Curdlan production byAgrobacterium sp. ATCC 31749 on an ethanol fermentation coproduct

Genetic Engineering of Agrobacterium Increases Curdlan Production through Increased Expression of the crdASC Genes

Curdlan is a water-insoluble polymer that has structure and gelling properties that are useful in a wide variety of applications such as in medicine, cosmetics, packaging and the food and building industries. The capacity to produce curdlan has been detected in certain soil-dwelling bacteria of various phyla, although the role of curdlan in their survival remains unclear. One of the major limitations of the extensive use of curdlan in industry is the high cost of production during fermentation, partly because production involves specific nutritional requirements such as nitrogen limitation. Engineering of the industrially relevant curdlan-producing strain Agrobacterium sp. ATTC31749 is a promising approach that could decrease the cost of production. Here, during investigations on curdlan production, it was found that curdlan was deposited as a capsule. Curiously, only a part of the bacterial population produced a curdlan capsule. This heterogeneous distribution appeared to be due to the activity of Pcrd, the native promoter responsible for the expression of the crdASC biosynthetic gene cluster. To improve curdlan production, Pcrd was replaced by a promoter (PphaP) from another Alphaproteobacterium, Rhodobacter sphaeroides. Compared to Pcrd, PphaP was stronger and only mildly affected by nitrogen levels. Consequently, PphaP dramatically boosted crdASC gene expression and curdlan production. Importantly, the genetic modification overrode the strict nitrogen depletion regulation that presents a hindrance for maximal curdlan production and from nitrogen rich, complex media, demonstrating excellent commercial potential for achieving high yields using cheap substrates under relaxed fermentation conditions.

A review presenting production, characterization, and applications of biopolymer curdlan in food and pharmaceutical sectors

Curdlan is an exopolysaccharide, specifically a homopolysaccharide, with a high molecular weight that is made up entirely of monomeric glucose molecules connected by β-1,3-glycosidic bonds. Curdlan was first isolated in 1962 by Harada and his colleagues from Alcaligenes faecalis var myxogenes 10C3. Microbial synthesis of this curdlan is mainly associated with soil bacteria. Preliminary screening of curdlan-producing microorganisms is done on aniline blue media. The aniline blue positive microorganisms are subjected to submerged fermentation for the production of curdlan. To improve the yield of curdlan produced, various optimization techniques are employed such as Plackett–Burman, response surface methodology, and others. Curdlan can be characterized by its morphology, gel strength, its infrared, and magnetic resonances among many other characteristics. Due to its distinctive physicochemical and rheological properties, it has gained immense popularity in the food, biomedical, and pharmaceutical sectors. However, curdlan’s functionality can be improved by chemically modifying curdlan to obtain grafted curdlan, hydrogels, and nanocomposites which are discussed in detail herewith. Curdlan was authorized to be used in the food industry by the United States Food and Drug Administration in 1996 and also in 1989 in Taiwan, Japan, and Korea. Over the years, many patents using curdlan have also been filed from different parts of the world. This review provides information about its structure, biosynthesis, production strategies, optimization, characterization, applications, and patents.

Graphic abstract

Effects of carbon sources on production and properties of curdlan using Agrobaterium sp. DH-2

Curdlan has wide potential application in the food and biomedical fields due to its unique thermal gel and biological activity. This study investigated the effect of six sugars including glucose, fructose, lactose, maltose, sucrose and xylose as carbon sources on production and properties of curdlan using Agrobacterium sp. DH-2. The maximum production (38.1 g/L and 37.4 g/L, respectively) and yield (0.58 g curdlan/g sucrose and 0.53 g curdlan/g maltose, respectively) of curdlan were achieved by sucrose and maltose, followed by glucose, fructose, lactose and xylose. Scanning electron micrographs showed that the surface of cells was smooth in strain growth phase, while cells were covered by curdlan matrix acted as a net in the curdlan synthesis phase. The highest glucosyltransferase activity (19.9 U/g biomass) corresponded to the maximum curdlan production using the sucrose medium. The molecular weight and gel strength of curdlan were influenced by the carbon sources. The curdlan from xylose medium resulted in a maximum molecular weight of 1.59 × 10⁶ Da and the highest gel strength of 989.2 g/cm², while the curdlan from sucrose medium resulted in a lowest molecular weight of 1.10 × 10⁶ Da and gel strength of 672.8 g/cm². The high molecular weight of curdlan had high gel strength.

Chemistry and microbial sources of curdlan with potential application and safety regulations as prebiotic in food and health

Curdlan - a homopolysaccharide is comprised of glucose using β-1,3-glycosidic bond and produced by different types of microorganisms as exopolysaccharide. Curdlan gel is stable during freezing and thawing processes which find several applications in food and pharmaceutical industries. It acts as a prebiotic, stabilizer and water-holding, viscosifying and texturing agent. Additionally, curdlan gel is used as a food factor to develop the new products e.g. milk fat substitute, non-fat whipped cream, retorting (freeze-drying) process of Tofu, low-fat sausage, and low-fat hamburger. However, a great variation exists among different countries regarding the regulatory aspects of curdlan as food additives, dietary components or prebiotic substances. Therefore, the present review paper aims to discuss safety issues and the establishment of common guidelines and legislation globally, focusing on the use the applications of curdlan in the food sector including the development of noodles, meat-based products, and fat-free dairy products. This review analyzes and describes in detail the potential of curdlan as a sustainable alternative additive in health and food industries, emphasizing on the chemical composition, production, properties, and potential applications.

Production of the Polysaccharide Curdlan by Agrobacterium species on Processing Coproducts and Plant Lignocellulosic Hydrolysates

This review examines the production of the biopolymer curdlan, synthesized by Agrobacterium species (sp.), on processing coproducts and plant lignocellulosic hydrolysates. Curdlan is a β-(1→3)-D-glucan that has various food, non-food and biomedical applications. A number of carbon sources support bacterial curdlan production upon depletion of nitrogen in the culture medium. The influence of culture medium pH is critical to the synthesis of curdlan. The biosynthesis of the β-(1→3)-D-glucan is likely controlled by a regulatory protein that controls the genes involved in the bacterial production of curdlan. Curdlan overproducer mutant strains have been isolated from Agrobacterium sp. ATCC 31749 and ATCC 31750 by chemical mutagenesis and different selection procedures. Several processing coproducts of crops have been utilized to support the production of curdlan. Of the processing coproducts investigated, cassava starch waste hydrolysate as a carbon source or wheat bran as a nitrogen source supported the highest curdlan production by ATCC 31749 grown at 30 °C. To a lesser extent, plant biomass hydrolysates have been explored as possible substrates for curdlan production by ATCC 31749. Prairie cordgrass hydrolysates have been shown to support curdlan production by ATCC 31749 although a curdlan overproducer mutant strain, derived from ATCC 31749, was shown to support nearly double the level of ATCC 31749 curdlan production under the same growth conditions.

Xanthan gum production from acid hydrolyzed broomcorn stem as a sole carbon source by Xanthomonas campestris

Xanthan gum is an exo-polysaccharide industrially produced by fermentation using simple sugars. In this study, broomcorn stem was introduced as a low-cost- and widely available carbon source for xanthan gum fermentation. Broomcorn stem was hydrolyzed using sulphuric acid to liberate reducing sugar which was then used as a carbon source for biosynthesis of xanthan gum by Xanthomonas campesteris. Effects of hydrolysis time (15, 30, 45 and 60 min), sulphuric acid concentration (2, 4, 6 and 8% v/v) and solid loading (3, 4, 5 and 6% w/v) on the yield of reducing sugar and consequent xanthan production were investigated. Maximum reducing sugar yield (55.2%) and xanthan concentration (8.9 g/L) were obtained from hydrolysis of 4% (w/v) broomcorn stem with 6% (v/v) sulphuric acid for 45 min. The fermentation product was identified and confirmed as xanthan gum using Fourier transform infrared spectroscopy analysis. Thermogrvimetric analysis showed that thermal stability of synthesized xanthan gum was similar to those reported in previous studies. The molecular weight of the produced xanthan (2.23 × 10⁶ g/mol) was determined from the intrinsic viscosity. The pyruvate and acetyl contents in xanthan gum were 4.21 and 5.04%, respectively. The chemical composition results indicated that this biopolymer contained glucose, mannose and glucoronic acid with molecular ratio of 1.8:1.5:1.0. The kinetics of batch fermentation was also investigated. The kinetic parameters of the model were determined by fermentation results and evaluated. The results of this study are noteworthy for the sustainable xanthan gum production from low-value agricultural waste.

Improved curdlan production with discarded bottom parts of Asparagus spear

BACKGROUND

This work evaluated the improvement of curdlan production of Agrobacterium sp. ATCC 31749 by using culture medium containing juice of discarded bottom part of green Asparagus spear (MJDA). Curdlan production was carried out using Agrobacterium sp. ATCC 31749 in flasks with different volumes of MJDA and its non-juice-adding control (CK) incubated in shaker at 30 °C, 200 rpm rotation for 168 h.

RESULTS

All MJDA media increased Agrobacterium sp. ATCC 31749 cell mass and enhanced the cells' ability to utilise sucrose, the carbon source for curdlan biosynthesis, and thereby produced higher concentration of curdlan than CK which is used for commercial production of curdlan. After 168 h of fermentation, 10% MJDA produced 40.2 g/l of curdlan whiles CK produced 21.1 g/l. Curdlan production was increased by 90.4% higher in 10% MJDA than CK. Curdlan produced by 10% MJDA contains 1.2 and 1.5 µg/ml of Asparagus flavonoids and saponins respectively as additives which have wide range of health benefits. The mass of sucrose needed to produce 1.0 g curdlan by Agrobacterium sp. ATCC 31749 in CK is 1.7-fold more than in 10% MJDA.

CONCLUSION

The results strongly revealed that 5-10% MJDA is a good curdlan fermentation media which increase curdlan production yield with cheaper cost of production and simultaneously reduce environmental waste resulting from the large scaled discarded bottom parts of green Asparagus spear during Asparagus production.

Metabolic engineering of Agrobacterium sp. ATCC31749 for curdlan production from cellobiose

Curdlan is a commercial polysaccharide made by fermentation of Agrobacterium sp. Its anticipated expansion to larger volume markets demands improvement in its production efficiency. Metabolic engineering for strain improvement has so far been limited due to the lack of genetic tools. This research aimed to identify strong promoters and to engineer a strain that converts cellobiose efficiently to curdlan. Three strong promoters were identified and were used to install an energy-efficient cellobiose phosphorolysis mechanism in a curdlan-producing strain. The engineered strains were shown with enhanced ability to utilize cellobiose, resulting in a 2.5-fold increase in titer. The availability of metabolically engineered strain capable of producing β-glucan from cellobiose paves the way for its production from cellulose. The identified native promoters from Agrobacterium open up opportunities for further metabolic engineering for improved production of curdlan and other products. The success shown here marks the first such metabolic engineering effort in this microbe.

Production of the polysaccharide curdlan by anAgrobacteriumstrain grown on a plant biomass hydrolysate

Production of the commercially available polysaccharide curdlan by Agrobacterium sp. strain ECP-1, isolated as a mutant strain from ATCC 31749, on a medium containing a hydrolysate of the plant prairie cordgrass with selected ammonium phosphate concentrations was investigated for a period of 144 h. Although several ammonium phosphate concentrations supported curdlan production by the strain, the optimal concentration after 120 or 144 h was 3.3 mmol·L–1. Only ammonium phosphate concentrations of 1.1 or 8.7 mmol·L–1failed to support curdlan production by the strain after 120 or 144 h. Biomass production by strain ECP-1 on the hydrolysate-containing medium after 120 or 144 h was comparable, independent of the ammonium phosphate concentration present. The curdlan yield from the cordgrass hydrolysate indicated that the grass was an effective plant biomass substrate for polysaccharide production.