Ethylene glycol is metabolized to ethanol and acetate and induces expression of bacterial microcompartments in Propionibacterium freudenreichii.

Ethylene glycol (EG, 1,2-ethanediol) is a two-carbon dihydroxy alcohol that can be derived from fermentation of plant-derived xylose and arabinose and which can be formed during food fermentations. Here we show that Propionibacterium freudenreichii DSM 20271 is able to convert EG in anaerobic conditions to ethanol and acetate in almost equimolar amounts. The metabolism of EG led to a moderate increase of biomass, indicating its metabolism is energetically favourable. A proteomic analysis revealed EG induced expression of the pdu-cluster, which encodes a functional bacterial microcompartment (BMC) involved in the degradation of 1,2-propanediol, with the presence of BMCs confirmed using transmission electron microscopy. Cross-examination of the proteomes of 1,2-propanediol and EG grown cells revealed PDU BMC-expressing cells have elevated levels of DNA repair proteins and cysteine biosynthesis proteins. Cells grown in 1,2-propanediol and EG also showed enhanced resistance against acid and bile salt-induced stresses compared to lactate-grown cells. Our analysis of whole genome sequences of selected genomes of BMC-encoding microorganisms able to metabolize EG with acetaldehyde as intermediate indicate a potentially broad-distributed role of the pdu operon in metabolism of EG. Based on our results we conclude EG is metabolized to acetate and ethanol with acetaldehyde as intermediate within BMCs in P. freudenreichii.

A web-based microbiological hazard identification tool for infant foods.

An integrated approach to identify and assess Microbiological Hazards (MHs) and mitigate risks in infant food chains is crucial to ensure safe foods for infants and young children. A systematic procedure was developed to identify MHs in specific infant foods. This includes five major steps: 1) relevant hazard-food pairing, 2) process inactivation efficiency, 3) recontamination possibility after processing, 4) MHs growth opportunity, and 5) MHs-food association level. These steps were integrated into an online tool called the Microbiological Hazards IDentification (MiID) decision support system (DSS), targeting food companies, governmental agencies and academia users, and is accessible at https://foodmicrobiologywur.shinyapps.io/Microbial_hazards_ID/. The MiID DSS was validated in four case studies, focussing on infant formula, fruit puree, cereal-based meals, and fresh fruits, each representing distinct products and processing characteristics. The results obtained through the application of the MiID DSS, compared with identification by food safety experts, consistently identified the top MHs in these food products. This process affirms its effectiveness in systematic hazard identification. The introduction of the MiID DSS helps to structure the first steps in HACCP (hazard analysis) and in risk assessment (hazard identification) to follow a structured and well-documented procedure, balancing the risk of overlooking relevant MHs or including too many irrelevant MHs. It is a valuable addition to risk analysis/assessment in infant food chains and has the potential for future extension. This includes the incorporation of newly acquired data related to infant foods via a semi-publicly hosted platform, or it can be adapted for hazard identification in general food products using a similar framework.

Microaerobic metabolism of lactate and propionate enhances vitamin B12 production in Propionibacterium freudenreichii.

BACKGROUND: Propionibacterium freudenreichii is used in biotechnological applications to produce vitamin B12. Although cultured mainly in anaerobic conditions, microaerobic conditions can greatly enhance biomass formation in P. freudenreichii. Since B12 yields may be coupled to biomass formation, microaerobic conditions show great potential for increasing B12 yields in P. freudenreichii. RESULTS: Here we show biomass formation increases 2.7 times for P. freudenreichii grown in microaerobic conditions on lactate versus anaerobic conditions (1.87 g/L vs 0.70 g/L). Consumption of lactate in microaerobic conditions resulted first in production of pyruvate, propionate and acetate. When lactate was depleted, pyruvate and propionate were oxidised with a concomitant sixfold increase in the B12 titer compared to anaerobic conditions, showing potential for propionate and pyruvate as carbon sources for B12 production. Consequently, a fed-batch reactor with anaerobically precultured lactate-grown cells was fed propionate in microaerobic conditions resulting in biomass increase and production of B12. Vitamin yields increased from 0.3 [Formula: see text] B12 per mmol lactate in anaerobic conditions to 2.4 [Formula: see text] B12 per mmol lactate and 8.4 [Formula: see text] B12 per mmol propionate in microaerobic conditions. Yield per cell dry weight (CDW) increased from 41 [Formula: see text] per g CDW in anaerobic conditions on lactate to 92 [Formula: see text] per g CDW on lactate and 184 [Formula: see text] per g CDW on propionate in microaerobic conditions. CONCLUSIONS: Here we have shown both B12 yield per substrate and per CDW were highest on cells oxidising propionate in microaerobic conditions, showing the potential of propionate for biotechnological production of vitamin B12 by P. freudenreichii.

Bacterial Microcompartment-Dependent 1,2-Propanediol Utilization of Propionibacterium freudenreichii.

Bacterial microcompartments (BMCs) are proteinaceous prokaryotic organelles that enable the utilization of substrates such as 1,2-propanediol and ethanolamine. BMCs are mostly linked to the survival of particular pathogenic bacteria by providing a growth advantage through utilization of 1,2-propanediol and ethanolamine which are abundantly present in the human gut. Although a 1,2-propanediol utilization cluster was found in the probiotic bacterium Propionibacterium freudenreichii, BMC-mediated metabolism of 1,2-propanediol has not been demonstrated experimentally in P. freudenreichii. In this study we show that P. freudenreichii DSM 20271 metabolizes 1,2-propanediol in anaerobic conditions to propionate and 1-propanol. Furthermore, 1,2-propanediol induced the formation of BMCs, which were visualized by transmission electron microscopy and resembled BMCs found in other bacteria. Proteomic analysis of 1,2-propanediol grown cells compared to L-lactate grown cells showed significant upregulation of proteins involved in propanediol-utilization (pdu-cluster), DNA repair mechanisms and BMC shell proteins while proteins involved in oxidative phosphorylation were down-regulated. 1,2-Propanediol utilizing cells actively produced vitamin B12 (cobalamin) in similar amounts as cells growing on L-lactate. The ability to metabolize 1,2-propanediol may have implications for human gut colonization and modulation, and can potentially aid in delivering propionate and vitamin B12 in situ.

Propionibacterium freudenreichii thrives in microaerobic conditions by complete oxidation of lactate to CO2.

In this study we show increased biomass formation for four species of food-grade propionic acid bacteria (Acidipropionibacterium acidipropionici, Acidipropionibacterium jensenii, Acidipropionibacterium thoenii and Propionibacterium freudenreichii) when exposed to oxygen, implicating functional respiratory systems. Using an optimal microaerobic condition, P. freudenreichii DSM 20271 consumed lactate to produce propionate and acetate initially. When lactate was depleted propionate was oxidized to acetate. We propose to name the switch from propionate production to consumption in microaerobic conditions the 'propionate switch'. When propionate was depleted the 'acetate switch' occurred, resulting in complete consumption of acetate. Both growth rate on lactate (0.100 versus 0.078 h-1 ) and biomass yield (20.5 versus 8.6 g* mol-1 lactate) increased compared to anaerobic conditions. Proteome analysis revealed that the abundance of proteins involved in the aerobic and anaerobic electron transport chains and major metabolic pathways did not significantly differ between anaerobic and microaerobic conditions. This implicates that P. freudenreichii is prepared for utilizing O2 when it comes available in anaerobic conditions. The ecological niche of propionic acid bacteria can conceivably be extended to environments with oxygen gradients from oxic to anoxic, so-called microoxic environments, as found in the rumen, gut and soils, where they can thrive by utilizing low concentrations of oxygen.

CRISPR-Cas genome engineering of esterase activity in Saccharomyces cerevisiae steers aroma formation.

OBJECTIVE: Saccharomyces cerevisiae is used worldwide for the production of ale-type beers. This yeast is responsible for the production of the characteristic fruity aroma compounds. Esters constitute an important group of aroma active secondary metabolites produced by S. cerevisiae. Previous work suggests that esterase activity, which results in ester degradation, may be the key factor determining the abundance of fruity aroma compounds. Here, we test this hypothesis by deletion of two S. cerevisiae esterases, IAH1 and TIP1, using CRISPR-Cas9 genome editing and by studying the effect of these deletions on esterase activity and extracellular ester pools. RESULTS: Saccharomyces cerevisiae mutants were constructed lacking esterase IAH1 and/or TIP1 using CRISPR-Cas9 genome editing. Esterase activity using 5-(6)-carboxyfluorescein diacetate (cFDA) as substrate was found to be significantly lower for ΔIAH1 and ΔIAH1ΔTIP1 mutants compared to wild type (WT) activity (P < 0.05 and P < 0.001, respectively). As expected, we observed an increase in relative abundance of acetate and ethyl esters and an increase in ethyl esters in ΔIAH1 and ΔTIP1, respectively. Interestingly, the double gene disruption mutant ΔIAH1ΔTIP1 showed an aroma profile comparable to WT levels, suggesting the existence and activation of a complex regulatory mechanism to compensate multiple genomic alterations in aroma metabolism.