A new model for the anaerobic fermentation of glycerol in enteric bacteria: trunk and auxiliary pathways in Escherichia coli

Anaerobic fermentation of glycerol in the Enterobacteriaceae family has long been considered a unique property of species that synthesize 1,3-propanediol (1,3-PDO). However, we have discovered that Escherichia coli can ferment glycerol in a 1,3-PDO-independent manner. We identified 1,2-propanediol (1,2-PDO) as a fermentation product and established the pathway that mediates its synthesis as well as its role in the metabolism of glycerol. We also showed that the trunk pathway responsible for the conversion of glycerol into glycolytic intermediates is composed of two enzymes: a type II glycerol dehydrogenase (glyDH-II) and a dihydroxyacetone kinase (DHAK), the former of previously unknown physiological role. Based on our findings, we propose a new model for glycerol fermentation in enteric bacteria in which: (i) the production of 1,2-PDO provides a means to consume reducing equivalents generated in the synthesis of cell mass, thus facilitating redox balance, and (ii) the conversion of glycerol to ethanol, through a redox-balanced pathway, fulfills energy requirements by generating ATP via substrate-level phosphorylation. The activity of the formate hydrogen-lyase and F(0)F(1)-ATPase systems were also found to facilitate the fermentative metabolism of glycerol, and along with the ethanol and 1,2-PDO pathways, were considered auxiliary or enabling. We demonstrated that glycerol fermentation in E. coli was not previously observed due to the use of medium formulations and culture conditions that impair the aforementioned pathways. These include high concentrations of potassium and phosphate, low concentrations of glycerol, alkaline pH, and closed cultivation systems that promote the accumulation of hydrogen gas.     

Efficient production of 1,3-propanediol from fermentation of crude glycerol with mixed cultures in a simple medium

The objective of this study was to examine the applicability of mixed cultures for 1,3-propanediol (1,3-PDO) production from crude glycerol. Three different sources of mixed cultures were tested, where the mixed culture from a municipal wastewater treatment plant showed the best results. 1,3-PDO can be produced as the main product in this mixed culture with typical organic acids like acetic and butyric acids as by-products. The yield was in the range of 0.56–0.76 mol 1,3-PDO per mol glycerol consumed depending on the glycerol concentration. A final product concentration as high as 70 g/L was obtained in fed-batch cultivation with a productivity of 2.6 g/L h. 1,3-PDO can be kept in the culture several days after termination of the fermentation without being degraded. Degradation tests showed that 1,3-PDO is degraded much slower than other compounds in the fermentation broth. In comparison to 1,3-PDO production in typical pure cultures, the process developed in this work with a mixed culture achieved the same levels of product titer, yield and productivity, but has the decisive advantage of operation under complete non-sterile conditions. Moreover, a defined fermentation medium without yeast extract can be used and nitrogen gassing can be omitted during cultivation, leading to a strong reduction of investment and production costs.     

Alkaline conditions stimulate the production of 1,3-propanediol in Lactobacillus panis PM1 through shifting metabolic pathways

A novel Lactobacillus panis PM1 isolate was found to be capable of converting glycerol to 1,3-propanediol (1,3-PDO), an increasingly valuable commodity chemical. In this study the effects of various process parameters, including glucose and glycerol concentrations, inoculum size, temperature, aeration, pH, and carbon source were examined to determine the optimal conditions for the production of 1,3-PDO using a culture method simulating late log to early stationary phases. Inoculum size did not influence the production of 1,3-PDO, and temperature variance showed similar 1,3-PDO production between 25 and 37 °C under the examined conditions. Glycerol concentration and pH played a primary role in the final concentration of 1,3-PDO. The highest production occurred at 150–250 mM glycerol when 50 mM glucose was available. Alkaline initial conditions (pH 9–10) stimulated the production of 1,3-PDO which concurrently occurred with increased acetic acid production. Under these conditions, 213.6 mM of 1,3-PDO were produced from 300 mM glycerol (conversion efficiency was 71 %). These observations indicated that the production of 1,3-PDO was associated with the shift of the metabolic end-product ethanol to acetic acid, and that this shift resulted in an excess concentration of NADH available for the processing of glycerol to 1,3-PDO.     

A robust soft sensor to monitor 1,3-propanediol fermentation process by Clostridium butyricum based on artificial neural network

With the aggravation of environmental pollution and energy crisis, the sustainable microbial fermentation process of converting glycerol to 1,3-propanediol (1,3-PDO) has become an attractive alternative. However, the difficulty in the online measurement of glycerol and 1,3-PDO creates a barrier to the fermentation process and then leads to the residual glycerol and therefore, its wastage. Thus, in the present study, the four-input artificial neural network (ANN) model was developed successfully to predict the concentration of glycerol, 1,3-PDO, and biomass with high accuracy. Moreover, an ANN model combined with a kinetic model was also successfully developed to simulate the fed-batch fermentation process accurately. Hence, a soft sensor from the ANN model based on NaOH-related parameters has been successfully developed which cannot only be applied in software to solve the difficulty of glycerol and 1,3-PDO online measurement during the industrialization process, but also offer insight and reference for similar fermentation processes.     

Adaptation dynamics of Clostridium butyricum in high 1,3-propanediol content media

Aim of the present study was to evaluate the effect of exogenous additions of 1,3-propanediol (1,3-PDO) on microbial growth and metabolites production of Clostridium butyricum VPI 1718 strain, during crude glycerol fermentation. Preliminary batch cultures in anaerobic Duran bottles revealed that early addition of 1,3-PDO caused growth cessation in rather low quantities (15?g/L), while 1,3-PDO additions during the middle exponential growth phase up to 70?g/L resulted in an almost linear decrease of the specific growth rate (μ), accompanied by reduced glycerol assimilation, with substrate consumption being used mainly for energy of maintenance requirements. During batch trials in a 3-L bioreactor, the strain proved able to withstand more than 70?g/L of both biologically produced and externally added 1,3-PDO, whereas glycerol assimilation and metabolite production were carried on at a lower rate. Adaptation of the strain in high 1,3-PDO concentration environments was validated during its continuous cultivation with pulses of 1,3-PDO in concentrations of 31 and 46?g/L, where no washout phenomena were noticed. As far as C. butyricum cellular lipids were concerned, during batch bioreactor cultivations, 1,3-PDO addition was found to favor the biosynthesis of unsaturated fatty acids. Also, fatty acid composition was studied during continuous cultures, in which additions of 1,3-PDO were performed at steady states. Lipids were globally more saturated compared to batch cultures, while by monitoring of the transitory phases, it was noticed that the gradual diol washout had an evident impact in the alteration of the fatty acid composition, by rendering them more unsaturated.     

Glycerol assimilation and production of 1,3-propanediol by Citrobacter amalonaticus Y19

Citrobacter amalonaticus Y19 (Y19) was isolated because of its ability for carbon monoxide-dependent hydrogen production (water–gas shift reaction). This paper reports the assimilation of glycerol and the production of 1,3-propanediol (1,3-PDO) by Y19. Genome sequencing revealed that Y19 contained the genes for the utilization of glycerol and 1,2-propanediol (pdu operon) along with those for the synthesis of coenzyme B₁₂ (cob operon). On the other hand, it did not possess the genes for the fermentative metabolism of glycerol of Klebsiella pneumoniae, which consists of both the oxidative (dhaD and dhaK) and reductive (dhaB and dhaT) pathways. In shake-flask cultivation under aerobic conditions, Y19 could grow well with glycerol as the sole carbon source and produced 1,3-PDO. The level of 1,3-PDO production was improved when vitamin B₁₂ was added to the culture medium under aerobic conditions. Under anaerobic conditions, cell growth and 1,3-PDO production on glycerol was also possible, but only when an exogenous electron acceptor, such as nitrate or fumarate, was added. This is the first report of the glycerol metabolism and 1,3-PDO production by C. amalonaticus Y19.     

Development of Klebsiella pneumoniae J2B as microbial cell factory for the production of 1,3-propanediol from glucose

1,3-Propanediol (1,3-PDO) is an important platform chemical which has a wide application in food, cosmetics, pharmaceutical and textile industries. Its biological production using recombinant Escherichia coli with glucose as carbon source has been commercialized by DuPont, but E. coli cannot synthesize coenzyme B₁₂ which is an essential and expensive cofactor of glycerol dehydratase, a core enzyme in 1,3-PDO biosynthesis. This study aims to develop a more economical microbial cell factory using Klebsiella pneumoniae J2B which can naturally synthesize coenzyme B₁₂. To this end, the heterologous pathway for the production of glycerol from dihydroxyacetone-3-phosphate (DHAP), a glycolytic intermediate, was introduced to J2B and, afterwards, the strain was extensively modified for carbon and energy metabolisms including: (i) removal of carbon catabolite repression, (ii) blockage of glycerol export across the cell membrane, (iii) improvement of NADH regeneration/availability, (iv) modification of TCA cycle and electron transport chain, (v) overexpression of 1,3-PDO module enzyme, and (vi) overexpression of glucose transporter. A total of 33 genes were modified and/or overexpressed, and one resulting strain could produce 814 mM (62 g/L) of 1,3-PDO with the yield of 1.27 mol/mol glucose in fed-batch bioreactor culture with a limited supplementation of coenzyme B₁₂ at 4 μM, which is ~10 fold less than that employed by DuPont. This study highlights the importance of balanced use of glucose in the production of carbon backbone of the target chemical (1,3-PDO) and regeneration of reducing power (NADH). This study also suggests that K. pneumoniae J2B is a promising host for the production of 1,3-PDO from glucose.     

肺炎克雷伯氏菌利用甘油生产1,3-PDO发酵优化的研究进展

发展可再生能源,尤其是生物能源,具有显著的能量收益和碳减排效益。随着石油等不可再生资源的减少,许多大宗传统石油化工产品正不断被使用可再生原料的生物制造产品替代。生物发酵法生产1,3-丙二醇（1,3-PDO）顺应了这一潮流,具有广阔的发展前景。提高微生物发酵竞争力,优化发酵法生产1,3-PDO水平,势必增加1,3-PDO的生产效益。对肺炎克雷伯氏菌（Klebsiella pneumoniae）发酵法进行1,3-PDO生产的代谢机理、菌株筛选和利用、发酵参数的选择和优化以及发酵工程策略的设计和监测等进行综述,为利用生物柴油副产物甘油生产有重要工业价值的1,3-PDO产品提供参考。     

Construction of glycerol synthesis pathway in <Emphasis Type="Italic">Klebsiella pneumoniae</Emphasis> for bioconversion of glucose into 1,3-propanediol

1,3-Propanediol (1,3-PDO) is an important three-carbon compound widely used in new polyester polymer materials. Natural organisms that can produce 1,3-PDO from glycerol were well studied. However, no natural microorganisms found could directly convert glucose to 1,3-PDO due to its insufficient glycerol synthesis pathway. In this study, two essential glycerol synthesis genes, CgGPD gene (encoding glycerol-3-phosphate dehydrogenase from Candida glycerinogenes) and ScGPP2 gene (encoding glycerol-3-phosphatase from Saccharomyces cerevisiae), were expressed in wild-type Klebsiella pneumoniae, a natural 1,3-PDO producers with reduction pathway for 1,3-PDO synthesis from glycerol. The results of fermentation, key enzyme activities, and metabolites analysis confirmed that recombinant K. pneumoniae now possessed a metabolic pathway capable of converting glucose to 1,3-PDO. The strain could produce 1,3-PDO from glucose with a final titer of 17.27 g/L with 40 g/L glucose in the medium, showing a 1.26-fold increase compared with 30 g/L glucose. Also, adding certain concentrations of glycerol could quickly initiate the 1,3-PDO synthetic pathway and promote the accumulation of 1,3-PDO, which could shorten the fermentation cycle. These results have important implications for further studies involving the use of one strain for bioconversion of glucose to 1,3-PDO.     

Systems metabolic engineering of Corynebacterium glutamicum for high-level production of 1,3-propanediol from glucose and xylose

Corynebacterium glutamicum is a versatile chassis which has been widely used to produce various amino acids and organic acids. In this study, we report the development of an efficient C. glutamicum strain to produce 1,3-propanediol (1,3-PDO) from glucose and xylose by systems metabolic engineering approaches, including (1) construction and optimization of two different glycerol synthesis modules; (2) combining glycerol and 1,3-PDO synthesis modules; (3) reducing 3-hydroxypropionate accumulation by clarifying a mechanism involving 1,3-PDO re-consumption; (4) reducing the accumulation of toxic 3-hydroxypropionaldehyde by pathway engineering; (5) engineering NADPH generation pathway and anaplerotic pathway. The final engineered strain can efficiently produce 1,3-PDO from glucose with a titer of 110.4 g/L, a yield of 0.42 g/g glucose, and a productivity of 2.30 g/L/h in fed-batch fermentation. By further introducing an optimized xylose metabolism module, the engineered strain can simultaneously utilize glucose and xylose to produce 1,3-PDO with a titer of 98.2 g/L and a yield of 0.38 g/g sugars. This result demonstrates that C. glutamicum is a potential chassis for the industrial production of 1,3-PDO from abundant lignocellulosic feedstocks.     

Development of bioelectrocatalytic activity stimulates mixed‐culture reduction of glycerol in a bioelectrochemical system

In a microbial bioelectrochemical system (BES), organic substrate such as glycerol can be reductively converted to 1,3-propanediol (1,3-PDO) by a mixed population biofilm growing on the cathode. Here, we show that 1,3-PDO yields positively correlated to the electrons supplied, increasing from 0.27 ± 0.13 to 0.57 ± 0.09 mol PDO mol⁻¹ glycerol when the cathodic current switched from 1 A m⁻² to 10 A m⁻². Electrochemical measurements with linear sweep voltammetry (LSV) demonstrated that the biofilm was bioelectrocatalytically active and that the cathodic current was greatly enhanced only in the presence of both biofilm and glycerol, with an onset potential of −0.46 V. This indicates that glycerol or its degradation products effectively served as cathodic electron acceptor. During long-term operation (> 150 days), however, the yield decreased gradually to 0.13 ± 0.02 mol PDO mol⁻¹ glycerol, and the current–product correlation disappeared. The onset potentials for cathodic current decreased to −0.58 V in the LSV tests at this stage, irrespective of the presence or absence of glycerol, with electrons from the cathode almost exclusively used for hydrogen evolution (accounted for 99.9% and 89.5% of the electrons transferred at glycerol and glycerol-free conditions respectively). Community analysis evidenced a decreasing relative abundance of Citrobacter in the biofilm, indicating a community succession leading to cathode independent processes relative to the glycerol. It is thus shown here that in processes where substrate conversion can occur independently of the electrode, electroactive microorganisms can be outcompeted and effectively disconnected from the substrate.     

Influence of dhaT mutation of K. pneumoniae on 1,3-propanediol fermentation

Metabolic role of 1,3-propanediol oxidoreductase (PDOR) in the production of 1,3-propanediol (1,3-PDO) with K. pneumonia was investigated by knocking out the coded gene dhaT. Fermentation with both the wide-type and mutant were studied in 5 l fermentor. A PDOR-deficient mutant K. pneumonia T1.9131 with 19% PDOR activity of the wild type was constructed. The cultures of the mutant indicated that PDOR inactivation had great effect on the other dha regulon enzymes: activity of glycerol dehydratase decreased by 70% while activity of glycerol dehydrogenase increased by 68%. Fed-batch fermentation showed that more metabolic flux of glycerol was directed to lactate and ethanol in the mutant. Lactate was identified as major metabolite and received an increase in the final concentration from 45 to 91 g l⁻¹, while the concentration of 1,3-PDO production dropped from 94 to 36 g l⁻¹. The results demonstrated PDOR was not indispensable in glycerol metabolism but was crucial in high 1,3-PDO productivity. It is postulated that a hypothetical oxidoreductase was expressed and replaced the function of PDOR. Blocking the pathway towards lactate and ethanol could be a plausible scheme to enhance 1,3-PDO productivity.     

以甘油为底物利用重组大肠杆菌生产聚3-羟基丙酸

【目的】解决前期研究中所构建的以甘油为底物合成聚3-羟基丙酸(P3HP)的代谢途径中存在两个主要的问题——细胞内还原力不平衡和质粒丢失,以提高P3HP的产量。【方法】克隆来源于肺炎克雷伯氏菌的1,3-丙二醇(1,3-PDO)氧化还原酶基因,构建P3HP和1,3-PDO联产的菌株,解决细胞内还原力不平衡的问题。利用自杀性载体系统介导的同源重组技术,将甘油脱水酶及其激活因子的基因整合到大肠杆菌基因组中,提高质粒的稳定性。同时,对发酵条件进行优化。【结果】菌种改造和发酵条件优化显著提高了P3HP产量,在摇瓶条件下到达2.7 g/L,比以前的报道提高2倍,并可同时得到2.4 g/L 1,3-PDO。【结论】该重组大肠杆菌合成P3HP的产量得到提高,具有较好的工业化生产前景。     

Production of 1,3-Propanediol from Glucose by Recombinant <Emphasis Type="Italic">Escherichia coli</Emphasis> BL21(DE3)

A range of recombinant strains of Escherichia coli were developed to produce 1,3-propanediol (1,3-PDO), an important C3 diol, from glucose. Two modules, the glycerol-producing pathway converting dihydroxyacetone phosphate to glycerol and the 1,3-PDO-producing pathway converting glycerol to 1,3-PDO, were introduced into E. coli. In addition, to avoid oxidative assimilation of the produced glycerol, glycerol oxidative pathway was deleted. Furthermore, to enhance the carbon flow to the Embden- Meyerhof-Parnas pathway, the Entner-Doudoroff pathway was disrupted by deleting 6-phosphogluconate dehydratase and 2-keto-3-deoxy-6-phosphogluconate aldolase. Finally, the acetate production pathway was removed to minimize the production of acetate, a major and toxic by-product. Flask experiments were carried out to examine the performance of the developed recombinant E. coli. The best strain could produce 1,3-PDO with a yield of 0.47 mol/mol glucose. Along with 1,3-PDO, glycerol was produced with a yield of 0.33 mol/mol glucose.     

Development of recombinant <Emphasis Type="Italic">Klebsiella pneumoniae</Emphasis> ∆<Emphasis Type="Italic">dhaT</Emphasis> strain for the co-production of 3-hydroxypropionic acid and 1,3-propanediol from glycerol

Klebsiella pneumoniae converts glycerol to the specialty chemical 1,3-propanediol (1,3-PDO), which is used for the production of polytrimethylene terepthalate (PTT). In this study, an NAD⁺-dependent gamma-glutamyl-gamma-aminobutyraldehyde dehydrogenase (PuuC) of K. pneumoniae DSM 2026, which oxidizes 3-hydroxypropionaldehyde to a platform chemical 3-hydroxypropionic acid (3-HP), was cloned and overexpressed in K. pneumoniae DSM 2026 for the co-production of 3-HP and 1,3-PDO from glycerol. In addition, the gene dhaT, encoding NADH-dependent 1,3-propanediol oxidoreductase (1,3-PDOR), was deleted from the chromosome for the balanced production of 3-HP and 1,3-PDO. The recombinant K. pneumoniae ∆dhaT, expressing puuC, produced 3.6 g 3-HP and 3.0 g 1,3-PDO per liter with an average yield of 81% on glycerol carbon in shake flask culture under microaerobic conditions. When a fed-batch culture was carried out under microaerobic conditions at pH 7.0 in a 5-l bioreactor, the recombinant K. pneumoniae ∆dhaT (puuC) strain produced 16.0 g 3-HP and 16.8 g 1,3-PDO per liter with a cumulative yield of 51% on glycerol carbon in 24 h. The production of 1,3-PDO in the dhaT-deletion mutant was attributed to the expression of NAD(P)H-dependent hypothetical oxidoreductase. This study demonstrates the feasibility of obtaining two commercially valuable chemicals, 3-HP and 1,3-PDO, at a significant scale.     

Converting crude glycerol to 1,3-propandiol using resting and immobilized Klebsiella sp. HE-2 cells

In this study, 1,3-propanediol (1,3-PDO) was produced from crude glycerol through the fermentation of resting and immobilized cells of a Klebsieblla sp. HE-2 strain isolated from a hydrogen producing anaerobic sludge collected in Southern Taiwan. The Klebsieblla sp. HE-2 cells were first grown on a fermentation medium (FM medium). The medium was then switched to resting-cell medium (RC medium) tailored to improve the production of 1,3-PDO. Using a glycerol-amended FM medium, the soluble metabolites consisted of 1,3-PDO, 2,3-butanediol, and ethanol and byproducts (such as acetic acid and lactic acid) at a content of 18, 28, 49, and 5% (of total soluble metabolites), respectively. When the culture was transferred from the FM medium to the RC medium, the concentration of 1,3-PDO was doubled from 5 g/L to 10 g/L. Using immobilized cells of Klebsieblla sp. HE-2 greatly improved the operational stability and reusability of the cells, as the immobilized cells could be used for 6 cycles without significant activity loss. The immobilized cells were able to directly utilize non-pretreated crude glycerol obtained from a local biodiesel manufacturing plant for 1,3-PDO production with an efficiency comparable to that obtained from using pure glycerol.     

Continuous production of 1,3-propanediol using raw glycerol with immobilized Clostridium beijerinckii NRRL B-593 in comparison to suspended culture

The continuous production of 1,3-propanediol (1,3-PDO) was investigated with Clostridium beijerinckii NRRL B-593 using raw glycerol without purification obtained from a biodiesel production process. Ceramic rings and pumice stones were used for cell immobilization in a packed-bed bioreactor. For comparison purpose, a control bioreactor with suspended culture was also run. The effect of hydraulic retention time (HRT) on the production of 1,3-PDO in both immobilized and suspended bioreactors were also investigated. The study revealed that HRT is an important factor for both immobilized and suspended systems and a HRT of 2 h is the best one in terms of volumetric production rate (g 1,3-PDO/L/h). Furthermore, cell immobilization had also obvious benefits especially for the robustness and the reliability of the production. The results indicated that cell immobilization achieved a 2.5-fold higher productivity in comparison to suspended cell system. Based on our results, continuous production of 1,3-PDO with immobilized cells is an efficient method, and raw glycerol can be utilized without any pretreatment.     

Cyanobacterial production of 1,3-propanediol directly from carbon dioxide using a synthetic metabolic pathway

Production of chemicals directly from carbon dioxide using light energy is an attractive option for a sustainable future. The 1,3-propanediol (1,3-PDO) production directly from carbon dioxide was achieved by engineered Synechococcus elongatus PCC 7942 with a synthetic metabolic pathway. Glycerol dehydratase catalyzing the conversion of glycerol to 3-hydroxypropionaldehyde in a coenzyme B₁₂-dependent manner worked in S. elongatus PCC 7942 without addition of vitamin B₁₂, suggesting that the intrinsic pseudovitamin B₁₂ served as a substitute of coenzyme B₁₂. The highest titers of 1,3-PDO (3.79±0.23 mM; 288±17.7 mg/L) and glycerol (12.62±1.55 mM; 1.16±0.14 g/L), precursor of 1,3-PDO, were reached after 14 days of culture under optimized conditions in this study.     

Disruption of glpF gene encoding the glycerol facilitator improves 1,3-propanediol production from glucose via glycerol in Escherichia coli

Engineered Escherichia coli has recently been applied to produce 1,3-propanediol (1,3-PDO) from glucose. A metabolic intermediate in the production pathway, glycerol, is partially secreted into the extracellular of E. coli through a glycerol facilitator encoded by glpF, and this secretion consequently decreases 1,3-PDO production. Therefore, we aimed to determine whether disrupting the glpF gene would improve 1,3-PDO production in E. coli. The intracellular glycerol concentration in a glpF-disruptant was 7·5 times higher than in a non-disruptant. The glpF-disrupted and non-disrupted E. coli strains produced 0·26 and 0·09 g l⁻¹ of 1,3-PDO, respectively, from 1% glucose after 72 h of cultivation. The specific growth rate (μ) and the 1,3-PDO yield from glucose (Y_P/S) in the disruptant were higher than those in the non-disruptant (ΔglpF, μ = 0·08 ± 0·00 h⁻¹, Y_P/S = 0·06 mol mol-glucose⁻¹; BW25113, μ = 0·06 ± 0·00 h⁻¹, Y_P/S = 0·02 mol mol-glucose⁻¹). Disruption of the glpF gene decreased the production of the by-product, acetic acid. These results indicated that disruption of glpF increased the intracellular concentration of glycerol and consequently increased 1,3-PDO production in E. coli.     

Isolation and characterization of novel 1,3-propanediol-producing Lactobacillus panis PM1 from bioethanol thin stillage

Conversion of glycerol to 1,3-propanediol (1,3-PDO) is an attractive option to increase the economic efficiency of the biofuel industry. A bacterial strain that produced 1,3-PDO in the presence of glycerol was isolated from thin stillage, the fermentation residue of bioethanol production. This 1,3-PDO-producing organism was identified as Lactobacillus panis through biochemical characteristics and by 16S rRNA sequencing. Characterization of the L. panis strain hereafter designated as PM1 revealed it was an aerotolerant acidophilic anaerobe able to grow over a wide range of temperatures; tolerant to high concentrations of sodium chloride, ethanol, acetic acid, and lactic acid; and resistant to many common antibiotics. L. panis PM1 could utilize glucose, lactose, galactose, maltose, xylose, and arabinose, but could not grow on sucrose or fructose. Production of 1,3-PDO by L. panis PM1 occurred only when glucose was available as the carbon source in the absence of oxygen. These metabolic characteristics strongly suggested NADH recycling for glucose metabolism is achieved through 1,3-PDO production by this strain. These characteristics classified L. panis PM1 within the group III heterofermentative lactic acid bacteria, which includes the well-characterized 1,3-PDO-producing strain, Lactobacillus reuteri. Metabolite production profiles showed that L. panis PM1 produced considerable amounts of succinic acid (~11–12 mM) from normal MRS medium, which distinguishes this strain from L. reuteri strains.