Complete and efficient conversion of plant cell wall hemicellulose into high-value bioproducts by engineered yeast

Advances in the metabolic engineering of Saccharomyces cerevisiae and Yarrowia lipolytica for the production of -carotene

β-Carotene is one kind of the most important carotenoids. The major functions of β-carotene include the antioxidant and anti-cardiovascular properties, which make it a growing market. Recently, the use of metabolic engineering to construct microbial cell factories to synthesize β-carotene has become the latest model for its industrial production. Among these cell factories, yeasts including Saccharomyces cerevisiae and Yarrowia lipolytica have attracted the most attention because of the: security, mature genetic manipulation tools, high flux toward carotenoids using the native mevalonate pathway and robustness for large-scale fermentation. In this review, the latest strategies for β-carotene biosynthesis, including protein engineering, promoters engineering and morphological engineering are summarized in detail. Finally, perspectives for future engineering approaches are proposed to improve β-carotene production.

Synthetically-primed adaptation of Pseudomonas putida to a non-native substrate D-xylose

To broaden the substrate scope of microbial cell factories towards renewable substrates, rational genetic interventions are often combined with adaptive laboratory evolution (ALE). However, comprehensive studies enabling a holistic understanding of adaptation processes primed by rational metabolic engineering remain scarce. The industrial workhorse Pseudomonas putida was engineered to utilize the non-native sugar D-xylose, but its assimilation into the bacterial biochemical network via the exogenous xylose isomerase pathway remained unresolved. Here, we elucidate the xylose metabolism and establish a foundation for further engineering followed by ALE. First, native glycolysis is derepressed by deleting the local transcriptional regulator gene hexR. We then enhance the pentose phosphate pathway by implanting exogenous transketolase and transaldolase into two lag-shortened strains and allow ALE to finetune the rewired metabolism. Subsequent multilevel analysis and reverse engineering provide detailed insights into the parallel paths of bacterial adaptation to the non-native carbon source, highlighting the enhanced expression of transaldolase and xylose isomerase along with derepressed glycolysis as key events during the process.

A versatile in situ cofactor enhancing system for meeting cellular demands for engineered metabolic pathways

Cofactor imbalance obstructs the productivities of metabolically engineered cells. Herein, we employed a minimally perturbing system, xylose reductase and lactose (XR/lactose), to increase the levels of a pool of sugar phosphates which are connected to the biosynthesis of NAD(P)H, FAD, FMN, and ATP in Escherichia coli. The XR/lactose system could increase the amounts of the precursors of these cofactors and was tested with three different metabolically engineered cell systems (fatty alcohol biosynthesis, bioluminescence light generation, and alkane biosynthesis) with different cofactor demands. Productivities of these cells were increased 2-4-fold by the XR/lactose system. Untargeted metabolomic analysis revealed different metabolite patterns among these cells, demonstrating that only metabolites involved in relevant cofactor biosynthesis were altered. The results were also confirmed by transcriptomic analysis. Another sugar reducing system (glucose dehydrogenase) could also be used to increase fatty alcohol production but resulted in less yield enhancement than XR. This work demonstrates that the approach of increasing cellular sugar phosphates can be a generic tool to increase in vivo cofactor generation upon cellular demand for synthetic biology.

Efficient Production of Triacetic Acid Lactone from Lignocellulose Hydrolysate by Metabolically Engineered Yarrowia lipolytica

Lignocellulose is a promising renewable feedstock for the bioproduction of high-value biochemicals. The poorly expressed xylose catabolic pathway was the bottleneck in the efficient utilization of the lignocellulose feedstock in yeast. Herein, multiple genetic and process engineering strategies were explored to debottleneck the conversion of xylose to the platform chemical triacetic acid lactone (TAL) in Yarrowia lipolytica. We identified that xylose assimilation generating more cofactor NADPH was favorable for the TAL synthesis. pH control improved the expression of acetyl-CoA carboxylase and generated more precursor malonyl-CoA. Combined with the suppression of the lipid synthesis pathway, 5.03 and 4.18 g/L TAL were produced from pure xylose and xylose-rich wheat straw hydrolysate, respectively. Our work removed the bottleneck of the xylose assimilation pathway and effectively upgraded wheat straw hydrolysate to TAL, which enabled us to build a sustainable oleaginous yeast cell factory to cost-efficiently produce green chemicals from low-cost lignocellulose by Y. lipolytica.

Unlocking the potential of Dongting Lake-grown Miscanthus lutarioriparius biomass: A comprehensive quality analysis and bioproduct application study

Miscanthus lutarioriparius grown in Dongting Lake has an annual biomass yield potential of 1 million tons. However, with the shutdown of its previous utilization for paper-making, abandoning this huge amount of biomass has caused serious economic, ecological, and social problems. Constructing an industrial cluster to continuously convert biomass into various bioproducts is a win-win measure to address this dilemma. With the increasing confirmation of the importance of biomass quality affecting the conservation process, fully understanding the biomass characteristics of Dongting Lake-grown M. lutarioriparius is crucial for building a scientific industrial cluster. The present work is designed to explore the variation in biomass quality across the entire Dongting Lake area. Results show that the biomass contented with Cd, Mn, Zn, and Cr has significant geographical differences, with a general trend of Southern Dongting Lake-grown biomass having a higher concentration than that from Eastern and Western Dongting Lake areas. Moreover, significant differences are found in terms of biomass ash content, lignin content, and the degree of polymerization of cellulose (DP). The biomass with low ash content is generally from the entire Eastern Dongting Lake area and the northern part of the Western Dongting Lake area. Virtually all Western Dongting Lake-grown biomass has a low lignin content (approximately 18 %). Regarding the spatial variation of DP, Eastern Dongting Lake-grown biomass has a higher DP (average at 585.33) than that in Southern (575.15) and then Western Dongting Lake (529.16). Based on these quality indicators, the biomass production potentials for bioethanol, biochar, and xylo-oligosaccharide were calculated and visualized. Results show that biomass from almost the entire Western and Eastern Dongting Lake area is suitable for bioethanol and xylo-oligosaccharide production, while biomass from the Southern Dongting Lake area for biochar production. These results provide scientific guidance for the future utilization of Dongting Lake-grown M. lutarioriparius biomass.

Assessing lignocellulosic biomass as a source of emergency foods

Catastrophes such as a nuclear war would generate atmospheric soot and reduce sunlight, making it difficult to grow crops. Under such conditions, people might turn to inedible plant biomass for nutrition, but the convertibility and nutritional content of this biomass have not been rigorously analyzed. We found that if plant biomass were converted into food at 30% efficiency, 6.7 kg of biomass per day would yield adequate carbohydrates, but contain potentially toxic or insufficient levels of other nutrients for a family of four. Therefore, exploiting biomass with low mineral content for carbohydrates and consuming other sources of protein, fat, and vitamins such as edible insects/single-cell proteins and vitamin supplements could provide a balanced diet in a global catastrophic environment.

A novel strategy for L-arginine production in engineered Escherichia coli

BACKGROUND

L-arginine is an important amino acid with applications in diverse industrial and pharmaceutical fields. N-acetylglutamate, synthesized from L-glutamate and acetyl-CoA, is a precursor of the L-arginine biosynthetic branch in microorganisms. The enzyme that produces N-acetylglutamate, N-acetylglutamate synthase, is allosterically inhibited by L-arginine. L-glutamate, as a central metabolite, provides carbon backbone for diverse biological compounds besides L-arginine. When glucose is the sole carbon source, the theoretical maximum carbon yield towards L-arginine is 96.7%, but the experimental highest yield was 51%. The gap of L-arginine yield indicates the regulation complexity of carbon flux and energy during the L-arginine biosynthesis. Besides endogenous biosynthesis, N-acetylglutamate, the key precursor of L-arginine, can be obtained by chemical acylation of L-glutamate with a high yield of 98%. To achieve high-yield production of L-arginine, we demonstrated a novel approach by directly feeding precursor N-acetylglutamate to engineered Escherichia coli.

RESULTS

We reported a new approach for the high yield of L-arginine production in E. coli. Gene argA encoding N-acetylglutamate synthase was deleted to disable endogenous biosynthesis of N-acetylglutamate. The feasibility of external N-acetylglutamate towards L-arginine was verified via growth assay in argA- strain. To improve L-arginine production, astA encoding arginine N-succinyltransferase, speF encoding ornithine decarboxylase, speB encoding agmatinase, and argR encoding an arginine responsive repressor protein were disrupted. Based on overexpression of argDGI, argCBH operons, encoding enzymes of the L-arginine biosynthetic pathway, ~ 4 g/L L-arginine was produced in shake flask fermentation, resulting in a yield of 0.99 mol L-arginine/mol N-acetylglutamate. This strain was further engineered for the co-production of L-arginine and pyruvate by removing genes adhE, ldhA, poxB, pflB, and aceE, encoding enzymes involved in the conversion and degradation of pyruvate. The resulting strain was shown to produce 4 g/L L-arginine and 11.3 g/L pyruvate in shake flask fermentation.

CONCLUSIONS

Here, we developed a novel approach to avoid the strict regulation of L-arginine on ArgA and overcome the metabolism complexity in the L-arginine biosynthesis pathway. We achieve a high yield of L-arginine production from N-acetylglutamate in E. coli. Co-production pyruvate and L-arginine was used as an example to increase the utilization of input carbon sources.

Direct Utilization of Peroxisomal Acetyl-CoA for the Synthesis of Polyketide Compounds in Saccharomyces cerevisiae

Polyketides are a class of natural products with many applications but are mainly appealing as pharmaceuticals. Heterologous production of polyketides in the yeast Saccharomyces cerevisiae has been widely explored because of the many merits of this model eukaryotic microorganism. Although acetyl-CoA and malonyl-CoA, the precursors for polyketide synthesis, are distributed in several yeast subcellular organelles, only cytosolic synthesis of polyketides has been pursued in previous studies. In this study, we investigate polyketide synthesis by directly using acetyl-CoA in the peroxisomes of yeast strain CEN.PK2-1D. We first demonstrate that the polyketide flaviolin can be synthesized in this organelle upon peroxisomal colocalization of native acetyl-CoA carboxylase and 1,3,6,8-tetrahydroxynaphthalene synthase (a type III polyketide synthase). Next, using the synthesis of the polyketide triacetic acid lactone as an example, we show that (1) a new peroxisome targeting sequence, pPTS1, is more effective than the previously reported ePTS1 for peroxisomal polyketide synthesis; (2) engineering peroxisome proliferation is effective to boost polyketide production; and (3) peroxisomes provide an additional acetyl-CoA reservoir and extra space to accommodate enzymes so that utilizing the peroxisomal pathway plus the cytosolic pathway produces more polyketide than the cytosolic pathway alone. This research lays the groundwork for more efficient heterologous polyketide biosynthesis using acetyl-CoA pools in subcellular organelles.

Improving solubility and copy number of taxadiene synthase to enhance the titer of taxadiene in Yarrowia lipolytica

Taxadiene is an important precursor for the biosynthesis of highly effective anticancer drug paclitaxel, but its microbial biosynthesis yield is very low. In this study, we employed Yarrowia lipolytica as a microbial host to produce taxadiene. First, a "push-pull" strategy was adopted to increase taxadiene production by 234%. Then taxadiene synthase was fused with five solubilizing tags respectively, leading a maximum increase of 62.3% in taxadiene production when fused with SUMO. Subsequently, a multi-copy iterative integration method was used to further increase taxadiene titer, achieving the maximum titer of 23.7 mg/L in shake flask culture after three rounds of integration. Finally, the taxadiene titer was increased to 101.4 mg/L by optimization of the fed-batch fermentation conditions. This is the first report of taxadiene biosynthesis accomplished in Y. lipolytica, serving as a good example for the sustainable production of taxadiene and other terpenoids in this oleaginous yeast.

Big data mining, rational modification, and ancestral sequence reconstruction inferred multiple xylose isomerases for biorefinery

The isomerization of xylose to xylulose is considered the most promising approach to initiate xylose bioconversion. Here, phylogeny-guided big data mining, rational modification, and ancestral sequence reconstruction strategies were implemented to explore new active xylose isomerases (XIs) for Saccharomyces cerevisiae. Significantly, 13 new active XIs for S. cerevisiae were mined or artificially created. Moreover, the importance of the amino-terminal fragment for maintaining basic XI activity was demonstrated. With the mined XIs, four efficient xylose-utilizing S. cerevisiae were constructed and evolved, among which the strain S. cerevisiae CRD5HS contributed to ethanol titers as high as 85.95 and 94.76 g/liter from pretreated corn stover and corn cob, respectively, without detoxifying or washing pretreated biomass. Potential genetic targets obtained from adaptive laboratory evolution were further analyzed by sequencing the high-performance strains. The combined XI mining methods described here provide practical references for mining other scarce and valuable enzymes.

Enhanced production of 3,4-dihydroxybutyrate from xylose by engineered yeast via xylonate re-assimilation under alkaline condition

To realize lignocellulose-based bioeconomy, efficient conversion of xylose into valuable chemicals by microbes is necessary. Xylose oxidative pathways that oxidize xylose into xylonate can be more advantageous than conventional xylose assimilation pathways because of fewer reaction steps without loss of carbon and ATP. Moreover, commodity chemicals like 3,4-dihydroxybutyrate and 3-hydroxybutyrolactone can be produced from the intermediates of xylose oxidative pathway. However, successful implementations of xylose oxidative pathway in yeast have been hindered because of the secretion and accumulation of xylonate which is a key intermediate of the pathway, leading to low yield of target product. Here, high-yield production of 3,4-dihydroxybutyrate from xylose by engineered yeast was achieved through genetic and environmental perturbations. Specifically, 3,4-dihydroxybutyrate biosynthetic pathway was established in yeast through deletion of ADH6 and overexpression of yneI. Also, inspired by the mismatch of pH between host strain and key enzyme of XylD, alkaline fermentations (pH ≥ 7.0) were performed to minimize xylonate accumulation. Under the alkaline conditions, xylonate was re-assimilated by engineered yeast and combined product yields of 3,4-dihydroxybutyrate and 3-hydroxybutyrolactone resulted in 0.791 mol/mol-xylose, which is highest compared with previous study. These results shed light on the utility of the xylose oxidative pathway in yeast.

Alleviating glucose repression and enhancing respiratory capacity to increase itaconic acid production

The Crabtree effect products ethanol and acetic acid can be used for itaconic acid (IA) production in Saccharomyces cerevisiae. However, both the IA synthesis and oxidative phosphorylation pathways were hampered by glucose repression when glucose was used as the substrate. This study aimed to improve IA titer by increasing gene expressions related to glucose derepression without impairing yeast growth on glucose. Engineering the acetyl-CoA synthesis pathway increased the titer of IA to 257 mg/L in a urea-based medium. Instead of entire pathway overexpression, we found that some signaling pathways regulating glucose repression were effective targets to improve IA production and respiratory capacity. As a consequence of the reduced inhibition, IA titer was further increased by knocking out a negative regulator of the mitochondrial retrograde signaling MKS1. SNF1/MIG1 signaling was disturbed by deleting the hexokinase HXK2 or an endoplasmic reticulum membrane protein GSF2. The shaking results showed that XYY286 (BY4741, HO::cadA, Y::Dz.ada, 208a::Mt.acs, Δhxk2, pRS415-cadA, pRS423-aac2) accumulated 535 mg/L IA in 168 h in the YSCGLU medium. qRT-PCR results verified that deletion of MKS1 or HXK2 upregulated the gene expressions of the IA synthesis and respiratory pathways during the growth on glucose.

Advanced one-pot deconstruction and valorization of lignocellulosic biomass into triacetic acid lactone using Rhodosporidium toruloides

BACKGROUND

Rhodosporidium toruloides is capable of co-utilization of complex carbon sources and robust growth from lignocellulosic hydrolysates. This oleaginous yeast is therefore an attractive host for heterologous production of valuable bioproducts at high titers from low-cost, deconstructed biomass in an economically and environmentally sustainable manner. Here we demonstrate this by engineering R. toruloides to produce the polyketide triacetic acid lactone (TAL) directly from unfiltered hydrolysate deconstructed from biomass with minimal unit process operations.

RESULTS

Introduction of the 2-pyrone synthase gene into R. toruloides enabled the organism to produce 2.4 g/L TAL from simple media or 2.0 g/L from hydrolysate produced from sorghum biomass. Both of these titers are on par with titers from other better-studied microbial hosts after they had been heavily engineered. We next demonstrate that filtered hydrolysates produced from ensiled sorghum are superior to those derived from dried sorghum for TAL production, likely due to the substantial organic acids produced during ensiling. We also demonstrate that the organic acids found in ensiled biomass can be used for direct synthesis of ionic liquids within the biomass pretreatment process, enabling consolidation of unit operations of in-situ ionic liquid synthesis, pretreatment, saccharification, and fermentation into a one-pot, separations-free process. Finally, we demonstrate this consolidation in a 2 L bioreactor using unfiltered hydrolysate, producing 3.9 g/L TAL.

CONCLUSION

Many steps involved in deconstructing biomass into fermentable substrate can be combined into a distinct operation, and directly fed to cultures of engineered R. toruloides cultures for subsequent valorization into gram per liter titers of TAL in a cost-effective manner.

Biorefineries: Achievements and challenges for a bio-based economy

Climate change, socioeconomical pressures, and new policy and legislation are driving a decarbonization process across industries, with a critical shift from a fossil-based economy toward a biomass-based one. This new paradigm implies not only a gradual phasing out of fossil fuels as a source of energy but also a move away from crude oil as a source of platform chemicals, polymers, drugs, solvents and many other critical materials, and consumer goods that are ubiquitous in our everyday life. If we are to achieve the United Nations' Sustainable Development Goals, crude oil must be substituted by renewable sources, and in this evolution, biorefineries arise as the critical alternative to traditional refineries for producing fuels, chemical building blocks, and materials out of non-edible biomass and biomass waste. State-of-the-art biorefineries already produce cost-competitive chemicals and materials, but other products remain challenging from the economic point of view, or their scaled-up production processes are still not sufficiently developed. In particular, lignin's depolymerization is a required milestone for the success of integrated biorefineries, and better catalysts and processes must be improved to prepare bio-based aromatic simple molecules. This review summarizes current challenges in biorefinery systems, while it suggests possible directions and goals for sustainable development in the years to come.

Cas9-Based Metabolic Engineering of Issatchenkia orientalis for Enhanced Utilization of Cellulosic Hydrolysates

Issatchenkia orientalis, exhibiting high tolerance against harsh environmental conditions, is a promising metabolic engineering host for producing fuels and chemicals from cellulosic hydrolysates containing fermentation inhibitors under acidic conditions. Although genetic tools for I. orientalis exist, they require auxotrophic mutants so that the selection of a host strain is limited. We developed a drug resistance gene (cloNAT)-based genome-editing method for engineering any I. orientalis strains and engineered I. orientalis strains isolated from various sources for xylose fermentation. Specifically, xylose reductase, xylitol dehydrogenase, and xylulokinase from Scheffersomyces stipitis were integrated into an intended chromosomal locus in four I. orientalis strains (SD108, IO21, IO45, and IO46) through Cas9-based genome editing. The resulting strains (SD108X, IO21X, IO45X, and IO46X) efficiently produced ethanol from cellulosic and hemicellulosic hydrolysates even though the pH adjustment and nitrogen source were not provided. As they presented different fermenting capacities, selection of a host I. orientalis strain was crucial for producing fuels and chemicals using cellulosic hydrolysates.

Advances in Metabolic Engineering Paving the Way for the Efficient Biosynthesis of Terpenes in Yeasts

Terpenes are a large class of secondary metabolites with diverse structures and functions that are commonly used as valuable raw materials in food, cosmetics, and medicine. With the development of metabolic engineering and emerging synthetic biology tools, these important terpene compounds can be sustainably produced using different microbial chassis. Currently, yeasts such as Saccharomyces cerevisiae and Yarrowia lipolytica have received extensive attention as potential hosts for the production of terpenes due to their clear genetic background and endogenous mevalonate pathway. In this review, we summarize the natural terpene biosynthesis pathways and various engineering strategies, including enzyme engineering, pathway engineering, and cellular engineering, to further improve the terpene productivity and strain stability in these two widely used yeasts. In addition, the future prospects of yeast-based terpene production are discussed in light of the current progress, challenges, and trends in this field. Finally, guidelines for future studies are also emphasized.

Chain flexibility of medicinal lipids determines their selective partitioning into lipid droplets

In guiding lipid droplets (LDs) to serve as storage vessels that insulate high-value lipophilic compounds in cells, we demonstrate that chain flexibility of lipids determines their selective migration in intracellular LDs. Focusing on commercially important medicinal lipids with biogenetic similarity but structural dissimilarity, we computationally and experimentally validate that LD remodeling should be differentiated between overproduction of structurally flexible squalene and that of rigid zeaxanthin and β-carotene. In molecular dynamics simulations, worm-like flexible squalene is readily deformed to move through intertwined chains of triacylglycerols in the LD core, whereas rod-like rigid zeaxanthin is trapped on the LD surface due to a high free energy barrier in diffusion. By designing yeast cells with either much larger LDs or with a greater number of LDs, we observe that intracellular storage of squalene significantly increases with LD volume expansion, but that of zeaxanthin and β-carotene is enhanced through LD surface broadening; as visually evidenced, the outcomes represent internal penetration of squalene and surface localization of zeaxanthin and β-carotene. Our study shows the computational and experimental validation of selective lipid migration into a phase-separated organelle and reveals LD dynamics and functionalization.

Lipid droplet (LD) is a highly dynamic organelle capable of regulating lipid metabolism, storage and transportation. Here, by combining molecular dynamics simulations and microbial LD engineering, the authors demonstrate that the structural flexibility of lipids is one of decisive factors in selective partitioning into LDs.

Reprogramming Microbial CO2-Metabolizing Chassis With CRISPR-Cas Systems

Global warming is approaching an alarming level due to the anthropogenic emission of carbon dioxide (CO₂). To overcome the challenge, the reliance on fossil fuels needs to be alleviated, and a significant amount of CO₂ needs to be sequestrated from the atmosphere. In this endeavor, carbon-neutral and carbon-negative biotechnologies are promising ways. Especially, carbon-negative bioprocesses, based on the microbial CO₂-metabolizing chassis, possess unique advantages in fixing CO₂ directly for the production of fuels and value-added chemicals. In order to fully uncover the potential of CO₂-metabolizing chassis, synthetic biology tools, such as CRISPR-Cas systems, have been developed and applied to engineer these microorganisms, revolutionizing carbon-negative biotechnology. Herein, we review the recent advances in the adaption of CRISPR-Cas systems, including CRISPR-Cas based genome editing and CRISPR interference/activation, in cyanobacteria, acetogens, and methanogens. We also envision future innovations via the implementation of rising CRISPR-Cas systems, such as base editing, prime editing, and transposon-mediated genome editing.

Metabolic engineering of oleaginous yeast Rhodotorula toruloides for overproduction of triacetic acid lactone

The plant‐sourced polyketide triacetic acid lactone (TAL) has been recognized as a promising platform chemical for the biorefinery industry. However, its practical application was rather limited due to low natural abundance and inefficient cell factories for biosynthesis. Here, we report the metabolic engineering of oleaginous yeast Rhodotorula toruloides for TAL overproduction. We first introduced a 2‐pyrone synthase gene from Gerbera hybrida (GhPS) into R. toruloides and investigated the effects of different carbon sources on TAL production. We then systematically employed a variety of metabolic engineering strategies to increase the flux of acetyl‐CoA by enhancing its biosynthetic pathways and disrupting its competing pathways. We found that overexpression of ATP‐citrate lyase (ACL1) improved TAL production by 45% compared to the GhPS overexpressing strain, and additional overexpression of acetyl‐CoA carboxylase (ACC1) further increased TAL production by 29%. Finally, we characterized the resulting strain I12‐ACL1‐ACC1 using fed‐batch bioreactor fermentation in glucose or oilcane juice medium with acetate supplementation and achieved a titer of 28 or 23 g/L TAL, respectively. This study demonstrates that R. toruloides is a promising host for the production of TAL and other acetyl‐CoA‐derived polyketides from low‐cost carbon sources.

A Review of the Recent Developments in the Bioproduction of Polylactic Acid and Its Precursors Optically Pure Lactic Acids

Lactic acid (LA) is an important organic acid with broad industrial applications. Considered as an environmentally friendly alternative to petroleum-based plastic with a wide range of applications, polylactic acid has generated a great deal of interest and therefore the demand for optically pure l- or d-lactic acid has increased accordingly. Microbial fermentation is the industrial route for LA production. LA bacteria and certain genetic engineering bacteria are widely used for LA production. Although some fungi, such as Saccharomyces cerevisiae, are not natural LA producers, they have recently received increased attention for LA production because of their acid tolerance. The main challenge for LA bioproduction is the high cost of substrates. The development of LA production from cost-effective biomasses is a potential solution to reduce the cost of LA production. This review examined and discussed recent progress in optically pure l-lactic acid and optically pure d-lactic acid fermentation. The utilization of inexpensive substrates is also focused on. Additionally, for PLA production, a complete biological process by one-step fermentation from renewable resources is also currently being developed by metabolically engineered bacteria. We also summarize the strategies and procedures for metabolically engineering microorganisms producing PLA. In addition, there exists some challenges to efficiently produce PLA, therefore strategies to overcome these challenges through metabolic engineering combined with enzyme engineering are also discussed.