Transcriptional Responses of Herbaspirillum seropedicae to Environmental Phosphate Concentration.

Herbaspirillum seropedicae is a nitrogen-fixing endophytic bacterium associated with important cereal crops, which promotes plant growth, increasing their productivity. The understanding of the physiological responses of this bacterium to different concentrations of prevailing nutrients as phosphate (Pi) is scarce. In some bacteria, culture media Pi concentration modulates the levels of intracellular polyphosphate (polyP), modifying their cellular fitness. Here, global changes of H. seropedicae SmR1 were evaluated in response to environmental Pi concentrations, based on differential intracellular polyP levels. Cells grown in high-Pi medium (50 mM) maintained high polyP levels in stationary phase, while those grown in sufficient Pi medium (5 mM) degraded it. Through a RNA-seq approach, comparison of transcriptional profiles of H. seropedicae cultures revealed that 670 genes were differentially expressed between both Pi growth conditions, with 57% repressed and 43% induced in the high Pi condition. Molecular and physiological analyses revealed that aspects related to Pi metabolism, biosynthesis of flagella and chemotaxis, energy production, and polyhydroxybutyrate metabolism were induced in the high-Pi condition, while those involved in adhesion and stress response were repressed. The present study demonstrated that variations in environmental Pi concentration affect H. seropedicae traits related to survival and other important physiological characteristics. Since environmental conditions can influence the effectiveness of the plant growth-promoting bacteria, enhancement of bacterial robustness to withstand different stressful situations is an interesting challenge. The obtained data could serve not only to understand the bacterial behavior in respect to changes in rhizospheric Pi gradients but also as a base to design strategies to improve different bacterial features focusing on biotechnological and/or agricultural purposes.

Enhancement of γ-aminobutyric acid (GABA) production by Lactobacillus brevis CRL 2013 based on carbohydrate fermentation.

Gamma aminobutyric acid (GABA) is a non-protein amino acid that is widely distributed in nature and its physiological importance goes beyond its role as an inhibitory neurotransmitter of the central nervous system in mammals. Since microbial fermentation is one of the most promising methods to obtain GABA, the production of this metabolite by several strains of lactic acid bacteria isolated from quinoa and amaranth sourdoughs was investigated. Lactobacillus brevis CRL 2013 produced the highest GABA levels, reaching 265 mM when optimal culture conditions were set up. The fermentative profile showed that CRL 2013 was able to catabolize carbohydrates through the phosphoketolase pathway yielding variable amounts of lactic acid, acetate and ethanol, which depended on the type of carbon source available and the presence of external electron acceptors such as fructose. Enhanced growth parameters and low GABA synthesis were associated to pentose fermentation. This impairment on GABA production machinery was partially overpassed by the addition of ethanol to the culture media. These results support the potential use of L. brevis CRL 2013 as a starter culture for the manufacture of GABA-enriched functional foods and provide further insights to the understanding of the GAD system regulation in lactic acid bacteria.

YebC, a putative transcriptional factor involved in the regulation of the proteolytic system of Lactobacillus.

The proteolytic system of Lactobacillus plays an essential role in bacterial growth, contributes to the flavor development of fermented products, and can release bioactive health-beneficial peptides during milk fermentation. In this work, a genomic analysis of all genes involved in the proteolytic system of L. delbrueckii subsp. lactis CRL 581 was performed. Genes encoding the cell envelope-associated proteinase, two peptide transport systems, and sixteen peptidases were identified. The influence of the peptide supply on the transcription of 23 genes involved in the proteolytic system of L. delbrueckii subsp. lactis was examined after cell growth in a chemically defined medium (CDM) and CDM supplemented with Casitone. prtL, oppA 1, optS, optA genes as well as oppDFBC and optBCDF operons were the most highly expressed genes in CDM; their expression being repressed 6- to 115-fold by the addition of peptides. The transcriptional analysis was confirmed by proteomics; the up-regulation of the PrtL, PepG, OppD and OptF proteins in the absence of peptides was observed while the DNA-binding protein YebC was up-regulated by peptides. Binding of YebC to the promoter region of prtL, oppA 1, and optS, demonstrated by electrophoretic mobility shift assays, showed that YebC acts as a transcriptional repressor of key proteolytic genes.

Lactic Acid Bacteria as Cell Factories for the Generation of Bioactive Peptides.

There is a growing interest in the incorporation of functional foods in the daily diet to achieve health promotion and disease risk reduction. Numerous studies have focused on the production of biologically active peptides as nutraceuticals and functional food ingredients due to their health benefits. These short peptides, displaying antihypertensive, antioxidant, mineral binding, immunomodulatory and antimicrobial activities are hidden in a latent state within the primary sequences of food proteins requiring enzymatic proteolysis for their release. While microbial fermentation is one of the major and economically most convenient processes used to generate bioactive peptides, lactic acid bacteria (LAB) are widely used as starter cultures for the production of diverse fermented foods. This article reviews the current knowledge on LAB as cell factories for the production of bioactive peptides from a variety of food protein sources. These microorganisms depend on a complex proteolytic system to ensure successful fermentation processes. In the dairy industry, LAB containing cell envelope-associated proteinases (CEPs) are employed as biocatalysts for the first step of casein breakdown releasing bioactive peptides during milk fermentation. A better understanding of the functionality and regulation of the proteolytic system of LAB opens up future opportunities for the production of novel food-derived compounds with potential health-promoting properties.

Characterization of the mature cell surface proteinase of Lactobacillus delbrueckii subsp. lactis CRL 581.

The cell envelope-associated proteinase (CEP) of Lactobacillus delbrueckii subsp. lactis CRL 581 (PrtL) has an essential role in bacterial growth, contributes to the flavor and texture development of fermented products, and can release bioactive health-beneficial peptides during milk fermentation. The genome of L. delbrueckii subsp. lactis CRL 581 possesses only one gene that encodes PrtL, which consists of 1924 amino acids and is a multidomain protein anchored to the cell via its W domain. PrtL was extracted from the cell under high ionic strength conditions using NaCl, suggesting an electrostatic interaction between the proteinase and the cell envelope. The released PrtL was purified and biochemically characterized; its activity was maximal at temperatures between 37 and 40 °C and at pH between 7 and 8. Under optimal conditions, PrtL exhibited higher affinity for succinyl-alanyl-alanyl-prolyl-phenylalanine-p-nitroanilide than for succinyl-alanyl-glutamyl-prolyl-phenylalanine-p-nitroanilide, while methoxy-succinyl-arginyl-prolyl-tyrosyl-p-nitroanilide was not degraded. A similar α- and ß-casein degradation pattern was observed with the purified and the cell envelope-bound proteinase. Finally, on the basis of its specificity towards caseins and the unique combination of amino acids at residues thought to be involved in substrate specificity, PrtL can be classified as a representative of a new group of CEP.

FAD binding properties of a cytosolic version of Escherichia coli NADH dehydrogenase-2.

Respiratory NADH dehydrogenase-2 (NDH-2) of Escherichia coli is a peripheral membrane-bound flavoprotein. By eliminating its C-terminal region, a water soluble truncated version was obtained in our laboratory. Overall conformation of the mutant version resembles the wild-type protein. Considering these data and the fact that the mutant was obtained as an apo-protein, the truncated version is an ideal model to study the interaction between the enzyme and its cofactor. Here, the FAD binding properties of this version were characterized using far-UV circular dichroism (CD), differential scanning calorimetry (DSC), limited proteolysis, and steady-state and dynamic fluorescence spectroscopy. CD spectra, thermal unfolding and DSC profiles did not reveal any major difference in secondary structure between apo- and holo-protein. In addition, digestion site accessibility and tertiary conformation were similar for both proteins, as seen by comparable chymotryptic cleavage patterns. FAD binding to the apo-protein produced a parallel increment of both FAD fluorescence quantum yield and steady-state emission anisotropy. On the other hand, addition of FAD quenched the intrinsic fluorescence emission of the truncated protein, indicating that the flavin cofactor should be closely located to the protein Trp residues. Analysis of the steady-state and dynamic fluorescence data confirms the formation of the holo-protein with a 1:1 binding stoichiometry and an association constant KA=7.0(±0.8)×10(4)M(-1). Taken together, the FAD-protein interaction is energetically favorable and the addition of FAD is not necessary to induce the enzyme folded state. For the first time, a detailed characterization of the flavin:protein interaction was performed among alternative NADH dehydrogenases.

Polyphosphate degradation in stationary phase triggers biofilm formation via LuxS quorum sensing system in Escherichia coli.

In most natural environments, association with a surface in a structure known as biofilm is the prevailing microbial life-style of bacteria. Polyphosphate (polyP), an ubiquitous linear polymer of hundreds of orthophosphate residues, has a crucial role in stress responses, stationary-phase survival, and it was associated to bacterial biofilm formation and production of virulence factors. In previous work, we have shown that Escherichia coli cells grown in media containing a critical phosphate concentration >37 mM maintained an unusual high polyP level in stationary phase. The aim of the present work was to analyze if fluctuations in polyP levels in stationary phase affect biofilm formation capacity in E. coli. Polymer levels were modulated by the media phosphate concentration or using mutant strains in polyP metabolism. Cells grown in media containing phosphate concentrations higher than 25 mM were defective in biofilm formation. Besides, there was a disassembly of 24 h preformed biofilm by the addition of high phosphate concentration to the medium. These phenotypes were related to the maintenance or re-synthesis of polyP in stationary phase in static conditions. No biofilm formation was observed in ppk(-)ppx(-) or ppk(-)ppx(-)/ppk(+) strains, deficient in polyP synthesis and hydrolysis, respectively. luxS and lsrK mutants, impaired in autoinducer-2 quorum sensing signal metabolism, were unable to form biofilm unless conditioned media from stationary phase wild type cells grown in low phosphate were used. We conclude that polyP degradation is required for biofilm formation in sufficient phosphate media, activating or triggering the production of autoinducer-2. According to our results, phosphate concentration of the culture media should be carefully considered in bacterial adhesion and virulence studies.

Amphipathic C-terminal region of Escherichia coli NADH dehydrogenase-2 mediates membrane localization.

Respiratory NADH dehydrogenase-2 (NDH-2) of Escherichia coli is a membrane-bound flavoprotein. Bioinformatics approaches suggested the involvement of NDH-2 C-terminal region in membrane anchorage. Here, we demonstrated that NDH-2 is a peripheral membrane protein and that its predicted C-terminal amphipathic Arg390-Ala406 helix is sufficient to bind the protein to lipid membranes. Additionally, a cytosolic NDH-2 protein (Trun-3), lacking the last 43 aminoacids, was purified and characterized. FAD cofactor was absent in purified Trun-3. Upon the addition of FAD, Trun-3 maximum velocity was similar to native NDH-2 rate with ferricyanide and MTT acceptors. However, Trun-3 activity was around 5-fold lower with quinones. No significant difference in K(m) values was observed for both enzymes. For the first time, an active and water soluble NDH-2 was obtained, representing a major improvement for structural/functional characterizations.