It's been known for years that many B vitamins and also K2 can be generated by the gut microbiome. Now a study shows that vitamin C can also be created "down there". Lactic acid bacteria and bifidobacteria can do it.
http://datateca.unad.edu.co/contenidos/401585/Bacteria_as_vitamin_suppliers.pdf
http://datateca.unad.edu.co/contenidos/401585/Bacteria_as_vitamin_suppliers.pdf