- Research article
- Open Access
Production of enterodiol from defatted flaxseeds through biotransformation by human intestinal bacteria
- Cheng-Zhi Wang†1,
- Xiao-Qing Ma†2,
- Dong-Hui Yang2Email author,
- Zhi-Rong Guo1,
- Gui-Rong Liu3, 5,
- Ge-Xin Zhao1,
- Jie Tang1,
- Ya-Nan Zhang1,
- Miao Ma2,
- Shao-Qing Cai2,
- Bao-Shan Ku4 and
- Shu-Lin Liu1, 3, 5Email author
© Wang et al; licensee BioMed Central Ltd. 2010
- Received: 5 August 2009
- Accepted: 16 April 2010
- Published: 16 April 2010
The effects of enterolignans, e.g., enterodiol (END) and particularly its oxidation product, enterolactone (ENL), on prevention of hormone-dependent diseases, such as osteoporosis, cardiovascular diseases, hyperlipemia, breast cancer, colon cancer, prostate cancer and menopausal syndrome, have attracted much attention. To date, the main way to obtain END and ENL is chemical synthesis, which is expensive and inevitably leads to environmental pollution. To explore a more economic and eco-friendly production method, we explored biotransformation of enterolignans from precursors contained in defatted flaxseeds by human intestinal bacteria.
We cultured fecal specimens from healthy young adults in media containing defatted flaxseeds and detected END from the culture supernatant. Following selection through successive subcultures of the fecal microbiota with defatted flaxseeds as the only carbon source, we obtained a bacterial consortium, designated as END-49, which contained the smallest number of bacterial types still capable of metabolizing defatted flaxseeds to produce END. Based on analysis with pulsed field gel electrophoresis, END-49 was found to consist of five genomically distinct bacterial lineages, designated Group I-V, with Group I strains dominating the culture. None of the individual Group I-V strains produced END, demonstrating that the biotransformation of substrates in defatted flaxseeds into END is a joint work by different members of the END-49 bacterial consortium. Interestingly, Group I strains produced secoisolariciresinol, an important intermediate of END production; 16S rRNA analysis of one Group I strain established its close relatedness with Klebsiella. Genomic analysis is under way to identify all members in END-49 involved in the biotransformation and the actual pathway leading to END-production.
Biotransformation is a very economic, efficient and environmentally friendly way of mass-producing enterodiol from defatted flaxseeds.
- Bacterial Consortium
- Fecal Specimen
Early in the 1980s, enterodiol (END) and enterolactone (ENL) were first detected in the serum, urine and bile of humans and several animals [1, 2]. They were classified as phytoestrogens due to their origins from plants and their estrogenic as well as antiestrogenic activities in humans. Epidemiologic and pharmacologic studies have shown that END and particularly its oxidation product ENL have preventive effects on osteoporosis, cardiovascular diseases, hyperlipemia, breast cancer, colon cancer, prostate cancer and menopausal syndrome [3–7]. Unlike other plant-derived lignans, they are also known as mammalian lignan or enterolignan, because they are mainly found in mammals. Numerous studies have indicated that END and ENL can be produced from several plants, such as flaxseed, by bacteria in the intestinal tract of humans and animals. Thompson et al. tested 68 common plant foods and found that flaxseed flour and its defatted meal produced the highest yield of END and ENL in vitro, up to 800 times higher than that from others .
Flaxseed is the dried seed of Linum usitatissimum L., which is widely distributed in northern China, with an annual output of 420,000 tons (ranking fourth in the world). The important precursors of END and ENL synthesis include secoisolariciresinol diglucoside (SDG), secoisolariciresinol (SECO), matairesinol (MAT), lariciresinol (LCS) and pinoresinol (PRS) [9–11]. Among these precursors, SDG is the most abundant lignan in flaxseed, with a content of around 6.1-13.3 mg g-1 (dry matter) in whole flaxseeds, and 11.7-24.1 mg g-1 (dry matter) in the defatted flour .
Although de novo synthesis of END and ENL has been reported , the processes of synthesis are very complex and expensive, requiring more than ten major steps. More importantly, the reagents used in the reactions for the synthesis include LiAlH4, MeOH and several other chemicals, which are toxic and harmful to the environment. Therefore, biotransformation of precursors in plants to END or ENL is highly desirable.
In China, flaxseeds are mainly used as oil crop. The defatted waste, though a rich source of lignans, is mostly used as animal feed. To establish a method for producing enterolignans from defatted flaxseeds by bacterial biotransformation, we screened human fecal samples and obtained cultures that can efficiently produce END. After 49 rounds of selection by successive subcultures of human fecal bacterial microbiota in media containing defatted flaxseeds as the only carbon source, we obtained a group of mixed bacteria that could metabolize flaxseeds to produce END under both anaerobic and aerobic culture conditions. In this paper, we report the method and discuss its potential applications for large scale production of enterolignans.
Determining culture media for bacterial production of END from defatted flaxseeds
To select the human intestinal bacteria that could efficiently metabolize flaxseed lignans to produce END without the need of strictly anaerobic culture conditions, we compared three types of culture media (A, B, C; see components in Methods).
Optimization of culture conditions for large-scale production of END
For large-scale production of END, we increased the volume of medium B from 3 ml to 2 liter with 40 g defatted flaxseeds in 4 liter Erlenmeyer flasks. In one of the Erlenmeyer flasks, 50 ml liquid paraffin was added on top of the culture medium; in another Erlenmeyer flask, no liquid paraffin was added, for comparison of effects of anaerobic vs aerobic culture conditions on END production. The culture was continued at 37°C for 6 days and then terminated for analysis of END production. Interestingly, cultures with or without liquid paraffin added on top of the culture had similar yields of END and the concentration of END reached 86.76 ± 4.19 mg l-1 in both cases, demonstrating that biotransformation of flaxseed lignans into END in our system did not require strict anaerobic conditions.
Enrichment of END
Selection of END-producing bacteria by successive subcultures
Pulsed field gel electrophoresis (PFGE) analysis of END-49
Phylogenetic characterization of Group I strains
The likely health values of enterolignans and, on the other hand, difficulties in its large scale industrial production at low cost and without environmental pollution call for biotransformation technologies to convert plant lignans to them. Numerous bacterial isolates that can conduct the biotransformation have been reported [8, 10, 12, 14–20, 23]. However, most of the reported bacteria require strict anaerobic conditions to grow and metabolize plant lignans to produce enterolignans, which significantly restricts large scale production. Here in this study, we report highly efficient production of END from defatted flaxseeds through biotransformation by human intestinal bacteria without having to culture the bacteria under anaerobic conditions. The method described here has four advantages. First, instead of pure lignans (SDG, SECO, MAT, etc.), defatted flaxseed flour was used as the substrate for END production. As flaxseeds are widely available around the world and the defatted by-products of flaxseeds are usually used as animal feeds or even treated as waste, our study provides a very economic and eco-friendly method of END production using these low cost materials. Second, the high efficiency of END production by our bacterial culture system without the need of strictly anaerobic conditions makes large scale production much easier. Third, no extra carbon source would be needed in the culture, which is especially advantageous, because the most energy-efficient carbon sources, e.g., glucose, normally repress the utilization of other energy sources by microorganisms. Therefore, in the absence of common carbon sources, the biotransformation of flaxseeds into END would be remarkably enhanced. Fourth, this method is entirely harmless to the environment, as the solvents used in this procedure were only water and ethanol, both of which could be recycled.
In this study, a bacterial consortium, END-49, was obtained from human intestinal microbiota through successive subcultures. END-49 was highly efficient in converting flaxseed lignans into END, producing up to 3.9 mg g-1, much higher than previously reported 0.6 mg g-1 (such as in ). END-49 consists of at least five genomically different bacterial lineages as estimated on the basis of PFGE analysis. As none of the single-colony isolated bacterial strains could produce END, we postulate that the biotransformation was conducted jointly by several different bacteria, including some or all the PFGE-resolved Group I-V strains and possibly some bacteria that escaped detection in this study. The Next-Generation sequencing technologies (e.g., 454 and SOLiD) may eventually help identify the END-producers by determining the whole genome sequences of all bacteria in the consortium and facilitate the elucidation of the pathways of END production by these bacteria.
END and ENL have two enantiomeric mirror image forms, which can be inter-converted by intestinal bacteria. In our study, END produced by "END-49" was (+)-form, consistent with the published work  in which SDG from flaxseed was transformed to (+)-ENL via (+)-SECO. Additionally, researchers have confirmed that the absolute configurations at C-2 and C-3 of END and ENL were not changed during the microbial metabolism . Therefore, obviously, in our study, SDG was converted to (+)-END by human intestinal microbiota via (+)-SECO as a metabolic intermediate.
The method described in this study had been optimized and could be used to obtain bacterial consortia that can convert plant lignans into END or related products. Using this method, we screened fecal specimens from 28 young adults and detected END or its dehydrogenized product in all cases (data not shown), consistent with previous reports that bacteria that can convert plant lignans into END or related products are common members of the human intestinal microbiota [28, 29] and they are readily obtainable for use in the bio-production of END.
Biotransformation is a very economic, efficient and environmentally friendly way of mass-producing enterodiol from defatted flaxseeds.
Chemicals and reagents
HPLC-grade acetonitrile was purchased from Merck KGaA Co. Ltd (Darmstadt, Germany), and purified water was provided by Hangzhou Wahaha Co. Ltd (Zhejiang, China). Analytical-grade methanol, n-butanol, petroleum ether, ethanol, KH2PO4 and K2HPO4 were purchased from Beijing Chemical Reagents Co. Ltd (Beijing, China). Enterodiol Standard was purchased from Sigma Chemical Co. (St. Louis, MO., USA). Amberlite XAD-2 macroporous resin (20-60 mesh size, 330 m2 g-1 average surface area) was purchased from Supelco, Sigma-Aldrich Co. Ltd (Bellefonte, USA). Optical rotations were measured in MeOH solutions with a DIP-360 automatic polarimeter (Jasco Co., Tokyo) at 25°C, and CD spectra were determined with a JASCO J 805 spectropolarimeter (Jasco Co.).
Flaxseed samples were collected from Bei-An County of Heilongjiang Province, China, and were identified as the dried seeds of Linum usitatissimum L. by author. Voucher specimens (sample no. 071024) were deposited was deposited in the herbarium of pharmacognosy research group, School of Pharmaceutical Sciences, Peking University Health Science Center. They were ground into powder (pass 40 mesh sieve) and then defatted by petroleum ether prior to use.
Culture media and bacterial culture
Cooked meat medium base and Luria-Bertani (LB) nutrient agar were purchased from Beijing Land Bridge technology Co. Ltd (Beijing, China). Medium A contained tryptone 30 g, yeast extract 5 g, beef powder 5 g, glucose 3 g, NaH2PO4 5 g and amidulin 2 g, and the volume was made up to 1 liter with distilled water. Medium B was designed to lack any carbon source in the medium except defatted flaxseeds (see below), containing the following reagents (in one liter): NaCl 3 g, KH2PO4 2.6 g, K2HPO4 1.85 g, 1% (v/v) reducing solution (30 g/l L-aminothiopropionic acid and 30 g/l sodium hyposulfite, dissolved in PBS), and 1 g NH4Cl. Medium C was the same as medium B except the absence of any nitrogen source.
Culture was conducted as follows: 0.3 g of defatted flaxseeds was added into each of tubes containing either medium A, B or C (3 ml), which were then sealed with liquid paraffin and autoclaved at 121°C for 15 min. Into the medium, 0.3 g of fresh human feces was added and incubated at 37°C for 72 h. Supernatant of the cultures was then inspected for the appearance of END.
Collection and processing of fecal samples
Initially, fresh fecal specimens (ca. 4.0 g each), obtained from 28 healthy young subjects (fourteen females and fourteen males, 22-33 years old), were suspended in 20 ml sterile phosphate buffer saline (PBS, 2.6 g l-1 KH2PO4, 1.85 g l-1 K2HPO4, PH 7.4) and 2 ml such fecal suspension was transferred to 20 ml medium, followed by incubation at 37°C for 36 h. During the fecal collection and culture preparation, no strictly anaerobic techniques or instruments were used. The fecal specimen that we used for END production was from a 33 years old female.
High-performance liquid chromatography (HPLC)
The HPLC system consisted of Agilent 1200 series HPLC apparatus (Agilent Technologies, USA), including high-pressure binary-gradient solvent-delivery pump, DAD detector, autosampler, thermostat column compartment and chemstation (9.01 edition). Zorbax SB-C18 column (4.6 mm × 250 mm, 5 μm) was used to analyze all of the samples. Mobile phase consisted of water (A) and acetonitrile (B) in a linear gradient change from 100% A to 50% A and 50% B in 30 min. Detection wavelength was 280 nm, and the temperature of the column oven was 25°C with a flow rate of 1.0 ml min-1.
Calibration of the END and SECO curves
The stock solutions of END standard (1.98 mg ml-1) and SECO standard (175.5 μg ml-1) were prepared by accurately weighing and transferring each of them into a volumetric flask (1 ml) and dissolving it in methanol. Solutions for END calibration (0.0198 ~ 1.98 mg ml-1) and SECO calibration (175.5 ~ 2.74 μg ml-1) were prepared by dilution of the stock solutions with methanol, with six dilution series being analyzed (1.98, 0.99, 0.396, 0.198, 0.099, 0.0198 mg ml-1) for END calibration and seven dilution series being analyzed (175.5, 87.75, 43.86, 21.94, 10.97, 5.48, 2.74 μg ml-1) for SECO calibration. For each calibration curve, independent dilutions were analyzed. The calibration equation of END was obtained by plotting HPLC peak areas (Y) versus the concentration of calibrators (X, mg ml-1), which was as follows: Y = 4433.46 X + 63.86 (R2 = 0.9999), with a good linearity over the range from 0.0198 mg ml-1 to 1.98 mg ml-1, and the calibration equation of SECO was obtained by plotting HPLC peak areas (Y) versus the concentration of calibrators (X, μg ml-1), which was as follows: Y = 12.59 X - 1.40 (R2 = 0.9998), with a good linearity over the range from 2.74 μg ml-1 to 175.5 μg ml-1.
Limits of detection and quantification
Stock solutions of END and SECO standards were separately diluted to make a series of solutions with methanol and analyzed by HPLC. On the basis of signal-to-noise ratio (S/N), the limits of detection (LOD) and quantification (LOQ) of END standard were determined to be 0.699 μg ml-1 (S/N = 3) and 1.398 μg ml-1 (S/N = 10), respectively. The LOD and LOQ of SECO standard were determined to be 0.690 μg ml-1 (S/N = 3) and 1.370 μg ml-1 (S/N = 10), respectively.
Sampling of the cultures
A volume of 200 μl of culture was sampled every 24 h and extracted with 400 μl n-butanol saturated with water. A portion of n-butanol extracts (320 μl) was transferred to a centrifuge tube and evaporated to dryness by N2. The residue was dissolved in 200 μl methanol and centrifuged for 3 min (12500 r min-1), and then 20 μl of the supernatant was filtered and analyzed by HPLC.
Successive passages of cultures for sustained production of END
A culture was started with a fecal specimen at 37°C and sampled every 24 hours for analysis by HPLC. As END could be detected in the culture as early as within the first 24 hours at concentrations of 31.45 ± 1.51 mg l-1 and the yields remained relatively stable for 6 days (starting to decline on day 9; data not shown), we used an interval of 6 days for successive passages of the culture by 1:10 dilutions in medium B without paraffin, as strict anaerobic culture conditions were not necessary (see above). A portion of the first fecal culture was stocked on day 6 from the initiation of the culture in 25% (v/v) glycerol at -80°C as "passage 1" (designated as END-1); a portion of each of all successive subcultures was stocked on the 6th day of the culture in the same way and was designated as END-2, END-3, and so on. To identify the bacteria that were involved in the biotransformation of flaxseed lignans into END, we first needed to select them out of the initial bacterial mixture in the fecal specimen. Our general strategy was to dilute the cultures in which END was produced and use the highest dilution of the bacterial culture that still produced END for successive passages in medium B, which would support only the bacteria that use defatted flaxseeds as a carbon source.
Pulsed field gel electrophoresis (PFGE)
The endonucleases I-CeuI, AvrII, XbaI and SpeI were purchased from New England Biolabs. PFGE was performed in a CHEF - DRII system (Bio-Rad). Preparation and digestion of high molecular weight genomic DNA, digestion of DNA in agarose blocks and separation of DNA by PFGE, were as reported [30, 31].
We thank Dr. Qi-De Han for his support throughout this project. This work was supported by grants from the National Natural Science Foundation of China to DHY (No.30672622) and SLL (NSFC No.30370774, 30870098 and 30970119), and a 985 Project grant of Peking University Health Science Center to SLL.
- Stitch SR, Toumba JK, Groen MB, Funke CW, Leemhuis J, Vink J, Woods GF: Excretion, isolation and structure of a new phenolic constituent of female urine. Nature. 1980, 287 (5784): 738-740. 10.1038/287738a0.View ArticlePubMedGoogle Scholar
- Setchell KD, Lawson AM, Mitchell FL, Adlercreutz H, Kirk DN, Axelson M: Lignans in man and in animal species. Nature. 1980, 287 (5784): 740-742. 10.1038/287740a0.View ArticlePubMedGoogle Scholar
- Wang LQ: Mammalian phytoestrogens: enterodiol and enterolactone. Journal of chromatography. 2002, 777 (1-2): 289-309. 10.1016/S1570-0232(02)00281-7.View ArticlePubMedGoogle Scholar
- Adlercreutz H, Mousavi Y, Clark J, Hockerstedt K, Hamalainen E, Wahala K, Makela T, Hase T: Dietary phytoestrogens and cancer: in vitro and in vivo studies. The Journal of steroid biochemistry and molecular biology. 1992, 41 (3-8): 331-337. 10.1016/0960-0760(92)90359-Q.View ArticlePubMedGoogle Scholar
- Kitts DD, Yuan YV, Wijewickreme AN, Thompson LU: Antioxidant activity of the flaxseed lignan secoisolariciresinol diglycoside and its mammalian lignan metabolites enterodiol and enterolactone. Molecular and cellular biochemistry. 1999, 202 (1-2): 91-100. 10.1023/A:1007022329660.View ArticlePubMedGoogle Scholar
- Lemay A, Dodin S, Kadri N, Jacques H, Forest JC: Flaxseed dietary supplement versus hormone replacement therapy in hypercholesterolemic menopausal women. Obstetrics and gynecology. 2002, 100 (3): 495-504. 10.1016/S0029-7844(02)02123-3.View ArticlePubMedGoogle Scholar
- Adlercreutz H: Lignans and human health. Critical reviews in clinical laboratory sciences. 2007, 44 (5-6): 483-525. 10.1080/10408360701612942.View ArticlePubMedGoogle Scholar
- Thompson LU, Robb P, Serraino M, Cheung F: Mammalian lignan production from various foods. Nutrition and cancer. 1991, 16 (1): 43-52. 10.1080/01635589109514139.View ArticlePubMedGoogle Scholar
- Axelson M, Sjovall J, Gustafsson BE, Setchell KD: Origin of lignans in mammals and identification of a precursor from plants. Nature. 1982, 298 (5875): 659-660. 10.1038/298659a0.View ArticlePubMedGoogle Scholar
- Borriello SP, Setchell KD, Axelson M, Lawson AM: Production and metabolism of lignans by the human faecal flora. The Journal of applied bacteriology. 1985, 58 (1): 37-43.View ArticlePubMedGoogle Scholar
- Heinonen S, Nurmi T, Liukkonen K, Poutanen K, Wahala K, Deyama T, Nishibe S, Adlercreutz H: In vitro metabolism of plant lignans: new precursors of mammalian lignans enterolactone and enterodiol. Journal of agricultural and food chemistry. 2001, 49 (7): 3178-3186. 10.1021/jf010038a.View ArticlePubMedGoogle Scholar
- Johnsson P, Kamal-Eldin A, Lundgren LN, Aman P: HPLC method for analysis of secoisolariciresinol diglucoside in flaxseeds. Journal of agricultural and food chemistry. 2000, 48 (11): 5216-5219. 10.1021/jf0005871.View ArticlePubMedGoogle Scholar
- Van Oeveren A, Jansen JFGA, Feringa BL: Enantioselective Synthesis of Natural Dibenzylbutyrolactone Lignans (-)-Enterolactone, (-)-Hinokinin, (-)-Pluviatolide, (-)-Enterodiol, and Furofuran Lignan (-)-Eudesmin via Tandem Conjugate Addition to gamma-Alkoxybutenolides. J Org Chem. 1994, 59 (20): 5999-6007. 10.1021/jo00099a033.View ArticleGoogle Scholar
- Clavel T, Henderson G, Alpert CA, Philippe C, Rigottier-Gois L, Dore J, Blaut M: Intestinal bacterial communities that produce active estrogen-like compounds enterodiol and enterolactone in humans. Applied and environmental microbiology. 2005, 71 (10): 6077-6085. 10.1128/AEM.71.10.6077-6085.2005.PubMed CentralView ArticlePubMedGoogle Scholar
- Clavel T, Borrmann D, Braune A, Dore J, Blaut M: Occurrence and activity of human intestinal bacteria involved in the conversion of dietary lignans. Anaerobe. 2006, 12 (3): 140-147. 10.1016/j.anaerobe.2005.11.002.View ArticlePubMedGoogle Scholar
- Clavel T, Henderson G, Engst W, Dore J, Blaut M: Phylogeny of human intestinal bacteria that activate the dietary lignan secoisolariciresinol diglucoside. FEMS microbiology ecology. 2006, 55 (3): 471-478. 10.1111/j.1574-6941.2005.00057.x.View ArticlePubMedGoogle Scholar
- Clavel T, Lippman R, Gavini F, Dore J, Blaut M: Clostridium saccharogumia sp. nov. and Lactonifactor longoviformis gen. nov., sp. nov., two novel human faecal bacteria involved in the conversion of the dietary phytoestrogen secoisolariciresinol diglucoside. Systematic and applied microbiology. 2007, 30 (1): 16-26. 10.1016/j.syapm.2006.02.003.View ArticlePubMedGoogle Scholar
- Jin JS, Zhao YF, Nakamura N, Akao T, Kakiuchi N, Min BS, Hattori M: Enantioselective dehydroxylation of enterodiol and enterolactone precursors by human intestinal bacteria. Biological & pharmaceutical bulletin. 2007, 30 (11): 2113-2119.View ArticleGoogle Scholar
- Jin JS, Kakiuchi N, Hattori M: Enantioselective oxidation of enterodiol to enterolactone by human intestinal bacteria. Biological & pharmaceutical bulletin. 2007, 30 (11): 2204-2206.View ArticleGoogle Scholar
- Jin JS, Zhao YF, Nakamura N, Akao T, Kakiuchi N, Hattori M: Isolation and characterization of a human intestinal bacterium, Eubacterium sp. ARC-2, capable of demethylating arctigenin, in the essential metabolic process to enterolactone. Biological & pharmaceutical bulletin. 2007, 30 (5): 904-911.View ArticleGoogle Scholar
- Wang LQ, Meselhy MR, Li Y, Qin GW, Hattori M: Human intestinal bacteria capable of transforming secoisolariciresinol diglucoside to mammalian lignans, enterodiol and enterolactone. Chemical & pharmaceutical bulletin. 2000, 48 (11): 1606-1610.View ArticleGoogle Scholar
- Xie LH, Akao T, Hamasaki K, Deyama T, Hattori M: Biotransformation of pinoresinol diglucoside to mammalian lignans by human intestinal microflora, and isolation of Enterococcus faecalis strain PDG-1 responsible for the transformation of (+)-pinoresinol to (+)-lariciresinol. Chemical & pharmaceutical bulletin. 2003, 51 (5): 508-515.View ArticleGoogle Scholar
- Xie LH, Ahn EM, Akao T, Abdel-Hafez AA, Nakamura N, Hattori M: Transformation of arctiin to estrogenic and antiestrogenic substances by human intestinal bacteria. Chemical & pharmaceutical bulletin. 2003, 51 (4): 378-384.View ArticleGoogle Scholar
- Liu SL, Hessel A, Sanderson KE: Genomic mapping with I-Ceu I, an intron-encoded endonuclease specific for genes for ribosomal RNA, in Salmonella spp., Escherichia coli, and other bacteria. Proceedings of the National Academy of Sciences of the United States of America. 1993, 90 (14): 6874-6878. 10.1073/pnas.90.14.6874.PubMed CentralView ArticlePubMedGoogle Scholar
- Liu SL, Sanderson KE: I-CeuI reveals conservation of the genome of independent strains of Salmonella typhimurium. Journal of bacteriology. 1995, 177 (11): 3355-3357.PubMed CentralPubMedGoogle Scholar
- Liu SL, Schryvers AB, Sanderson KE, Johnston RN: Bacterial phylogenetic clusters revealed by genome structure. Journal of bacteriology. 1999, 181 (21): 6747-6755.PubMed CentralPubMedGoogle Scholar
- Liu SL, Liu GR, Li SX, Liu WQ, Zheng JF, Zhu WF, Gu HX, Guo XK, Sanderson KE, Zhou YG: Bacterial genome structure: a molecular marker to reveal phylogenetic clusters. Journal of Peking University [Med]. 2002, 34 (5): 457-463.Google Scholar
- Clavel T, Dore J, Blaut M: Bioavailability of lignans in human subjects. Nutrition research reviews. 2006, 19 (2): 187-196. 10.1017/S0954422407249704.View ArticlePubMedGoogle Scholar
- Possemiers S, Bolca S, Eeckhaut E, Depypere H, Verstraete W: Metabolism of isoflavones, lignans and prenylflavonoids by intestinal bacteria: producer phenotyping and relation with intestinal community. FEMS microbiology ecology. 2007, 61 (2): 372-383. 10.1111/j.1574-6941.2007.00330.x.View ArticlePubMedGoogle Scholar
- Liu SL, Sanderson KE: A physical map of the Salmonella typhimurium LT2 genome made by using XbaI analysis. Journal of bacteriology. 1992, 174 (5): 1662-1672.PubMed CentralPubMedGoogle Scholar
- Liu SL: Physical mapping of Salmonella genomes. Methods in molecular biology (Clifton, NJ). 2007, 394: 39-58. full_text.View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.