Adaptation to high salt in Lactobacillus: role of peptides and proteolytic enzymes

Cultivable microbial diversity, peptide profiles, and bio-functional properties in Parmigiano Reggiano cheese

Introduction

Lactic acid bacteria (LAB) communities shape the sensorial and functional properties of artisanal hard-cooked and long-ripened cheeses made with raw bovine milk like Parmigiano Reggiano (PR) cheese. While patterns of microbial evolution have been well studied in PR cheese, there is a lack of information about how this microbial diversity affects the metabolic and functional properties of PR cheese.

Methods

To fill this information gap, we characterized the cultivable fraction of natural whey starter (NWS) and PR cheeses at different ripening times, both at the species and strain level, and investigated the possible correlation between microbial composition and the evolution of peptide profiles over cheese ripening.

Results and discussion

The results showed that NWS was a complex community of several biotypes belonging to a few species, namely, Streptococcus thermophilus, Lactobacillus helveticus, and Lactobacillus delbrueckii subsp. lactis. A new species-specific PCR assay was successful in discriminating the cheese-associated species Lacticaseibacillus casei, Lacticaseibacillus paracasei, Lacticaseibacillus rhamnosus, and Lacticaseibacillus zeae. Based on the resolved patterns of species and biotype distribution, Lcb. paracasei and Lcb. zeae were most frequently isolated after 24 and 30 months of ripening, while the number of biotypes was inversely related to the ripening time. Peptidomics analysis revealed more than 520 peptides in cheese samples. To the best of our knowledge, this is the most comprehensive survey of peptides in PR cheese. Most of them were from β-caseins, which represent the best substrate for LAB cell-envelope proteases. The abundance of peptides from β-casein 38-88 region continuously increased during ripening. Remarkably, this region contains precursors for the anti-hypertensive lactotripeptides VPP and IPP, as well as for β-casomorphins. We found that the ripening time strongly affects bioactive peptide profiles and that the occurrence of Lcb. zeae species is positively linked to the incidence of eight anti-hypertensive peptides. This result highlighted how the presence of specific LAB species is likely a pivotal factor in determining PR functional properties.

Transcriptomic Responses to Nitrite Degradation by Limosilactobacillus fermentum RC4 and Effect of ndh Gene Overexpression on Nitrite Degradation

The excessive nitrite residue may increase cell damage and cancer risk. Limosilactobacillu fermentum RC4 exhibited excellent nitrite degradation ability. Herein, the molecular mechanism of nitrite degradation by L. fermentum RC4 was studied by integrating scanning electron microscopy analysis, transcriptomics, and gene overexpression. The results demonstrated that the gene profile of RC4 cultured in MRS broth with 0, 100, and 300 mg/L NaNO2 varied considerably; RC4 responded to nitrite degradation by regulating pyruvate metabolism, energy synthesis, nitrite metabolism, redox equilibrium, protein protection, and signaling. High nitrite concentrations affected the morphology of RC4 with a longer phenotype, rough and wrinkle cell and reduced cell surface hydrophobicity. Moreover, an up-regulated expression of gene ndh encoding NADH dehydrogenase, which provides electrons for nitrite reduction by catalyzing NADH, was identified when RC4 was exposed to nitrite. Overexpression of ndh in RC4 increased the nitrite degradation rate by 2-9.5% in MRS broth with 100 mg/L NaNO2. Thus, the findings of this study could be helpful for the application of L. fermentum to reduce nitrite residues and improve food safety in fermented food products.

Health-Promoting Nature of Lactococcus lactis IBB109 and Lactococcus lactis IBB417 Strains Exhibiting Proliferation Inhibition and Stimulation of Interleukin-18 Expression in Colorectal Cancer Cells

Lactic acid bacteria (LAB) are Gram-positive bacteria which are considered for use as adjuvant therapeutics in management of various disease ailments, including obesity, irritable bowel syndrome, lactose intolerance and cancer. To investigate the possible use of Lactococcus lactis strains from our collection in treatment of gastrointestinal cancer, we tested them for the ability to arrest proliferation of human colorectal adenocarcinoma cells (Caco-2). Results of the BrdU assay showed that the anti-proliferative activity of L. lactis cells is strain-specific. We found that particularly, two strains, L. lactis IBB109 and L. lactis IBB417, exhibited the most potent inhibitory effect. Moreover, both strains triggered interleukin 18 gene expression, normally inhibited in Caco-2 (cancer) cells. To examine the probiotic potential of the two strains, we tested them for bile salts and acid tolerance, as well as adhesion properties. Both isolates exhibited probiotic potential—they survived in the presence of 0.3% bile salts and tolerated exposure to low pH and osmotic stress. Notably, we found that L. lactis IBB417 displayed better adherence to mucus and Caco-2 cells than L. lactis IBB109. Additionally, by microdilution tests we confirmed that both strains are sensitive to all nine antibiotics of human and veterinary importance listed by the European Food Safety Authority. Finally, by in silico investigations of whole genome sequencing data, we revealed the genetic features of L. lactis IBB109 and L. lactis IBB417 that can be associated with functional (e.g., adhesion and carbohydrate metabolic genes) and safety (e.g., virulence and antibiotic resistance) aspects of the strains, confirming their health-promoting potential.

Use of Stress Signals of Their Attached Bacteria to Monitor Sympagic Algae Preservation in Canadian Arctic Sediments

Based on the strong aggregation of sympagic (ice-associated) algae and the high mortality or inactivity of bacteria attached to them, it was previously hypothesized that sympagic algae should be significant contributors to the export of carbon to Arctic sediments. In the present work, the lipid content of 30 sediment samples collected in the Canadian Arctic was investigated to test this hypothesis. The detection of high proportions of trans vaccenic fatty acid (resulting from cis-trans isomerase (CTI) activity of bacteria under hypersaline conditions) and 10S-hydroxyhexadec-8(trans)-enoic acid (resulting from 10S-DOX bacterial detoxification activity in the presence of deleterious free palmitoleic acid) confirmed: (i) the strong contribution of sympagic material to some Arctic sediments, and (ii) the impaired physiological status of its associated bacterial communities. Unlike terrestrial material, sympagic algae that had escaped zooplanktonic grazing appeared relatively preserved from biotic degradation in Arctic sediments. The expected reduction in sea ice cover resulting from global warming should cause a shift in the relative contributions of ice-associated vs. pelagic algae to the seafloor, and thus to a strong modification of the carbon cycle.

Combination of osmotic stress and sugar stress response mechanisms is essential for Gluconacetobacter diazotrophicus tolerance to high-sucrose environments

Sugar-rich environments represent an important challenge for microorganisms. The osmotic and molecular imbalances resulting from this condition severely limit microbial metabolism and growth. Gluconacetobacter diazotrophicus is one of the most sugar-tolerant prokaryotes, able to grow in the presence of sucrose concentrations up to 30%. However, the mechanisms that control its tolerance to such conditions remain poorly exploited. The present work investigated the key mechanisms of tolerance to high sugar in G. diazotrophicus. Comparative proteomics was applied to investigate the main functional pathways regulated in G. diazotrophicus when cultivated in the presence of high sucrose. Among 191 proteins regulated by high sucrose, regulatory pathways related to sugar metabolism, nutrient uptake, compatible solute synthesis, amino acid metabolism, and proteolytic system were highlighted. The role of these pathways on high-sucrose tolerance was investigated by mutagenesis analysis, which revealed that the knockout mutants zwf::Tn5 (sugar metabolism), tbdr::Tn5 (nutrient uptake), mtlK::Tn5 (compatible solute synthesis), pepN::Tn5 (proteolytic system), metH::Tn5 (amino acid metabolism), and ilvD::Tn5 (amino acid metabolism) became more sensitive to high sucrose. Together, our results identified mechanisms involved in response to high sugar in G. diazotrophicus, shedding light on the combination of osmotolerance and sugar-tolerance mechanisms. KEY POINTS: • G. diazotrophicus intensifies glycolysis to metabolize the excess of sugar. • G. diazotrophicus turns down the uptake of nutrients in response to high sugar. • G. diazotrophicus requires amino acid availability to resist high sugar.

Characterization and biological properties of peptides isolated from dried fermented cow milk products by RP-HPLC: Amino acid composition, antioxidant, antihypertensive, and antidiabetic properties

This study aimed to assess the biological properties of peptide fractions isolated from dried fermented dairy products (jameed) as influenced by processing. Peptide fractions were separated by reversed-phase high-performance liquid chromatography (RP-HPLC) from salted (Sa) and unsalted (Us) cow milk jameed after drying the fermented curd by sun drying (Sd) or freeze-drying (Fd) and were characterized for their antioxidant capacity and inhibitory activity toward angiotensin I-converting enzyme (ACE) and α-amylase. Sd samples showed more numerous peptide peaks in RP-HPLC chromatograms than Fd samples, regardless of the salt content. High antioxidant activity was evidenced in several peptide fractions from FdUs jameed (including fractions 1, 2, 4, 7, 8, 9, and 10), SdUs jameed (1, 2, 5, 7, and 9), and FdSa jameed (2, 5, 6, and 9). By contrast, peptide fractions from SdSa (1, 2, 3, 5, 8, and 9), SdUs (4, 5, and 10), and FdUs (5, 6, and 8) jameed displayed the highest ACE inhibitory activity. Similarly, the highest inhibition of α-amylase was obtained with fractions from SdSa (1, 2, 3, 4, 5, 6, 8, and 9), SdUs (2 and 6), and FdUs (1, 7 and 9) jameed. A significant negative correlation was evidenced between antioxidant activity and anti-α-amylase activity of peptide fractions from SdSa jameed. These findings demonstrate that cow milk jameed is a source of bioactive peptides with antioxidant, anti-ACE, and anti-α-amylase properties in vitro, which can be tailored by adjusting the salt content and the drying conditions. PRACTICAL APPLICATION: This study shows that cow milk jameed, a staple fermented food in several Mediterranean countries, can serve as a useful source of multifunctional bioactive peptides with potential antioxidant, hypotensive, and hypoglycemic effects, which may help prevent and manage chronic health conditions such as hypertension, type 2 diabetes, and the metabolic syndrome. The bioactivities of certain peptide fractions were enhanced by lowering the salt content of jameed or by the drying method. The relatively simple RP-HPLC method described in this study can be used to isolate the peptide fractions of interest for further characterization and use as functional ingredients.

A review on enzyme-producing lactobacilli associated with the human digestive process: From metabolism to application

Complex carbohydrates, proteins, and other food components require a longer digestion process to be absorbed by the lining of the alimentary canal. In addition to the enzymes of the gastrointestinal tract, gut microbiota, comprising a large range of bacteria and fungi, has complementary action on the production of digestive enzymes. Within this universe of "hidden soldiers", lactobacilli are extensively studied because of their ability to produce lactase, proteases, peptidases, fructanases, amylases, bile salt hydrolases, phytases, and esterases. The administration of living lactobacilli cells has been shown to increase nutrient digestibility. However, it is still little known how these microbial-derived enzymes act in the human body. Enzyme secretion may be affected by variations in temperature, pH, and other extreme conditions faced by the bacterial cells in the human body. Besides, lactobacilli administration cannot itself be considered the only factor interfering with enzyme secretion, human diet (microbial substrate) being determinant in their metabolism. This review highlights the potential of lactobacilli to release functional enzymes associated with the digestive process and how this complex metabolism can be explored to contribute to the human diet. Enzymatic activity of lactobacilli is exerted in a strain-dependent manner, i.e., within the same lactobacilli species, there are different enzyme contents, leading to a large variety of enzymatic activities. Thus, we report current methods to select the most promising lactobacilli strains as sources of bioactive enzymes. Finally, a patent landscape and commercial products are described to provide the state of art of the transfer of knowledge from the scientific sphere to the industrial application.

Viability and stress state of bacteria associated with primary production or zooplankton-derived suspended particulate matter in summer along a transect in Baffin Bay (Arctic Ocean)

In the framework of the GreenEdge Project (whose the general objective is to understand the dynamic of the phytoplankton spring bloom in Arctic Ocean), lipid composition and viability and stress state of bacteria were monitored in sea ice and suspended particulate matter (SPM) samples collected in 2016 along a transect from sea ice to open water in Baffin Bay (Arctic Ocean). Lipid analyses confirmed the dominance of diatoms in the bottommost layer of ice and suggested (i) the presence of a strong proportion of micro-zooplankton in SPM samples collected at the western ice covered St 403 and St 409 and (ii) a high proportion of macro-zooplankton (copepods) in SPM samples collected at the eastern ice covered St 413 and open water St 418. The use of the propidium monoazide (PMA) method allowed to show a high bacterial mortality in sea ice and in SPM material collected in shallower waters at St 409 and St 418. This mortality was attributed to the release of bactericidal free fatty acids by sympagic diatoms under the effect of light stress. A strong cis-trans isomerization of bacterial MUFAs was observed in the deeper SPM samples collected at the St 403 and St 409. It was attributed to the ingestion of bacteria stressed by salinity in brine channels of ice by sympagic bacterivorous microzooplankton (ciliates) incorporating trans fatty acids of their preys before to be released in the water column during melting. The high trans/cis ratios also observed in SPM samples collected in the shallower waters at St 413 and St 418 suggest the presence of positively or neutrally buoyant extracellular polymeric substances (EPS)-rich particles retained in sea ice and discharged (with bacteria stressed by salinity) in seawater after the initial release of algal biomass. Such EPS particles, which are generally considered as ideal vectors for bacterial horizontal distribution in the Arctic, appeared to contain a high proportion of dead and non-growing bacteria.

Lactic acid production using cheese whey based medium in a stirred tank reactor by a ccpA mutant of Lacticaseibacillus casei

In lactobacilli, CcpA is known to modulate the expression of genes involved in sugar metabolism, stress response and aerobic adaptation. This study aimed to evaluate a ccpA mutant of Lacticaseibacillus casei BL23 to increase lactic acid production using cheese whey. The ccpA derivative (BL71) showed better growth than the L. casei wild-type in the whey medium. In a stirred tank reactor, at 48 h, lactate production by BL71 was eightfold higher than that by BL23. In batch fermentations, the final values reached were 44.23 g L^-1 for BL71 and 27.58 g L^-1 for BL23. Due to a decrease in the delay of lactate production in the mutant, lactate productivity increased from 0.17 g (L.h)^-1 with BL23 to 0.80 g (L.h)^-1 with BL71. We found that CcpA would play additional roles in nitrogen metabolism by the regulation of the proteolytic system. BL71 displayed higher activity of the PepX, PepQ and PrtP enzymes than BL23. Analysis of prtP expression confirmed this deregulation in BL71. Promoter analysis of the prtP gene revealed CcpA binding sites with high identity to the cre consensus sequence and the interaction of CcpA with this promoter was confirmed in vitro. We postulate that deregulation of the proteolytic system in BL71 allows a better exploitation of nitrogen resources in cheese whey, resulting in enhanced fermentation capacity. Therefore, the ccpA gene could be a good target for future technological developments aimed at effective and inexpensive lactate production from dairy industrial wastes.

Protease produced by Lactobacillus brevis enhanced nutritional values of African yam beans and demonstrated improvement in the growth and blood indices of albino rats

This study evaluates the nutritional values of African yam bean hydrolyzed with protease from Lactobacillus brevis and afterward assess its effect on the growth and blood parameters of albino rats. The nutritional compositions of AYB hydrolyzed with partially purified protease from L. brevis were determined by standard chemical methods. The protease-hydrolyzed AYB was thereafter formulated into feeds with different inclusion levels (20, 40 and 60%), which was used to feed albino rats for 27 days. After the feeding trial, the blood of anesthetized albino rats was collected using the cardiac puncture method, and the hematological parameters were determined by standard biochemical methods. The AYB hydrolyzed with partially purified protease had the highest percentage crude protein with a value of 31.2% when compared with boiled, soaked and boiled, and raw sample with the values of 20.9, 20.9 and 19.9%, respectively. The treatment of AYB with purified protease also resulted in an increase of vitamins and some essential amino acids when compared with unhydrolyzed AYB. The group of rats fed with 60% hydrolyzed AYB had the highest percentage average weight gain of approximately 144%, while the values recorded for the groups fed with commercial feed and unhydrolyzed AYB were approximately 86 and 101%, respectively. The hematological analysis revealed that the hemoglobin (Hb) and packed cell volume (PCV) of the group fed with 40% hydrolyzed AYB of 14 g/L and 38% respectively, were significantly higher than the rats fed with commercial feed with values of 10 g/L and 32%, respectively. Thus, enzyme-hydrolyzed AYB might be a suitable alternative to animal protein with good functional properties.

Peptide Profiling and Biological Activities of 12-Month Ripened Parmigiano Reggiano Cheese

Proteolysis degree, biological activities, and water-soluble peptide patterns were evaluated in 12 month-ripened Parmigiano Reggiano (PR) cheeses collected in different dairy farms and showing different salt and fat content. Samples classified in high-salt and high-fat group (HH) generally showed lower proteolysis degree than samples having low-salt and low-fat content (LL). This positive correlation between salt/fat reduction and proteolysis was also confirmed by the analysis of biological activities, as the LL group showed higher average values of angiotensin-converting enzyme (ACE)-inhibitory and antioxidant activities. UHPLC/HR-MS allowed the identification of 805 unique peptides: LL and HH groups shared 59.3% of these peptides, while 20.9% and 19.9% were LL and HH specific, respectively. Frequency analysis of peptides identified a core of 183 peptides typical of 12-month ripened PR cheeses (corresponding to the 22.7% of total peptides), but no significant differences were detected in peptide patterns between LL and HH groups. Forty bioactive peptides, including 18 ACE-inhibitors and 12 anti-microbial peptides, were identified, of which 25 firstly found in PR cheese. Globally, this work contributed to unraveling the potentially healthy benefits of peptides fraction in PR cheese and provided prior evidence that PR with reduced fat/salt content showed the highest antihypertensive and antioxidant activities.

Hydroxypropyl β-cyclodextrin improving multiple stresses tolerance of Lactococcus lactis subsp. lactis

L. lactis is known as industrial starter in the fermentation of dairy and meat products, and it plays an important role in human health as an edible probiotic. During industrial production, L. lactis often experiences different stresses that delay the growth and decrease the survival in some serious conditions. In this study, the protective effects of hydroxypropyl β-cyclodextrin (HP β-CD) on L. lactis under multiple stresses were investigated. The microbial cells were treated with different stresses including heat, NaCl, cold, and H₂ O₂ stresses, and the results were showed by measuring the OD₆₀₀ or spot plating method. The growth and tolerance were improved when HP β-CD was added during different stress conditions, better than that of trehalose. Besides, the scanning electron microscopic and fluorescence spectrum studies showed that HP β-CD could combine with L. lactis to protect the cell structure, suggesting that HP β-CD may act as a protective agent of L. lactis. Therefore, HP β-CD could be considered as a potential protective agent to be applied in food industry, and its protective mechanism on L. lactis still needs further investigation.

Cultivable non-starter lactobacilli from ripened Parmigiano Reggiano cheeses with different salt content and their potential to release anti-hypertensive peptides

The impact of salt and fat intake on human health drives the consumer's attention towards dairy food with reduced salt and fat contents. How changes in salt and fat content modulate dairy LAB population and the associated proteolytic activities have been poorly studied. Here, non-starter LAB populations from 12 Parmigiano Reggiano (PR) cheeses (12-month ripened), clustered in low salt and fat content (LL-PR) and high salt and fat content (HH-PR) groups, were investigated and identified at specie-level with molecular assays. Lactobacillus rhamnosus was dominant in HH-PR samples, whereas Lactobacillus paracasei in LL-PR samples. (GTG)₅ rep-PCR analysis discriminated 11 and 12 biotypes for L. rhamnosus and L. paracasei isolates, respectively. Screening for proteolytic activity identified L. rhamnosus strains more proteolytic than L. paracasei, and, within L. rhamnosus species, HH-PR strains were generally more proteolytic than LL-PR strains. Two L. rhamnosus representatives, namely strain 0503 from LL-PR and strain 2006 from HH-PR, were functionally characterized in cow milk fermentation assay. HH-PR strain 2006 overcame LL-PR strain 0503 in acidification performance, leading to a fermented milk with higher angiotensin I-converting enzyme inhibitory and antioxidant activities. L. rhamnosus 2006 was more prone to release VPP, while L. rhamnosus 0503 released higher amount of IPP. This study provides evidences that salt/fat content affects NSLAB cultivable fraction and the associated proteolytic ability resulting in a complex occurrence of bioactive peptides featuring health-promoting properties.

Metabolic modulations of Pseudomonas graminis in response to H2O2 in cloud water

In cloud water, microorganisms are exposed to very strong stresses especially related to the presence of reactive oxygen species including H₂O₂ and radicals, which are the driving force of cloud chemistry. In order to understand how the bacterium Pseudomonas graminis isolated from cloud water respond to this oxidative stress, it was incubated in microcosms containing a synthetic solution of cloud water in the presence or in the absence of H₂O₂. P. graminis metabolome was examined by LC-MS and NMR after 50 min and after 24 hours of incubation. After 50 min, the cells were metabolizing H₂O₂ while this compound was still present in the medium, and it was completely biodegraded after 24 hours. Cells exposed to H₂O₂ had a distinct metabolome as compared to unexposed cells, revealing modulations of certain metabolic pathways in response to oxidative stress. These data indicated that the regulations observed mainly involved carbohydrate, glutathione, energy, lipid, peptides and amino-acids metabolisms. When cells had detoxified H₂O₂ from the medium, their metabolome was not distinguishable anymore from unexposed cells, highlighting the capacity of resilience of this bacterium. This work illustrates the interactions existing between the cloud microbial metabolome and cloud chemistry.

Proteins as Nano-Carriers for Bioactive Compounds. The Case of 7S and 11S Soy Globulins and Folic Acid Complexation

Isolated 7S and 11S globulins obtained from defeated soy flour were complexated with folic acid (FA) in order to generate nano-carriers for this important vitamin in human nutrition. Fluorescence spectroscopy and dynamic light scattering were applied to follow the nano-complexes formation and for their characterization. Fluorescence experimental data were modeled by the Stern-Volmer and a modified double logarithm approach. The results obtained confirmed static quenching. The number of binding sites on the protein molecule was ~1. The values obtained for the binding constants suggest a high affinity between proteins and FA. Particle size distribution allowed to study the protein aggregation phenomenon induced by FA bound to the native proteins. Z-average manifested a clear trend to protein aggregation. 11S-FA nano-complexes resulted in more polydispersity. ζ-potential of FA nano-complexes did not show a remarkable change after FA complexation. The biological activity of nano-complexes loaded with FA was explored in terms of their capacity to enhance the biomass formation of Lactobacillus casei BL23. The results concerning to nano-complexes inclusion in culture media showed higher bacterial growth. Such a result was attributed to the entry of the acid by the specific receptors concomitantly by the peptide receptors. These findings have technological impact for the use of globulins-FA based nano-complexes in nutraceutical, pharmaceutical and food industries.

Cigarette smoking is associated with an altered vaginal tract metabolomic profile

Cigarette smoking has been associated with both the diagnosis of bacterial vaginosis (BV) and a vaginal microbiota lacking protective Lactobacillus spp. As the mechanism linking smoking with vaginal microbiota and BV is unclear, we sought to compare the vaginal metabolomes of smokers and non-smokers (17 smokers/19 non-smokers). Metabolomic profiles were determined by gas and liquid chromatography mass spectrometry in a cross-sectional study. Analysis of the 16S rRNA gene populations revealed samples clustered into three community state types (CSTs) ---- CST-I (L. crispatus-dominated), CST-III (L. iners-dominated) or CST-IV (low-Lactobacillus). We identified 607 metabolites, including 12 that differed significantly (q-value < 0.05) between smokers and non-smokers. Nicotine, and the breakdown metabolites cotinine and hydroxycotinine were substantially higher in smokers, as expected. Among women categorized to CST-IV, biogenic amines, including agmatine, cadaverine, putrescine, tryptamine and tyramine were substantially higher in smokers, while dipeptides were lower in smokers. These biogenic amines are known to affect the virulence of infective pathogens and contribute to vaginal malodor. Our data suggest that cigarette smoking is associated with differences in important vaginal metabolites, and women who smoke, and particularly women who are also depauperate for Lactobacillus spp., may have increased susceptibilities to urogenital infections and increased malodor.

Metabolomic study of the response to cold shock in a strain of Pseudomonas syringae isolated from cloud water

INTRODUCTION

Active microorganisms have been recently discovered in clouds, thus demonstrating the capacity of microorganisms to exist in harsh environments, including exposure to UV and oxidants, osmotic and cold shocks, etc. It is important to understand how microorganisms respond to and survive such stresses at the metabolic level.

OBJECTIVES

The objective of this work is to assess metabolome modulation in a strain of Pseudomonas syringae isolated from cloud water and facing temperature downshift from 17 to 5 °C by identifying key molecules and pathways of the response/adaptation to cold shock.

METHODS

Bacterial extracts from suspensions of cells grown at 17 °C and further incubated in microcosms at 5 and 17 °C to mimic cloud conditions were analysed by combining LC-MS and NMR; the results were evaluated in comparison to similar suspensions kept at constant temperature. The differences in the metabolome profiles were deciphered using multivariate statistics (PLS-DA).

RESULTS

Key cold shock biomarkers were observed, including cryoprotectants (trehalose, glucose, glycerol, carnitine, glutamate), antioxidants (glutathione and carnitine) and their precursors, alkaloids (bellendine and slaframine) and metabolites involved in energy metabolism (ATP, carbohydrates). Furthermore, new short peptides (nine dipeptides and a tetrapeptide) were found that have no known function.

CONCLUSIONS

This study shows that in response to cold temperatures, Pseudomonas syringae PDD-32b-74 demonstrates numerous metabolism modifications to counteract the impacts of low temperatures.

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.

Characterization of prophages containing "evolved" Dit/Tal modules in the genome of Lactobacillus casei BL23

Lactic acid bacteria (LAB) have many applications in food and industrial fermentations. Prophage induction and generation of new virulent phages is a risk for the dairy industry. We identified three complete prophages (PLE1, PLE2, and PLE3) in the genome of the well-studied probiotic strain Lactobacillus casei BL23. All of them have mosaic architectures with homologous sequences to Streptococcus, Lactococcus, Lactobacillus, and Listeria phages or strains. Using a combination of quantitative real-time PCR, genomics, and proteomics, we showed that PLE2 and PLE3 can be induced-but with different kinetics-in the presence of mitomycin C, although PLE1 remains as a prophage. A structural analysis of the distal tail (Dit) and tail associated lysin (Tal) baseplate proteins of these prophages and other L. casei/paracasei phages and prophages provides evidence that carbohydrate-binding modules (CBM) located within these "evolved" proteins may replace receptor binding proteins (RBPs) present in other well-studied LAB phages. The detailed study of prophage induction in this prototype strain in combination with characterization of the proteins involved in host recognition will facilitate the design of new strategies for avoiding phage propagation in the dairy industry.

Influence of osmotic stress on the profile and gene expression of surface layer proteins in Lactobacillus acidophilus ATCC 4356

In this work, we studied the role of surface layer (S-layer) proteins in the adaptation of Lactobacillus acidophilus ATCC 4356 to the osmotic stress generated by high salt. The amounts of the predominant and the auxiliary S-layer proteins SlpA and SlpX were strongly influenced by the growth phase and high-salt conditions (0.6 M NaCl). Changes in gene expression were also observed as the mRNAs of the slpA and slpX genes increased related to the growth phase and presence of high salt. A growth stage-dependent modification on the S-layer protein profile in response to NaCl was observed: while in control conditions, the auxiliary SlpX protein represented less than 10 % of the total S-layer protein, in high-salt conditions, it increased to almost 40 % in the stationary phase. The increase in S-layer protein synthesis in the stress condition could be a consequence of or a way to counteract the fragility of the cell wall, since a decrease in the cell wall thickness and envelope components (peptidoglycan layer and lipoteichoic acid content) was observed in L. acidophilus when compared to a non-S-layer-producing species such as Lactobacillus casei. Also, the stationary phase and growth in high-salt medium resulted in increased release of S-layer proteins to the supernatant medium. Overall, these findings suggest that pre-growth in high-salt conditions would result in an advantage for the probiotic nature of L. acidophilus ATCC 4356 as the increased amount and release of the S-layer might be appropriate for its antimicrobial capacity.

Stress responses in probiotic Lactobacillus casei

Survival in harsh environments is critical to both the industrial performance of lactic acid bacteria (LAB) and their competitiveness in complex microbial ecologies. Among the LAB, members of the Lactobacillus casei group have industrial applications as acid-producing starter cultures for milk fermentations and as specialty cultures for the intensification and acceleration of flavor development in certain bacterial-ripened cheese varieties. They are amongst the most common organisms in the gastrointestinal (GI) tract of humans and other animals, and have the potential to function as probiotics. Whether used in industrial or probiotic applications, environmental stresses will affect the physiological status and properties of cells, including altering their functionality and biochemistry. Understanding the mechanisms of how LAB cope with different environments is of great biotechnological importance, from both a fundamental and applied perspective: hence, interaction between these strains and their environment has gained increased interest in recent years. This paper presents an overview of the important features of stress responses in Lb. casei, and related proteomic or gene expression patterns that may improve their use as starter cultures and probiotics.

Osmoprotection of Bacillus subtilis through import and proteolysis of proline-containing peptides

Bacillus subtilis can attain cellular protection against the detrimental effects of high osmolarity through osmotically induced de novo synthesis and uptake of the compatible solute l-proline. We have now found that B. subtilis can also exploit exogenously provided proline-containing peptides of various lengths and compositions as osmoprotectants. Osmoprotection by these types of peptides is generally dependent on their import via the peptide transport systems (Dpp, Opp, App, and DtpT) operating in B. subtilis and relies on their hydrolysis to liberate proline. The effectiveness with which proline-containing peptides confer osmoprotection varies considerably, and this can be correlated with the amount of the liberated and subsequently accumulated free proline by the osmotically stressed cell. Through gene disruption experiments, growth studies, and the quantification of the intracellular proline pool, we have identified the PapA (YqhT) and PapB (YkvY) peptidases as responsible for the hydrolysis of various types of Xaa-Pro dipeptides and Xaa-Pro-Xaa tripeptides. The PapA and PapB peptidases possess overlapping substrate specificities. In contrast, osmoprotection by peptides of various lengths and compositions with a proline residue positioned at their N terminus was not affected by defects in the PapA and PapB peptidases. Taken together, our data provide new insight into the physiology of the osmotic stress response of B. subtilis. They illustrate the flexibility of this ubiquitously distributed microorganism to effectively exploit environmental resources in its acclimatization to sustained high-osmolarity surroundings through the accumulation of compatible solutes.

Mining the human gut microbiome for novel stress resistance genes

With the rapid advances in sequencing technologies in recent years, the human genome is now considered incomplete without the complementing microbiome, which outnumbers human genes by a factor of one hundred. The human microbiome, and more specifically the gut microbiome, has received considerable attention and research efforts over the past decade. Many studies have identified and quantified "who is there?," while others have determined some of their functional capacity, or "what are they doing?" In a recent study, we identified novel salt-tolerance loci from the human gut microbiome using combined functional metagenomic and bioinformatics based approaches. Herein, we discuss the identified loci, their role in salt-tolerance and their importance in the context of the gut environment. We also consider the utility and power of functional metagenomics for mining such environments for novel genes and proteins, as well as the implications and possible applications for future research.

Functional metagenomics reveals novel salt tolerance loci from the human gut microbiome

Metagenomics is a powerful tool that allows for the culture-independent analysis of complex microbial communities. One of the most complex and dense microbial ecosystems known is that of the human distal colon, with cell densities reaching up to 10(12) per gram of faeces. With the majority of species as yet uncultured, there are an enormous number of novel genes awaiting discovery. In the current study, we conducted a functional screen of a metagenomic library of the human gut microbiota for potential salt-tolerant clones. Using transposon mutagenesis, three genes were identified from a single clone exhibiting high levels of identity to a species from the genus Collinsella (closest relative being Collinsella aerofaciens) (COLAER_01955, COLAER_01957 and COLAER_01981), a high G+C, Gram-positive member of the Actinobacteria commonly found in the human gut. The encoded proteins exhibit a strong similarity to GalE, MurB and MazG. Furthermore, pyrosequencing and bioinformatic analysis of two additional fosmid clones revealed the presence of an additional galE and mazG gene, with the highest level of genetic identity to Akkermansia muciniphila and Eggerthella sp. YY7918, respectively. Cloning and heterologous expression of the genes in the osmosensitive strain, Escherichia coli MKH13, resulted in increased salt tolerance of the transformed cells. It is hoped that the identification of atypical salt tolerance genes will help to further elucidate novel salt tolerance mechanisms, and will assist our increased understanding how resident bacteria cope with the osmolarity of the gastrointestinal tract.

Proteomic analyses to reveal the protective role of glutathione in resistance of Lactococcus lactis to osmotic stress

Previously, we have shown that glutathione can protect Lactococcus lactis against oxidative stress and acid stress. In this study, we show that glutathione taken up by L. lactis SK11 can protect this organism against osmotic stress. When exposed to 5 M NaCl, L. lactis SK11 cells containing glutathione exhibited significantly improved survival compared to the control cells. Transmission electron microscopy showed that the integrity of L. lactis SK11 cells containing glutathione was maintained for at least 24 h, whereas autolysis of the control cells occurred within 2 h after exposure to this osmotic stress. Comparative proteomic analyses using SK11 cells containing or not containing glutathione that were exposed or not exposed to osmotic stress were performed. The results revealed that 21 of 29 differentially expressed proteins are involved in metabolic pathways, mainly sugar metabolism. Several glycolytic enzymes of L. lactis were significantly upregulated in the presence of glutathione, which might be the key for improving the general stress resistance of a strain. Together with the results of previous studies, the results of this study demonstrated that glutathione plays important roles in protecting L. lactis against multiple environmental stresses; thus, glutathione can be considered a general protectant for improving the robustness and stability of dairy starter cultures.

Metabolomic characterization of the salt stress response in Streptomyces coelicolor

The humicolous actinomycete Streptomyces coelicolor routinely adapts to a wide variety of habitats and rapidly changing environments. Upon salt stress, the organism is also known to increase the levels of various compatible solutes. Here we report the results of the first high-resolution metabolomics time series analysis of various strains of S. coelicolor exposed to salt stress: the wild type, mutants with progressive knockouts of the ectoine biosynthesis pathway, and two stress regulator mutants (with disruptions of the sigB and osaB genes). Samples were taken from cultures at 0, 4, 8, and 24 h after salt stress treatment and analyzed by liquid chromatography-mass spectrometry with an LTQ Orbitrap XL mass spectrometer. The results suggest that a large fraction of amino acids is upregulated in response to the salt stress, as are proline/glycine-containing di- and tripeptides. Additionally we found that 5'-methylthioadenosine, a known inhibitor of polyamine biosynthesis, is downregulated upon salt stress. Strikingly, no major differences between the wild-type cultures and the two stress regulator mutants were found, indicating a considerable robustness of the metabolomic response to salt stress, compared to the more volatile changes in transcript abundance reported earlier.

Metabolic impact and potential exploitation of the stress reactions in lactobacilli

Lactic acid bacteria (LAB) are a functionally related group of organisms known primarily for their bioprocessing roles in food and beverages. The largest variety of metabolic properties is found in the group of lactobacilli the vast majority of which has been isolated in cereal environments, namely sourdoughs, in which their role ranges from sporadic contaminants to major fermentative flora. Growth or survival in each of these environmental niches depends on the ability of the organism to sense and respond to varying conditions such as temperature, pH, nutrients availability and cell population density. Fermentation process conditions, including temperature range, dough yield, oxygen, pH as well as the amount and composition of starter cultures, determine the cells' metabolic response. In fact, the exposure of microbial cells to stressful conditions during fermentation involves a broad transcriptional response with many induced or repressed genes. The complex network of such responses, involving several metabolic activities will reflect upon the metabolome of the fermentative flora, and thus on the composition and organoleptic properties of the final products. This review shall provide insight into stress response mechanisms and delineate the vast potential residing in the exploitation of the stress dependent metabolome of LAB focusing on bacteria of the sourdough environment as one of the richest sources of lactobacilli.

High salt stress in Bacillus subtilis: involvement of PBP4* as a peptidoglycan hydrolase

The study was focused on the role of the penicillin binding protein PBP4* of Bacillus subtilis during growth in high salinity rich media. Using pbpE-lacZ fusion, we found that transcription of the pbpE gene is induced in stationary phase and by increased salinity. This increase was also corroborated at the translation level for PBP4* by western blot. Furthermore, we showed that a strain harboring gene disruption in the structural gene (pbpE) for the PBP4* endopeptidase resulted in a salt-sensitive phenotype and increased sensitivity to cell envelope active antibiotics (vancomycin, penicillin and bacitracin). Since the pbpE gene seems to be part of a two-gene operon with racX, a racX::pRV300 mutant was obtained. This mutant behaved like the wild-type strain with respect to high salt. Electron microscopy showed that high salt and mutation of pbpE resulted in cell wall defects. Whole cells or purified peptidoglycan from WT cultures grown in high salt medium showed increased autolysis and susceptibility to mutanolysin. We demonstrate through zymogram analysis that PBP4* has murein hydrolyze activity. All these results support the hypothesis that peptidoglycan is modified in response to high salt and that PBP4* contributes to this modification.

Tolerance to high osmolality of the lactic acid bacterium Oenococcus oeni and identification of potential osmoprotectants

Growth of the lactic acid bacterium Oenococcus oeni under hyperosmotic constraint was investigated in a chemically defined medium. The bacterium could grow on media with an elevated osmolality, preferably below 1.5 Osm kg(-)(1) H(2)O. At osmolalities comprised between 0.6 and 1.5 Osm kg(-)(1) H(2)O, the growth deficit elicited by the sugars glucose and fructose was slightly more severe than with salts (NaCl or KCl). In contrast to what was observed in other lactic acid bacteria, proline, glycine betaine and related molecules were unable to relieve inhibition of growth of O. oeni under osmotic constraint. This was correlated to the absence of sequences homologous to the genes coding for glycine betaine and/or proline transporters described in Lactococcus lactis and Lactobacillus plantarum. The amino acid aspartate proved to be osmoprotective under electrolyte and non-electrolyte stress. Examination of the role of peptides during osmoregulation showed that proline- and glutamate-containing peptides were protective under salt-induced stress, and not under sugar-induced stress. Under high salt, PepQ a cytoplasmic prolidase that specifically liberated proline from di-peptides increased activity, while PepX (X-prolyl-dipeptidyl aminopeptidase) and PepI (iminopeptidase) activities were unaffected. Our data suggest that proline- and glutamate-containing peptides may contribute to the adaptation of O. oeni to high salt through their intracellular hydrolysis and/or direct accumulation.

Effect of sugars and growth media on the dehydration of Lactobacillus delbrueckii ssp. bulgaricus

AIMS

The efficiency of trehalose, sucrose and maltose to protect Lactobacillus bulgaricus during drying has been evaluated in bacteria grown at low water activity.

METHODS AND RESULTS

Bacteria were grown in MRS (control), and in MRS supplemented with sucrose (MRS-sucrose) or with polyethyleneglycol (PEG) (MRS-PEG) as low water activity media. The growth in low water activity media (MRS-sucrose and MRS-PEG) prior to drying enhanced the effectiveness of trehalose as thermoprotectant during drying. The efficiency of sucrose was improved when bacteria were grown in MRS-sucrose. On the other hand, the growth in both low water activity media did not affect the efficiency of maltose. The damage produced during dehydration has been evaluated by means of growth kinetics in milk. The preservation of bacteria dehydrated with sucrose, after growing them in MRS-sucrose, appears to be as efficient as the dehydration with trehalose.

CONCLUSIONS

The growth of L. bulgaricus in low water activity media enhances the protective action of trehalose and sucrose. SIGNIFICANCE AND IMPACT OF THE THE STUDY: These results may aid the dairy industry to improve the recovery of the starters at low costs after preservation processes.

Role of anionic phospholipids in the adaptation of Bacillus subtilis to high salinity

The importance of the content of anionic phospholipids [cardiolipin (CL) and phosphatidylglycerol (PG)] in the osmotic adaptation and in the membrane structure of Bacillus subtilis cultures was investigated. Insertion mutations in the three putative cardiolipin synthase genes (ywiE, ywnE and ywjE) were obtained. Only the ywnE mutation resulted in a complete deficiency in cardiolipin and thus corresponds to a true clsA gene. The osmotolerance of a clsA mutant was impaired: although at NaCl concentrations lower than 1.2 M the growth curves were similar to those of its wild-type control, at 1.5 M NaCl (LBN medium) the lag period increased and the maximal optical density reached was lower. The membrane of the clsA mutant strain showed an increased PG content, at both exponential and stationary phase, but no trace of CL in either LB or LBN medium. As well as the deficiency in CL synthesis, the clsA mutant showed other differences in lipid and fatty acids content compared to the wild-type, suggesting a cross-regulation in membrane lipid pathways, crucial for the maintenance of membrane functionality and integrity. The biophysical characteristics of membranes and large unilamellar vesicles from the wild-type and clsA mutant strains were studied by Laurdan's steady-state fluorescence spectroscopy. At physiological temperature, the clsA mutant showed a decreased lateral lipid packing in the protein-free vesicles and isolated membranes compared with the wild-type strain. Interestingly, the lateral lipid packing of the membranes of both the wild-type and clsA mutant strains increased when they were grown in LBN. In a conditional IPTG-controlled pgsA mutant, unable to synthesize PG and CL in the absence of IPTG, the osmoresistance of the cultures correlated with their content of anionic phospholipids. The transcriptional activity of the clsA and pgsA genes was similar and increased twofold upon entry to stationary phase or under osmotic upshift. Overall, these results support the involvement of the anionic phospholipids in the growth of B. subtilis in media containing elevated NaCl concentrations.

Cell wall modifications during osmotic stress in Lactobacillus casei

AIMS

To study the modification of the cell wall of Lactobacillus casei ATCC 393 grown in high salt conditions.

METHODS AND RESULTS

Differences in the overall structure of cell wall between growth in high salt (MRS + 1 mol l(-1) NaCl; N condition) and control (MRS; C condition) conditions were determined by transmission electronic microscopy and analytical procedures. Lactobacillus casei cells grown in N condition were significantly larger than cells grown under unstressed C condition. Increased sensitivity to mutanolysin and antibiotics with target in the cell wall was observed in N condition. Purified cell wall also showed the increased sensitivity to lysis by mutanolysin. Analysis of peptidoglycan (PG) from stressed cells showed that modification was at the structural level in accordance with a decreased PG cross-link involving penicillin-binding proteins (PBP). Nine PBP were first described in this species and these proteins were expressed in low percentages or presented a modified pattern of saturation with penicillin G (Pen G) during growth in high salt. Three of the essential PBP were fully saturated in N condition at lower Pen G concentrations than in C condition, suggesting differences in functionality in vivo.

CONCLUSIONS

The results show that growth in high salt modified the structural properties of the cell wall.

SIGNIFICANCE AND IMPACT OF STUDY

Advances in understanding the adaptation to high osmolarity, in particular those involving sensitivity to lysis of lactic acid bacteria.