Substrate specificity of glucose dehydrogenase (GDH) of Enterobacter asburiae PSI3 and rock phosphate solubilization with GDH substrates as C sources

NaCl Modifies Biochemical Traits in Bacterial Endophytes Isolated from Halophytes: Towards Salinity Stress Mitigation Using Consortia

Bacterial endophytes (120) were isolated from six halophytes (Distichlis spicata, Cynodon dactylon, Eragrostis obtusiflora, Suaeda torreyana, Kochia scoparia, and Baccharis salicifolia). These halophiles were molecularly identified and characterized with or without NaCl conditions. Characterization was based on tests such as indole acetic acid (IAA), exopolysaccharides (EPS), and siderophores (SID) production; solubilization of phosphate (P), potassium (K), zinc (Zn), and manganese (Mn); mineralization of phytate; enzymatic activity (acid and alkaline phosphatase, phytases, xylanases, and chitinases) and the mineralization/solubilization mechanisms involved (organic acids and sugars). Moreover, compatibility among bacteria was assessed. Eleven halophiles were characterized as highly tolerant to NaCl (2.5 M). The bacteria isolated were all different from each other. Two belonged to Bacillus velezensis and one to B. pumilus while the rest of bacteria were identified up to the genus level as belonging to Bacillus, Halobacillus, Halomonas, Pseudomonas, Nesterenkonia, and three strains of Oceanobacillus. The biochemical responses of nutrient solubilization and enzymatic activity were different between bacteria and were influenced by the presence of NaCl. Organic acids were involved in P mineralization and nutrient solubilization. Tartaric acid was common in the solubilization of P, Zn, and K. Maleic and vanillic acid were only detected in Zn and K solubilization, respectively. Furthermore, sugars appeared to be involved in the solubilization of nutrients; fructose was detected in the solubilization tests. Therefore, these biochemical bacterial characteristics should be corroborated in vivo and tested as a consortium to mitigate saline stress in glycophytes under a global climate change scheme that threatens to exacerbate soil salinity.

Genomic insight of phosphate solubilization and plant growth promotion of two taxonomically distinct winter crops by Enterobacter sp. DRP3

AIMS

Study of rhizospheric microbiome-mediated plant growth promotional attributes currently highlighted as a key tool for the development of suitable bio-inoculants for sustainable agriculture purposes. In this context, we have conducted a detailed study regarding the characterization of phosphate solubilizing potential by plant growth-promoting bacteria that have been isolated from the rhizosphere of a pteridophyte Dicranopteris sp., growing on the lateritic belt of West Bengal.

METHODS AND RESULTS

We have isolated three potent bacterial strains, namely DRP1, DRP2, and DRP3 from the rhizoids-region of Dicranopteris sp. Among the isolated strains, DRP3 is found to have the highest phosphate solubilizing potentiality and is able to produce 655.89 and 627.58 µg ml-1 soluble phosphate by solubilizing tricalcium phosphate (TCP) and Jordan rock phosphate, respectively. This strain is also able to solubilize Purulia rock phosphate moderately (133.51 µg ml-1). Whole-genome sequencing and further analysis of the studied strain revealed the presence of pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase gdh gene along with several others that were well known for their role in phosphate solubilization. Further downstream, quantitative reverse transcriptase PCR-based expression study revealed 1.59-fold upregulation of PQQ-dependent gdh gene during the solubilization of TCP. Root colonization potential of the studied strain on two taxonomically distinct winter crops viz. Cicer arietinum and Triticum aestivum has been checked by using scanning electron microscopy. Other biochemical analyses for plant growth promotion traits including indole acetic acid production (132.02 µg ml-1), potassium solubilization (3 mg l-1), biofilm formation, and exopolymeric substances productions (1.88-2.03 µg ml-1) also has been performed.

CONCLUSION

This study highlighted the active involvement of PQQ-dependent gdh gene during phosphate solubilization from any Enterobacter group. Moreover, our study explored different roadmaps for sustainable farming methods and the preservation of food security without endangering soil health in the future.

A comprehensive comparative genomic analysis revealed that plant growth promoting traits are ubiquitous in strains of Stenotrophomonas

Stenotrophomonas strains, which are often described as plant growth promoting (PGP) bacteria, are ubiquitous in many environments. A total of 213 genomes of strains of Stenotrophomonas were analyzed using comparative genomics to better understand the ecological roles of these bacteria in the environment. The pan-genome of the 213 strains of Stenotrophomonas consists of 27,186 gene families, including 710 core gene families, 11,039 unique genes and 15,437 accessory genes. Nearly all strains of Stenotrophomonas harbor the genes for GH3-family cellulose degradation and GH2- and GH31-family hemicellulose hydrolase, as well as intact glycolysis and tricarboxylic acid cycle pathways. These abilities suggest that the strains of this genus can easily obtain carbon and energy from the environment. The Stenotrophomonas strains can respond to oxidative stress by synthesizing catalase, superoxide dismutase, methionine sulfoxide reductase, and disulfide isomerase, as well as managing their osmotic balance by accumulating potassium and synthesizing compatible solutes, such as betaine, trehalose, glutamate, and proline. Each Stenotrophomonas strain also contains many genes for resistance to antibiotics and heavy metals. These genes that mediate stress tolerance increase the ability of Stenotrophomonas strains to survive in extreme environments. In addition, many functional genes related to attachment and plant colonization, growth promotion and biocontrol were identified. In detail, the genes associated with flagellar assembly, motility, chemotaxis and biofilm formation enable the strains of Stenotrophomonas to effectively colonize host plants. The presence of genes for phosphate-solubilization and siderophore production and the polyamine, indole-3-acetic acid, and cytokinin biosynthetic pathways confer the ability to promote plant growth. These strains can produce antimicrobial compounds, chitinases, lipases and proteases. Each Stenotrophomonas genome contained 1-9 prophages and 17-60 genomic islands, and the genes related to antibiotic and heavy metal resistance and the biosynthesis of polyamines, indole-3-acetic acid, and cytokinin may be acquired by horizontal gene transfer. This study demonstrates that strains of Stenotrophomonas are highly adaptable for different environments and have strong potential for use as plant growth-promoting bacteria.

UHPLC-MS/MS and QRT-PCR profiling of PGP agents and Rhizobium spp. of induced phytohormones for growth promotion in mungbean (var. Co4

In present study, five potential strains with different plant growth promotion (PGP) characteristics were used. By considering various PGP properties of different bacterial strains, several treatments based on various combinations were developed and studied on mungbean (var. Co4). The quantification of the phytohormones was performed on ultrahigh-performance liquid chromatograph coupled to heated electrospray ionization tandem mass spectrometry (UHPLC/HESI-MS/MS). Indole 3-acetic acid (IAA) and Indole 3-butyric acid (IBA) were quantified in positive ionization mode while Gibberellic acid (GA₃) and salicylic acid (SA) were quantified in negative ionization mode. Among all the treatments two penta combinations of consortia 1 (Rhizobium + Azospirillum + Pseudomonas + Bacillus spp. + Bacillus licheniformis) and consortia 2 (Rhizobium + Azotobacter + Pseudomonas + Bacillus spp. + Bacillus licheniformis) were found most effective. Higher amount of IAA (1.043 μg g⁻¹), IBA (0.036 μg g⁻¹), GA₃ (1.999 μg g⁻¹) and SA (0.098 μg g⁻¹) Fresh weight (FW) were found in treated adolescent root tissues of consortia 2 as compared to consortia 1. Moreover, transcriptional level of the plant hormones were 2–4 fold higher in the relative gene expression study of three genes: ARF (Auxin responsive factors), ERF-IF (Ethylene-responsive Initiation Factors) and GAI (Gibberellic-Acid Insensitive) in consortia 2, on the 15^th, 30^th and 45^th day using quantitative real time-Polymerase chain reaction (qRT-PCR). Furthermore, Yield attributing characters like, the number of nodules plant⁻¹, number of pods plant⁻¹, weight of nodule and seed yield plant⁻¹ were also increased as compared to the control. As a result, the current research elucidated that penta combinations consortium of Rhizobium sp. and rhizobacteria can be developed as a single delivery system biofertilizer for enhancing mungbean productivity.

Genomic Analysis of the Endophytic Stenotrophomonas Strain 169 Reveals Features Related to Plant-Growth Promotion and Stress Tolerance

Plant-associated Stenotrophomonas isolates have great potential for plant growth promotion, especially under stress conditions, due to their ability to promote tolerance to abiotic stresses such as salinity or drought. The endophytic strain Stenotrophomonas sp. 169, isolated from a field-grown poplar, increased the growth of inoculated in vitro plants, with a particular effect on root development, and was able to stimulate the rooting of poplar cuttings in the greenhouse. The strain produced high amounts of the plant growth-stimulating hormone auxin under in vitro conditions. The comparison of the 16S rRNA gene sequences and the phylogenetic analysis of the core genomes showed a close relationship to Stenotrophomonas chelatiphaga and a clear separation from Stenotrophomonas maltophilia. Whole genome sequence analysis revealed functional genes potentially associated with attachment and plant colonization, growth promotion, and stress protection. In detail, an extensive set of genes for twitching motility, chemotaxis, flagella biosynthesis, and the ability to form biofilms, which are connected with host plant colonization, could be identified in the genome of strain 169. The production of indole-3-acetic acid and the presence of genes for auxin biosynthesis pathways and the spermidine pathway could explain the ability to promote plant growth. Furthermore, the genome contained genes encoding for features related to the production of different osmoprotective molecules and enzymes mediating the regulation of stress tolerance and the ability of bacteria to quickly adapt to changing environments. Overall, the results of physiological tests and genome analysis demonstrated the capability of endophytic strain 169 to promote plant growth. In contrast to related species, strain 169 can be considered non-pathogenic and suitable for biotechnology applications.

Role of microorganisms in bioleaching of rare earth elements from primary and secondary resources

In an era of environmental degradation, and water, and mineral scarcity, enhancing microbial function in sustainable mining has become a prerequisite for the future of the green economy. In recent years, the extensive use of rare earth elements (REEs) in green and smart technologies has led to an increase in the focus on recovery and separation of REEs from ore matrices. However, the recovery of REEs using traditional methods is complex and energy intensive, leading to the requirement to develop processes which are more economically feasible and environmentally friendly. The use of phosphate solubilizing microorganisms for bioleaching of REEs provides a biotechnical approach for the recovery of REEs from primary and secondary sources. However, managing and understanding the microbial-mineral interactions in order to develop a successful method for bioleaching of REEs still remains a major challenge. This review focuses on the use of microbes for the bioleaching of REEs and highlights the importance of genomic studies in order to narrow down potential microorganisms for the optimal extraction of REEs.

Carbohydrate uptake in Advenella mimigardefordensis strain DPN7T is mediated by periplasmic sugar oxidation and a TRAP-transport system

In this study, we investigated an SBP (DctP_Am ) of a tripartite ATP-independent periplasmic transport system (TRAP) in Advenella mimigardefordensis strain DPN7^T . Deletion of dctP_Am as well as of the two transmembrane compounds of the tripartite transporter, dctQ and dctM, impaired growth of A. mimigardefordensis strain DPN7^T , if cultivated on mineral salt medium supplemented with d-glucose, d-galactose, l-arabinose, d-fucose, d-xylose or d-gluconic acid, respectively. The wild type phenotype was restored during complementation studies of A. mimigardefordensis ΔdctP_Am using the broad host vector pBBR1MCS-5::dctP_Am . Furthermore, an uptake assay with radiolabeled [¹⁴ C(U)]-d-glucose clearly showed that the deletion of dctP_Am , dctQ and dctM, respectively, disabled the uptake of this aldoses in cells of either mutant strain. Determination of K_D performing thermal shift assays showed a shift in the melting temperature of DctP_Am in the presence of d-gluconic acid (K_D 11.76 ± 1.3 µM) and the corresponding aldonic acids to the above-mentioned carbohydrates d-galactonate (K_D 10.72 ± 1.4 µM), d-fuconic acid (K_D 13.50 ± 1.6 µM) and d-xylonic acid (K_D 8.44 ± 1.0 µM). The sugar (glucose) dehydrogenase activity (E.C.1.1.5.2) in the membrane fraction was shown for all relevant sugars, proving oxidation of the molecules in the periplasm, prior to transport.

Ensifer meliloti overexpressing Escherichia coli phytase gene (appA) improves phosphorus (P) acquisition in maize plants

The Escherichia coli phytase gene appA encoding enzyme AppA was cloned in a broad host range plasmid pBBR1MCS2 (lac promoter), termed pVA1, and transformed into the Ensifer meliloti 1020. Transformation of pVA1 in Ensifer meliloti {E. m (pVA1)} increased its phosphatase and phytase activity by ∼9- and ∼50-fold, respectively, compared to the transformants containing empty plasmid as control {E. m (pBBR1MCS2)}. The western blot experiments using rabbit anti-AppA antibody showed that AppA is translocated into the periplasm of the host after its expression. Ensifer meliloti harboring AppA protein {E. m (pVA1)} and {E. m (pBBR1MCS2)} could acidify the unbuffered phytate minimal media (pH 8.0) containing Ca-phytate or Na-phytate as sole organic P (Po) source to below pH 5.0 and released P. However, both {E. m (pVA1)} and {E. m (pBBR1MCS2)} neither dropped pH of the medium nor released P when the medium was buffered at pH 8.0 using Tris-Cl, indicating that acidification of medium was important for the enzymatic hydrolysis of phytate. Further experiments proved that maize plants inoculated with {E. m. (pVA1)} showed increase in growth under sterile semi solid agar (SSA) medium containing Na-phytate as sole P source. The present study could be helpful in generating better transgenic bioinoculants harboring phosphate mineralization properties that ultimately promote plant growth.

Self-monitoring of tear glucose: the development of a tear based glucose sensor as an alternative to self-monitoring of blood glucose

Tear glucose sensing for diabetes management has long been sought as an alternative to more invasive self-monitoring of blood glucose (SMBG).

Whole genome sequencing and analysis of plant growth promoting bacteria isolated from the rhizosphere of plantation crops coconut, cocoa and arecanut

Coconut, cocoa and arecanut are commercial plantation crops that play a vital role in the Indian economy while sustaining the livelihood of more than 10 million Indians. According to 2012 Food and Agricultural organization's report, India is the third largest producer of coconut and it dominates the production of arecanut worldwide. In this study, three Plant Growth Promoting Rhizobacteria (PGPR) from coconut (CPCRI-1), cocoa (CPCRI-2) and arecanut (CPCRI-3) characterized for the PGP activities have been sequenced. The draft genome sizes were 4.7 Mb (56% GC), 5.9 Mb (63.6% GC) and 5.1 Mb (54.8% GB) for CPCRI-1, CPCRI-2, CPCRI-3, respectively. These genomes encoded 4056 (CPCRI-1), 4637 (CPCRI-2) and 4286 (CPCRI-3) protein-coding genes. Phylogenetic analysis revealed that both CPCRI-1 and CPCRI-3 belonged to Enterobacteriaceae family, while, CPCRI-2 was a Pseudomonadaceae family member. Functional annotation of the genes predicted that all three bacteria encoded genes needed for mineral phosphate solubilization, siderophores, acetoin, butanediol, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, chitinase, phenazine, 4-hydroxybenzoate, trehalose and quorum sensing molecules supportive of the plant growth promoting traits observed in the course of their isolation and characterization. Additionally, in all the three CPCRI PGPRs, we identified genes involved in synthesis of hydrogen sulfide (H2S), which recently has been proposed to aid plant growth. The PGPRs also carried genes for central carbohydrate metabolism indicating that the bacteria can efficiently utilize the root exudates and other organic materials as energy source. Genes for production of peroxidases, catalases and superoxide dismutases that confer resistance to oxidative stresses in plants were identified. Besides these, genes for heat shock tolerance, cold shock tolerance and glycine-betaine production that enable bacteria to survive abiotic stress were also identified.

Screening of a glucoside 3-dehydrogenase-producing strain, Sphingobacterium faecium, based on a high-throughput screening method and optimization of the culture conditions for enzyme production

The objective of this study was to screen glucoside 3-dehydrogenase (G3DH)-producing strain based on a high-throughput G3DH screening method. Optimization of culture conditions of the isolated strain was also applied in this study. This screening method employed electron transfer reaction in 96-well microtiter plates, α-methyl-D-glucoside, galactose, 2-deoxy-D-glucose, and 3-O-methyl-D-glucose were used as substrates. Using this screening method, one out of 78 strains isolated from different soil samples was obtained with high G3DH activity. The accuracy of the screening method was proved by alkaline treatment analysis of 3-keto sugars. The isolated strain was identified as Sphingobacterium faecium ZJF-D6 by phenotypic characterization and 16S rDNA sequence analysis. The culture conditions of S. faecium for G3DH production were optimized. Sucrose was found as the most suitable carbon source for the G3DH production. The highest G3DH production and cell growth were achieved using the medium at the initial pH of 7.0 at 25 °C for 36 h with activity of 8.03 × 10(-2) U/mL culture. This strain appears promising for potential application in the industry to produce 3-keto sugars. To our knowledge, this is the first report on S. faecium for G3DH production. The method described herein represents a useful tool for the high-throughput isolation of G3DH.

Altitudinal gradient of microbial biomass phosphorus and its relationship with microbial biomass carbon, nitrogen, and rhizosphere soil phosphorus on the eastern slope of Gongga Mountain, SW China

Microbial biomass phosphorus (MBP) is one of the most active forms of phosphorus (P) in soils. MBP plays an important role in the biogeochemical P cycle. To explore MBP distribution and its relationship with other factors, the MBP and rhizosphere soil P concentrations and fractions in six vegetation zones on the eastern slope of Gongga Mountain in SW China were investigated. The MBP distribution followed a parabolic pattern with altitude and the concentration was highest in the subalpine dark coniferous forest (SDC) zone, which was approximately 3500 m above sea level (asl). Below 3500 m asl, the MBP distribution was controlled by precipitation and vegetation type. In addition, temperature, precipitation and vegetation type controlled the MBP distribution at elevations above 3500 m asl. No specific distribution pattern was determined regarding rhizosphere soil P fractions. However, MBP was significantly correlated with the unavailable P fraction in the rhizosphere rather than with the available P fraction. This result suggests that the relationships between the rhizosphere soil P fractions and the MBP depend on time. The microbial biomass element ratios were relatively consistent on the eastern slope of Gongga Mountain. However, variations in the microbial biomass element rations were observed in six of the vegetation zones. The mean C:N:P ratio was 9.0∶1.3∶1. Overall, vegetation type resulted in the observed fluctuations of the microbial biomass element ratio.

2-ketogluconic acid secretion by incorporation of Pseudomonas putida KT 2440 gluconate dehydrogenase (gad) operon in Enterobacter asburiae PSI3 improves mineral phosphate solubilization

Enterobacter asburiae PSI3 is known to efficiently solubilize rock phosphate by secretion of approximately 50 mM gluconic acid in Tris-buffered medium in the presence of 75 mM glucose and in a mixture of seven aldosugars each at 15 mM concentration, mimicking alkaline vertisol soils. Efficacy of this bacterium in the rhizosphere requires P release in the presence of low amount of sugars. To achieve this, E. asburiae PSI3 has been manipulated to express gluconate dehydrogenase (gad) operon of Pseudomonas putida KT 2440 to produce 2-ketogluconic acid. E. asburiae PSI3 harboring gad operon had 438 U of GAD activity, secreted 11.63 mM 2-ketogluconic and 21.65 mM gluconic acids in Tris-rock phosphate-buffered medium containing 45 mM glucose. E. asburiae PSI3 gad transformant solubilized 0.84 mM P from rock phosphate in TRP-buffered liquid medium. In the presence of a mixture of seven sugars each at 12 mM, the transformant brought about a drop in pH to 4.1 and released 0.53 mM P.

Review of glucose oxidases and glucose dehydrogenases: a bird's eye view of glucose sensing enzymes

The evolution from first-generation through third-generation glucose sensors has witnessed the appearance of a number of very diverse oxidoreductases, which vary tremendously in terms of origin, structure, substrate specificity, cofactor used as primary electron acceptor, and acceptable final electron acceptor. This article summarizes our present knowledge of redox enzymes currently utilized in commercially available glucose monitoring systems to promote a fuller appreciation of enzymatic properties and principles employed in blood glucose monitoring to help avoid potential errors.

Repression of mineral phosphate solubilizing phenotype in the presence of weak organic acids in plant growth promoting fluorescent pseudomonads

Two phosphate solubilizing bacteria (PSB), M3 and SP1, were obtained from the rhizosphere of mungbean and sweet potato, respectively and identified as strains of Pseudomonas aeruginosa. Their rock phosphate (RP) solubilizing abilities were found to be due to secretion high amount of gluconic acid. In the presence of malate and succinate, individually and as mixture, the P solubilizing ability of both the strains was considerably reduced. This was correlated with a nearly 80% decrease in the activity of the glucose dehydrogenase (GDH) but not gluconate dehydrogenase (GAD) in both the isolates. Thus, GDH enzyme, catalyzing the periplasmic production of gluconic acid, is under reverse catabolite repression control by organic acids in P. aeruginosa M3 and SP1. This is of relevance in rhizospheric conditions and is a new explanation for the lack of field efficacy of such PSB.

Phosphate-solubilizing and plant-growth-promoting Pseudomonas aeruginosa PS1 improves greengram performance in quizalafop-p-ethyl and clodinafop amended soil

The quizalafop-p-ethyl- and clodinafop-tolerant phosphate-solubilizing and plant-growth-promoting Pseudomonas aeruginosa PS1 isolated from the rhizospheric soils of mustard was used to determine its phosphate-solubilizing activity and other plant-growth-promoting traits both in the presence and absence of technical grade quizalafop-p-ethyl and clodinafop under in vitro conditions. Quizalafop-p-ethyl (at 40, 80, and 120 ppb) and clodinafop (at 400, 800, and 1200 ppb) reduced the P-solubilizing activity, synthesis of indole-3-acetic acid, and siderophores progressively with increasing concentrations of each herbicide. Hydrogen cyanide and ammonia synthesisized by this strain, however, did not change. Furthermore, the effects of three concentrations each of quizalafop-p-ethyl [40 (recommended dose), 80, and 120 ppb] and clodinafop [400 (recommended dose), 800, and 1200 ppb] were evaluated on plant-growth-promoting Pseudomonas aeruginosa strain PS1 inoculated greengram plants, grown in sandy clay loam soil. Generally, all of the concentrations of both quizalafop-p-ethyl and clodinafop showed phytotoxicity and severely affected the growth, symbiosis, grain yield, and nutrient uptake by greengram plants. The toxicity of quizalafop-p-ethyl and clodinafop enhanced gradually with the increase in the dose rate of herbicides. Quizalafop-p-ethyl was more toxic than clodinafop. In contrast, herbicide-tolerant P. aeruginosa strain PS1 when used with herbicides increased the measured parameters at all concentrations. Both quizalafop-p-ethyl at 120 ppb and clodinafop at 400 ppb increased total chlorophyll content, leghemoglobin, root N, shoot N, root P, shoot P, seed yield, and seed protein, relative to the uninoculated control. The study suggests that the phytotoxicity of herbicides to legumes could be reduced by applying the growth-promoting herbicide-tolerant strain of Pseudomonas aeruginosa PS1.

Complete fermentation of xylose and methylglucuronoxylose derived from methylglucuronoxylan by Enterobacter asburiae strain JDR-1

Acid pretreatment is commonly used to release pentoses from the hemicellulose fraction of cellulosic biomass for bioconversion. The predominant pentose in the hemicellulose fraction of hardwoods and crop residues is xylose in the polysaccharide methylglucuronoxylan, in which as many as one in six of the beta-1,4-linked xylopyranose residues is substituted with alpha-1,2-linked 4-O-methylglucuronopyranose. Resistance of the alpha-1,2-methylglucuronosyl linkages to acid hydrolysis results in release of the aldobiuronate 4-O-methylglucuronoxylose, which is not fermented by bacterial biocatalysts currently used for bioconversion of hemicellulose. Enterobacter asburiae strain JDR-1, isolated from colonized hardwood (sweetgum), efficiently ferments both methylglucuronoxylose and xylose, producing predominantly ethanol and acetate. (13)C-nuclear magnetic resonance studies defined the Embden-Meyerhof pathway for metabolism of glucose and the pentose phosphate pathway for xylose metabolism. Rates of substrate utilization, product formation, and molar growth yields indicated methylglucuronoxylose is transported into the cell and hydrolyzed to release methanol, xylose, and hexauronate. Enterobacter asburiae strain JDR-1 is the first microorganism described that ferments methylglucuronoxylose generated along with xylose during the acid-mediated saccharification of hemicellulose. Genetic definition of the methylglucuronoxylose utilization pathway may allow metabolic engineering of established gram-negative bacterial biocatalysts for complete bioconversion of acid hydrolysates of methylglucuronoxylan. Alternatively, Enterobacter asburiae strain JDR-1 may be engineered for the efficient conversion of acid hydrolysates of hemicellulose to biofuels and chemical feedstocks.

Metabolic channeling of glucose towards gluconate in phosphate-solubilizing Pseudomonas aeruginosa P4 under phosphorus deficiency

Most phosphate-solubilizing bacteria (PSB), including the Pseudomonas species, release P from sparingly soluble mineral phosphates by producing high levels of gluconic acid from extracellular glucose, in a reaction catalyzed by periplasmic glucose dehydrogenase, which is an integral component of glucose catabolism of pseudomonads. To investigate the differences in the glucose metabolism of gluconic acid-producing PSB pseudomonads and low gluconic acid-producing/non-PSB strains, several parameters pertaining to growth and glucose utilization under P-sufficient and P-deficient conditions were monitored for the PSB isolate Pseudomonas aeruginosa P4 (producing approximately 46 mM gluconic acid releasing 437 microM P) and non-PSB P. fluorescens 13525. Our results show interesting differences in the channeling of glucose towards gluconate and other catabolic end-products like pyruvate and acetate with respect to P status for both strains. However, PSB strain P. aeruginosa P4, apart from exhibiting better growth under both low and high Pi conditions, differed from P. fluorescens 13525 in its ability to accumulate gluconate under P-solubilizing conditions. These alterations in growth, glucose utilization and acid secretion are correlated with glucose dehydrogenase, glucose-6-phosphate dehydrogenase and pyruvate carboxylase activities. The ability to shift glucose towards a direct oxidative pathway under P deficiency is speculated to underlie the differential gluconic acid-mediated P-solubilizing ability observed amongst pseudomonads.

Glucose dehydrogenase of a rhizobacterial strain of Enterobacter asburiae involved in mineral phosphate solubilization shares properties and sequence homology with other members of enterobacteriaceae

Glucose dehydrogenase (GDH) of Gram-negative bacteria is a membrane bound enzyme catalyzing the oxidation of glucose to gluconic acid and is involved in the solubilization of insoluble mineral phosphate complexes. A 2.4 kb glucose dehydrogenase gene (gcd) of Enterobacter asburiae sharing extensive homology to the gcd of other enterobacteriaceae members was cloned in a PCR-based directional genome walking approach and the expression confirmed in Escherichia coli YU423 on both MacConkey glucose agar and hydroxyapatite (HAP) containing media. Mineral phosphate solubilization by the cloned E. asburiae gcd was confirmed by the release of significant amount of phosphate in HAP containing liquid medium. gcd was over expressed in E. coli AT15 (gcd::cm) and the purified recombinant protein had a high affinity to glucose, and oxidized galactose and maltose with lower affinities.The enzyme was highly sensitive to heat and EDTA, and belonged to Type I, similar to GDH of E. coli.

Effect of Carbon Substrates on Rock Phosphate Solubilization by Bacteria from Composts and Macrofauna

Five of the 207 isolates from different composts, farm waste compost (FWC), rice straw compost (RSC), Gliricidia vermicompost (GVC), and macrofauna, showed rock phosphate (RP) solubilization in buffered medium in plate culture. When tested in RP broth medium, all five strains, Enterobacter cloacae EB 27, Serratia marcescens EB 67, Serratia sp. EB 75, Pseudomonas sp. CDB 35, and Pseudomonas sp. BWB 21, showed gluconic acid production and solubilized RP. Based on cellulose-degrading and P-solubilizing ability, two strains were selected for further studies. In the presence of different carbon sources, both strains showed a drop in pH and solubilized RP. P released was maximum with glucose (1212 and 522 micromol) and minimum with cellobiose (455 and 306 micromol) by S. marcescens EB 67 and Pseudomonas sp. CDB 35, respectively. Glucose dehydrogenase (GDH) activity was 63 and 77% with galactose and 35 and 46% with cellobiose when compared to glucose (100%) by EB 67 and CDB 35, respectively. Both strains solubilized RP in the presence of different crop residues. EB 67 and CDB 35 showed maximum cellulase activity (0.027 units) in the presence of rice straw and a mixture of rice straw and root. P solubilized from RP in the presence of pigeonpea root was 134 and 140 micromol with EB 67 and CDB 35. Significantly, these bacteria isolated from composts and macrofauna solubilized rock phosphate in the presence of various pure carbon substrates and crop residues and their importance in soil/rhizosphere conditions is discussed.