Characterization of yeasts with high L[+]-lactic acid production: lactic acid specific soft-agar overlay (LASSO) and TAFE-patterns

The transcriptomic response of a wine strain of Lachancea thermotolerans to oxygen deprivation

The yeast Lachancea thermotolerans is of significant biotechnological interest, and selected strains of this species have become commonly used starter cultures in wine fermentation. However, the impact of this species on wine is frequently limited by the rapid dominance of Saccharomyces cerevisiae strains which are better adapted to wine alcoholic fermentation conditions. Previous studies have shown that the major limiting factor for L. thermotolerans competitive performance in the wine ecosystem is oxygen availability, and not ethanol levels as had been previously suggested. Here we investigated the transcriptional response of L. thermotolerans to anaerobiosis in wine fermentation conditions. The data show that L. thermotolerans broadly redirects gene expression towards genes involved in central carbon metabolism, lipid metabolism, remodeling of the cell wall as well as autophagy. Furthermore, the induction of genes that are likely involved in the generation of lactate indicates a redirection of metabolic flux towards this metabolite. The data provide the first insight into the oxygen-dependent response of L. thermotolerans and suggest potential genetic targets to improve lactate production and/or anaerobic fermentation performance of this yeast.

New insights into the variability of lactic acid production in Lachancea thermotolerans at the phenotypic and genomic level

Non-conventional yeasts are increasingly applied in fermented beverage industry to obtain distinctive products with improved quality. Among these yeasts, Lachancea thermotolerans has multiple features of industrial relevance, especially the production of l(+)-lactic acid (LA), useful for the biological acidification of wine and beer. Since few information is available on this peculiar activity, the current study aimed to explore the physiological and genetic variability among L. thermotolerans strains. From a strain collection, mostly isolated from wine, a huge phenotypic diversity was acknowledged and allowed the selection of a high (SOL13) and a low (COLC27) LA producer. Comparative whole-genome sequencing of these two selected strains and the type strain CBS 6340^T showed a high similarity in terms of gene content and functional annotation. Notwithstanding, target gene-based analysis revealed variations between high and low producers in the key gene sequences related to LA accumulation. More in-depth investigation of the core promoters and expression analysis of the genes ldh, encoding lactate dehydrogenase, indicated the transcriptional regulation may be the principal cause behind phenotypic differences. These findings highlighted the usefulness of whole-genome sequencing coupled with expression analysis. They provided crucial genetic insights for a deeper investigation of the intraspecific variability in LA production pathway.

The evolution of Lachancea thermotolerans is driven by geographical determination, anthropisation and flux between different ecosystems

The yeast Lachancea thermotolerans (formerly Kluyveromyces thermotolerans) is a species with remarkable, yet underexplored, biotechnological potential. This ubiquist occupies a range of natural and anthropic habitats covering a wide geographic span. To gain an insight into L. thermotolerans population diversity and structure, 172 isolates sourced from diverse habitats worldwide were analysed using a set of 14 microsatellite markers. The resultant clustering revealed that the evolution of L. thermotolerans has been driven by the geography and ecological niche of the isolation sources. Isolates originating from anthropic environments, in particular grapes and wine, were genetically close, thus suggesting domestication events within the species. The observed clustering was further validated by several means including, population structure analysis, F-statistics, Mantel’s test and the analysis of molecular variance (AMOVA). Phenotypic performance of isolates was tested using several growth substrates and physicochemical conditions, providing added support for the clustering. Altogether, this study sheds light on the genotypic and phenotypic diversity of L. thermotolerans, contributing to a better understanding of the population structure, ecology and evolution of this non-Saccharomyces yeast.

Sequential gene integration for the engineering of Kluyveromyces marxianus

The attributes of the yeast Kluyveromyces marxianus (rapid growth rate at high temperature, utilization of a wide range of inexpensive carbon sources) make it a promising industrial host for the synthesis of protein and non-protein products. However, no stable multicopy plasmids are currently available for long-term culture of K. marxianus. To allow the stable genetic/metabolic engineering of K. marxianus, a method for integrating precise numbers of the same or different genes was developed for this yeast. A K. marxianus URA3 deletion mutant was constructed and the URA3 blaster (UB) reusable selection cassette from Saccharomyces cerevisiae was used to select sequential, untargeted chromosomal insertions of the Bacillus megaterium lactate dehydrogenase (LDH) gene. Following excision of the UB cassette from the chromosomes, the integrating vector was retransformed into the strain and a second copy of LDH was inserted, demonstrating the success of this method for sequential gene integrations in K. marxianus. LDH activity and lactic acid concentration increased with each gene insertion, further illustrating the success of this method.

Production ofD-Lactic Acid by Bacterial Fermentation of Rice Starch

D-Lactic acid was synthesized by the fermentation of rice starch using microorganisms. Two species: Lactobacillus delbrueckii and Sporolactobacillus inulinus were found to be active in producing D-lactic acid of high optical purity after an intensive screening test for D-lactic acid bacteria using glucose as substrate. Rice powder used as the starch source was hydrolyzed with a combination of enzymes: alpha-amylase, beta-amylase, and pullulanase to obtain rice saccharificate consisting of maltose as the main component. Its average gross yield was 82.5%. Of the discovered D-lactic acid bacteria, only Lactobacillus delbrueckii could ferment both maltose and the rice saccharificate. After optimizing the fermentation of the rice saccharificate using this bacterium, pilot scale fermentation was conducted to convert the rice saccharificate into D-lactic acid with a D-content higher than 97.5% in a yield of 70%. With this yield, the total yield of D-lactic acid from brown rice was estimated to be 47%, which is almost equal to the L-lactic acid yield from corn. The efficient synthesis of D-lactic acid can open a way to the large scale application of high-melting poly(lactic acid) that is a stereocomplex of poly(L-lactide) and poly(D-lactide). Schematic representation of the production of D-lactic acid starting from brown rice as described here.

Mixed lactic acid-alcoholic fermentation by Saccharomyces cerevisiae expressing the Lactobacillus casei L(+)-LDH

We describe the construction of a Saccharomyces cerevisiae strain expressing the gene encoding the L(+)-lactate dehydrogenase [L(+)-LDH)] from Lactobacillus casei. The recombinant strain is able to perform a mixed lactic acid-alcoholic fermentation. Yeast cells expressing the L(+)-LDH gene from the yeast alcohol dehydrogenase (ADH1) promoter on a multicopy plasmid simultaneously convert glucose to both ethanol and lactate, with up to 20% of the glucose transformed into L(+)-lactate. Such strains may be used in every field where both biological acidification and alcoholic fermentation are required.