Glycine betaine, carnitine, and choline enhance salinity tolerance and prevent the accumulation of sodium to a level inhibiting growth of Tetragenococcus halophila

The protective role of potassium in the adaptation of Pseudomonas protegens SN15-2 to hyperosmotic stress

Pseudomonas protegens is an important biocontrol agent with the ability to suppress plant pathogens and promote plant growth. P. protegens' ability to endure hyperosmotic stress is crucial to its effectiveness as a biocontrol agent. This study elucidated potassium's role and mechanism of action in enabling the hyperosmotic tolerance of P. protegens. Potassium was observed to significantly improve the growth of P. protegens under hyperosmotic conditions. Four functionally redundant potassium transporters, KdpA1, KdpA2, TrkH, and Kup, were identified in P. protegens, of which KdpA2 and TrkH were particularly important for its growth under hyperosmotic conditions. Potassium enhanced the biofilm formation and cell membrane stability of P. protegens under hyperosmotic conditions. In addition, we revealed that K+ stimulates the expression of several genes related to DNA damage repair in P. protegens under hyperosmotic conditions. Further experiments revealed that the DNA repair-related recG induced by potassium contributes to P. protegens' hyperosmotic tolerance. We also found that the sigma factor RpoN participates in the hyperosmotic adaptation of P. protegens. Furthermore, we revealed that the opuCABCD operon, whose expression is induced by potassium through RpoN, serves as the key pathway through which betaine, choline, and carnitine improve the hyperosmotic tolerance of P. protegens.

Halophilic lactic acid bacteria - Play a vital role in the fermented food industry

Halophilic lactic acid bacteria have been widely found in various high-salt fermented foods. The distribution of these species in salt-fermented foods contributes significantly to the development of the product's flavor. Besides, these bacteria also have the ability to biosynthesize bioactive components which potentially apply to different areas. In this review, insights into the metabolic properties, salt stress responses, and potential applications of these bacteria have been have been elucidated. The purpose of this review highlights the important role of halophilic lactic acid bacteria in improving the quality and safety of salt-fermented products and explores the potential application of these bacteria.

Simulating compatible solute biosynthesis using a metabolic flux model of the biomining acidophile, Acidithiobacillus ferrooxidans ATCC 23270

Halotolerant, acidophilic, bioleaching microorganisms are crucial to biomining operations that utilize saline water. Compatible solutes play an important role in the adaptation of these microorganisms to saline environments. Acidithiobacillus ferrooxidans ATCC 23270, an iron- and sulfur-oxidizing acidophilic bacterium, synthesizes trehalose as its native compatible solute but is still sensitive to salinity. Recently, halotolerant bioleaching bacteria were found to use ectoine as their key compatible solute. Previously, bioleaching bacteria were recalcitrant to genetic manipulation; however, recent advancements in genetic tools and techniques allow successful genetic modification of A. ferrooxidans ATCC 23270. Therefore, this study aimed to test, in silico, the effect of native and synthetic compatible solute biosynthesis by A. ferrooxidans ATCC 23270 on its growth and metabolism. Metabolic network flux modelling was used to provide a computational framework for the prediction of metabolic fluxes during production of native and synthetic compatible solutes by A. ferrooxidans ATCC 23270, in silico. Complete pathways for trehalose biosynthesis by the bacterium are proposed and captured in the updated metabolic model including a newly discovered UDP-dependent trehalose synthesis pathway. Finally, the effect of nitrogen sources on compatible solute production was simulated and showed that using nitrogen gas as the sole nitrogen source enables the ectoine-producing 'engineered' microbe to oxidize up to 20% more ferrous iron in comparison to the native microbe that only produces trehalose. Therefore, the predictive outcomes of the model have the potential to guide the design and optimization of a halotolerant strain of A. ferrooxidans ATCC 23270 for saline bioleaching operations.

Biodegradation of Choline NTF2 by Pantoea agglomerans in Different Osmolarity. Characterization and Environmental Implications of the Produced Exopolysaccharide

A specific microorganism, Pantoea agglomerans uam8, was isolated from the ionic liquid (IL) Choline NTF2 and identified by molecular biology. A biodegradation study was performed at osmolarity conditions (0.2, 0.6, 1.0 M). These had an important influence on the growth of the strain, exopolysaccharide (EPS) production, and biodegradation (1303 mg/L max production and 80% biodegradation at 0.6 M). These conditions also had an important influence on the morphology of the strain and its EPSs, but not in the chemical composition. The EPS (glucose, mannose and galactose (6:0.5:2)) produced at 0.6 M was further characterized using different techniques. The obtained EPSs presented important differences in the behavior of the emulsifying activity for vegetable oils (olive (86%), sunflower (56%) and coconut (90%)) and hydrocarbons (diesel (62%), hexane (60%)), and were compared with commercial emulsifiers. The EPS produced at 0.6 M had the highest emulsifying activity overall. This EPS did not show cytotoxicity against the tested cell line (<20%) and presented great advantages as an antioxidant (1,1-diphenyl-2-picryl-hydrazyl radical (DPPH) (85%), hydroxyl radical (OH) (99%), superoxide anion (O2-) (94%), chelator (54%), and antimicrobial product (15 mm). The osmolarity conditions directly affected the capacity of the strain to biodegrade IL and the subsequently produced EPS. Furthermore, the EPS produced at 0.6 M has potential for environmental applications, such as the removal of hazardous materials by emulsification, whilst resulting in positive health effects such as antioxidant activity and non-toxicity.

Examining the impact of Tetragenococcus halophilus, Zygosaccharomyces rouxii, and Starmerella etchellsii on the quality of soy sauce: a comprehensive review of microbial population dynamics in fermentation

Soy sauce is a popular fermented seasoning due to its distinct flavor and rich umami taste. Its traditional production involves two stages: solid-state fermentation and moromi (brine fermentation). During moromi, the dominant microbial population in the soy sauce mash changes, which is called microbial succession and is essential for the formation of soy sauce flavor compounds. Research has identified the sequence of succession, starting with Tetragenococcus halophilus, then Zygosaccharomyces rouxii, and lastly, Starmerella etchellsii. Factors such as the environment, microbial diversity, and interspecies relationships drive this process. Salt and ethanol tolerance influence microbial survival, while nutrients in the soy sauce mash support the cells in resisting external stress. Different microbial strains have varying abilities to survive and respond to external factors during fermentation, which impacts soy sauce quality. In this review, we would examine the factors behind the succession of common microbial populations in the soy sauce mash and explore how microbial succession affects soy sauce quality. The insights gained can help better manage the dynamic changes in microbes during fermentation, leading to improved production efficiency.

Characterization of GshAB of Tetragenococcus halophilus: a two-domain glutathione synthetase

The γ-glutamyl tripeptide glutathione (γ-Glu-Cys-Gly) is a low molecular thiol that acts as antioxidant in response to oxidative stress in eukaryotes and prokaryotes. γ-Glutamyl dipeptides including γ-Glu-Cys, γ-Glu-Glu, and γ-Glu-Gly also have kokumi activity. Glutathione is synthesized by first ligating Glu with Cys by γ-glutamylcysteine ligase (Gcl/GshA), and then the resulting dipeptide γ-glutamylcysteine is ligated with Gly by glutathione synthetase (Gs/GshB). GshAB/GshF enzymes that contain both Gcl and Gs domains are capable of catalyzing both reactions. The current study aimed to characterize GshAB from Tetragenococcus halophilus after heterologous expression in Escherichia coli. The optimal conditions for GshAB from T. halophilus were pH 8.0 and 25 °C. The substrate specificity of the Gcl reaction of GshAB was also determined. GshAB has a high affinity to Cys. γ-Glu-Cys was the only dipeptide generated when Glu, Cys, Gly, and other amino acids were present in the reaction system. This specificity differentiates GshAB from T. halophilus from Gcl of heterofermentative lactobacilli and GshAB of Streptococcus agalactiae, which also use amino acids other than Cys as glutamyl-acceptor. Quantification of gshAB in cDNA libraries from T. halophilus revealed that gshAB was overexpressed in response to oxidative stress but not in response to acid, osmotic, or cold stress. In conclusion, GshAB in T. halophilus served as part of the oxidative stress response but this study did not provide any evidence for a contribution to the resistance to other stressors.Key points Glutathione synthesis in Tetragenococcus halophilus is carried out by the two-domain enzyme GshAB. GshAB is inhibited by glutathione and is highly specific for Cys as acceptor. T. halophilus synthesizes glutathione in response to oxidative stress.

Orchestrated Response of Intracellular Zwitterionic Metabolites in Stress Adaptation of the Halophilic Heterotrophic Bacterium Pelagibaca bermudensis

Osmolytes are naturally occurring organic compounds that protect cells against various forms of stress. Highly polar, zwitterionic osmolytes are often used by marine algae and bacteria to counteract salinity or temperature stress. We investigated the effect of several stress conditions including different salinities, temperatures, and exposure to organic metabolites released by the alga Tetraselmis striata on the halophilic heterotrophic bacterium Pelagibaca bermudensis. Using ultra-high-performance liquid chromatography (UHPLC) on a ZIC-HILIC column and high-resolution electrospray ionization mass spectrometry, we simultaneously detected and quantified the eleven highly polar compounds dimethylsulfoxonium propionate (DMSOP), dimethylsulfoniopropionate (DMSP), gonyol, cysteinolic acid, ectoine, glycine betaine (GBT), carnitine, sarcosine, choline, proline, and 4-hydroxyproline. All compounds are newly described in P. bermudensis and potentially involved in physiological functions essential for bacterial survival under variable environmental conditions. We report that adaptation to various forms of stress is accomplished by adjusting the pattern and amount of the zwitterionic metabolites.

Shift of Choline/Betaine Pathway in Recombinant Pseudomonas for Cobalamin Biosynthesis and Abiotic Stress Protection

The B12-producing strains Pseudomonas nitroreducens DSM 1650 and Pseudomonas sp. CCUG 2519 (both formerly Pseudomonas denitrificans), with the most distributed pathway among bacteria for exogenous choline/betaine utilization, are promising recombinant hosts for the endogenous production of B12 precursor betaine by direct methylation of bioavailable glycine or non-proteinogenic β-alanine. Two plasmid-based de novo betaine pathways, distinguished by their enzymes, have provided an expression of the genes encoding for N-methyltransferases of the halotolerant cyanobacterium Aphanothece halophytica or plant Limonium latifolium to synthesize the internal glycine betaine or β-alanine betaine, respectively. These betaines equally allowed the recombinant pseudomonads to grow effectively and to synthesize a high level of cobalamin, as well as to increase their protective properties against abiotic stresses to a degree comparable with the supplementation of an exogenous betaine. Both de novo betaine pathways significantly enforced the protection of bacterial cells against lowering temperature to 15 °C and increasing salinity to 400 mM of NaCl. However, the expression of the single plant-derived gene for the β-alanine-specific N-methyltransferase additionally increased the effectiveness of exogenous glycine betaine almost twofold on cobalamin biosynthesis, probably due to the Pseudomonas' ability to use two independent pathways, their own choline/betaine pathway and the plant β-alanine betaine biosynthetic pathway.

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.

Utilization and accumulation of compatible solutes in Halomonas pacifica: a species of moderately halophilic bacteria isolated from a saline lake in South Libya

When grown in high salt concentrations, halophilic bacteria often accumulate compatible solutes, which have major applications in biotechnology because they stabilize cells and proteins. Four Gram-negative bacterial strains, belonging to the family Halomonadaceae, were isolated from Qaberoun and Um-Alma lakes in South Libya using high-salinity medium. The strains were identified using 16S rRNA gene sequencing as belonging to Halomonas pacifica (strain ABQ1), Halomonas venusta (ABQ2), Halomonas elongata (ABU1) and Halomonas salifodinae (ABU2). H. pacifica ABQ1 is a moderate halophile (salinity range 0.05 to 2.5 M NaCl), with a broad tolerance to pH (7 to 9) and temperature (25–37 °C). Addition of the compatible solutes glycine betaine (betaine) and ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidine carboxylic acid) to the medium had a positive effect on growth of H. pacifica at 2 M NaCl. In rich LB medium, betaine was the major compatible solute accumulated, with ectoine only being accumulated at salinities in excess of 1 M NaCl. In minimal M9 medium, betaine was not produced, but increasing amounts of ectoine were synthesized with increasing salinity, and hydroxyectoine [(4S,5S)−5-hydroxy-2-methyl-1,4,5,6-tetrahydropyrimidine-4-carboxylic acid] was also synthesized when the cells were grown in very high salt. We have thus identified H. pacifica as a producer of ectoine and hydroxyectoine, with more being produced at higher salinities. As industrial demand for these compatible solutes continues to increase, this system has biotechnological potential.

Transcriptome analysis of L-leucine-producing Corynebacterium glutamicum under the addition of trimethylglycine

It has been widely reported that the addition of trimethylglycine (betaine) decreases osmotic pressure inhibition for cell growth, leading to increased production of amino acids. However, the underlying mechanism is unclear. To determine the global metabolic differences that occur under the addition of trimethylglycine, transcriptome analysis was performed. Transcriptome analysis of Corynebacterium glutamicum JL1211 revealed that 272 genes exhibited significant changes under trimethylglycine addition. We performed Gene Ontology (GO) and KEGG enrichment pathway analyses on these differentially expressed genes (DEGs). Significantly upregulated genes were mainly involved in the regulation of ABC transporters, especially phosphate transporters and sulfur metabolism. The three phosphate transporter genes pstC, pstA and pstB were upregulated by 13.06-fold, 29.80-fold and 30.49-fold, respectively. Notably, the transcriptional levels of the cysD, cysN, cysH and sir genes were upregulated by 81.5-fold, 57.3-fold, 77.6-fold and 125.4-fold, respectively, consistent with assimilatory sulfate reduction under the addition of trimethylglycine. The upregulation of ilvBN and leuD genes might result in increased L-leucine formation. The data indicated changes in the transcriptome of C. glutamicum with trimethylglycine treatment, thus providing a mechanism supporting the application of trimethylglycine in the production of L-leucine and other amino acids by C. glutamicum strains.

The diversity among the species Tetragenococcus halophilus including new isolates from a lupine seed fermentation

BACKGROUND

Tetragenococcus (T.) halophilus can be isolated from a variety of fermented foods, such as soy sauce, different soy pastes, salted fish sauce and from cheese brine or degraded sugar beet thick juice. This species contributes by the formation of short chain acids to the flavor of the product. Recently, T. halophilus has been identified as a dominant species in a seasoning sauce fermentation based on koji made with lupine seeds.

RESULTS

In this study we characterized six strains of T. halophilus isolated from lupine moromi fermentations in terms of their adaptation towards this fermentation environment, salt tolerance and production of biogenic amines. Phylogenic and genomic analysis revealed three distinctive lineages within the species T. halophilus with no relation to their isolation source, besides the lineage of T. halophilus subsp. flandriensis. All isolated strains from lupine moromi belong to one lineage in that any of the type strains are absent. The strains form lupine moromi could not convincingly be assigned to one of the current subspecies. Taken together with strain specific differences in the carbohydrate metabolism (arabinose, mannitol, melibiose, gluconate, galactonate) and amino acid degradation pathways such as arginine deiminase pathway (ADI) and the agmatine deiminase pathway (AgDI) the biodiversity in the species of T. halophilus is greater than expected. Among the new strains, some strains have a favorable combination of traits wanted in a starter culture.

CONCLUSIONS

Our study characterized T. halophilus strains that were isolated from lupine fermentation. The lupine moromi environment appears to select strains with specific traits as all of the strains are phylogenetically closely related, which potentially can be used as a starter culture for lupine moromi. We also found that the strains can be clearly distinguished phylogenetically and phenotypically from the type strains of both subspecies T. halophilus subsp. halophilus and T. halophilus subsp. flandriensis.

Integrated production of optically pure l-lactic acid from paper mill sludge by simultaneous saccharification and co-fermentation (SSCF)

Paper mill sludge (PMS) raises critical environmental issues due to its disposal problem, but its high sugar content and well-dispersed structure make it a great feedstock for biochemical production. The technical feasibility of integrating cellulase enzyme production into lactic acid (LA) fermentation from PMS was investigated in this study. The low ash content of PMS suggests a great potential for cellulase production. The enzyme produced using PMS without any treatment gave an activity of 7.8 FPU/ml, a performance comparable to the commercial enzyme, Cellic CTec 2. The LA yield from PMS with in-house enzyme was 64.7% and 73.7% at the enzyme loading of 10 and 15 FPU/g-glucan, respectively. The LA obtained was optically pure L- isomer with over 99% purity. The optimal condition of LA production by Bacillus coagulans was found to be 50 °C and pH 5.3 (with 50 g/L CaCO₃). The nutrient effect of yeast extract (YE) and corn steep liquor (CSL) was substrate dependent, and CSL could substitute YE as an inexpensive nutrient when using PMS as a substrate.

Characterization of DinJ-YafQ toxin-antitoxin module in Tetragenococcus halophilus: activity, interplay, and evolution

Tetragenococcus halophilus is a moderately halophilic lactic acid bacterium widely used in high-salt food fermentation because of its coping ability under various stress conditions. Bacterial toxin-antitoxin (TA) modules are widely distributed and play important roles in stress response, but those specific for genus Tetragenococcus have never been explored. Here, a bona fide TA module named DinJ₁-YafQ₁^tha was characterized in T. halophilus. The toxin protein YafQ₁^tha acts as a ribonuclease, and its overexpression severely inhibits Escherichia coli growth. These toxic effects can be eliminated by introducing DinJ₁^tha, indicating that YafQ₁^tha activity is blocked by the formed DinJ₁-YafQ₁^tha complex. In vivo and in vitro assays showed that DinJ₁^tha alone or DinJ₁-YafQ₁^tha complex can repress the transcription of dinJ₁-yafQ₁^tha operon by binding directly to the promoter sequence. In addition, dinJ₁-yafQ₁^tha is involved in plasmid maintenance and stress response, and its transcriptional level is regulated by various stresses. These findings reveal the possible roles of DinJ₁-YafQ₁^tha system in the stress adaptation processes of T. halophilus during fermentation. A single antitoxin DinJ₂^tha without a cognate toxin protein was also found. Its sequence shows low similarity to that of DinJ₁^tha, indicating that this antitoxin may have evolved from a different ancestor. Moreover, DinJ₂^tha can cross-interact with noncognate toxin YafQ₁^tha and cross-regulate with dinJ₁-yafQ₁^tha operon. In summary, DinJ-YafQ^tha characterization may be helpful in investigating the key roles of TA systems in T. halophilus and serves as a foundation for further research. KEY POINTS: • dinJ₁-yafQ₁^tha is the first functional TA module characterized in T. halophilus and upregulated significantly upon osmotic and acidic stress. • DinJ₂^tha can exhibit physical and transcriptional interplay with DinJ₁-YafQ₁^tha. • dinJ₂^tha may be acquired from bacteria in distant affiliation and inserted into the T. halophilus genome through horizontal gene transfer.

Effects of CcpA against salt stress in Lactiplantibacillus plantarum as assessed by comparative transcriptional analysis

Lactiplantibacillus plantarum is frequently exposed to salt stress during industrial applications. Catabolite control protein (CcpA) controls the transcription of many genes, but its role in the response to salt stress remains unclear. In this study, we used transcriptome analyses to investigate differences in the logarithmic growth phases of Lactiplantibacillus plantarum ST-III and its ccpA-knockout mutant when grown with or without salt and glycine betaine (GB). The deletion of ccpA significantly affected bacterial growth under different conditions. Among the comparisons, the highest proportion of differentially expressed genes (64%) was observed in the comparison between the wild-type and ccpA mutant grown with NaCl, whereas the lowest proportion (6%) was observed in the comparison between the ccpA mutant strain cultures grown with NaCl alone or with GB together. Transcriptomic analyses showed that CcpA could regulate GB uptake, activate iron uptake, produce acetyl-CoA, and affect fatty acid composition to maintain membrane lipid homeostasis in the adaptation of high-salinity conditions. Conclusively, these results demonstrate the importance of CcpA as a master regulator of these processes in response to salt stress, and provide new insights into the complex regulatory network of lactic acid bacteria. KEY POINTS: • The absence of CcpA significantly affected growth of L. plantarum and its response to salt stress. • CcpA regulates compatible solutes absorption and ions transport to resist salt stress. • CcpA alters fatty acids composition to maintain membrane lipid homeostasis towards salt stress.

NaCl Concentration-Dependent Aminoglycoside Resistance of Halomonas socia CKY01 and Identification of Related Genes

Among various species of marine bacteria, those belonging to the genus Halomonas have several promising applications and have been studied well. However, not much information has been available on their antibiotic resistance. In our efforts to learn about the antibiotic resistance of strain Halomonas socia CKY01, which showed production of various hydrolases and growth promotion by osmolytes in previous study, we found that it exhibited resistance to multiple antibiotics including kanamycin, ampicillin, oxacillin, carbenicillin, gentamicin, apramycin, tetracycline, and spectinomycin. However, the H. socia CKY01 resistance pattern to kanamycin, gentamicin, apramycin, tetracycline, and spectinomycin differed in the presence of 10% NaCl and 1% NaCl in the culture medium. To determine the mechanism underlying this NaCl concentration-dependent antibiotic resistance, we compared four aminoglycoside resistance genes under different salt conditions while also performing time-dependent reverse transcription PCR. We found that the aph2 gene encoding aminoglycoside phosphotransferase showed increased expression under the 10% rather than 1% NaCl conditions. When these genes were overexpressed in an Escherichia coli strain, pETDuet-1::aph2 showed a smaller inhibition zone in the presence of kanamycin, gentamicin, and apramycin than the respective control, suggesting aph2 was involved in aminoglycoside resistance. Our results demonstrated a more direct link between NaCl and aminoglycoside resistance exhibited by the H. socia CKY01 strain.

Characterization of the two nonidentical ArgR regulators of Tetragenococcus halophilus and their regulatory effects on arginine metabolism

The halophilic lactic acid bacterium Tetragenococcus halophilus has been widely used in high-salinity fermentation processes of food. Previous studies have indicated that the catabolism of arginine may contribute to the osmotic stress adaptation of T. halophilus. Unusually, in the chromosome of T. halophilus, preceding the arginine deiminase (ADI) operon, locate two co-transcribed genes, both encoding an ArgR regulator; similar structure was rarely found and the roles of the regulators have not been demonstrated. In the current study, regulatory roles of these two nonidentical ArgR regulators on the arginine metabolism of T. halophilus were investigated. The results show that these two regulators play different roles in arginine metabolism, ArgR1 acts as a negative regulator of the ADI pathway by binding to the promoter sequences and repressing the transcription of genes, and the addition of arginine or hyper-osmotic stress conditions can abolish the ArgR1 repression, whereas ArgR2 negatively regulates the genes involved in arginine biosynthesis. Our study found that despite the commonly known roles of the ArgR regulators as the activator of arginine catabolism and the repressor of arginine biosynthesis, which are found in most studied bacteria possessed one ArgR regulator, the two nonidentical ArgR regulators of T. halophilus both act as repressors, and the repression by which is regulated when sensing changes of environments. By revealing the regulation of arginine metabolism, the current study provides molecular insights and potential tools for future applications of halophiles in biotechnology. KEY POINTS: • The expression of the ADI pathway of T. halophilus is regulated by carbon sources and osmotic stress. • The arginine metabolism process of T. halophilus is fine-tuned by the two ArgR regulators. • The ADI pathway may contribute to the osmotic stress adaptation by generating more energy and accumulating citrulline which acts as compatible solute.

The local transcriptional regulators SacR1 and SacR2 act as repressors of fructooligosaccharides metabolism in Lactobacillus plantarum

Background

In Lactobacillus plantarum, fructooligosaccharides (FOS) metabolism is controlled by both global and local regulatory mechanisms. Although catabolite control protein A has been identified as a global regulator of FOS metabolism, the functions of local regulators remain unclear. This study aimed to elucidate the roles of two local regulators, SacR1 and SacR2, in the regulation of FOS metabolism in L. plantarum both in vitro and in vivo.

Results

The inactivation of sacR1 and sacR2 affected the growth and production of metabolites for strains grown on FOS or glucose, respectively. A reverse transcription-quantitative PCR analysis of one wild-type and two mutant strains (ΔsacR1 and ΔsacR2) of L. plantarum identified SacR1 and SacR2 as repressors of genes relevant to FOS metabolism in the absence of FOS, and these genes could be induced or derepressed by the addition of FOS. The analysis predicted four potential transcription factor binding sites (TFBSs) in the putative promoter regions of two FOS-related clusters. The binding of SacR1 and SacR2 to these TFBSs both in vitro and in vivo was verified using electrophoretic mobility shift assays and chromatin immunoprecipitation, respectively. A consensus sequence of WNNNNNAACGNNTTNNNNNW was deduced for the TFBSs of SacR1 and SacR2.

Conclusion

Our results identified SacR1 and SacR2 as local repressors for FOS metabolism in L. plantarum. The regulation is achieved by the binding of SacR1 and SacR2 to TFBSs in the promoter regions of FOS-related clusters. The results provide new insights into the complex network regulating oligosaccharide metabolism by lactic acid bacteria.

Intracellular osmoprotectant concentrations determine Propionibacterium freudenreichii survival during drying

Propionibacterium freudenreichii is a beneficial bacterium widely used in food as a probiotic and as a cheese-ripening starter. In these different applications, it is produced, dried, and stored before being used. Both freeze-drying and spray-drying were considered for this purpose. Freeze-drying is a discontinuous process that is energy-consuming but that allows high cell survival. Spray-drying is a continuous process that is more energy-efficient but that can lead to massive bacterial death related to heat, osmotic, and oxidative stresses. We have shown that P. freudenreichii cultivated in hyperconcentrated rich media can be spray-dried with limited bacterial death. However, the general stress tolerance conferred by this hyperosmotic constraint remained a black box. In this study, we modulated P. freudenreichii growth conditions and monitored both osmoprotectant accumulation and stress tolerance acquisition. Changing the ratio between the carbohydrates provided and non-protein nitrogen during growth under osmotic constraint modulated osmoprotectant accumulation. This, in turn, was correlated with P. freudenreichii tolerance towards different stresses, on the one hand, and towards freeze-drying and spray-drying, on the other. Surprisingly, trehalose accumulation correlated with spray-drying survival and glycine betaine accumulation with freeze-drying. This first report showing the ability to modulate the trehalose/GB ratio in osmoprotectants accumulated by a probiotic bacterium opens new perspectives for the optimization of probiotics production.

Comparative Transcriptional Analysis of Lactobacillus plantarum and Its ccpA-Knockout Mutant Under Galactooligosaccharides and Glucose Conditions

Galactooligosaccharides (GOS) are documented prebiotic compounds, but knowledge of the metabolic and regulatory mechanisms of GOS utilization by lactic acid bacteria is still limited. Here we used transcriptome and physiological analyses to investigate the differences in the logarithmic growth phase of Lactobacillus plantarum and L. plantarum ΔccpA metabolizing GOS or glucose as the sole source of carbohydrate. In total, 489 genes (16%) were differentially transcribed in the wild-type L. plantarum grown on glucose and GOS and the value is decreased to 7% due to the loss of ccpA. Only 6% genes were differentially expressed when the wild-type and the ccpA mutant were compared on GOS. Transcriptome data revealed that the carbon sources significantly affected the expression of several genes, and some of the genes were mediated by CcpA. In particular, lac and gal gene clusters resembled the corresponding clusters in L. acidophilus NCFM that are involved in GOS metabolism, indicating that these clusters may be participating in GOS utilization. Moreover, reverse transcription-PCR analysis showed that GOS-related gene clusters were organized in five independent polycistronic units. In addition, many commonalities were found between fructooligosaccharides and GOS metabolism in L. plantarum, including differentially expressed genes involved in oligosaccharide metabolism, conversion of metabolites, and changes in fatty acid biosynthesis. Overall, our findings provide new information on gene transcription and the metabolic mechanism associated with GOS utilization, and confirm that CcpA plays an important role in carbon metabolism regulation in L. plantarum.

Identification of a GntR family regulator BusRTha and its regulatory mechanism in the glycine betaine ABC transport system of Tetragenococcus halophilus

Glycine betaine is one of the most effective compatible solutes of the halophilic lactic acid bacterium Tetragenococcus halophilus, the transportation of which is essential for its survival under salinity stress condition. In the current study, we attempted to define a glycine betaine ABC transporter system of T. halophilus, busA_Tha, which plays an important role in adapting to salinity condition. The expression of busA_Tha enhanced the growth of the recombinant strain under high salinity. BusR_Tha, a transcription regulator that represses the expression of busA_Tha, was characterized, and the repression was abrogated under high salinity. The binding of the regulator was demonstrated through electrophoretic mobility shift assays, and the binding sites were characterized as 5'-AAA(T/G)TGAC(C/A)(G/A)T(C/A)C-3'. This is the first studied transcription regulator of T. halophilus, and our findings provide insights into the molecular mechanism of halophilic life and tools for further application of halophiles as chassis in industrial biotechnology.

Morphological, transcriptional, and metabolic analyses of osmotic-adapted mechanisms of the halophilic Aspergillus montevidensis ZYD4 under hypersaline conditions

Halophilic fungi in hypersaline habitats require multiple cellular responses for high-salinity adaptation. However, the exact mechanisms behind these adaptation processes remain to be slightly known. The current study is aimed at elucidating the morphological, transcriptomic, and metabolomic changes of the halophilic fungus Aspergillus montevidensis ZYD4 under hypersaline conditions. Under these conditions, the fungus promoted conidia formation and suppressed cleistothecium development. Furthermore, the fungus differentially expressed genes (P < 0.0001) that controlled ion transport, amino acid transport and metabolism, soluble sugar accumulation, fatty acid β-oxidation, saturated fatty acid synthesis, electron transfer, and oxidative stress tolerance. Additionally, the hypersalinized mycelia widely accumulated metabolites, including amino acids, soluble sugars, saturated fatty acids, and other carbon- and nitrogen-containing compounds. The addition of metabolites-such as neohesperidin, biuret, aspartic acid, alanine, proline, and ornithine-significantly promoted the growth (P ≤ 0.05) and the morphological adaptations of A. montevidensis ZYD4 grown in hypersaline environments. Our study demonstrated that morphological shifts, ion equilibrium, carbon and nitrogen metabolism for solute accumulation, and energy production are vital to halophilic fungi so that they can build tolerance to high-salinity environments.

Quantitative RNA-seq Analysis Unveils Osmotic and Thermal Adaptation Mechanisms Relevant for Ectoine Production in Chromohalobacter salexigens

Quantitative RNA sequencing (RNA-seq) and the complementary phenotypic assays were implemented to investigate the transcriptional responses of Chromohalobacter salexigens to osmotic and heat stress. These conditions trigger the synthesis of ectoine and hydroxyectoine, two compatible solutes of biotechnological interest. Our findings revealed that both stresses make a significant impact on C. salexigens global physiology. Apart from compatible solute metabolism, the most relevant adaptation mechanisms were related to “oxidative- and protein-folding- stress responses,” “modulation of respiratory chain and related components,” and “ion homeostasis.” A general salt-dependent induction of genes related to the metabolism of ectoines, as well as repression of ectoine degradation genes by temperature, was observed. Different oxidative stress response mechanisms, secondary or primary, were induced at low and high salinity, respectively, and repressed by temperature. A higher sensitivity to H₂O₂ was observed at high salinity, regardless of temperature. Low salinity induced genes involved in “protein-folding-stress response,” suggesting disturbance of protein homeostasis. Transcriptional shift of genes encoding three types of respiratory NADH dehydrogenases, ATP synthase, quinone pool, Na⁺/H⁺ antiporters, and sodium-solute symporters, was observed depending on salinity and temperature, suggesting modulation of the components of the respiratory chain and additional systems involved in the generation of H⁺ and/or Na⁺ gradients. Remarkably, the Na⁺ intracellular content remained constant regardless of salinity and temperature. Disturbance of Na⁺- and H⁺-gradients with specific ionophores suggested that both gradients influence ectoine production, but with differences depending on the solute, salinity, and temperature conditions. Flagellum genes were strongly induced by salinity, and further induced by temperature. However, salt-induced cell motility was reduced at high temperature, possibly caused by an alteration of Na⁺ permeability by temperature, as dependence of motility on Na⁺-gradient was observed. The transcriptional induction of genes related to the synthesis and transport of siderophores correlated with a higher siderophore production and intracellular iron content only at low salinity. An excess of iron increased hydroxyectoine accumulation by 20% at high salinity. Conversely, it reduced the intracellular content of ectoines by 50% at high salinity plus high temperature. These findings support the relevance of iron homeostasis for osmoadaptation, thermoadaptation and accumulation of ectoines, in C. salexigens.

CcpA-Dependent Carbon Catabolite Repression Regulates Fructooligosaccharides Metabolism in Lactobacillus plantarum

Fructooligosaccharides (FOSs) metabolism in Lactobacillus plantarum is controlled by two gene clusters, and the global regulator catabolite control protein A (CcpA) may be involved in the regulation. To understand the mechanism, this study focused on the regulation relationships of CcpA toward target genes and the binding effects on the catabolite responsive element (cre). First, reverse transcription-PCR analysis of the transcriptional organization of the FOS-related gene clusters showed that they were organized in three independent polycistronic units. Diauxic growth, hierarchical utilization of carbohydrates and repression of FOS-related genes were observed in cultures containing FOS and glucose, suggesting carbon catabolite repression (CCR) control in FOS utilization. Knockout of ccpA gene eliminated these phenomena, indicating the principal role of this gene in CCR of FOS metabolism. Furthermore, six potential cre sites for CcpA binding were predicted in the regions of putative promoters of the two clusters. Direct binding was confirmed by electrophoretic mobility shift assays in vitro and chromatin immunoprecipitation in vivo. The results of the above studies suggest that CcpA is a vital regulator of FOS metabolism in L. plantarum and that CcpA-dependent CCR regulates FOS metabolism through the direct binding of CcpA toward the cre sites in the promoter regions of FOS-related clusters.

Survival strategy of the salt-tolerant lactic acid bacterium, Tetragenococcus halophilus, to counteract koji mold, Aspergillus oryzae, in soy sauce brewing

In soy sauce brewing, the results of the fermentation of lactic acid greatly affect the quality of soy sauce. The soy sauce moromi produced with Aspergillus oryzae RIB40 allows the growth of Tetragenococcus halophilus NBRC 12172 but not T. halophilus D10. We isolated and identified heptelidic acid (HA), an inhibitor of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), produced by A. oryzae RIB40 as the growth inhibitor of the salt-tolerant lactic acid bacteria. The growth inhibition of T. halophilus D10 by HA was suggested to be associated with the direct inhibition of GAPDH activity under high salt environment. The difference in the susceptibility to HA among various strains of T. halophilus was caused by the mutations in the gene encoding GAPDH.

Draft genome sequence of Exiguobacterium aurantiacum strain PN47 isolate from saline ponds, known as "Salar del Huasco", located in the Altiplano in the North of Chile

In this report, we present a draft genome of 2,886,173bp of an Exiguobacterium aurantiacum strain PN47 isolate from the sediment of a saline pond named "Salar del Huasco" in the Altiplano in the North of Chile. Strain PN47 encodes adaptive characteristics enabling survival in extreme environmental conditions of high heavy metal and salt concentrations and high alkalinity.

Betaine Improves Polymer-Grade D-Lactic Acid Production by Sporolactobacillus inulinus Using Ammonia as Green Neutralizer

The traditional CaCO3-based fermentation process generates huge amount of insoluble CaSO4 waste. To solve this problem, we have developed an efficient and green D-lactic acid fermentation process by using ammonia as neutralizer. The 106.7 g/L of D-lactic acid production and 0.89 g per g of consumed sugar were obtained by Sporolactobacillus inulinus CASD with a high optical purity of 99.7% by adding 100 mg/L betaine in the simple batch fermentation process. The addition of betaine was experimentally proven to protect cell at high concentration of ammonium ion, increase the D-lactate dehydrogenase specific activity and thus promote the production of D-lactic acid.

Draft Genome Sequences of Tetragenococcus muriaticus Strains 3MR10-3 and PMC-11-5 Isolated from Thai Fish Sauce during Natural Fermentation

Tetragenococcus muriaticus strains 3MR10-3 and PMC-11-5 are homofermentative halophilic lactic acid bacteria isolated from Thai fish sauce during natural fermentation. Their draft genomes were sequenced. Our interest in these organisms is related to their impact on fish sauce flavor and their high osmotolerance.

A partial proteome reference map of Tetragenococcus halophilus and comparative proteomic and physiological analysis under salt stress

Tetragenococcus halophilus, a moderately halophilic Gram-positive lactic acid bacteria, was widely existed in many food fermentation systems, where salt stress is an environmental condition commonly encountered.

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.

The metabolism and biotechnological application of betaine in microorganism

Glycine betaine (betaine) is widely distributed in nature and can be found in many microorganisms, including bacteria, archaea, and fungi. Due to its particular functions, many microorganisms utilize betaine as a functional chemical and have evolved different metabolic pathways for the biosynthesis and catabolism of betaine. As in animals and plants, the principle role of betaine is to protect microbial cells against drought, osmotic stress, and temperature stress. In addition, the role of betaine in methyl group metabolism has been observed in a variety of microorganisms. Recent studies have shown that betaine supplementation can improve the performance of microbial strains used for the fermentation of lactate, ethanol, lysine, pyruvate, and vitamin B12, during which betaine can act as stress protectant or methyl donor for the biosynthesis of structurally complex compounds. In this review, we summarize the transport, synthesis, catabolism, and functions of betaine in microorganisms and discuss potential engineering strategies that employ betaine as a methyl donor for the biosynthesis of complex secondary metabolites such as a variety of vitamins, coenzymes, and antibiotics. In conclusion, the biocompatibility, C/N ratio, abundance, and comprehensive metabolic information of betaine collectively indicate that this molecule has great potential for broad applications in microbial biotechnology.

Paths and determinants for Penicillium janthinellum to resist low and high copper

Copper (Cu) tolerance was well understood in fungi yeasts but not in filamentous fungi. Filamentous fungi are eukaryotes but unlike eukaryotic fungi yeasts, which are a collection of various fungi that are maybe classified into different taxa but all characterized by growth as filamentous hyphae cells and with a complex morphology. The current knowledge of Cu resistance of filamentous fungi is still fragmental and therefore needs to be bridged. In this study, we characterized Cu resistance of Penicillium janthinellum strain GXCR and its Cu-resistance-decreasing mutants (EC-6 and UC-8), and conducted sequencing of a total of 6 transcriptomes from wild-type GXCR and mutant EC-6 grown under control and external Cu. Taken all the results together, Cu effects on the basal metabolism were directed to solute transport by two superfamilies of solute carrier and major facilitator, the buffering free CoA and Acyl-CoA pool in the peroxisome, F-type H(+)-transporting ATPases-based ATP production, V-type H(+)-transporting ATPases-based transmembrane transport, protein degradation, and alternative splicing of pre-mRNAs. Roles of enzymatic and non-enzymatic antioxidants in resistance to low and high Cu were defined. The backbone paths, signaling systems, and determinants that involve resistance of filamentous fungi to high Cu were determined, discussed and outlined in a model.

Carnitine in bacterial physiology and metabolism

Carnitine is a quaternary amine compound found at high concentration in animal tissues, particularly muscle, and is most well studied for its contribution to fatty acid transport into mitochondria. In bacteria, carnitine is an important osmoprotectant, and can also enhance thermotolerance, cryotolerance and barotolerance. Carnitine can be transported into the cell or acquired from metabolic precursors, where it can serve directly as a compatible solute for stress protection or be metabolized through one of a few distinct pathways as a nutrient source. In this review, we summarize what is known about carnitine physiology and metabolism in bacteria. In particular, recent advances in the aerobic and anaerobic metabolic pathways as well as the use of carnitine as an electron acceptor have addressed some long-standing questions in the field.

Comparative transcriptomic analysis reveals novel genes and regulatory mechanisms of Tetragenococcus halophilus in response to salt stress

Tetragenococcus halophilus, a moderately halophilic Gram-positive bacterium, was isolated from Chinese style soy sauce. This species is a valuable resource for investigating salt tolerance mechanisms and improving salinity resistance in microorganisms. RNA-seq was used to sequence T. halophilus samples treated with 0 M (T1), 1 M (T2), and 3.5 M NaCl (T3). Comparative transcriptomic analyses of the different treatments were performed using gene ontology and Kyoto encyclopedia of genes and genome. The comparison of T1 and T2 by RNA-seq revealed that genes involved in transcription, translation, membrane system, and division were highly up-regulated under optimum salt condition. The comparison of T2 and T3 showed that genes related to heat shock proteins or the ATP-binding cassette transport systems were significantly up-regulated under maximum-salt condition. In addition, a considerable proportion of the significantly differently expressed genes identified in this study are novel. These data provide a crucial resource that may determine specific responses to salt stress in T. halophilus.

Betaine and beet molasses enhance L-lactic acid production by Bacillus coagulans

Lactic acid is an important chemical with various industrial applications, and it can be efficiently produced by fermentation, in which Bacillus coagulans strains present excellent performance. Betaine can promote lactic acid fermentation as an effective osmoprotectant. Here, positive effect of betaine on fermentation by B. coagulans is revealed. Betaine could enhance lactic acid production by protecting l-LDH activity and cell growth from osmotic inhibition, especially under high glucose concentrations and with poor organic nitrogen nutrients. The fermentation with 0.05 g/L betaine could produce 17.9% more lactic acid compared to the fermentation without betaine. Beet molasses, which is rich in sucrose and betaine, was utilized in a co-feeding fermentation and raised the productivity by 22%. The efficient lactic acid fermentation by B. coagulans is thus developed by using betaine and beet molasses.

Bacterial ecology of PDO Coppa and Pancetta Piacentina at the end of ripening and after MAP storage of sliced product

The objective of this study was to evaluate the microbiota of two typical Italian PDO delicatessens Coppa and Pancetta Piacentina, produced in Piacenza area (Italy). Classical and molecular approaches were employed, in order to acquire knowledge on their bacterial ecology and its evolution after slicing and MAP storing; thus, the biodiversity of characteristic bacterial community, already present or introduced during such procedures, was studied in both full ripened and sliced samples from two producers (A and B) of the PDO district, packaged under MAP and stored at 2 and 8 °C for 30 days. The microbiota of the two kinds of Italian delicatessen demonstrated peculiar differences, particularly regarding the staphylococci and lactic acid bacteria (LAB) ratio. Moreover, some species within these two groups appeared to be linked to the kind of product: Leuconostoc, Lactobacillus versmoldensis and Staphylococcus saprophyticus were found only in Pancetta while Lactobacillus pentosus, Staphylococcus equorum, Staphylococcus xylosus, Staphylococcus sciuri and Macrococcus caseolyticus occurred only in Coppa. Also, both delicatessens from producer A were richer in LAB compared to those of producer B and the opposite applied for staphylococci. Interestingly, Tetragenococcus halophilus was detectable in all the samples and its presence in the sausage environment has been reported only for Capocollo. Storage did not substantially modify the microbiota composition, the only changes being the relative abundance of same sequences; S. xylosus was prevalent before slicing process and S. equorum at the end of MAP storage at both 2 °C and 8 °C. Concerning microbial contamination during the slicing process, our results suggest that the adopted procedures assure high hygienic quality standard of these typical products, with exception of a contamination by Psychrobacter psychrophilus in Coppa B. The possible origin of species rarely or never reported in the sausage environment and detected in this study is discussed.

Not only osmoprotectant: betaine increased lactate dehydrogenase activity and L-lactate production in lactobacilli

Lactobacilli are commonly used for industrial production of polymer-grade L-lactic acid. The present study tested the Tween 80 alternative betaine in L-lactate production by several industrial lactobacilli. In flask fermentation of Lactobacillus casei, Lactobacillus buchneri, Lactobacillus lactis and Lactobacillus rhamnosus, the betaine addition (2g/l) had similar osmoprotectant effect with Tween 80 but had increased the lactate dehydrogenase activities and L-lactate production than Tween 80 control. In fed-batch fermentation of L. casei, betaine supplementation improved the L-lactic acid titer to 190 g/l, the yield to 95.5% (g L-lactic acid/g glucose), the productivity to 2.6g/lh, and the optical purity to 97.0%. The results demonstrated that supplementation of Tween 80 alternative - betaine in the fermentation medium is feasible for industrial l-lactic acid fermentation by lactobacilli, which will improve the lactate production but will not increase the process costs and modify any process conditions.

Involvement of OpcR, a GbsR-type transcriptional regulator, in negative regulation of two evolutionarily closely related choline uptake genes in Bacillus subtilis

The osmoprotectant glycine betaine can be generated intracellularly from conversion of the exogenous precursor choline by enzymes encoded by the gbsAB operon in Bacillus subtilis. Uptake of choline from outside B. subtilis cells is mediated through two evolutionarily closely related ATP-binding cassette transporters, OpuB and OpuC. Expression of the opuB operon and of the opuC operon is known to be osmoinducible. Here, we show that choline exerts a suppressive effect on opuC expression during normal growth and under osmotic stress. In the absence of the choline-responsive repressor GbsR, opuB expression is also suppressed by choline. We also report that a gene (formerly yvbF, now designated opcR) located immediately upstream of the opuC operon negatively regulates transcription of the opuC operon and, in the absence of GbsR, also that of the opuB operon. An inverted repeat (TTGTAAA-N8-TTTACAA) that overlaps with the -35 hexamer of the promoters of both operons has been identified as the OpcR operator. OpcR belongs to the GbsR-type transcriptional regulators. Its orthologues with unknown function are present in some other Bacillus species. Moreover, deletion analyses revealed that a region located further upstream of the promoters of the opuB operon and the opuC operon is critical for expression of both operons during normal growth and under osmotic stress. Osmotic induction of these two operons appears not to be OpcR mediated. OpcR is not a choline-responsive repressor. The possible biological role of OpcR is discussed.

Glycine betaine catabolism contributes to Pseudomonas syringae tolerance to hyperosmotic stress by relieving betaine-mediated suppression of compatible solute synthesis

Many bacteria can accumulate glycine betaine for osmoprotection and catabolize it as a growth substrate, but how they regulate these opposing roles is poorly understood. In Pseudomonas syringae B728a, expression of the betaine catabolism genes was reduced by an osmotic upshift to an intermediate stress level, consistent with betaine accumulation, but was increased by an upshift to a high stress level, as confirmed by an accompanying increase in degradation of radiolabeled betaine. Deletion of the gbcAB betaine catabolism genes reduced osmotolerance at a high osmolarity, and this reduction was due to the relief of betaine-mediated suppression of compatible solute synthesis. This conclusion was supported by the findings that, at high osmolarity, the ΔgbcAB mutant accumulated high betaine levels and low endogenous solutes and exhibited reduced expression of the solute synthesis genes. Moreover, the ΔgbcAB mutant and a mutant deficient in the synthesis of the compatible solutes NAGGN and trehalose exhibited similar reductions in osmotolerance and also in fitness on bean leaves. Activation of betaine catabolism at high osmotic stress resulted, in part, from induction of gbdR, which encodes the transcriptional activator GbdR. Betaine catabolism was subject to partial repression by succinate under hyperosmotic stress conditions, in contrast to strong repression in the absence of stress, suggesting that betaine functions both in nutrition and as an intracellular signal modulating solute synthesis under hyperosmotic stress conditions. Collectively, these results begin to provide a detailed mechanistic understanding of how P. syringae transitions from reliance on exogenously derived betaine to the use of endogenous solutes during adaptation to hyperosmotic conditions.

Gene cloning and biochemical characterization of 4-N-trimethylaminobutyraldehyde dehydrogenase II from Pseudomonas sp. 13CM

The gene encoding 4-N-trimethylaminobutyraldehyde dehydrogenase (TMABaldehyde-DH) from Pseudomonas sp. 13CM, responsible for the conversion of 4-N-trimethylaminobutyraldehyde (TMABaldehyde) to γ-butyrobetaine in the carnitine biosynthesis pathway, isolated by shotgun cloning and expressed in Escherichia coli DH5α. The recombinant TMABaldehyde-DH was purified 19.5 fold to apparent homogeneity by hydrophobic and affinity chromatography and biochemically characterized. The enzyme was found to be a trimer with identical 52 kDa subunits. The isoelectric point was found to be 4.5. Optimum pH and temperature were found respectively as pH 9.5 and 40 °C. The Km values for TMABaldehyde, 4-dimethylaminobutyraldehyde, and NAD+ were respectively, 0.31, 0.62, and 1.16 mM. The molecular and catalytic properties differed from those of TMABaldehyde-DH I, which was discovered initially in Pseudomonas sp. 13CM. The new enzyme, designated TMABaldehyde-DH II, structural gene was inserted into an expression vector pET24b (+) and over-expressed in E. coli BL21 (DE3) under the control of a T7 promoter. The recombinant TMABaldehyde-DH from Pseudomonas sp. 13CM can now be obtained in large quantity necessary for further biochemical characterization and applications.