Adaptation of the wine bacterium Oenococcus oeni to ethanol stress: role of the small heat shock protein Lo18 in membrane integrity

The role of membrane physiology in sHSP Lo18-lipid interaction and lipochaperone activity

To cope with environmental stresses, organisms, including lactic acid bacteria such as O. oeni, produce stress proteins called HSPs. In wine, O. oeni is constantly confronted by stress affecting its membrane fluidity. To survive through in these deleterious conditions, O. oeni synthesizes Lo18, a unique, small HSP which acts as a molecular chaperone and a lipochaperone. The molecular mechanism underlying its lipochaperone activity, particularly regarding membrane lipid composition, remains poorly understood. In this context, Lo18 lipochaperone activity and the associated modification in protein structure were studied during interaction with different liposomes from O. oeni cultures representing unstressed, stressed and stressed-adapted physiological states. The results showed that the presence of the membrane (whatever its nature) induces a modification of Lo18's structure. Also, the presence of oleic acid and/or phosphatidylglycerol is important to favor Lo18-membrane interaction, allowing lipochaperone activity. This research enhances understanding of sHSP-membrane interactions in bacterial systems.

Deletions of conserved extracytoplasmic function sigma factors-encoding genes in Streptomyces have a major impact on secondary metabolism

BACKGROUND

Ethanol shock significantly affects expression of over 1200 genes in Streptomyces venezuelae NRRL B-65,442, including those involved in secondary metabolite biosynthesis and a cryptic gene pepX, which encodes a 19-amino acid peptide with an unknown function.

RESULTS

To establish a possible correlation between the PepX peptide and secondary metabolism in S. venezuelae, its gene was deleted, followed by analyses of the transcriptome and secondary metabolome of the mutant. Although the secondary metabolome of the pepX mutant was not strongly affected, pepX deletion, similar to ethanol shock, mostly resulted in downregulated expression of secondary metabolite biosynthesis gene clusters (BGCs). At the same time, there was a reverse correlation between the expression of certain extracytoplasmic function sigma factors (ECFs) and several BGCs. Individual deletions of three selected ECF-coding genes conserved in Streptomyces that were upregulated upon both pepX deletion and ethanol shock, had a profound positive effect on the expression of BGCs, which also correlated with the overproduction of specific secondary metabolites. Deletion of one such ECF-coding gene in a marine sponge-derived Streptomyces sp. also significantly altered the secondary metabolite profile, suggesting an important role of this ECF in the regulation of secondary metabolism.

CONCLUSIONS

These findings pave the way for the activation or upregulation of BGCs in Streptomyces bacteria harboring genes for ECFs homologous to those identified in this study, hereby assisting in the discovery of novel bioactive secondary metabolites.

Mixed biofilm formation by Oenococcus oeni and Saccharomyces cerevisiae: A new strategy for the wine fermentation process

Bacterial biofilms have attracted much attention in the food industry since this phenotype increases microbial resistance to environmental stresses. In wine-making, the biofilm produced by Oenococcus oeni is able to persist in this harsh environment and perform malolactic fermentations. Certain viticultural practices are interested in the simultaneous triggering of alcoholic fermentation by yeasts of the species Saccharomyces cerevisiae and malolactic fermentation by lactic acid bacteria. As yet, no data is available on the ability of these micro-organisms to produce mixed biofilms and promote fermentations. Here, the ability of S. cerevisiae and O. oeni to form mixed biofilms on different surfaces found in vinification was observed and analyzed using scanning electron microscopy experiments. Then, following co-inoculation with biofilm or planktonic cells microvinifications were carried out to demonstrate that the mixed biofilms developed on oak allow the efficient completion of fermentations because of their high resistance to stress. Finally, comparisons of the different metabolic profiles obtained by LC-MS were made to assess the impact of the mode of life of biofilms on wine composition.

Significant influence of four highly conserved amino-acids in lipochaperon-active sHsps on the structure and functions of the Lo18 protein

To cope with environmental stresses, bacteria have developed different strategies, including the production of small heat shock proteins (sHSP). All sHSPs are described for their role as molecular chaperones. Some of them, like the Lo18 protein synthesized by Oenococcus oeni, also have the particularity of acting as a lipochaperon to maintain membrane fluidity in its optimal state following cellular stresses. Lipochaperon activity is poorly characterized and very little information is available on the domains or amino-acids key to this activity. The aim in this paper is to investigate the importance at the protein structure and function level of four highly conserved residues in sHSP exhibiting lipochaperon activity. Thus, by combining in silico, in vitro and in vivo approaches the importance of three amino-acids present in the core of the protein was shown to maintain both the structure of Lo18 and its functions.

Is the lipochaperone activity of sHSP a key to the stress response encoded in its primary sequence?

Several strategies have been put in place by organisms to adapt to their environment. One of these strategies is the production of stress proteins such as sHSPs, which have been widely described over the last 30 years for their role as molecular chaperones. Some sHSPs have, in addition, the particularity to exert a lipochaperone role by interacting with membrane lipids to maintain an optimal membrane fluidity. However, the mechanisms involved in this sHSP-lipid interaction remain poorly understood and described rather sporadically in the literature. This review gathers the information concerning the structure and function of these proteins available in the literature in order to highlight the mechanism involved in this interaction. In addition, analysis of primary sequence data of sHSPs available in database shows that sHSPs can interact with lipids via certain amino acid residues present on some β sheets of these proteins. These residues could have a key role in the structure and/or oligomerization dynamics of sHPSs, which is certainly essential for interaction with membrane lipids and consequently for maintaining optimal cell membrane fluidity.

Molecular chaperone function of three small heat-shock proteins from a model probiotic species

Small heat-shock proteins (sHSP) are ubiquitous ATP-independent chaperones that prevent irreversible aggregation of heat-damaged denaturing proteins. Lactiplantibacillus plantarum is a widespread Gram-positive bacterium with probiotic claims and vast potential for agro-food, biotechnological and biomedical applications. L. plantarum possesses a family of three sHSP, which were previously demonstrated to be involved in its stress tolerance mechanisms. Here, the three L. plantarum sHSP were heterologously expressed, purified and shown to have a chaperone activity in vitro, measuring their capacity to suppress protein aggregation, as assayed spectrophotometrically by light scattering. Their anti-aggregative capacity was found to be differently influenced by pH. Differences were also found relative to their holdase function and their capacity to modulate liposome membrane fluidity, suggesting interplays between them and indicating diversified activities. This is the first study assessing the chaperone action of sHSP from a probiotic model. The different roles of the three sHSP can increase L. plantarum's capabilities to survive the various types of stress characterising the diverse habitats of this highly adaptable species. Reported evidence supports the interest in L. plantarum as one of the model species for bacteria that have three different sHSP-encoding genes in their genomes.

Targeted Metabolomics and High-Throughput RNA Sequencing-Based Transcriptomics Reveal Massive Changes in the Streptomyces venezuelae NRRL B-65442 Metabolism Caused by Ethanol Shock

The species Streptomyces venezuelae is represented by several distinct strains with variable abilities to biosynthesize structurally diverse secondary metabolites. In this work, we examined the effect of ethanol shock on the transcriptome and metabolome of Streptomyces venezuelae NRRL B-65442 using high-throughput RNA sequencing (RNA-seq) and high-resolution liquid chromatography-tandem mass spectrometry (LC-MS/MS). Ethanol shock caused massive changes in the gene expression profile, differentially affecting genes for secondary metabolite biosynthesis and central metabolic pathways. Most of the data from the transcriptome analysis correlated well with the metabolome changes, including the overproduction of jadomycin congeners and a downshift in the production of desferrioxamines, legonoxamine, foroxymithin, and a small cryptic ribosomally synthesized peptide. Some of the metabolome changes, such as the overproduction of chloramphenicol, could not be explained by overexpression of the cognate biosynthetic genes but correlated with the expression profiles of genes for precursor biosynthesis. Changes in the transcriptome were also observed for several genes known to play a role in stress response in other bacteria and included at least 10 extracytoplasmic function σ factors. This study provides important new insights into the stress response in antibiotic-producing bacteria and will help to understand the complex mechanisms behind the environmental factor-induced regulation of secondary metabolite biosynthesis. IMPORTANCE Streptomyces spp. are filamentous Gram-positive bacteria known as versatile producers of secondary metabolites, of which some have been developed into human medicines against infections and cancer. The genomes of these bacteria harbor dozens of gene clusters governing the biosynthesis of secondary metabolites (BGCs), of which most are not expressed under laboratory conditions. Detailed knowledge of the complex regulation of BGC expression is still lacking, although certain growth conditions are known to trigger the production of previously undetected secondary metabolites. In this work, we investigated the effect of ethanol shock on the production of secondary metabolites by Streptomyces venezuelae and correlated these findings with the expression of cognate BGCs and primary metabolic pathways involved in the generation of cofactors and precursors. The findings of this study set the stage for the rational manipulation of bacterial genomes aimed at enhanced production of industrially important bioactive natural products.

Genomic Analysis of an Excellent Wine-Making Strain Oenococcus oeni SD-2a

Oenococcus oeni is an important microorganism in wine-making-related engineering, and it improves wine quality and stability through malolactic fermentation. Although the genomes of more than 200 O. oeni strains have been sequenced, only a few include completed genome maps. Here, the genome sequence of O. oeni SD-2a, isolated from Shandong, China, has been determined. It is a fully assembled genome sequence of this strain. The complete genome is 1,989,703 bp with a G+C content of 37.8% without a plasmid. The genome includes almost all the essential genes involved in central metabolic pathways and the stress genes reported in other O. oeni strains. Some natural competence-related genes, like comEA, comEC, comFA, comG operon, and comFC, suggest that O. oeni SD-2a may have natural transformation potential. A comparative genomics analysis revealed 730 gene clusters in O. oeni SD-2a homologous to those in four other lactic acid bacteria species (O. oeni PSU-1, O. oeni CRBO-11381, Lactiplantibacillus plantarum UNQLp11, and Pediococcus pentosaceus KCCM40703). A collinearity analysis showed poor collinearity between O. oeni SD-2a and

O. oeni PSU-1, indicating great differences in their evolutionary histories. The results provide general knowledge of O. oeni SD-2a and lay the foundation for specific gene function analyses.

Exploring the diversity of bacteriophage specific to Oenococcus oeni and Lactobacillus spp and their role in wine production

The widespread existence of bacteriophage has been of great interest to the biological research community and ongoing investigations continue to explore their diversity and role. They have also attracted attention and in-depth research in connection to fermented food processing, in particular from the dairy and wine industries. Bacteriophage, mostly oenophage, may in fact be a 'double edged sword' for winemakers: whilst they have been implicated as a causal agent of difficulties with malolactic fermentation (although not proven), they are also beginning to be considered as alternatives to using sulphur dioxide to prevent wine spoilage. Investigation and characterisation of oenophage of Oenococcus oeni, the main species used in winemaking, are still limited compared to lactococcal bacteriophage of Lactococcus lactis and Lactiplantibacillus plantarum (formally Lactobacillus plantarum), the drivers of most fermented dairy products. Interestingly, these strains are also being used or considered for use in winemaking. In this review, the genetic diversity and life cycle of phage, together with the debate on the consequent impact of phage predation in wine, and potential control strategies are discussed. KEY POINTS: • Bacteriophage detected in wine are diverse. • Many lysogenic bacteriophage are found in wine bacteria. • Phage impact on winemaking can depend on the stage of the winemaking process. • Bacteriophage as potential antimicrobial agents against spoilage organisms.

Wine aroma profile modification by Oenococcus oeni strains from Rioja Alavesa region: selection of potential malolactic starters

Previously six selected Oenococcus oeni strains (P2A, P3A, P3G, P5A, P5C and P7B) have been submitted to further characterization in order to clarify their potential as malolactic starters. Laboratory scale vinifications gave an insight of the most vigorous strains: both P2A and P3A strains were able to conclude malolactic fermentation (MLF) in less than 15 days. The remaining strains showed good viability and were able to successfully finish MLF in the established analysis time, except for the strain P5A, which viability was totally lost after inoculation. Also spontaneous fermentation was not initiated. None of the strains was biogenic amine producer; however, P5C strain significantly increased the concentration of volatile phenol-precursor hydroxycinnamic acids after MLF. Regarding the evolution of wine aromatic compounds, main changes were detected for both ethyl and acetate esters after MLF; however, key aromatic compounds including alcohols, terpenes or acids were also found to significantly increase. Principal component analysis classified the strains in two distinct groups, each one correlated with different key volatile compounds. P2A, P3A, P3G and P5C strains were mainly linked to esters, while P7B and the commercial strain Viniflora OENOS showed higher score for diverse compounds as hexanoic acid, β-damascenone, linalool or 2-phenylethanol. These results confirmed the specific impact of each strain on wine aroma profile, which could lead to the production of wines with individual characteristics, in which the reliability and safety of MLF is also ensured.

The Small Ones Matter-sHsps in the Bacterial Chaperone Network

Small heat shock proteins (sHsps) are an evolutionarily conserved class of ATP-independent chaperones that form the first line of defence during proteotoxic stress. sHsps are defined not only by their relatively low molecular weight, but also by the presence of a conserved α-crystallin domain, which is flanked by less conserved, mostly unstructured, N- and C-terminal domains. sHsps form oligomers of different sizes which deoligomerize upon stress conditions into smaller active forms. Activated sHsps bind to aggregation-prone protein substrates to form assemblies that keep substrates from irreversible aggregation. Formation of these assemblies facilitates subsequent Hsp70 and Hsp100 chaperone-dependent disaggregation and substrate refolding into native species. This mini review discusses what is known about the role and place of bacterial sHsps in the chaperone network.

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.

Network of Entamoeba histolytica HSP18.5 dimers formed by two overlapping [IV]-X-[IV] motifs

Small heat shock proteins (sHSPs) are ATP-independent molecular chaperones with low molecular weight that prevent the aggregation of proteins during stress conditions and maintain protein homeostasis in the cell. sHSPs exist in dynamic equilibrium as a mixture of oligomers of various sizes with a constant exchange of subunits between them. Many sHSPs form cage-like assemblies that may dissociate into smaller oligomers during stress conditions. We carried out the functional and structural characterization of a small heat shock protein, HSP18.5, from Entamoeba histolytica (EhHSP18.5). It showed a pH-dependent change in its oligomeric state, which varied from a tetramer to larger than 48-mer. EhHSP18.5 protected Nde I and lysozyme substrates from temperature and chemical stresses, respectively. The crystal structure of EhHSP18.5 was determined at a resolution of 3.28 Å in C2221 cell with four subunits in the asymmetric unit forming two non-metazoan sHSP-type dimers. Unlike the reported cage-like structures, EhHSP18.5 formed a network of linear chains of molecules in the crystal. Instead of a single [IV]-X-[IV] motif, EhHSP18.5 has two overlapping I/V-X-I/V sequences at the C-terminus giving rise to novel interactions between the dimers. Negative staining Electron Microscopy images of EhHSP18.5 showed the presence of multiple oligomers: closed structures of various sizes and long tube-like structures.

Small but mighty: a functional look at bacterial sHSPs

Small heat shock proteins (sHSPs) are widespread in every kingdom of life, being indispensable for protein quality control networks. Alongside canonical chaperone functions, sHSPs seem to have been a very plastic scaffold for acquiring multiple related functions across evolution. This review aims to summarize what is known about sHSPs functioning in the Bacteria Kingdom.

Effect of exogenous proline on the ethanolic tolerance and malolactic performance of Oenococcus oeni

The use of malolactic starter cultures, often offer no guarantee of microbiological success due to the chemical and physical factors (pH, ethanol, SO₂, nutrient availability) that occur during the winemaking process. This study was born with the aim of improving the performance of the lactic acid bacteria used as a starter culture in the de-acidification of wines. Two commercial strains of Oenococcus oeni, were used. Was evaluated the effect of exogenous l-proline added during the bacterial growth, on the improvement of their survival in the presence of different ethanol concentrations and their ability to degrade l-malic acid in synthetic wine with the presence of 12% (v/v) and 13% (v/v) of ethanol. The results showed that l-proline improve ethanol tolerance and so the malolactic performances of O. oeni. This work represents an important strategy to ensure good vitality and improve the performance of the malolactic starter.

Heterologous expression of ctsR from Oenococcus oeni enhances the acid-ethanol resistance of Lactobacillus plantarum

Oenococcus oeni is a lactic acid bacterium that is widely used in wine-making to conduct malolactic fermentation (MLF). During MLF, O. oeni undergoes acid and ethanol stress that impairs its growth. In order to investigate the role that the ctsR gene plays in acid-ethanol stress, the ctsR gene from O. oeni was expressed heterologously in L. plantarum. The transcription level of the ctsR gene and 10 additional stress response genes in L. plantarum were analyzed by RT-qPCR. Physiological assays to assess ROS accumulation, cell membrane integrity, intracellular ATP and GSH levels, Ca2+/Mg2+-ATPase and Na+/K+-ATPase activities were also performed. Results showed that the recombinant strain WCFS1-CtsR exhibited stronger growth performance than the control strain WCFS1-Vector, and the expression of ctsR, clp, and hsp genes were significantly increased under acid-ethanol stress. Furthermore, WCFS1-CtsR displayed 1.08-, and 1.39-fold higher ATP and GSH concentrations, respectively, compared to the corresponding values for WCFS1-Vector under acid-ethanol stress. ROS accumulation and PI value of WCFS1-CtsR were decreased by 46.52% and 42.80%, respectively, compared to the control strain. In addition, the two ATPase activities in WCFS1-CtsR increased significantly compared with WCFS1-Vector. This is the first report demonstrating that ctsR gene enhances the acid-ethanol tolerance of L. plantarum.

Heterologous Expression of Argininosuccinate Synthase From Oenococcus oeni Enhances the Acid Resistance of Lactobacillus plantarum

Oenococcus oeni can survive well in wine (an acid-stress environment) and dominate malolactic fermentation (MLF). To demonstrate a possible role of argininosuccinate synthase gene (argG) in the acid tolerance response of O. oeni, a related argG gene was inserted into a plasmid pMG36e and heterologously expressed in Lactobacillus plantarum SL09, a wine isolate belonging to a species of relevant importance in MLF. The expression levels of the argG gene in L. plantarum were analyzed by RT-qPCR, argininosuccinate synthase (ASS) activity and cell properties (amino acids, pH, H⁺-ATPase activity, and ATP levels) were determined at pH 3.7 in comparison with that at pH 6.3. Results showed that the recombinant strain L. plantarum SL09 (pMG36eargG) exhibited stronger growth performance compared with the control strain (without argG gene), and the expression levels of hsp1, cfa, atp, the citrate and malate metabolic genes were apparently increased under acid stress. In addition, the recombinant strain exhibited 11.0-, 2.0-, 1.9-fold higher ASS activity, H⁺-ATPase activity and intracellular ATP level, compared with the corresponding values for control strain during acid-stresses condition, which may take responsible for the acid tolerance enhancement of the recombinant strain. This is the first work report on heterologous expression of argG gene, and the results presented in this study will be beneficial for the research on acid stress response of O. oeni.

CtsR, the Master Regulator of Stress-Response in Oenococcus oeni, Is a Heat Sensor Interacting With ClpL1

Oenococcus oeni is a lactic acid bacterium responsible for malolactic fermentation of wine. While many stress response mechanisms implemented by O. oeni during wine adaptation have been described, little is known about their regulation. CtsR is the only regulator of stress response genes identified to date in O. oeni. Extensively characterized in Bacillus subtilis, the CtsR repressor is active as a dimer at 37°C and degraded at higher temperatures by a proteolytic mechanism involving two adapter proteins, McsA and McsB, together with the ClpCP complex. The O. oeni genome does not encode orthologs of these adapter proteins and the regulation of CtsR activity remains unknown. In this study, we investigate CtsR function in O. oeni by using antisense RNA silencing in vivo to modulate ctsR gene expression. Inhibition of ctsR gene expression by asRNA leads to a significant loss in cultivability after heat shock (58%) and acid shock (59%) highlighting the key role of CtsR in the O. oeni stress response. Regulation of CtsR activity was studied using a heterologous expression system to demonstrate that O. oeni CtsR controls expression and stress induction of the O. oeni hsp18 gene when produced in a ctsR-deficient B. subtilis strain. Under heat stress conditions, O. oeni CtsR acts as a temperature sensor and is inactivated at growth temperatures above 33°C. Finally, using an E. coli bacterial two-hybrid system, we showed that CtsR and ClpL1 interact, suggesting a key role for ClpL1 in controlling CtsR activity in O. oeni.

Heterologous expression of Oenococcus oeni sHSP20 confers temperature stress tolerance in Escherichia coli

Small heat shock proteins (sHSPs) are heat shock proteins sized 12-43 kDa that can protect proteins from denaturation, particularly under high temperature; sHSPs thus increase the heat tolerance capability of an organisms enabling survival in adverse climates. sHSP20 is overexpressed in Oenococcus oeni in response to low temperatures. However, we found that overexpression of sHSP20 in Escherichia coli BL21 increased the microbial survival ratio at 50 °C by almost 2 h. Adding sHSP20 to the glutamate dehydrogenase solution significantly increased the stability of the enzyme at high temperature (especially at 60-70 °C), low pH values (especially below 6.0), and high concentration of metal ions of Ga2+, Zn2+, Mn2+, and Fe3+. Notably, the coexpression of sHSP20 significantly enhanced soluble expression of laccase from Phomopsis sp. XP-8 (CCTCCM209291) in E. coli without codon optimization, as well as the activity and heat stability of the expressed enzyme. In addition to the chaperone activity of sHSP20 in the gene containing host in vivo and the enzyme heat stability in vitro, our study indicated the capability of coexpression of sHSP20 to increase the efficiency of prokaryotic expression of fungal genes and the activity of expressed enzymes. Graphical abstract ᅟ.

Oenococcus oeni Exopolysaccharide Biosynthesis, a Tool to Improve Malolactic Starter Performance

Oenococcus oeni is the lactic acid bacterium that most commonly drives malolactic fermentation (MLF) in wine. Though the importance of MLF in terms of wine microbial stability and sensory improvement is well established, it remains a winemaking step not so easy to control. O. oeni displays many adaptation tools to resist the harsh wine conditions which explain its natural dominance at this stage of winemaking. Previous findings showed that capsular polysaccharides and endogenous produced dextran increased the survival rate and the conservation time of malolactic starters. In this paper, we showed that exopolysaccharides specific production rates were increased in the presence of single stressors relevant to wine (pH, ethanol). The transcription of the associated genes was investigated in distinct O. oeni strains. The conditions in which eps genes and EPS synthesis were most stimulated were then evaluated for the production of freeze dried malolactic starters, for acclimation procedures and for MLF efficiency. Sensory analysis tests on the resulting wines were finally performed.

Ethanol stress in Oenococcus oeni: transcriptional response and complex physiological mechanisms

Oenococcus oeni is the dominant species able to cope with a hostile environment of wines, comprising cumulative effects of low pH, high ethanol and SO₂ content, nonoptimal growth temperatures and growth inhibitory compounds. Ethanol tolerance is a crucial feature for the activity of O. oeni cells in wine because ethanol acts as a disordering agent of its cell membrane and negatively affects metabolic activity; it damages the membrane integrity, decreases cell viability and, as other stress conditions, delays the start of malolactic fermentation with a consequent alteration of wine quality. The cell wall, cytoplasmic membrane and metabolic pathways are the main sites involved in physiological changes aimed to ensure an adequate adaptive response to ethanol stress and to face the oxidative damage caused by increasing production of reactive oxygen species. Improving our understanding of the cellular impact of ethanol toxicity and how the cell responds to ethanol stress can facilitate the development of strategies to enhance microbial ethanol tolerance; this allows to perform a multidisciplinary endeavour requiring not only an ecological study of the spontaneous process but also the characterization of useful technological and physiological features of the predominant strains in order to select those with the highest potential for industrial applications.

Application of directed evolution to develop ethanol tolerant Oenococcus oeni for more efficient malolactic fermentation

Malolactic fermentation (MLF) is an important step in winemaking, which can be notoriously unreliable due to the fastidious nature of Oenococcus oeni. This study aimed to use directed evolution (DE) to produce a more robust strain of O. oeni having the ability to withstand high ethanol concentrations. DE involves an organism mutating and potentially adapting to a high stress environment over the course of extended cultivation. A continuous culture of O. oeni was established and exposed to progressively increasing ethanol content such that after approximately 330 generations, an isolate from this culture was able to complete MLF in high ethanol content medium earlier than its parent. The ethanol tolerance of a single isolate, A90, was tested to confirm the phenotype and its fermentation performance in wine. In order to investigate the genotypic differences in the evolved strain that led to the ethanol tolerance phenotype, the relative expression of a number of known stress response genes was compared between SB3 and A90. Notably, there was increase in hsp18 expression in 20% (v/v) ethanol by both strains with A90 exhibiting a higher degree of expression. This study is the first to use directed evolution for O. oeni strain improvement and confirms that this technique can be used successfully for the development of new candidate strains for the wine industry. This study also adds to the current knowledge on the genetic basis of ethanol tolerance in this bacterium.

Transcriptome responses of Lactobacillus acetotolerans F28 to a short and long term ethanol stress

Lactobacillus acetotolerans is a major microbe contributing to the Chinese liquor fermentation with unknown function. It can be grown well in a high concentration of ethanol. RNA sequencing (RNA-seq) was performed on L. acetotolerans F28 growing in 12% ethanol to determine important genetic mechanisms for both a short and long term adaption to this environment. A genome-wide transcriptional analysis revealed that the most important genetic elements for L. acetotolerans F28 grown in ethanol are related to high levels of stress response and fatty acid biosynthesis, and a reduction of amino acid transport and metabolism after both a short and long time stress. The fatty acid methyl ester analyses showed that most fatty acids were increased in L. acetotolerans F28 after exposure to ethanol while the unsaturated fatty acid octadecenoic acid (C18:1) was significantly increased. The increasing unsaturated fatty acid biosynthesis in L. acetotolerans F28 might enhance cell membrane fluidity and protect the cells against high concentration of ethanol. Overall, the transcriptome and functional analysis indicated that the elevated stress response and fatty acid biosynthesis, and the decrease of amino acid transport and metabolism might play important roles for L. acetotolerans F28 to adapt to environmental ethanol.

Genomics of lactic acid bacteria: Current status and potential applications

Lactic acid bacteria (LAB) are widely used for the production of a variety of foods and feed raw materials where they contribute to flavor and texture of the fermented products. In addition, specific LAB strains are considered as probiotic due to their health-promoting effects in consumers. Recently, the genome sequencing of LAB is booming and the increased amount of published genomics data brings unprecedented opportunity for us to reveal the important traits of LAB. This review describes the recent progress on LAB genomics and special emphasis is placed on understanding the industry-related physiological features based on genomics analysis. Moreover, strategies to engineer metabolic capacity and stress tolerance of LAB with improved industrial performance are also discussed.

Membrane chaperoning by members of the PspA/IM30 protein family

PspA, IM30 (Vipp1) and LiaH, which all belong to the PspA/IM30 protein family, form high molecular weight oligomeric structures. For all proteins membrane binding and protection of the membrane structure and integrity has been shown or postulated. Here we discuss the possible membrane chaperoning activity of PspA, IM30 and LiaH and propose that larger oligomeric structures bind to stressed membrane regions, followed by oligomer disassembly and membrane stabilization by protein monomers or smaller/different oligomeric scaffolds.

Stress Physiology of Lactic Acid Bacteria

Lactic acid bacteria (LAB) are important starter, commensal, or pathogenic microorganisms. The stress physiology of LAB has been studied in depth for over 2 decades, fueled mostly by the technological implications of LAB robustness in the food industry. Survival of probiotic LAB in the host and the potential relatedness of LAB virulence to their stress resilience have intensified interest in the field. Thus, a wealth of information concerning stress responses exists today for strains as diverse as starter (e.g., Lactococcus lactis), probiotic (e.g., several Lactobacillus spp.), and pathogenic (e.g., Enterococcus and Streptococcus spp.) LAB. Here we present the state of the art for LAB stress behavior. We describe the multitude of stresses that LAB are confronted with, and we present the experimental context used to study the stress responses of LAB, focusing on adaptation, habituation, and cross-protection as well as on self-induced multistress resistance in stationary phase, biofilms, and dormancy. We also consider stress responses at the population and single-cell levels. Subsequently, we concentrate on the stress defense mechanisms that have been reported to date, grouping them according to their direct participation in preserving cell energy, defending macromolecules, and protecting the cell envelope. Stress-induced responses of probiotic LAB and commensal/pathogenic LAB are highlighted separately due to the complexity of the peculiar multistress conditions to which these bacteria are subjected in their hosts. Induction of prophages under environmental stresses is then discussed. Finally, we present systems-based strategies to characterize the "stressome" of LAB and to engineer new food-related and probiotic LAB with improved stress tolerance.

Effect of Biofilm Formation by Oenococcus oeni on Malolactic Fermentation and the Release of Aromatic Compounds in Wine

The winemaking process involves the alcoholic fermentation of must, often followed by malolactic fermentation (MLF). The latter, mainly carried out by the lactic acid bacterium Oenococcus oeni, is used to improve wine quality when acidity reduction is required. Moreover, it prevents microbial spoilage and improves the wine’s organoleptic profile. Prior observations showed that O. oeni is able to resist several months in harsh wine conditions when adhered on oak barrels. Since biofilm is a prevailing microbial lifestyle in natural environments, the capacity of O. oeni to form biofilms was investigated on winemaking material such as stainless steel and oak chips. Scanning Electron Microscopy and Confocal Laser Scanning Microscopy showed that O. oeni was able to adhere to these surfaces and form spatially organized microcolonies embedded in extracellular substances. To assess the competitive advantage of this mode of life in wine, the properties of biofilm and planktonic cells were compared after inoculation in a fermented must (pH 3.5 or 3.2 and 12% ethanol) The results indicated that the biofilm culture of O. oeni conferred (i) increased tolerance to wine stress, and (ii) functional performance with effective malolactic activities. Relative gene expression focusing on stress genes and genes involved in EPS synthesis was investigated in a mature biofilm and emphasized the role of the matrix in increased biofilm resistance. As oak is commonly used in wine aging, we focused on the O. oeni biofilm on this material and its contribution to the development of wine color and the release of aromatic compounds. Analytical chromatography was used to target the main oak aging compounds such as vanillin, gaiacol, eugenol, whisky-lactones, and furfural. The results reveal that O. oeni biofilm developed on oak can modulate the wood-wine transfer of volatile aromatic compounds during MLF and aging by decreasing furfural, gaiacol, and eugenol in particular. This work showed that O. oeni forms biofilms consisting of stress-tolerant cells capable of efficient MLF under winemaking conditions. Therefore surface-associated behaviors should be considered in the development of improved strategies for the control of MLF in wine.

Exopolysaccharides produced by lactic acid bacteria: from health-promoting benefits to stress tolerance mechanisms

A wide range of lactic acid bacteria (LAB) is able to produce capsular or extracellular polysaccharides, with various chemical compositions and properties. Polysaccharides produced by LAB alter the rheological properties of the matrix in which they are dispersed, leading to typically viscous and "ropy" products. Polysaccharides are involved in several mechanisms such as prebiosis and probiosis, tolerance to stress associated to food process, and technological properties of food. In this paper, we summarize the beneficial properties of exopolysaccharides (EPS) produced by LAB with particular attention to prebiotic properties and to the effect of exopolysaccharides on the LAB-host interaction mechanisms, such as bacterial tolerance to gastrointestinal tract conditions, ability of ESP-producing probiotics to adhere to intestinal epithelium, their immune-modulatory activity, and their role in biofilm formation. The pro-technological aspect of exopolysaccharides is discussed, focusing on advantageous applications of EPS in the food industry, i.e., yogurt and gluten-free bakery products, since it was found that these microbial biopolymers positively affect the texture of foods. Finally, the involvement of EPS in tolerance to stress conditions that are commonly encountered in fermented beverages such as wine is discussed.

Protein homeostasis-more than resisting a hot bath

Maintenance of protein homeostasis is essential for survival of all organisms. In bacteria, the protein quality control system has a broad physiological impact beyond heat shock resistance, being involved in virulence, antibiotic resistance, as well as protection against environmental stresses. Its contribution to rejuvenation and growth arrest suggests interference with protein quality control to be a novel antimicrobial strategy. Remarkably, a protein quality control module originating from environmental strains has been found to be horizontally transferred to predominant clonal groups of bacteria providing exquisite thermotolerance to recently emerged global pathogens suggesting that novel features related to protein homeostasis contribute to the transition to new environments.

The Antisense RNA Approach: a New Application for In Vivo Investigation of the Stress Response of Oenococcus oeni, a Wine-Associated Lactic Acid Bacterium

Oenococcus oeni is a wine-associated lactic acid bacterium mostly responsible for malolactic fermentation in wine. In wine, O. oeni grows in an environment hostile to bacterial growth (low pH, low temperature, and ethanol) that induces stress response mechanisms. To survive, O. oeni is known to set up transitional stress response mechanisms through the synthesis of heat stress proteins (HSPs) encoded by the hsp genes, notably a unique small HSP named Lo18. Despite the availability of the genome sequence, characterization of O. oeni genes is limited, and little is known about the in vivo role of Lo18. Due to the lack of genetic tools for O. oeni, an efficient expression vector in O. oeni is still lacking, and deletion or inactivation of the hsp18 gene is not presently practicable. As an alternative approach, with the goal of understanding the biological function of the O. oeni hsp18 gene in vivo, we have developed an expression vector to produce antisense RNA targeting of hsp18 mRNA. Recombinant strains were exposed to multiple stresses inducing hsp18 gene expression: heat shock and acid shock. We showed that antisense attenuation of hsp18 affects O. oeni survival under stress conditions. These results confirm the involvement of Lo18 in heat and acid tolerance of O. oeni. Results of anisotropy experiments also confirm a membrane-protective role for Lo18, as previous observations had already suggested. This study describes a new, efficient tool to demonstrate the use of antisense technology for modulating gene expression in O. oeni.