Identification of proteins induced at low pH in Lactococcus lactis

Functional diversity of AI-2/LuxS system in lactic acid bacteria: Impacts on biofilm formation and environmental resilience

A key component of microbial communication, autoinducer-2 (AI-2) signaling, affects several physiological processes, including environmental adaptation and biofilm formation in lactic acid bacteria (LAB). The multifarious contribution of AI-2, synthesized by LuxS, in improving biofilms and tolerance to hostile conditions in LAB has been investigated in this review. The evolutionary conservation and diversity of AI-2 are shown by a phylogenetic analysis of luxS gene among several LAB species. Furthermore, AI-2 signaling in LAB improves resistance to unfavorable environmental factors, including pH fluctuations, temperature extremes, and antimicrobial agents. Lactic acid bacteria could set off defenses against harmful impacts from environmental stresses.

Potential Virulence and Survival of Acid-Resistant Bacillus cereus KS100Bc Isolated from Foodservice Establishments in Korea

Bacillus cereus is an opportunistic pathogen causing food poisoning manifested by diarrheal or emetic syndromes. This study aimed to examine the potential virulence and survival of acid-resistant B. cereus strains isolated from foodservice establishments in schools. All isolates were positive for genes encoding nonhemolytic enterotoxin complexes, except for one strain that lacked nheC. More than 80% of the strains harbored enterotoxigenic genes. In contrast, cesB, which encodes an emetic toxin, was not detected. Among the isolates, the biofilm-forming activity of KS100Bc strain was 8.4 times higher than that of the type strain; therefore, KS100Bc was selected for further analyses. The KS100Bc survived in simulated gastric fluid (SGF) at pH 2.0 for 120 min, which is a major host barrier against exogenous microorganisms. The parameter Kmax of the log-linear + tail model for ATCC 14579 was up to1.31-fold higher than that for KS100Bc in SGF, indicating that it is difficult to inactivate KS100Bc in acidic gastric environments. Although whole-genome sequencing identified nucleotide sequences with high homology (>99%) between ATCC 14579 and KS100Bc, significant differences were observed in phenotypes such as biofilm formation and survival in SGF. The transcriptional profiles were compared to examine the differences in the response to acid shock. KS100Bc exhibited increased expression of genes associated with pH homeostasis, oxidative response, metabolic rearrangements, and general stress response with over twofold (P value < 0.05) relative gene expression compared with the type strain. These results suggest that identifying the characteristics of the acid-adapted isolate can broaden our understanding of therapeutic strategies and provide new directions for controlling and preventing the spread of an outbreak.

Histidine transport is essential for the growth of Staphylococcus aureus at low pH

Staphylococcus aureus is an opportunistic pathogen capable of causing many different human diseases. During colonization and infection, S. aureus will encounter a range of hostile environments, including acidic conditions such as those found on the skin and within macrophages. However, little is known about the mechanisms that S. aureus uses to detect and respond to low pH. Here, we employed a transposon sequencing approach to determine on a genome-wide level the genes required or detrimental for growth at low pH. We identified 31 genes that were essential for the growth of S. aureus at pH 4.5 and confirmed the importance of many of them through follow up experiments using mutant strains inactivated for individual genes. Most of the genes identified code for proteins with functions in cell wall assembly and maintenance. These data suggest that the cell wall has a more important role than previously appreciated in promoting bacterial survival when under acid stress. We also identified several novel processes previously not linked to the acid stress response in S. aureus. These include aerobic respiration and histidine transport, the latter by showing that one of the most important genes, SAUSA300_0846, codes for a previously uncharacterized histidine transporter. We further show that under acid stress, the expression of the histidine transporter gene is increased in WT S. aureus. In a S. aureus SAUSA300_0846 mutant strain expression of the histidine biosynthesis genes is induced under acid stress conditions allowing the bacteria to maintain cytosolic histidine levels. This strain is, however, unable to maintain its cytosolic pH to the same extent as a WT strain, revealing an important function specifically for histidine transport in the acid stress response of S. aureus.

Proteomic Profiling and Stress Response in Pediococcus acidilactici under Acetic Acid

Lactic acid bacteria are indispensable functional microorganisms for cereal vinegar brewing, but cell activities are inhibited by the dominant acetic acid stress. Herein, an acetic-acid-tolerant strain isolated previously was identified as Pediococcus acidilactici, which also exhibited good resistance to other stresses during vinegar brewing. Proteomics analysis evidenced that differentially expressed proteins involved in the glycolysis and gluconeogenesis pathway, pyruvate metabolism, and sugar phosphotransferase system were all downregulated. Meanwhile, saturation of fatty acids and antioxidant enzymes was strengthened. The effects of several proteins on the resistance of P. acidilactici and Lactobacillus lactis relied on the types of strain and stress. AccA and AcpP participating in fatty acid metabolism and biosynthesis and Mnc related to stress response were found to protect cells by modifying fatty acid compositions and reinforcing the antioxidant defense system. Our works deepen the mechanisms of P. acidilactici under acetic acid and offer targets for engineering cell tolerance.

Differences in Acid Stress Response of Lacticaseibacillus paracasei Zhang Cultured from Solid-State Fermentation and Liquid-State Fermentation

Liquid-state fermentation (LSF) and solid-state fermentation (SSF) are two forms of industrial production of lactic acid bacteria (LAB). The choice of two fermentations for LAB production has drawn wide concern. In this study, the tolerance of bacteria produced by the two fermentation methods to acid stress was compared, and the reasons for the tolerance differences were analyzed at the physiological and transcriptional levels. The survival rate of the bacterial agent obtained from solid-state fermentation was significantly higher than that of bacteria obtained from liquid-state fermentation after spray drying and cold air drying. However, the tolerance of bacterial cells obtained from liquid-state fermentation to acid stress was significantly higher than that from solid-state fermentation. The analysis at physiological level indicated that under acid stress, cells from liquid-state fermentation displayed a more solid and complete membrane structure, higher cell membrane saturated fatty acid, more stable intracellular pH, and more stable activity of ATPase and glutathione reductase, compared with cells from solid-state fermentation, and these physiological differences led to better tolerance to acid stress. In addition, transcriptomic analysis showed that in the cells cultured from liquid-state fermentation, the genes related to glycolysis, inositol phosphate metabolism, and carbohydrate transport were down-regulated, whereas the genes related to fatty acid synthesis and glutamate metabolism were upregulated, compared with those in cells from solid-state fermentation. In addition, some genes related to acid stress response such as cspA, rimP, rbfA, mazF, and nagB were up-regulated. These findings provide a new perspective for the study of acid stress tolerance of L. paracasei Zhang and offer a reference for the selection of fermentation methods of LAB production.

Adaptive Evolution of Lactococcus Lactis to Thermal and Oxidative Stress Increases Biomass and Nisin Production

High values of agitation and temperature lead to stressful conditions in the fermentations of Lactococcus lactis due to its aero-tolerant and mesophilic nature. Here, the adaptive laboratory evolution (ALE) technique was applied to increase biomass and nisin production yields by enhancing L. lactis subsp. lactis robustness at higher growth temperature and aeration rates. In two separate ALE experiments, after 162 serial transfers, optimum agitation and growth temperature of L. lactis were shifted from 40 rpm and 30 °C to 200 rpm and 37 °C, respectively. Oxidative and acid resistance were enhanced in the evolved strain. Whole-genome sequencing revealed the emergence of five single-nucleotide polymorphisms in the genome of the evolved strain in jag, DnaB, ArgR, cation transporter genes, and one putative protein. The evolved strain of L. lactis in this study has more industrial desirable features and improved nisin production capability and can act more efficiently in nisin production in stressful conditions.

Superoxide dismutase: an updated review on its health benefits and industrial applications

Many short-lived and highly reactive oxygen species, such as superoxide anion (O2-) and hydrogen peroxide (H2O2), are toxic or can create oxidative stress in cells, a response involved in the pathogenesis of numerous diseases depending on their concentration, location, and cellular conditions. Superoxide dismutase (SOD) activities as an endogenous and exogenous cell defense mechanism include the potential use in treating various diseases, improving the potential use in treating various diseases, and improving food-stuffs preparation dietary supplements human nutrition. Published work indicates that SOD regulates oxidative stress, lipid metabolism, inflammation, and oxidation in cells. It can prevent lipid peroxidation, the oxidation of low-density lipoprotein in macrophages, lipid droplets' formation, and the adhesion of inflammatory cells into endothelial monolayers. It also expresses antioxidant effects in numerous cancer-related processes. Additionally, different forms of SOD may also augment food processing and pharmaceutical applications, exhibit anticancer, antioxidant, and anti-inflammatory effects, and prevent arterial problems by protecting the proliferation of vascular smooth muscle cells. Many investigations in this review have reported the therapeutic ability and physiological importance of SOD. Because of their antioxidative effects, SODs are of great potential in the medicinal, cosmetic, food, farming and chemical industries. This review discusses the findings of human and animal studies that support the advantages of SOD enzyme regulations to reduce the formation of oxidative stress in various ways.

Updates on understanding of probiotic lactic acid bacteria responses to environmental stresses and highlights on proteomic analyses

Probiotics are defined as live microorganisms that improve the health of the host when administered in adequate quantities. Nonetheless, probiotics encounter extreme environmental conditions during food processing or along the gastrointestinal tract. This review discusses different environmental stresses that affect probiotics during food preparation, storage, and along the alimentary canal, including high temperature, low temperature, low and alkaline pH, oxidative stress, high hydrostatic pressure, osmotic pressure, and starvation. The understanding of how probiotics deal with environmental stress and thrive provides useful information to guide the selection of the strains with enhanced performance in specific situations, in food processing or during gastrointestinal transit. In most cases, multiple biological functions are affected upon exposure of the cell to environmental stress. Sensing of sublethal environmental stress can allow for adaptation processes to occur, which can include alterations in the expression of specific proteins.

Long-Term Transcriptional Activity at Zero Growth of a Cosmopolitan Rare Biosphere Member

Microbial diversity in the environment is mainly concealed within the rare biosphere (all species with <0.1% relative abundance). While dormancy explains a low-abundance state very well, the mechanisms leading to rare but active microorganisms remain elusive. We used environmental systems biology to genomically and transcriptionally characterize "Candidatus Desulfosporosinus infrequens," a low-abundance sulfate-reducing microorganism cosmopolitan to freshwater wetlands, where it contributes to cryptic sulfur cycling. We obtained its near-complete genome by metagenomics of acidic peat soil. In addition, we analyzed anoxic peat soil incubated under in situ-like conditions for 50 days by Desulfosporosinus-targeted qPCR and metatranscriptomics. The Desulfosporosinus population stayed at a constant low abundance under all incubation conditions, averaging 1.2 × 106 16S rRNA gene copies per cm³ soil. In contrast, transcriptional activity of "Ca. Desulfosporosinus infrequens" increased at day 36 by 56- to 188-fold when minor amendments of acetate, propionate, lactate, or butyrate were provided with sulfate, compared to the no-substrate-control. Overall transcriptional activity was driven by expression of genes encoding ribosomal proteins, energy metabolism, and stress response but not by expression of genes encoding cell growth-associated processes. Since our results did not support growth of these highly active microorganisms in terms of biomass increase or cell division, they had to invest their sole energy for maintenance, most likely counterbalancing acidic pH conditions. This finding explains how a rare biosphere member can contribute to a biogeochemically relevant process while remaining in a zero-growth state over a period of 50 days.IMPORTANCE The microbial rare biosphere represents the largest pool of biodiversity on Earth and constitutes, in sum of all its members, a considerable part of a habitat's biomass. Dormancy or starvation is typically used to explain the persistence of low-abundance microorganisms in the environment. We show that a low-abundance microorganism can be highly transcriptionally active while remaining in a zero-growth state for at least 7 weeks. Our results provide evidence that this zero growth at a high cellular activity state is driven by maintenance requirements. We show that this is true for a microbial keystone species, in particular a cosmopolitan but permanently low-abundance sulfate-reducing microorganism in wetlands that is involved in counterbalancing greenhouse gas emissions. In summary, our results provide an important step forward in understanding time-resolved activities of rare biosphere members relevant for ecosystem functions.

RECTA: Regulon Identification Based on Comparative Genomics and Transcriptomics Analysis

Regulons, which serve as co-regulated gene groups contributing to the transcriptional regulation of microbial genomes, have the potential to aid in understanding of underlying regulatory mechanisms. In this study, we designed a novel computational pipeline, regulon identification based on comparative genomics and transcriptomics analysis (RECTA), for regulon prediction related to the gene regulatory network under certain conditions. To demonstrate the effectiveness of this tool, we implemented RECTA on Lactococcus lactis MG1363 data to elucidate acid-response regulons. A total of 51 regulons were identified, 14 of which have computational-verified significance. Among these 14 regulons, five of them were computationally predicted to be connected with acid stress response. Validated by literature, 33 genes in Lactococcus lactis MG1363 were found to have orthologous genes which were associated with six regulons. An acid response related regulatory network was constructed, involving two trans-membrane proteins, eight regulons (llrA, llrC, hllA, ccpA, NHP6A, rcfB, regulons #8 and #39), nine functional modules, and 33 genes with orthologous genes known to be associated with acid stress. The predicted response pathways could serve as promising candidates for better acid tolerance engineering in Lactococcus lactis. Our RECTA pipeline provides an effective way to construct a reliable gene regulatory network through regulon elucidation, and has strong application power and can be effectively applied to other bacterial genomes where the elucidation of the transcriptional regulation network is needed.

Metabolomic and proteomic analysis of D-lactate-producing Lactobacillus delbrueckii under various fermentation conditions

As an important feedstock monomer for the production of biodegradable stereo-complex poly-lactic acid polymer, D-lactate has attracted much attention. To improve D-lactate production by microorganisms such as Lactobacillus delbrueckii, various fermentation conditions were performed, such as the employment of anaerobic fermentation, the utilization of more suitable neutralizing agents, and exploitation of alternative nitrogen sources. The highest D-lactate titer could reach 133 g/L under the optimally combined fermentation condition, increased by 70.5% compared with the control. To decipher the potential mechanisms of D-lactate overproduction, the time-series response of intracellular metabolism to different fermentation conditions was investigated by GC-MS and LC-MS/MS-based metabolomic analysis. Then the metabolomic datasets were subjected to weighted correlation network analysis (WGCNA), and nine distinct metabolic modules and eight hub metabolites were identified to be specifically associated with D-lactate production. Moreover, a quantitative iTRAQ-LC-MS/MS proteomic approach was employed to further analyze the change of intracellular metabolism under the combined fermentation condition, identifying 97 up-regulated and 42 down-regulated proteins compared with the control. The in-depth analysis elucidated how the key factors exerted influence on D-lactate biosynthesis. The results revealed that glycolysis and pentose phosphate pathways, transport of glucose, amino acids and peptides, amino acid metabolism, peptide hydrolysis, synthesis of nucleotides and proteins, and cell division were all strengthened, while ATP consumption for exporting proton, cell damage, metabolic burden caused by stress response, and bypass of pyruvate were decreased under the combined condition. These might be the main reasons for significantly improved D-lactate production. These findings provide the first omics view of cell growth and D-lactate overproduction in L. delbrueckii, which can be a theoretical basis for further improving the production of D-lactate.

Integrating omics to unravel the stress-response mechanisms in probiotic bacteria: Approaches, challenges, and prospects

Identifying the stress-response mechanism of probiotic bacteria has always captivated the interest of food producers. It is crucial to identify probiotic bacteria that have increased stress tolerance to survive during production, processing, and storage of food products. However, in order to achieve high resistance to environmental factors, there is a need to better understand stress-induced responses and adaptive mechanisms. With advances in bacterial genomics, there has been an upsurge in the application of other omic platforms such as transcriptomics, proteomics, metabolomics, and some more recent ones such as interactomics, fluxomics, and phenomics. These omic technologies have revolutionized the functional genomics and their application. There have been several studies implementing various omic technologies to investigate the stress responses of probiotic bacteria. Integrated omics has the potential to provide in-depth information about the mechanisms of stress-induced responses in bacteria. However, there remain challenges in integrating information from different omic platforms. This review discusses current omic techniques and challenges faced in integrating various omic platforms with focus on their use in stress-response studies.

Analyses of the probiotic property and stress resistance-related genes of Lactococcus lactis subsp. lactis NCDO 2118 through comparative genomics and in vitro assays

Lactococcus lactis subsp. lactis NCDO 2118 was recently reported to alleviate colitis symptoms via its anti-inflammatory and immunomodulatory activities, which are exerted by exported proteins that are not produced by L. lactis subsp. lactis IL1403. Here, we used in vitro and in silico approaches to characterize the genomic structure, the safety aspects, and the immunomodulatory activity of this strain. Through comparative genomics, we identified genomic islands, phage regions, bile salt and acid stress resistance genes, bacteriocins, adhesion-related and antibiotic resistance genes, and genes encoding proteins that are putatively secreted, expressed in vitro and absent from IL1403. The high degree of similarity between all Lactococcus suggests that the Symbiotic Islands commonly shared by both NCDO 2118 and KF147 may be responsible for their close relationship and their adaptation to plants. The predicted bacteriocins may play an important role against the invasion of competing strains. The genes related to the acid and bile salt stresses may play important roles in gastrointestinal tract survival, whereas the adhesion proteins are important for persistence in the gut, culminating in the competitive exclusion of other bacteria. Finally, the five secreted and expressed proteins may be important targets for studies of new anti-inflammatory and immunomodulatory proteins. Altogether, the analyses performed here highlight the potential use of this strain as a target for the future development of probiotic foods.

Strain-Dependent Transcriptome Signatures for Robustness in Lactococcus lactis

Recently, we demonstrated that fermentation conditions have a strong impact on subsequent survival of Lactococcus lactis strain MG1363 during heat and oxidative stress, two important parameters during spray drying. Moreover, employment of a transcriptome-phenotype matching approach revealed groups of genes associated with robustness towards heat and/or oxidative stress. To investigate if other strains have similar or distinct transcriptome signatures for robustness, we applied an identical transcriptome-robustness phenotype matching approach on the L. lactis strains IL1403, KF147 and SK11, which have previously been demonstrated to display highly diverse robustness phenotypes. These strains were subjected to an identical fermentation regime as was performed earlier for strain MG1363 and consisted of twelve conditions, varying in the level of salt and/or oxygen, as well as fermentation temperature and pH. In the exponential phase of growth, cells were harvested for transcriptome analysis and assessment of heat and oxidative stress survival phenotypes. The variation in fermentation conditions resulted in differences in heat and oxidative stress survival of up to five 10-log units. Effects of the fermentation conditions on stress survival of the L. lactis strains were typically strain-dependent, although the fermentation conditions had mainly similar effects on the growth characteristics of the different strains. By association of the transcriptomes and robustness phenotypes highly strain-specific transcriptome signatures for robustness towards heat and oxidative stress were identified, indicating that multiple mechanisms exist to increase robustness and, as a consequence, robustness of each strain requires individual optimization. However, a relatively small overlap in the transcriptome responses of the strains was also identified and this generic transcriptome signature included genes previously associated with stress (ctsR and lplL) and novel genes, including nanE and genes encoding transport proteins. The transcript levels of these genes can function as indicators of robustness and could aid in selection of fermentation parameters, potentially resulting in more optimal robustness during spray drying.

Adaptation in Bacillus cereus: From Stress to Disease

Bacillus cereus is a food-borne pathogen that causes diarrheal disease in humans. After ingestion, B. cereus experiences in the human gastro-intestinal tract abiotic physical variables encountered in food, such as acidic pH in the stomach and changing oxygen conditions in the human intestine. B. cereus responds to environmental changing conditions (stress) by reversibly adjusting its physiology to maximize resource utilization while maintaining structural and genetic integrity by repairing and minimizing damage to cellular infrastructure. As reviewed in this article, B. cereus adapts to acidic pH and changing oxygen conditions through diverse regulatory mechanisms and then exploits its metabolic flexibility to grow and produce enterotoxins. We then focus on the intricate link between metabolism, redox homeostasis, and enterotoxins, which are recognized as important contributors of food-borne disease.

Development of a SCAR (sequence-characterised amplified region) marker for acid resistance-related gene in Lactobacillus plantarum

A sequence characterised amplified region marker was developed to determine an acid resistance-related gene in Lactobacillus plantarum. A random amplified polymorphic DNA marker named S116-680 was reported to be closely related to the acid resistance of the strains. The DNA band corresponding to this marker was cloned and sequenced with the induction of specific designed PCR primers. The results of PCR test helped to amplify a clear specific band of 680 bp in the tested acid-resistant strains. S116-680 marker would be useful to explore the acid-resistant mechanism of L. plantarum and to screen desirable malolactic fermentation strains.

Coping with low pH: molecular strategies in neutralophilic bacteria

As part of their life cycle, neutralophilic bacteria are often exposed to varying environmental stresses, among which fluctuations in pH are the most frequent. In particular, acid environments can be encountered in many situations from fermented food to the gastric compartment of the animal host. Herein, we review the current knowledge of the molecular mechanisms adopted by a range of Gram-positive and Gram-negative bacteria, mostly those affecting human health, for coping with acid stress. Because organic and inorganic acids have deleterious effects on the activity of the biological macromolecules to the point of significantly reducing growth and even threatening their viability, it is not unexpected that neutralophilic bacteria have evolved a number of different protective mechanisms, which provide them with an advantage in otherwise life-threatening conditions. The overall logic of these is to protect the cell from the deleterious effects of a harmful level of protons. Among the most favoured mechanisms are the pumping out of protons, production of ammonia and proton-consuming decarboxylation reactions, as well as modifications of the lipid content in the membrane. Several examples are provided to describe mechanisms adopted to sense the external acidic pH. Particular attention is paid to Escherichia coli extreme acid resistance mechanisms, the activity of which ensure survival and may be directly linked to virulence.

Metabolic and transcriptional analysis of acid stress in Lactococcus lactis, with a focus on the kinetics of lactic acid pools

The effect of pH on the glucose metabolism of non-growing cells of L. lactis MG1363 was studied by in vivo NMR in the range 4.8 to 6.5. Immediate pH effects on glucose transporters and/or enzyme activities were distinguished from transcriptional/translational effects by using cells grown at the optimal pH of 6.5 or pre-adjusted to low pH by growth at 5.1. In cells grown at pH 5.1, glucose metabolism proceeds at a rate 35% higher than in non-adjusted cells at the same pH. Besides the upregulation of stress-related genes (such as dnaK and groEL), cells adjusted to low pH overexpressed H(+)-ATPase subunits as well as glycolytic genes. At sub-optimal pHs, the total intracellular pool of lactic acid reached approximately 500 mM in cells grown at optimal pH and about 700 mM in cells grown at pH 5.1. These high levels, together with good pH homeostasis (internal pH always above 6), imply intracellular accumulation of the ionized form of lactic acid (lactate anion), and the concomitant export of the equivalent protons. The average number, n, of protons exported with each lactate anion was determined directly from the kinetics of accumulation of intra- and extracellular lactic acid as monitored online by (13)C-NMR. In cells non-adjusted to low pH, n varies between 2 and 1 during glucose consumption, suggesting an inhibitory effect of intracellular lactate on proton export. We confirmed that extracellular lactate did not affect the lactate: proton stoichiometry. In adjusted cells, n was lower and varied less, indicating a different mix of lactic acid exporters less affected by the high level of intracellular lactate. A qualitative model for pH effects and acid stress adaptation is proposed on the basis of these results.

Engineering trehalose synthesis in Lactococcus lactis for improved stress tolerance

Trehalose accumulation is a common cell defense strategy against a variety of stressful conditions. In particular, our team detected high levels of trehalose in Propionibacterium freudenreichii in response to acid stress, a result that led to the idea that endowing Lactococcus lactis with the capacity to synthesize trehalose could improve the acid tolerance of this organism. To this end, we took advantage of the endogenous genes involved in the trehalose catabolic pathway of L. lactis, i.e., trePP and pgmB, encoding trehalose 6-phosphate phosphorylase and β-phosphoglucomutase, respectively, which enabled the synthesis of trehalose 6-phosphate. Given that L. lactis lacks trehalose 6-phosphate phosphatase, the respective gene, otsB, from the food-grade organism P. freudenreichii was used to provide the required activity. The trehalose yield was approximately 15% in resting cells and in mid-exponential-phase cells grown without pH control. The intracellular concentration of trehalose reached maximal values of approximately 170 mM, but at least 67% of the trehalose produced was found in the growth medium. The viability of mutant and control strains was examined after exposure to heat, cold or acid shock, and freeze-drying. The trehalose-producing strains showed improved tolerance (5- to 10-fold-higher survivability) to acid (pH 3) and cold shock (4°C); there was also a strong improvement in cell survival in response to heat shock (45°C), and no protection was rendered against dehydration. The insight provided by this work may help the design of food-grade strains optimized for the dairy industry as well as for oral drug delivery.

Dynamic analysis of the Lactococcus lactis transcriptome in cheeses made from milk concentrated by ultrafiltration reveals multiple strategies of adaptation to stresses

Lactococcus lactis is used extensively for the production of various cheeses. At every stage of cheese fabrication, L. lactis has to face several stress-generating conditions that result from its own modification of the environment as well as externally imposed conditions. We present here the first in situ global gene expression profile of L. lactis in cheeses made from milk concentrated by ultrafiltration (UF-cheeses), a key economical cheese model. The transcriptomic response of L. lactis was analyzed directly in a cheese matrix, starting from as early as 2 h and continuing for 7 days. The growth of L. lactis stopped after 24 h, but metabolic activity was maintained for 7 days. Conservation of its viability relied on an efficient proteolytic activity measured by an increasing, quantified number of free amino acids in the absence of cell lysis. Extensive downregulation of genes under CodY repression was found at day 7. L. lactis developed multiple strategies of adaptation to stressful modifications of the cheese matrix. In particular, expression of genes involved in acidic- and oxidative-stress responses was induced. L. lactis underwent unexpected carbon limitation characterized by an upregulation of genes involved in carbon starvation, principally due to the release of the CcpA control. We report for the first time that in spite of only moderately stressful conditions, lactococci phage is repressed under UF-cheese conditions.

Adaptation to cold and proteomic responses of the psychrotrophic biopreservative Lactococcus piscium strain CNCM I-4031

There is considerable interest in the use of psychrotrophic bacteria for food biopreservation and in the understanding of cold adaptation mechanisms. The psychrotrophic biopreservative Lactococcus piscium strain CNCM I-4031 was studied for its growth behavior and proteomic responses after cold shock and during cold acclimation. Growth kinetics highlighted the absence of growth latency after cold shock, suggesting a very high promptness in cold adaptation, a behavior that has never been described before for lactic acid bacteria (LAB). A comparative proteomic analysis was applied with two-dimensional gel electrophoresis (2-DE), and upregulated proteins were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Both cold shock and cold acclimation triggered the upregulation of proteins involved in general and oxidative stress responses and fatty acid and energetic metabolism. However, 2-DE profiles and upregulated proteins were different under both conditions, suggesting a sequence of steps in cold adaptation. In addition, the major 7-kDa Csp protein was identified in the L. piscium CNCM I-4031 genome but was not cold regulated. The implication of the identified cold shock proteins and cold acclimation proteins in efficient cold adaptation, the possible regulation of a histidyl phosphocarrier protein, and the roles of a constitutive major 7-kDa Csp are discussed.

Regulation of CtsR activity in low GC, Gram+ bacteria

CtsR is the global transcriptional regulator of the core protein quality networks in low GC, Gram+ bacteria. Balancing these networks during environmental stress is of considerable importance for moderate survival of the bacteria, and also for virulence of pathogenic species. Therefore, inactivation of the CtsR repressor is one of the major cellular responses for fast and efficient adaptation to different protein stress conditions. Historically, CtsR inactivation was mainly studied for the heat stress response, and recently it has been shown that CtsR is an intrinsic thermosensor. Moreover, it has been demonstrated that CtsR degradation is regulated by a two-step mechanism during heat stress, dependent on the arginine kinase activity of McsB. Interestingly, CtsR is also inactivated during oxidative stress, but by a thiol-dependent regulatory pathway. These observations suggest that dual activity control of CtsR activity has developed during the course of evolution.

Transcriptomic response of Lactococcus lactis in mixed culture with Staphylococcus aureus

The mechanisms of interaction between Lactococcus lactis and the food pathogen Staphylococcus aureus are of crucial importance, as one major role of lactic acid bacteria (LAB) in fermented foods is to inhibit undesirable and pathogenic flora. It was never questioned if the presence of a pathogen can actively modify the gene expression patterns of LAB in a shared environment. In this study, transcriptome and biochemical analyses were combined to assess the dynamic response of L. lactis in a mixed culture with S. aureus. The presence of S. aureus hardly affected the growth of L. lactis but dramatically modified its gene expression profile. The main effect was related to earlier carbon limitation and a concomitantly lower growth rate in the mixed culture due to the consumption of glucose by both species. More specific responses involved diverse cellular functions. Genes associated with amino acid metabolism, ion transport, oxygen response, menaquinone metabolism, and cell surface and phage expression were differentially expressed in the mixed culture. This study led to new insights into possible mechanisms of interaction between L. lactis and S. aureus. Moreover, new and unexpected effects of L. lactis on the virulence of S. aureus were discovered, as described elsewhere (S. Even, C. Charlier, S. Nouaille, N. L. Ben Zakour, M. Cretenet, F. J. Cousin, M. Gautier, M. Cocaign-Bousquet, P. Loubière, and Y. Le Loir, Appl. Environ. Microbiol. 75:4459-4472, 2009).

Comprehensive proteome analysis of Mycobacterium ulcerans and quantitative comparison of mycolactone biosynthesis

Mycobacterium ulcerans is the causative agent of Buruli ulcer, a rapidly emerging human disease in which mycolactone, a cytotoxic and immunosuppressive macrocyclic polyketide, is responsible for massive skin destruction. The genome sequencing of M. ulcerans has recently been accomplished (http://genolist.pasteur.fr/BuruList/) enabling the first proteome study of this important human pathogen. Here, we present a comprehensive proteome analysis of different subcellular fractions and culture supernatant of in vitro grown M. ulcerans. By a combination of gel-based and gel-free techniques for protein and peptide separation with subsequent analysis by MS, we identified 1074 different proteins, corresponding to 25% of the protein-coding DNA sequence. Interestingly, new information was obtained about central metabolism and lipid biosynthesis, and as many as 192 conserved hypothetical proteins were found. Comparative analysis of the wild-type strain and an isogenic mycolactone-deficient mutant, by 2-DE and iTRAQ labeling of the cytoplasmic fraction, revealed differences in the expression profiles of proteins involved in lipid metabolism and information pathways, as well as stress responses.

RNA arbitrarily primed PCR and fourier transform infrared spectroscopy reveal plasticity in the acid tolerance response of Streptococcus macedonicus

We have previously reported that an acid tolerance response (ATR) can be induced in Streptococcus macedonicus cells at mid-log phase after autoacidification, transient exposure to acidic pH, or acid habituation, as well as at stationary phase. Here, we compared the transcriptional profiles of these epigenetic phenotypes, by RNA arbitrarily primed PCR (RAP-PCR), and their whole-cell chemical compositions, by Fourier transform infrared spectroscopy (FT-IR). RAP-PCR fingerprints revealed significant differences among the phenotypes, indicating that gene expression during the ATR is influenced not only by the growth phase but also by the treatments employed to induce the response. The genes coding for the mannose-specific IID component, the 1,2-diacylglycerol 3-glucosyltransferase, the 3-oxoacyl-acyl carrier protein, the large subunit of carbamoyl-phosphate synthase, and a hypothetical protein were found to be induced at least under some of the acid-adapting conditions. Furthermore, principal component analysis of the second-derivative-transformed FT-IR spectra segregated S. macedonicus phenotypes individually in all spectral regions that are characteristic for major cellular constituents like the polysaccharides of the cell wall, fatty acids of the cell membrane, proteins, and other compounds that absorb in these regions. These findings provide evidence for major changes in cellular composition due to acid adaptation that were clearly different to some extent among the phenotypes. Overall, our data demonstrate the plasticity in the ATR of S. macedonicus, which reflects the inherent ability of the bacterium to adjust the response to the distinctiveness of the imposed stress condition, probably to maximize its adaptability.

Impact of luxS and suppressor mutations on the gastrointestinal transit of Lactobacillus rhamnosus GG

It is generally believed that probiotic bacteria need to survive gastrointestinal transit to exert a health-promoting effect. In this study, a genuine luxS mutant and a luxS mutant containing unknown suppressor mutations of the probiotic strain Lactobacillus rhamnosus GG were compared to the wild type for survival and persistence in the murine gastrointestinal tract. The LuxS enzyme, catalyzing the production of the autoinducer-2 signaling molecule, also forms an integral part of the activated methyl cycle and the metabolism of methionine and cysteine. The genuine luxS mutant CMPG5412 showed drastically reduced persistence in mice, which was related to less survival in simulated gastric juice, indicating that LuxS metabolism is crucial for the gastric stress resistance of L. rhamnosus GG. The suppressor mutations in the other luxS mutant, CMPG5413, appear to compensate for the metabolic defects of the luxS mutation and to restore the resistance to gastric juice but cause a defect in adherence, biofilm formation, and exopolysaccharide production. The shorter residence time of this suppressor mutant in the murine gastrointestinal tract indicates a role for biofilm formation and exopolysaccharides in the persistence capacity of L. rhamnosus GG.