Complex medium enriched for production of VFAs through lactic acid formation and consumption and is linked with lactic acid bacteria and Megasphaera
Evaluating the pathways for the complex medium enrichment showed a best fit when glucose was catabolised through the heterofermentative (69%) and homofermentative (31%) pathway (Table S5). Lactate was subsequently fermented into propionate, butyrate, valerate, H2 and CO2. The secondary lactate fermentation was confirmed in a batch experiment with the enrichment culture and replacing glucose with lactate (Figure S5). The microbial community analysis revealed a dominance of Lactobacillus , Lactococcus andMegasphaera (Figure 3B, 3C and 3E).
Lactococcus species are known homofermentative lactic acid bacteria, while Lactobacillus and Leuconostoc species can also be heterofermentative (Madigan and Martinko, 2006).Megasphaera elsdenii is known to produce acetate, propionate, butyrate and valerate from lactate (Prabhu et al., 2012).Megasphaera is known to convert lactate in the intestinal tract of cows, pigs and humans (Duncan et al., 2004) and is linked to lactate-mediated medium-chain length carboxylate production microbiomes (Spirito et al., 2014). Megasphaera elsdenii interestingly prefers lactate uptake over glucose uptake, taking up limited amounts of glucose when lactate is present (Marounek et al., 1989), which argues for the Megasphaera species having a mainly lactate consuming role in the community.