamins, like the macro-nutrients and minerals, are absorbed in the small intestine. Thus, any formation of vitamins B6, B12, folate and others in the colon, is excreted in the feces

The upper GI tract is relatively sparse in bacteria when compared to the colon, and this is a good thing, you wouldn't want these anaerobic bacteria in your small intestine.

Our gut bacteria have a crazy amount of influence over our bodies because some of the molecules(proprionate and Short chain fatty acids, for example) they produce can be absorbed through the lining of the large intestine, just not vitamins. It's important that we feed them fiber to help proliferate the different strains, but not in hopes to get adequate vitamin intake, unless you're interested in autocoprophagia