Assessing secoisolariciresinol diglucoside metabolism in the rumen

Secoisolariciresinol diglucoside, a lignan found in many plants, predominantly in flaxseed. The plant lignan can play active role enhancing antioxidant capacity, however, it need to be metabolized into enterolignans, process which the gut microbiota is responsible. The rumen is a known efficient environment for lignan metabolism, which comprises the breakdown of SDG into secoisolariciresinol, then to enterodiol and enterolactone. The product enterolactone increases concentration in the rumen as flax is added in the diet of ruminants and the compound can be transferred to milk, benefiting human health by the functional food intake, preventing many oxidative-related disorders such as cardiovascular diseases, atherosclerosis, Alzheimer's disease, breast-cancer and others. Although the knowledge on lignans metabolism in the rumen, little information is available regarding the microorganisms linked to secoisolariciresinol diglucoside breakdown. The main bottleneck is that just few species of the rumen microbiome can be cultured in laboratory conditions, and thus, metagenomic approaches are needed to access the rumen ecology by-passing the in vitro culture. Therefore, a fosmid library was constructed using rumen microbes DNA inserts from the rumen of cows fed up to 15g/100g (dry matter basis) of flax meal, resulting on an 11,520 fosmid clones library, using Escherichia coli strain EPI300 as host bacteria. The library was screened for secoisolariciresinol diglucoside breakdown capacity using high-performance liquid chromatography and positive clones followed to plasmid DNA purification, deep sequencing and bioinformatic data analysis. Two out of 30 library plates showed effective secoisolariciresinol diglucoside breakdown, however, no metabolism products were identified and no putative gene linked to direct secoisolariciresinol diglucoside breakdown was identified from clones genome. To our knowledge this is the first fosmid library constructed to prospect rumen microorganisms with active role in plant lignans metabolism, thus, more studies regarding the subject must be carried out to broaden genes and enzymes involved in the process.