Ergothioneine-producing engineered bacteria and construction method and use thereof

Abstract

Ergothioneine-producing engineered bacteria and a construction method and use thereof are provided. The construction method includes: integrating a constitutive promoter PrrnD and an ergothioneine-synthesizing gene cluster egtABCDE derived from Mycolicibacterium neoaurum into a genome of Escherichia coli as a starting strain; and further mutating glutamate at a position 271 in an ATP phosphoribosyltransferase (HisG) gene of a strain produced above into lysine to produce the ergothioneine-producing engineered bacteria. With glucose as a main raw material, a fermentation culture can be conducted with the engineered bacteria to produce ergothioneine. The fermentation with the engineered bacteria to synthesize ergothioneine has advantages such as simple process and high production efficiency, can avoid the use of organic solvents and antibiotics in large quantities, and is suitable for industrial production.

Claims

1. A construction method of ergothioneine-producing engineered bacteria, comprising: 1) integrating a constitutive promoter PrrnD and an ergothioneine-synthesizing gene cluster MnegtABCDE derived from Mycolicibacterium neoaurum into a genome of Escherichia coli BL21 (DE3) as a starting strain, wherein the step 1) is as follows: subjecting the ergothioneine-synthesizing gene cluster MnegtABCDE derived from the Mycolicibacterium neoaurum to codon optimization for the Escherichia coli BL21 (DE3) to produce an optimized sequence, adding the constitutive promoter PrrnD preceding the optimized sequence to produce a MnegtABCDE gene cluster fragment carrying the constitutive promoter PrrnD, and integrating the MnegtABCDE gene cluster fragment to a motA locus in a genome of Escherichia coli BL21 (DE3), wherein a sgRNA primer for the motA locus is set forth in SEQ ID NO: 9; the optimized sequence produced after the codon optimization for the Escherichia coli is set forth in SEQ ID NO: 1, and the sequence of the constitutive promoter PrrnD is set forth in SEQ ID NO: 2; and 2) mutating glutamate at a position 271 in an ATP phosphoribosyltransferase HisG gene of a strain produced in the step 1) into lysine to produce the ergothioneine-producing engineered bacteria, wherein the step 2) is as follows: 2-1) with the genome of the Escherichia coli BL21 (DE3) as a template, conducting polymerase chain reaction (PCR) amplification to produce a hisG gene, wherein primers adopted for the PCR amplification are a primer hisG-F shown in SEQ ID NO: 12 and a primer hisG-R shown in SEQ ID NO: 13; 2-2) ligating the hisG gene to a linearized vector pET-24a(+) through a recombination reaction to produce a recombinant vector pET-24a(+)-hisG; and conducting whole-plasmid PCR with a primer hisGE271K-F shown in SEQ ID NO: 16 and a primer hisGE271K-R shown in SEQ ID NO: 17 to produce a recombinant vector pET-24a(+)-hisG.sup.E271K; 2-3) with the recombinant vector pET-24a(+)-hisG.sup.E271K as a template, conducting PCR amplification to produce a hisG.sup.E271K gene fragment; 2-4) with a pTarget plasmid as a template, constructing a pTargetF-sghisG plasmid targeting the hisG gene; 2-5) electrotransforming a pCas plasmid carrying a transporter gene into the strain obtained in the step 1) to produce an engineered bacteria EGT01/pCas, then further preparing to produce an engineered bacteria EGT01/pCas electrocompetent cell, and electrotransforming the hisG.sup.E271K gene fragment and the pTargetF-sghisG plasmid simultaneously into the engineered bacteria EGT01/pCas electrocompetent cell to produce Escherichia coli EGT01-hisG.sup.E271K/pCas/pTargetF-sghisG; and 2-6) subjecting the Escherichia coli EGT01-hisG.sup.E271K/pCas/pTargetF-sghisG to a screening culture to produce Escherichia coli EGT01-hisG.sup.E271K with the pTargetF-sghisG plasmid and the pCas plasmid removed, wherein the Escherichia coli EGT01-hisG.sup.E271K is the ergothioneine-producing engineered bacteria.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a synthesis route of ergothioneine by engineered bacteria;

(2) FIG. 2 is a schematic diagram of modification of HisG to release the feedback inhibition on L-histidine; and

(3) FIG. 3 shows a finished product of ergothioneine in an embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(4) A general idea of the construction of the ergothioneine-producing engineered bacteria of the present disclosure is as follows: A constitutive promoter PrrnD and an ergothioneine-synthesizing gene cluster MnegtABCDE (undergoing codon optimization for Escherichia coli) derived from Mycolicibacterium neoaurum are integrated to a motA locus of a genome of Escherichia coli BL21 (DE3) as a starting strain to produce a recombinant strain BL21(DE3)motA::MnegtABCDE, which is denoted as an engineered strain EGT01. The above process is intended to construct an ergothioneine synthesis route (as shown in FIG. 1). Further, glutamate at a position 271 in ATP phosphoribosyltransferase HisG in the EGT01 as a host cell is mutated into lysine through a target gene site-directed mutation for releasing the feedback inhibition on L-histidine (as shown in FIG. 2) to produce a recombinant strain EGT01 his (2-IK which is denoted an engineered strain EGT02.

(5) The present disclosure is described in further detail below with reference to specific implementations. The embodiments are merely demonstrations of the contents of the present disclosure and do not limit the scope of the present disclosure.

(6) The embodiments involve various general reaction systems and conditions. The reaction systems and conditions are described as follows:

(7) PCR system (DNA fragment, whole-plasmid amplification): 5PrimeSTAR buffer: 10 L, dNTP: 4 L, upstream and downstream primers: each 1 L, a DNA template (genomic or plasmid DNA carrying a target gene): 0.5 L, high-fidelity hot-start PCR DNA polymerase (HS PrimeStar DNA polymerase): 0.5 L, and ddH.sub.2O: making up to the total volume of 50 L.

(8) PCR conditions (DNA fragment, whole-plasmid amplification): pre-denaturation at 95 C. for 5 min; denaturation at 95 C. for 30 s; annealing at 55 C. for 30 s; extension at 72 C. (an extension time is calculated according to 1 kb/min), 32 cycles; extension at 72 C. for 10 min; and storage at 4 C.

(9) PCR system (colony PCR verification): 2Rapid Taq Master Mix: 10 L, upstream and downstream primers: each 0.4 L, ddH.sub.2O: 9.2 L, and DNA template: appropriate amount, with a total volume of 20 L.

(10) PCR conditions (colony PCR verification): pre-denaturation at 95 C. for 10 min; denaturation at 95 C. for 15 s; annealing at 58 C. for 15 s; extension at 72 C. (an extension time is calculated according to 4 kb/min), 28 cycles; extension at 72 C. for 2 min; and storage at 4 C.

(11) Recombination reaction system: 2GenRec Assembly Master Mix: 10 L, linearized vector: 0.03 mol, target gene fragment: 0.09 mol, and ddH.sub.2O: making up to the total volume of 20 L.

(12) Recombination reaction conditions: A prepared reaction solution is placed in a 50 C. water bath to allow a reaction for 50 min.

Example 1 Construction of Engineered Bacteria EGT01

(13) An ergothioneine-synthesizing gene cluster MnegtABCDE derived from Mycolicibacterium neoaurum (GenBank accession No: NZ_CP074376.1, location=1102768 . . . 1104015, 1104012 . . . 1105292, 1105285 . . . 1105971, 1105968 . . . 1106906, and 1106903 . . . 1107985) was subjected to codon optimization for Escherichia coli to produce an optimized sequence shown in SEQ ID NO: 1. A constitutive promoter PrrnD (a sequence of the PrrnD was shown in SEQ ID NO: 2) was added preceding the optimized sequence to produce a gene fragment, and the gene fragment was chemically synthesized. With the synthesized gene as a template, the primers egt-F and egt-R in Table 1 were used to conduct PCR amplification to produce a MnegtABCDE gene cluster fragment carrying the constitutive promoter PrrnD.

(14) TABLE-US-00001 TABLE1 Primername SEQIDNO: Sequence(5-3) egt-F SEQIDNO:3 CGAAATCGCGGCAGACAGAAAAA AAGATCAAAAAAATACTTGTGC egt-R SEQIDNO:4 CAGCGCCAGACTGTTTTAGCTAG ACACAGCTACC

(15) Primers motA-U-F/motA-U-R and motA-D-F/motA-D-R were designed according to a sequence of the motA gene (GenBank accession No: NZ_CP053602.1, location=1923468 . . . 1924355). With a genome of Escherichia coli BL21 (DE3) as a template, PCR amplification was conducted with the designed primers to produce gene fragments motA-U and motA-D, respectively. With the gene fragments motA-U, motA-D, and MnegtABCDE as templates and the motA-U-F and motA-D-R as primers, fusion PCR amplification was conducted to produce a MnegtABCDE gene cluster expression cassette. Sequences of the primers adopted in the above process were shown in Table 2.

(16) TABLE-US-00002 TABLE2 Primername SEQIDNO: Sequence(5-3) motA-U-F SEQIDNO:5 GTGCTTATCTTATTAGGTTACC motA-U-R SEQIDNO:6 TTTTTGATCTTTTTTTCTGTCT GCCGCGATTTCG motA-D-F SEQIDNO:7 GGTAGCTGTGTCTAGCTAAAAC AGTCTGGCGCTG motA-D-R SEQIDNO:8 TCATGCTTCCTCGGTTG

(17) Whole-plasmid PCR amplification was conducted with a pTarget plasmid as a template and sgmotA-F (sgRNA primer for the motA locus) and pTarget-R as primers. A resulting PCR product was enzymatically digested, purified, ligated, and chemically transformed into Escherichia coli JM109 competent cells. Transformed competent cells were coated on an LB solid medium plate including 30 mg/L of spectinomycin, and cultured at 37 C. to produce single colonies. Colony PCR verification was conducted with the primers testsg-motA-F and pTarget-R to select a correct strain, namely, a strain carrying a pTargetF-sgmotA plasmid targeting the motA gene. Sequences of the primers adopted in the above process were shown in Table 3.

(18) TABLE-US-00003 TABLE3 Primername SEQIDNO: Sequence(5-3) sgmotA-F SEQIDNO:9 CAAGCATGACGCTATCCGCG GTTTTAGAGCTAGAAATAGC pTarget-R SEQIDNO:10 ACTAGTATTATAGCTAGCAC TGAGC testsg-motA-F SEQIDNO:11 CAAGCATGACGCTATCCGCG

(19) Escherichia coli BL21 (DE3) was activated on an LB solid medium plate, then inoculated into an LB liquid medium, and cultured overnight to produce a seed culture. The seed culture was transferred to 25 mL of an LB liquid medium at 1%, cultured at 30 C. and 200 rpm until OD.sub.600 was 0.5, and then incubated in an ice bath for 30 min. A resulting bacterial solution was centrifuged at 4 C. and 4,000 rpm for 10 min to collect bacteria. The bacteria were washed three times with a pre-cooled 10% glycerol solution, then resuspended with 300 L of a 10% glycerol solution, and dispensed in sterile 1.5 mL EP tubes with 80 L/tube to obtain Escherichia coli BL21 (DE3) electrocompetent cells.

(20) A plasmid pCas carrying a transporter gene was electrotransformed into the Escherichia coli BL21 (DE3) electrocompetent cells. Transformed competent cells were coated on an LB solid medium plate including 30 mg/L of kanamycin and cultured at 37 C. to produce single colonies BL21 (DE3)/pCas. Electrocompetent cells were prepared with the Escherichia coli BL21 (DE3)/pCas. The MnegtABCDE gene cluster expression cassette and the pTargetF-sgmotA plasmid were simultaneously electrotransformed into the electrocompetent cells. Transformed competent cells were coated on an LB solid medium plate including 30 mg/L of kanamycin and spectinomycin, and cultured at 30 C. to produce single colonies. Colony PCR verification and sequencing verification were conducted with the primers motA-U-F and motA-D-R in Table 2. A correct strain was Escherichia coli BL21 (DE3) motA::MnegtABCDE/pCas/pTargetF-sgmotA.

(21) The Escherichia coli BL21 (DE3) motA::MnegtABCDE/pCas/pTargetF-sgmotA was inoculated into a kanamycin-containing LB liquid medium, IPTG was added at a final concentration of 0.5 mM, and the Escherichia coli was cultured at 30 C. and 200 rpm under shaking for 2 h. A resulting bacterial solution was coated on an LB solid medium plate including 30 mg/L of kanamycin, and cultured at 30 C. Single colonies without spectinomycin resistance were selected to obtain Escherichia coli BL21 (DE3) motA: MnegtABCDE/pCas with the pTargetF-sgmotA plasmid removed.

(22) The Escherichia coli BL21 (DE3) motA::MnegtABCDE/pCas was inoculated into an antibiotic-free LB liquid medium and cultured at 42 C. and 200 rpm under shaking for 2 h. A resulting bacterial solution was coated on an antibiotic-free LB solid medium plate and cultured at 42 C. Single colonies without kanamycin resistance were selected to obtain Escherichia coli BL21 (DE3) motA::MnegtABCDE with the pCas plasmid removed, which were the engineered bacteria EGT01.

Example 2 Construction of Engineered Bacteria EGT02

(23) Primers hisG-F/hisG-R were designed according to a sequence of the hisG gene (GenBank accession No: NZ_CP053602.1, location=1987293 . . . 1988192). With a genome of Escherichia coli BL21 (DE3) as a template, PCR amplification was conducted with the designed primers to produce a gene fragment hisG. PCR amplification was conducted with a vector pET-24a(+) as a template and pET-F and pET-R as primers to produce a linearized vector pET-24a(+). Sequences of the primers adopted in the above process were shown in Table 4.

(24) TABLE-US-00004 TABLE4 Primername SEQIDNO: Sequence(5-3) hisG-F SEQIDNO:12 CTTTAAGAAGGAGATATACATA TGACAGACAACTCTCGTTTAC hisG-R SEQIDNO:13 CTCAGCTTCCTTTCGGGCTTTG TCACTCCATCATCTTCTCAATC pET-F SEQIDNO:14 CAAAGCCCGAAAGGAAGCTGAG pET-R SEQIDNO:15 ATGTATATCTCCTTCTTAAAG

(25) A hisG gene was ligated to the linearized vector pET-24a(+) through a recombination reaction, and sequencing verification was conducted to produce a recombinant vector pET-24a(+)-hisG. Whole-plasmid PCR was conducted with the primers hisGE271K-F and hisGE271K-R in Table 5 to produce a recombinant vector pET-24a(+)-hisG.sup.E271K. The template plasmid was digested and then transformed into Escherichia coli, and a resulting plasmid was extracted and sequenced for verification. With the recombinant vector pET-24a(+)-hisG.sup.E271K as a template, PCR amplification was conducted using the primers hisG-F/hisG-R in Table 4 to produce a gene fragment hisG.sup.E271K.

(26) TABLE-US-00005 TABLE5 Primername SEQIDNO: Sequence(5-3) hisGE271K-F SEQIDNO:16 GGTCAGTAGCAAAACCCTGTTCTG hisGE271K-R SEQIDNO:17 GGTTTTGCTACTGACCATGTGC

(27) Whole-plasmid PCR amplification was conducted with a pTarget plasmid as a template and sghisG-F (sgRNA primer for the hisG locus) and the pTarget-R in Table 3 as primers. A resulting PCR product was enzymatically digested, purified, ligated, and chemically transformed into Escherichia coli JM109 competent cells. Transformed competent cells were coated on an LB solid medium plate including 30 mg/L of spectinomycin, and cultured at 37 C. to produce single colonies. Colony PCR verification was conducted with the primers testsg-hisG-F and the pTarget-R in Table 3 to select a correct strain, namely, a strain carrying a pTargetF-sghisG plasmid targeting the hisG gene. Sequences of the primers adopted in the above process were shown in Table 6.

(28) TABLE-US-00006 TABLE6 Primername SEQIDNO: Sequence(5-3) sghisG-F SEQIDNO:18 TGAGTCATCACTTAAACGGC GTTTTAGAGCTAGAAATAGC testsg-hisG-F SEQIDNO:19 TGAGTCATCACTTAAACGGC

(29) Engineered bacteria EGT01/pCas electrocompetent cells were prepared according to the method in Example 1. The hisG.sup.E271K gene fragment and the pTargetF-sghisG plasmid were electrotransformed simultaneously into the competent cells.

(30) Transformed competent cells were coated on an LB solid medium plate including 30 mg/L of kanamycin and spectinomycin, and cultured at 30 C. to produce single colonies. Colony PCR verification and sequencing verification were conducted with the primers hisG-F and hisG-R. A correct strain was Escherichia coli EGT01-hisG.sup.E271K/pCas/pTargetF-sghisG.

(31) The Escherichia coli EGT01-hisG.sup.E271K/pCas/pTargetF-sghisG was inoculated into a kanamycin-containing LB liquid medium, IPTG was added at a final concentration of 0.5 mM, and the Escherichia coli was cultured at 30 C. and 200 rpm under shaking for 2 h. A resulting bacterial solution was coated on an LB solid medium plate including 30 mg/L of kanamycin, and cultured at 30 C. Single colonies without spectinomycin resistance were selected to obtain Escherichia coli EGT01-hisG.sup.E271K/pCas with the pTargetF-sghisG plasmid removed.

(32) The Escherichia coli EGT01-hisG.sup.E271K/pCas was inoculated into an antibiotic-free LB liquid medium and cultured at 42 C. and 200 rpm under shaking for 2 h. A resulting bacterial solution was coated on an antibiotic-free LB solid medium plate and cultured at 42 C. Single colonies without kanamycin resistance were selected to obtain Escherichia coli EGT01-hisG.sup.E271K with the pCas plasmid removed, which were the engineered bacteria EGT02.

(33) Examples 3 to 5 and Comparative Examples 1 to 3 were fermentation tests of related strains. Medium formulas involved in these examples were as follows:

(34) LB liquid medium: yeast extract: 5 g/L, tryptone: 10 g/L, and sodium chloride: 10 g/L.

(35) LB solid medium: yeast extract: 5 g/L, tryptone: 10 g/L, sodium chloride: 10 g/L, and agar: 15 g/L.

(36) Seed medium for ergothioneine: glucose: 20 g/L to 25 g/L, yeast extract: 4 g/L to 6 g/L, peptone: 2 g/L to 4 g/L, dipotassium phosphate: 6 g/L to 8 g/L, magnesium sulfate heptahydrate: 1 g/L to 2 g/L, ferrous sulfate heptahydrate: 40 mg/L to 60 mg/L, manganese sulfate monohydrate: 10 mg/L to 20 mg/L, and vitamin B1: 2 mg/L to 5 mg/L.

(37) Fermentation medium for ergothioneine: glucose: 20 g/L to 25 g/L, yeast extract: 1 g/L to 5 g/L, ammonium sulfate: 5 g/L to 7 g/L, dipotassium phosphate: 8 g/L to 10 g/L, citric acid monohydrate: 2 g/L to 4 g/L, magnesium sulfate heptahydrate: 1 g/L to 2 g/L, ferrous sulfate heptahydrate: 150 mg/L to 200 mg/L, manganese sulfate monohydrate: 10 mg/L to 20 mg/L, vitamin B1: 2 mg/L to 5 mg/L, methionine: 2 g/L to 4 g/L, and trace elements: 1 mL/L to 2 mL/L. The trace elements were as follows: sodium molybdate dihydrate: 2.5 g/L, nickel sulfate hexahydrate: 2.5 g/L, calcium chloride dihydrate: 2 g/L, copper sulfate pentahydrate: 0.75 g/L, aluminum sulfate octahydrate: 2.25 g/L, cobalt chloride hexahydrate: 2.5 g/L, zinc chloride: 0.5 g/L, and boric acid: 3 g/L.

(38) First supplemented medium for ergothioneine: glucose: 600 g/L to 800 g/L and yeast extract: 8 g/L to 10 g/L.

(39) Second supplemented medium for ergothioneine: methionine: 10 g/L to 15 g/L and cysteine: 6 g/L to 8 g/L.

(40) The materials used for the preparation of media all were commercially-available analytically-pure reagents.

(41) The high-performance liquid chromatography (HPLC) analysis for ergothioneine in each example and comparative example of the present disclosure was conducted as follows:

(42) (1) Instrument: Shimadzu LC-2050 high-performance liquid chromatograph.

(43) (2) Chromatographic column: Thermo Fisher Scientific Hypersil ODS-2, 2504.6 mm, and 5 m.

(44) (3) Method parameters: a mobile phase: 5% (v/v) methanol aqueous solution; a flow rate: 1 mL/min; a wavelength of an ultraviolet detector: 257 nm; a column temperature: 35 C.; an injection volume: 5 L; and a detection time: 15 min.

(45) (4) Standard curve establishment method: 0.0500 g of an ergothioneine standard sample (commercially-available 99% ergothioneine) was accurately weighed, dissolved with ultrapure water, and diluted to 50 mL to produce a 1 g/L standard sample stock solution. 1 mL, 2 mL, 3 mL, 4 mL, and 5 mL of the stock solution each were taken and diluted with ultrapure water to 10 mL to produce standard sample solutions with concentrations of 0.1 g/L, 0.2 g/L, 0.3 g/L, 0.4 g/L, and 0.5 g/L, respectively. The standard sample solutions each were filtered with a 0.22 m water phase needle filter membrane and tested by HPLC. A standard curve was plotted with a concentration as an x-coordinate and a peak area as a y-coordinate, and an equation of the standard curve was calculated.

(46) (5) Detection of ergothioneine contents in finished products: 0.01 g (accurate to 0.00001 g) of an ergothioneine reference was accurately weighed, placed in a 50 mL volumetric flask, dissolved with ultrapure water, diluted to a specified scale, thoroughly shaken, and filtered with a 0.22 m filter membrane to produce a reference solution. 0.01 g (accurate to 0.00001 g) of an ergothioneine sample was accurately weighed, placed in a 50 mL volumetric flask, dissolved with ultrapure water, diluted to a specified scale, thoroughly shaken, and filtered with a 0.22 m filter membrane to produce a sample solution. Two sample solutions were prepared in parallel. The reference solution and the sample solutions each were injected according to the chromatographic conditions, and chromatograms were recorded. A content of ergothioneine in a sample was calculated by an external standard single-point method:
content of ergothioneine in a sample=(AsmrPr)/(Arms)100% where As represents a peak area of ergothioneine in a sample solution, Ar represents a peak area of ergothioneine in the reference solution, ms represents a weight of an ergothioneine sample (g), mr represents a weight of the ergothioneine reference (g), and Pr represents a content of the ergothioneine reference. An arithmetic mean value of results of two parallel measurements was taken as a reported value. An absolute difference between the results of two parallel measurements was not greater than 2% of the arithmetic mean value.

Example 3 Fermentation of the Engineered Bacteria EGT02 in a Shake Flask

(47) The engineered bacteria EGT02 were streaked on an LB solid medium plate for activation and cultured at 37 C. for 12 h, and inoculated into 10 mL of a seed medium and cultured at 37 C. and 220 rpm under shaking for 12 h to produce a seed culture. The seed culture was inoculated into 50 mL of a fermentation medium at an inoculum size of 5%, and cultured at 37 C. and 220 rpm under shaking for 48 h. A titer of ergothioneine in a fermentation broth was determined by HPLC to be 0.74 g/L.

Comparative Example 1 Fermentation of the Engineered Bacteria EGT01 in a Shake Flask

(48) A shake-flask fermentation experiment was conducted with the engineered bacteria EGT01 according to the conditions in Example 3. A titer of ergothioneine in a fermentation broth produced after 48 h of fermentation was determined by HPLC to be 0.19 g/L.

Example 4 Fermentation of the Engineered Bacteria EGT02 in a 5 L Tank

(49) The engineered bacteria EGT02 were streaked on an LB solid medium plate for activation and cultured at 37 C. for 12 h, and inoculated into 10 mL of a seed medium and cultured at 37 C. and 220 rpm under shaking for 12 h to produce a primary seed culture. The primary seed culture was inoculated into 100 mL of a seed medium at an inoculum size of 2%, and cultured at 37 C. and 220 rpm under shaking for 5 h to produce a secondary seed culture. The secondary seed culture was inoculated into 2 L of a fermentation medium at an inoculum size of 8%, and cultured. During the culture, a temperature was 37 C., an initial rotational speed was 250 rpm, a ventilation rate was 2 L/min (1 vvm), and a pH was controlled at 7.00.05 with 25% to 28% ammonia water. After the inoculation, a dissolved oxygen was controlled at 30% through dissolved oxygen-stirring connected control. A sample was collected every 2 h and tested for OD.sub.600 and glucose. When a glucose concentration in an initial medium was lower than 1 g/L, the feeding of the first supplemented medium was started to maintain a glucose concentration in a fermentation broth at 1 g/L to 2 g/L. When an OD.sub.600 value reached 10, the feeding of the second supplemented medium was started at a flow rate of 20 mL/h. Fermentation was completed at 48 h. A titer of ergothioneine in a fermentation broth was determined by HPLC to be 6.22 g/L.

Comparative Example 2 Fermentation of the Engineered Bacteria EGT01 in a 5 L Tank

(50) A 5 L tank fermentation experiment was conducted with the engineered bacteria EGT01 according to the conditions in Example 4. A titer of ergothioneine in a fermentation broth produced after 48 h of fermentation was determined by HPLC to be 2.49 g/L.

Example 5 Fermentation of the Engineered Bacteria EGT02 in a 50 L Tank

(51) The engineered bacteria EGT02 were streaked on an LB solid medium plate for activation and cultured at 37 C. for 12 h, and inoculated into 10 mL of a seed medium and cultured at 37 C. and 220 rpm under shaking for 12 h to produce a primary seed culture. The primary seed culture was inoculated into 400 mL of a seed medium at an inoculum size of 2%, and cultured at 37 C. and 220 rpm under shaking for 5 h to produce a secondary seed culture. The secondary seed culture was inoculated into 20 L of a fermentation medium at an inoculum size of 8%, and cultured. During the culture, a temperature was 37 C., an initial rotational speed was 200 rpm, a ventilation rate was 1.2 m.sup.3/h (1 vvm), and a pH was controlled at 7.00.05 with 25% to 28% ammonia water. After the inoculation, a dissolved oxygen was controlled at 30% through dissolved oxygen-stirring connected control. A sample was collected every 2 h and tested for OD.sub.600 and glucose. When a glucose concentration in an initial medium was lower than 1 g/L, the feeding of the first supplemented medium was started to maintain a glucose concentration in a fermentation broth at 1 g/L to 2 g/L. When an OD.sub.600 value reached 10, the feeding of the second supplemented medium was started at a flow rate of 200 mL/h. Fermentation was completed at 48 h. A titer of ergothioneine in a fermentation broth was determined by HPLC to be 6.80 g/L.

Comparative Example 3 Fermentation of the Engineered Bacteria EGT01 in a 50 L Tank

(52) A 50 L tank fermentation experiment was conducted with the engineered bacteria EGT01 according to the conditions in Example 5. A titer of ergothioneine in a fermentation broth produced after 48 h of fermentation was determined by HPLC to be 2.91 g/L.

Example 6 Separation and Purification of Ergothioneine in a Fermentation Broth

(53) The ergothioneine-containing fermentation broth produced in Example 5 was filtered through a 50 nm ceramic membrane with a pressure controlled at 0.45 MPa to 0.55 MPa and a temperature controlled at lower than 40 C., and pure water was added for replacement until a yield was 98.2%. The ceramic membrane filtration could remove cells and insoluble solids in the fermentation broth to produce a clear liquid. The clear liquid produced by the ceramic membrane was ultra-filtered through a 5,000 D ultrafiltration membrane with a pressure controlled at 0.45 MPa to 0.55 MPa and a temperature controlled at lower than 40 C., and pure water was added for replacement until a yield was 98.5%. The ultrafiltration membrane filtration could remove macromolecular proteins and sugars to produce a clear liquid. The clear liquid produced by the ultrafiltration membrane was decolorized with an LX-98 decolorization resin, and pure water was added for replacement until a yield was 99.3%. A decolorized liquid and a replaced liquid each were collected. The decolorized liquid and the replaced liquid each were vacuum-concentrated at a temperature of 65 C. until an ergothioneine content was 202 g/L. Absolute ethanol was added in a volume 5 times a volume of a concentrate to the concentrate, and stirring was conducted at room temperature to allow crystallization for 16 h. A resulting crystallization mixture was subjected to suction filtration to produce a wet crystal. The wet crystal was washed with absolute ethanol 3 times, and then dried at 65 C. for 12 h to produce a finished product of ergothioneine (FIG. 3). A content of ergothioneine in the finished product was determined by HPLC to be 99.55%.

(54) The above embodiments merely represent several implementations of the present disclosure, and the descriptions thereof are specific and detailed, but should not be construed as limiting the patent scope of the present disclosure. It should be noted that those of ordinary skills in the art can further make several variations and modifications without departing from the inventive concept of the present disclosure, and such variations and modifications all fall within the claimed scope of the present disclosure.