Propionate production from carbon monoxide by synthetic co-cultures of Acetobacterium wieringae spp. and propionigenic bacteria

Gas fermentation is a promising way for converting CO-rich gases to chemicals. We studied the use of synthetic co-cultures composed of carboxydotrophic and propionigenic bacteria to convert CO to propionate. So far isolated carboxydotrophs cannot directly ferment CO to propionate, and therefore this co-cultivation approach was investigated. Four distinct synthetic co-cultures were constructed, consisting of: Acetobacterium wieringae (DSM 1911T) and Pelobacter propionicus (DSM 2379T); Ac. wieringae (DSM 1911T) and Anaerotignum neopropionicum (DSM 3847T); Ac. wieringae strain JM and P. propionicus (DSM 2379T); Ac. wieringae strain JM and An. neopropionicum (DSM 3847T). Propionate was produced by all the co-cultures, with the highest titer (∼24 mM) measured in the co-culture composed of Ac. wieringae strain JM + An. neopropionicum, which also produced isovalerate (∼4 mM), butyrate (∼1 mM), and isobutyrate (0.3 mM). This co-culture was further studied using proteogenomics. As expected, enzymes involved in the Wood-Ljungdahl pathway in Ac. wieringae strain JM, which are responsible for the conversion of CO to ethanol and acetate, were detected; the proteome of An. neopropionicum confirmed the conversion of ethanol to propionate via the acrylate pathway. In addition, proteins related to amino acid metabolism and stress response were highly abundant during co-cultivation, which raises the hypothesis that amino acids are exchanged by the two microorganisms accompanied by isovalerate and isobutyrate production. This highlights the importance of explicitly looking at fortuitous microbial interactions during co-cultivation to fully understand co-cultures behavior. IMPORTANCE Syngas fermentation has great potential for the sustainable production of chemicals from wastes (via prior gasification) and flue gases containing CO/CO2. Research efforts need to be driven to expanding the product portfolio of gas fermentation, which is currently limited to mainly acetate and ethanol. This study provides the basis for a microbial process to produce propionate from CO using synthetic co-cultures composed of acetogenic and propionigenic bacteria and elucidates the metabolic pathways involved. Furthermore, based on proteomics results, we hypothesize that the two bacterial species engage in an interaction that results in amino acid exchange, which subsequently promotes isovalerate and isobutyrate production. These findings provide a new understanding of gas fermentation and a co-culturing strategy for expanding the product spectrum of microbial conversion of CO/CO2.

Distribution, organization and expression of genes concerned with anaerobic lactate-utilization in human intestinal bacteria

Lactate accumulation in the human gut is linked to a range of deleterious health impacts. However, lactate is consumed and converted to the beneficial short chain fatty acids butyrate and propionate by indigenous lactate-utilizing bacteria. To better understand the underlying genetic basis for lactate utilization, transcriptomic analysis was performed for two prominent lactate-utilizing species from the human gut, Anaerobutyricum soehngenii and Coprococcus catus, during growth on lactate, hexose sugar, or hexose plus lactate. In A. soehngenii L2-7, six genes of the lct cluster including NAD-independent D-lactate dehydrogenase (i-LDH) were co-ordinately upregulated during growth on equimolar D and L-lactate (DL-lactate). Upregulated genes included an acyl-CoA dehydrogenase related to butyryl-CoA dehydrogenase, which may play a role in transferring reducing equivalents between reduction of crotonyl-CoA and oxidation of lactate. Genes upregulated in C. catus GD/7 included a six-gene cluster (lap) encoding propionyl CoA-transferase, a putative lactoyl-CoA epimerase, lactoyl-CoA dehydratase and lactate permease, and two unlinked acyl-CoA dehydrogenase genes that are candidates for acryloyl-CoA reductase. An i-LDH homolog in C. catus is encoded by a separate, partial lct, gene cluster, but not upregulated on lactate. While C. catus converts three mols of DL-lactate via the acrylate pathway to two mols propionate and one mol acetate, some of the acetate can be re-used with additional lactate to produce butyrate. A key regulatory difference is that while glucose partially repressed lct cluster expression in A. soehngenii, there was no repression of lactate utilization genes by fructose in the non-glucose utilizer C. catus. This implies that bacteria such as C. catus might be more important in curtailing lactate accumulation in the gut.

Lactate and Acrylate Metabolism by Megasphaera elsdenii under Batch and Steady-State Conditions

ABSTRACTThe growth ofMegasphaera elsdeniion lactate with acrylate and acrylate analogues was studied under batch and steady-state conditions. Under batch conditions, lactate was converted to acetate and propionate, and acrylate was converted into propionate. Acrylate analogues 2-methyl propenoate and 3-butenoate containing a terminal double bond were similarly converted into their respective saturated acids (isobutyrate and butyrate), while crotonate and lactate analogues 3-hydroxybutyrate and (R)-2-hydroxybutyrate were not metabolized. Under carbon-limited steady-state conditions, lactate was converted to acetate and butyrate with no propionate formed. As the acrylate concentration in the feed was increased, butyrate and hydrogen formation decreased and propionate was increasingly generated, while the calculated ATP yield was unchanged.M. elsdeniimetabolism differs substantially under batch and steady-state conditions. The results support the conclusion that propionate is not formed during lactate-limited steady-state growth because of the absence of this substrate to drive the formation of lactyl coenzyme A (CoA) via propionyl-CoA transferase. Acrylate and acrylate analogues are reduced under both batch and steady-state growth conditions after first being converted to thioesters via propionyl-CoA transferase. Our findings demonstrate the central role that CoA transferase activity plays in the utilization of acids byM. elsdeniiand allows us to propose a modified acrylate pathway forM. elsdenii.

Impact of pH on Lactate Formation and Utilization by Human Fecal Microbial Communities

ABSTRACT The human intestine harbors both lactate-producing and lactate-utilizing bacteria. Lactate is normally present at <3 mmol liter−1 in stool samples from healthy adults, but concentrations up to 100 mmol liter−1 have been reported in gut disorders such as ulcerative colitis. The effect of different initial pH values (5.2, 5.9, and 6.4) upon lactate metabolism was studied with fecal inocula from healthy volunteers, in incubations performed with the addition of dl-lactate, a mixture of polysaccharides (mainly starch), or both. Propionate and butyrate formation occurred at pH 6.4; both were curtailed at pH 5.2, while propionate but not butyrate formation was inhibited at pH 5.9. With the polysaccharide mix, lactate accumulation occurred only at pH 5.2, but lactate production, estimated using l-[U-13C]lactate, occurred at all three pH values. Lactate was completely utilized within 24 h at pH 5.9 and 6.4 but not at pH 5.2. At pH 5.9, more butyrate than propionate was formed from l-[U-13C]lactate in the presence of polysaccharides, but propionate, formed mostly by the acrylate pathway, was the predominant product with lactate alone. Fluorescent in situ hybridization demonstrated that populations of Bifidobacterium spp., major lactate producers, increased approximately 10-fold in incubations with polysaccharides. Populations of Eubacterium hallii, a lactate-utilizing butyrate-producing bacterium, increased 100-fold at pH 5.9 and 6.4. These experiments suggest that lactate is rapidly converted to acetate, butyrate, and propionate by the human intestinal microbiota at pH values as low as 5.9, but at pH 5.2 reduced utilization occurs while production is maintained, resulting in lactate accumulation.