Influence of culture pH on freeze-drying viability of Oenococcus oeni and its relationship with fatty acid composition

Insights into the mechanisms of L. salivarius CECT5713 resistance to freeze-dried storage

Ligilactobacillus salivarius is a lactic acid bacterium exhibiting several health benefits. However, it is sensitive to freeze-drying and storage in the dried state, thus limiting its commercial exploitation. Our objective was to identify markers of cell resistance by applying multiscale characterization to L. salivarius CECT5713 cell populations exhibiting different resistance to freeze-dried storage. Cells were produced under two different sets of production conditions differing in the culture parameters (temperature, neutralizing solution, and harvesting time) and the protective formulation composition. The culturability, membrane integrity, and cell biochemical composition assessed by Fourier transform infrared (FTIR) micro-spectroscopy were evaluated after freezing, freeze-drying, and subsequent storage at 37 °C. Membrane properties (fatty acid composition, membrane fluidity, and phospholipid organization), as well as matrix physical properties (glass transition temperature and water activity), were determined. The most resistant cells to freeze-dried storage exhibited the highest cyclic fatty acid content and the most rigid membrane. Freeze-drying and storage induced damage to membrane integrity, proteins, nucleic acids, and constituents of the peptidoglycan cell wall. From the FTIR spectra analysis, we propose the minimization of the variations of the 1058 and 1714 cm-1 vibration bands (that arise mainly from symmetric C-O-C stretching and CO stretching, respectively) induced by the freeze-drying process as a marker of storage stability. We confirmed that a matrix with a glass transition temperature at least 50 °C higher than the storage temperature is crucial for L. salivarius CECT5713 storage stability. In addition, this work explored promising FTIR methods for a better understanding of the protection mechanisms involved.

Non-inhibitory levels of oxygen during cultivation increase freeze-drying stress tolerance in Limosilactobacillus reuteri DSM 17938

The physiological effects of oxygen on Limosilactobacillus reuteri DSM 17938 during cultivation and the ensuing properties of the freeze-dried probiotic product was investigated. On-line flow cytometry and k-means clustering gating was used to follow growth and viability in real time during cultivation. The bacterium tolerated aeration at 500 mL/min, with a growth rate of 0.74 ± 0.13 h^-1 which demonstrated that low levels of oxygen did not influence the growth kinetics of the bacterium. Modulation of the redox metabolism was, however, seen already at non-inhibitory oxygen levels by 1.5-fold higher production of acetate and 1.5-fold lower ethanol production. A significantly higher survival rate in the freeze-dried product was observed for cells cultivated in presence of oxygen compared to absence of oxygen (61.8% ± 2.4% vs. 11.5% ± 4.3%), coinciding with a higher degree of unsaturated fatty acids (UFA:SFA ratio of 10 for air sparged vs. 3.59 for N₂ sparged conditions.). Oxygen also resulted in improved bile tolerance and boosted 5'nucleotidase activity (370 U/L vs. 240 U/L in N₂ sparged conditions) but lower tolerance to acidic conditions compared bacteria grown under complete anaerobic conditions which survived up to 90 min of exposure at pH 2. Overall, our results indicate the controlled supply of oxygen during production may be used as means for probiotic activity optimization of L. reuteri DSM 17938.

Flow cytometric analysis reveals culture condition dependent variations in phenotypic heterogeneity of Limosilactobacillus reuteri

Optimisation of cultivation conditions in the industrial production of probiotics is crucial to reach a high-quality product with retained probiotic functionality. Flow cytometry-based descriptors of bacterial morphology may be used as markers to estimate physiological fitness during cultivation, and can be applied for online monitoring to avoid suboptimal growth. In the current study, the effects of temperature, initial pH and oxygen levels on cell growth and cell size distributions of Limosilactobacillus reuteri DSM 17938 were measured using multivariate flow cytometry. A pleomorphic behaviour was evident from the measurements of light scatter and pulse width distributions. A pattern of high growth yielding smaller cells and less heterogeneous populations could be observed. Analysis of pulse width distributions revealed significant morphological heterogeneities within the bacterial cell population under non-optimal growth conditions, and pointed towards low temperature, high initial pH, and high oxygen levels all being triggers for changes in morphology towards cell chain formation. However, cell size did not correlate to survivability after freeze-thaw or freeze-drying stress, indicating that it is not a key determinant for physical stress tolerance. The fact that L. reuteri morphology varies depending on cultivation conditions suggests that it can be used as marker for estimating physiological fitness and responses to its environment.

Effect of oleic acid on the viability of different freeze-dried Lactiplantibacillus plantarum strains

Freeze drying is one of the most convenient ways to preserve microorganisms, but in the freeze-drying process, strains will inevitably suffer varying degrees of damage under different conditions. The deterioration of cell membrane integrity is one of the main forms of damage. The type and ratio of fatty acids in the cell membrane affect its characteristics. Therefore, it is worth investigating whether certain fatty acids can increase freeze-drying resistance. In this study, we found that adding a low concentration of oleic acid to a cryoprotectant could increase survival rate of strains of Lactiplantibacillus plantarum following freeze drying, and the optimal concentration of oleic acid was determined to be 0.001%. When 0.001% oleic acid was added to phosphate-buffered saline, the freeze-drying survival rate of L. plantarum increased by up to 6.63 times. Adding 0.001% oleic acid to sorbitol, the survival rate of L. plantarum increased by as much as 3.65 times. The 0.001% oleic acid-sucrose cryoprotectant resulted in a freeze-drying survival rate of L. plantarum of about 90%, a 2.26-fold improvement compared with sucrose alone. Although the effect of oleic acid depends on the cryoprotectants used and the strain treated, addition of oleic acid showed significant improvement overall. Further experiments showed that adding a low concentration of oleic acid to the cryoprotectants improved the freeze-drying survival rate of L. plantarum by maintaining cell membrane integrity and lactate dehydrogenase activity. Our findings provide a new strategy for safeguarding bacterial viability in commonly used cryoprotectants by the addition of a common food ingredient, which may be extensively applied in the food industry.

Whey permeate as a substrate for the production of freeze-dried Lactiplantibacillus plantarum to be used as a malolactic starter culture

The aim of this work was to obtain freeze-dried biomass of the native Patagonian Lactiplantibacillus plantarum strain UNQLp 11 from a whey permeate (WP)-based medium and compare it with the growth in commercial MRS broth medium. Survival and activity of the freeze-dried Lb. plantarum strain were investigated after inoculation in wine as a starter culture for malolactic fermentation (MLF). The effect of storage and rehydration condition of the dried bacteria and the nutrient supplementation of wine were also studied. The freeze-dried cultures from WP and those grown in MRS showed similar survival results. Rehydration in MRS broth for 24 h and the addition of a rehydration medium to wine as nutrient supplementation improved the survival under wine harsh conditions and guaranteed the success of MLF. Storage at 4 °C under vacuum was the best option, maintaining high cell viability for at least 56 days, with malic acid consumption higher than 90% after 7 days of inoculation in a wine-like medium. These results represent a significant advance for sustainable production of dried malolactic starter cultures in an environmentally friendly process, which is low cost and easy to apply in winemaking under harsh physicochemical conditions.

Influences of acid and ethanol stresses on Oenococcus oeni SD-2a and its proteomic and transcriptional responses

BACKGROUND

During winemaking, malolactic fermentation (MLF) is usually induced by Oenococcus oeni owing to its high resistance to wine stress factors. To ensure a controlled and efficient MLF process, starter cultures are inoculated in wine. In previous studies, O. oeni strains with sub-lethal acid or ethanol stresses showed higher freeze-drying vitality and better MLF performance. To explore the mechanisms involved, influences of acid and ethanol stresses on O. oeni SD-2a were investigated in this study to gain a better understanding of the cross-protection responses.

RESULTS

The results showed that acid and ethanol stresses both caused damage to cell membranes and decreased cellular adenosine triphosphate concentration. At the same time, acid stress increased the uptake of glutathione, while ethanol stress led to cell depolarization. The results of comparative proteomic analysis highlighted that heat shock protein was induced with almost all acid and ethanol stresses. In addition, the expression of stress-relevant genes (hsp20, clpP, trxA, ctsR, recO, usp) increased greatly with ethanol and acid stress treatments. Finally, the viability of O. oeni was improved with acid and ethanol pretreatments after freeze-drying.

CONCLUSIONS

This study demonstrated that acid and ethanol stresses had mixed influences on O. oeni SD-2a. Some physiological and molecular changes would contribute to a more stress-tolerant state of O. oeni, thereby improving the viability of lyophilized cells. © 2020 Society of Chemical Industry.

Altered Metabolic Strategies: Elaborate Mechanisms Adopted by Oenococcus oeni in Response to Acid Stress

Oenococcus oeni plays a key role in inducing malolactic fermentation in wine. Acid stress is often encountered under wine conditions. However, the lack of systematic studies of acid resistance mechanisms limits the downstream fermentation applications. In this study, the acid responses of O. oeni were investigated by combining transcriptome, metabolome, and genome-scale metabolic modeling approaches. Metabolite profiling highlighted the decreased abundance of nucleotides under acid stress. The gene-metabolite bipartite network showed negative correlations between nucleotides and genes involved in ribosome assembly, translation, and post-translational processes, suggesting that stringent response could be activated under acid stress. Genome-scale metabolic modeling revealed marked flux rerouting, including reallocation of pyruvate, attenuation of glycolysis, utilization of carbon sources other than glucose, and enhancement of nucleotide salvage and the arginine deiminase pathway. This study provided novel insights into the acid responses of O. oeni, which will be useful for designing strategies to address acid stress in wine malolactic fermentation.

Freeze-Drying of Lactic Acid Bacteria: A Stepwise Approach for Developing a Freeze-Drying Protocol Based on Physical Properties

Freeze-drying or lyophilization has become a reference process for preserving lactic acid bacteria. The development of stable freeze-dried lactic acid bacteria (LAB) requires maintaining the biological activity of the cells and the macroscopic porous structure while increasing the efficiency of the manufacturing process. Physical properties of protective solutions, such as glass transition and collapse temperatures, are key elements not only for process optimization but also for the stability of freeze-dried LAB. This chapter provides a stepwise approach for developing a protective formulation for the long-term preservation of LAB and an efficient freeze-drying process. Methods for determining glass transition and collapse temperatures of protective solutions and cell suspensions, as well as water activity and water content of freeze-dried products, are described.

Identification and Characterization of the Small Heat Shock Protein Hsp20 from Oenococcus oeni SD-2a

Oenococcus oeni can exert its function in hostile wine conditions during the malolactic fermentation process. Therefore, it is an important microbial resource for exploring resistance genes. Hsp20 is an important small heat shock protein from O. oeni. The conserved consensus motif "A-x-x-x-x-G-x-L" of Hsp20 announced its role as a member of the small heat shock protein family. The hsp20 gene from O. oeni SD-2a was cloned to create the recombinant plasmid pTriEx-Hsp20. The recombinant plasmid was transformed into Escherichia coli BL21(DE3) competent cells, and the Hsp20 protein was induced by isopropyl-β-D-thiogalactoside (IPTG). The hsp20 gene from O. oeni SD-2a was successfully expressed, and a 20-kDa fusion protein was identified by SDS-PAGE. The purified Hsp20 protein was obtained using Ni-affinity chromatography. Additionally, BL21(DE3)/Hsp20 and BL21(DE3)/Ctrl were treated at high temperatures of 42 and 52 °C, at pH values of 2.0-12.0, under oxidative shock with 0.1% (v/v) and 0.2% (v/v) H₂O₂, and under an osmotic shock of 430 and 860 mM NaCl to compare the effects of heterologous expression of the Hsp20 protein from O. oeni SD-2a for stress resistance. Notably, Hsp20 overexpression showed enhanced resistance than the control strain did when confronted with different elevated stress conditions. The results demonstrated heterologous expression of the hsp20 gene from O. oeni SD-2a significantly improved the resistance of the host E. coli bacteria against stress conditions.

C18:1 Improves the Freeze-Drying Resistance of Lactobacillus plantarum by Maintaining the Cell Membrane

Increasing knowledge about lactic acid bacteria as fermentation starters and probiotics to improve health has led to a growing awareness of their application potential. Despite a long history of applying cryoprotectants, the maintenance of probiotic viability is still a major challenge. In this study, we implemented a strategy and explored its mechanisms in detail. We found that the survival rates after freeze-drying were positively correlated with the relative concentration of the octadecenoic acid (C18:1) and with the ratio of unsaturated to saturated FAs (U/S ratio). The addition of C18:1 significantly improved the survival of L. plantarum after freeze-drying. Contrary to the most commonly used cryoprotectants, the addition of C18:1 did not affect the glass transition temperature or collapse temperature. We predicted that the cell membrane characteristics would be significantly degraded during the drying stage, but C18:1 can effectively maintain the cell membrane integrity and fluidity. Our experiments confirmed those predictions, and simultaneously found that the enzyme activities of key enzymes of glucose metabolism were increased compared with the control group. These finding indicate that C18:1 might serve as a lyoprotectant to maintain the cell membrane integrity and fluidity, and thereby increasing the survival rate of L. plantarum after freeze-drying. This study constitutes a strategy to safeguard bacterial viability.

Evidence of the genetic diversity and clonal population structure of Oenococcus oeni strains isolated from different wine-making regions of China

Studies of the genetic diversity and population structure of Oenococcus oeni (O. oeni) strains from China are lacking compared to other countries and regions. In this study, amplified fragment length polymorphism (AFLP) and multilocus sequence typing (MLST) methods were used to investigate the genetic diversity and regional evolutionary patterns of 38 O. oeni strains isolated from different wine-making regions in China. The results indicated that AFLP was markedly more efficient than MLST for typing O. oeni strains. AFLP distinguished 37 DNA patterns compared to 7 sequence types identified using MLST, corresponding to discriminatory indices of 0.999 and 0.602, respectively. The AFLP results revealed a high level of genetic diversity among the O. oeni strains from different regions of China, since two subpopulations and an intraspecific homology higher than 60% were observed. Phylogenetic analysis of the O. oeni strains using the MLST method also identified two major phylogroups, which were differentiated into two distinct clonal complexes by minimum spanning tree analysis. Neither intragenic nor intergenic recombination verified the existence of the clonal population structure of the O. oeni strains.

Selection and Validation of Reference Genes for Quantitative Real-Time PCR Normalization Under Ethanol Stress Conditions in Oenococcus oeni SD-2a

The powerful Quantitative real-time PCR (RT-qPCR) was widely used to assess gene expression levels, which requires the optimal reference genes used for normalization. Oenococcus oeni (O. oeni), as the one of most important microorganisms in wine industry and the most resistant lactic acid bacteria (LAB) species to ethanol, has not been investigated regarding the selection of stable reference genes for RT-qPCR normalization under ethanol stress conditions. In this study, nine candidate reference genes (proC, dnaG, rpoA, ldhD, ddlA, rrs, gyrA, gyrB, and dpoIII) were analyzed to determine the most stable reference genes for RT-qPCR in O. oeni SD-2a under different ethanol stress conditions (8, 12, and 16% (v/v) ethanol). The transcript stabilities of these genes were evaluated using the algorithms geNorm, NormFinder, and BestKeeper. The results showed that dnaG and dpoIII were selected as the best reference genes across all experimental ethanol conditions. Considering single stress experimental modes, dpoIII and dnaG would be suitable to normalize expression level for 8% ethanol shock treatment, while the combination of gyrA, gyrB, and rrs would be suitable for 12% ethanol shock treatment. proC and gyrB revealed the most stable expression in 16% ethanol shock treatment. This study selected and validated for the first time the reference genes for RT-qPCR normalization in O. oeni SD-2a under ethanol stress conditions.

Transcriptomic Analysis of Oenococcus oeni SD-2a Response to Acid Shock by RNA-Seq

Oenococcus oeni can be applied to conduct malolactic fermentation (MLF), but also is the main species growing naturally in wine. Due to the high stress tolerance, it is an interesting model for investigating acid response mechanisms. In this study, the changes in the transcriptome of O.oeni SD-2a during the adaptation period have been studied. RNA-seq was introduced for the transcriptomic analysis of O. oeni samples treated with pH 4.8 and pH 3.0 at 0 and 1 h, respectively. Gene ontology (GO) and Kyoto encyclopedia of genes and genome (KEGG) were performed to compare the transcriptome data between different treatments. From GO analysis, the majority of differentially expressed genes (DEGs) (pH 3.0_1 h-VS-pH 4.8_1 h, pH 3.0_1 h-VS-pH 4.8_0 h, and pH 4.8_1 h-VS-pH 4.8_0 h) were found to be involved in the metabolic process, catalytic activity, cellular process, and binding. KEGG analysis reveals that the most functional gene categories affected by acid are membrane transport, amino acid metabolism and carbohydrate metabolism. Some genes, like the heat shock protein Hsp20, malate transporter and malate permease, were also over-expressed in response to acid stress. In addition, a considerable proportion of gene indicate a significantly different expression in this study, are novel, which needs to be investigated further. These results provide a new viewpoint and crucial resource on the acid stress response in O. oeni.

Effect of protective agents and previous acclimation on ethanol resistance of frozen and freeze-dried Lactobacillus plantarum strains

The aim of this work was to study the protective effect of sucrose, trehalose and glutamate during freezing and freeze-drying of three oenological Lactobacillus plantarum strains previously acclimated in the presence of ethanol. The efficiency of protective agents was assessed by analyses of membrane integrity and bacterial cultivability in a synthetic wine after the preservation processes. No significant differences in the cultivability, with respect to the controls cells, were observed after freezing at -80 °C and -20 °C, and pre-acclimated cells were more resistant to freeze-drying than non-acclimated ones. The results of multiparametric flow cytometry showed a significant level of membrane damage after freeze-drying in two of the three strains. The cultivability was determined after incubation in wine-like medium containing 13 or 14% v/v ethanol at 21 °C for 24 h and the results were interpreted using principal component analysis (PCA). Acclimation was the most important factor for preservation, increasing the bacterial resistance to ethanol after freezing and freeze-drying. Freeze-drying was the most drastic method of preservation, followed by freezing at -20 °C. The increase of ethanol concentration from 6 to 10% v/v in the acclimation medium improved the recovery of two of the three strains. In turn, the increase of ethanol content in the synthetic wine led to a dramatic decrease of viable cells in the three strains investigated. The results of this study indicate that a successful inoculation of dehydrated L. plantarum in wine depends not only on the use of protective agents, but also on the cell acclimation process prior to preservation, and on the ethanol content of wine.

Production of freeze-dried yeast culture for the brewing of traditional sorghum beer, tchapalo

Freeze‐drying is a well‐known dehydration method widely used to preserve microorganisms. In order to produce freeze‐dried yeast starter culture for the brewing purpose of African sorghum beer, we tested protective agents (sucrose, glucose, glycerol) in combination with support materials (millet, maize, sorghum, and cassava flours) at 1:1 ratio (v/v). The yeast strains Saccharomyces cerevisiae F_12–7 and Candida tropicalis C_0–7 previously isolated from sorghum beer were used in a mixed culture at a ratio of 2:1 (C. tropicalis/S. cerevisiae). After the freeze‐drying, the residual water contents were between 0.78 –2.27%, 0.55 –4.09%, and 0.40–2.61%, respectively, with sucrose, glucose and glycerol. The dried yeasts viabilities were between 4.0% and 10.6%. Among the protective agents used, sucrose was found to be the best protectant giving cell viabilities of 8.4–10.6%. Considering the support materials, millet flour was the best support after drying. When the freeze‐dried yeast powders were stored at 4°C and room temperature (25–28°C) for up to 3 months, the survival rates were the highest with cassava flour as the support material.

Scalable temperature induced stress for the large-scale production of functionalized Bifidobacteria

The application of sub-lethal stresses is known to be an efficient strategy to enhance survival of probiotic bacteria during drying processes. In this context, we previously showed that the application of heat stress upon the entry into stationary phase increased significantly the viability of Bifidobacterium bifidum. However, this heat shock has been considered only in small-scale bioreactor and no information is available for a possible scaling-up strategy. Five different operating scales (0.2, 2, 20, 200 and 2000 L) have thus been tested and the results showed that the viability of B. bifidum increases from 3.15 to 6.57 folds, depending on the scale considered. Our observations pointed out the fact that the heat stress procedure is scalable according to the main outcome, i.e., increases in cell viability, but other factors have to be taken into account. Among these factors, dissolved carbon dioxide seems to play a significant role, since it explains the differences observed between the test performed at laboratory scale and in industrial conditions.

Membrane fatty acid composition and fluidity are involved in the resistance to freezing of Lactobacillus buchneri R1102 and Bifidobacterium longum R0175

Determinations of membrane fatty acid composition and fluidity were used together with acidification activity and viability measurements to characterize the physiological state after freezing of Lactobacillus buchneri R1102 and Bifidobacterium longum R0175 cells harvested in the exponential and stationary growth phases. For both strains, lower membrane fluidity was achieved in cells harvested in the stationary growth phase. This change was linked to a lower unsaturated-to-saturated fatty acid ratio for both strains and a higher cyclic-to-saturated fatty acid ratio for L. buchneri R1102 alone. These membrane properties were linked to survival and to maintenance of acidification activity of the cells after freezing, which differed according to the strain and the growth phase. Survival of B. longum R0175 was increased by 10% in cells with low membrane fluidity and high relative saturated fatty acid contents, without any change in acidification activity. Acidification activity was more degraded (70 min) in L. buchneri R1102 cells displaying low membrane fluidity and high saturated and cyclic fatty acid levels. Finally, this study showed that membrane modifications induced by the growth phase differed among bacterial strains in terms of composition. By lowering membrane fluidity, these modifications could be beneficial for survival of B. longum R0175 during the freezing process but detrimental for maintenance of acidification activity of L. buchneri R1102.