王彥蘇,周佳宇,謝星光,丁成龍,戴傳超,*
(1.南京師范大學生命科學學院,江蘇省微生物資源產業(yè)化工程技術研究中心,江蘇 南京 210023;2.江蘇省農業(yè)科學院畜牧研究所,江 蘇 南京 210014)
隱蔽型真菌毒素的形成及降解方法的研究進展
王彥蘇1,周佳宇1,謝星光1,丁成龍2,戴傳超1,*
(1.南京師范大學生命科學學院,江蘇省微生物資源產業(yè)化工程技術研究中心,江蘇 南京 210023;2.江蘇省農業(yè)科學院畜牧研究所,江 蘇 南京 210014)
真菌毒素廣泛分布于霉變的或受霉菌污染的糧食、谷物、飼料中,對人類和家畜健康造成嚴重威脅。植物和微生物可以降解真菌毒素為弱毒性的毒素,但是這種降解并不徹底,隱蔽型的真菌毒素依然存在于植物體內或環(huán)境中。本文主要從真菌毒素的分布及危害、植物生長和儲存過程中降解真菌毒素的不完全性、微生物轉化或降解真菌毒素形成“隱蔽型真菌毒素”、隱蔽型真菌毒素降解過程這四個方面進行論述,希望為今后真菌毒素的完全降解研究提供參考。
真菌毒素降解;植物轉化;微生物降解;隱蔽型真菌毒素
真菌毒素是絲狀真菌產生的有毒次級代謝產物。莊稼收獲的前后容易感染真菌,如果谷物儲存條件潮濕并且溫度有利于真菌生長,就會有大量真菌毒素的產生。主要產毒素的真菌屬有:鐮刀菌屬(Fusarium)、青霉屬(Penicillium)、曲霉屬(Aspergillus)、鏈格孢霉屬(Alternar ia)。儲存谷物中也常常會有真菌毒素,污染了真菌毒素的食品或飼料對人體或動物有一定危害。許多方法可以減弱或消除真菌毒素:通過非營養(yǎng)吸附化合物如:活性碳、膨潤土、沸石等,可以減少動物胃腸道對毒素的吸收,最終可以減少毒素轉移到動物產品中[1];一些動植物或微生物可以通過改變真菌生長環(huán)境條件抑制真菌毒素的產生,來自植物宿主的次級代謝物質可以參與調節(jié)真菌毒素合成途徑,減少毒素積累;植物在生長和儲存過程中會降解一部分真菌毒素,微生物也可以通過吸附、水解、還原、糖苷化、甲?;确绞剑瑢φ婢舅仄鸬浇舛咀饔?。但是用以上方法處理過的食品看似安全,實際上卻存在著被植物或微生物代謝轉化的隱蔽型真菌毒素。隱蔽型真菌毒素不容易被檢測到,常常以共軛形式存在,如Deoxynivalenol(DON)的糖苷式(deoxynivalenol-3-D-glucopyranoside,D3G),Zearalenone(ZON)的硫化物Zearalenone-14-sulfate(Z14S),存在一定毒性,危害人體或動物健康,對于形成的隱蔽型真菌毒素的完全降解方 法也很少有研究。因此隱蔽型真菌毒素的檢測技術、解毒機制和降解方法還需要進一步深入研究。
真菌毒素是絲狀真菌產生的有毒次級代謝產物,主要的真菌毒素有:玉米赤霉烯酮、單端孢霉烯族毒素、黃曲霉毒素、赭曲霉毒素、麥角生物堿、伏馬菌素、展青霉素等,容易污染玉米、麥類等谷物、食品、飼料。Richard等[2]在成熟玉米青貯樣品中檢測到7種真菌毒素,如黃曲霉毒素B1、橘霉素、脫氧雪腐鐮刀菌烯醇、伏馬菌素B1、膠霉毒素、赭曲霉素、玉米赤霉烯酮。小麥貯藏中比較常見的真菌污染有棒曲霉(Aspergillus alternata)(14.6%)、煙曲霉(Aspergillus fumigatus)(10.5%)、黑曲霉(Aspergillus niger)(8.3%)、橘青霉(Penicillium citrinum)(3.8%)[3]。大麥儲存過程中,主要被鐮刀菌(Fusarium)污染,鐮刀菌所產生的毒素有伏馬菌素、脫氧雪腐鐮刀菌烯醇等。Sulyok等[4]在小麥和玉米中分離出39 種毒素,其中包括單端孢霉烯族毒素類、玉米烯酮、伏馬菌素、麥角生物堿等,如果食用了感染絲狀真菌的糧食和飼料,會對人體或動物造成一定危害。DON是單端孢霉烯族類毒素B中最主要的一種,能引起胃腸障礙、造血機能障礙、臟器出血,并具有致癌、致突變作用。DON的糖基化衍生物DON-3-glucoside(DON3Glc)被發(fā)現存在于谷物和啤酒中[5-6]。Shephard等[7]發(fā)現在家庭釀造的大麥啤酒可以檢測出伏馬菌素B1(fumonisins,FB1)、B2(FB2)和B3(FB3)。Siegel等[8]在啤酒和飲料中檢測到交鏈孢菌酮酸(tenuazonic acid,TA),平均含量達到11 μg/kg,最高達到了175 μg/kg。在飼料中,特別是青貯飼料中,如果青貯原料感染真菌或青貯過程不嚴格厭氧,造成真菌繁殖,會使青貯飼料中存在真菌毒素,對家畜健康造成危害[9]。Lepom[10]研究發(fā)現,如果使用感染了大刀鐮刀霉(Fusarium culmorum)的玉米進行青貯,11 d后檢測不到F. culmorum,但是該菌所產生的玉米烯酮(zearalenone)含量卻在整個12 周內沒有變化。
真菌毒素和其他異源物質一樣,可以被活體植物代謝,形成共軛的真菌毒素。病原菌在感染植物過程中,病原菌細胞壁降解酶基因表達上調,降解植物細胞壁侵染植物細胞[11]。Hariprasad等[12]研究發(fā)現,綠色蔬菜可吸收并通過疏水組織運輸真菌毒素到地上部分。植物種子可以通過被 動運輸,在水通道蛋白的參與下,吸收真菌毒素,并通過植物蒸騰作用吸收到葉部,也可積累到植物根內皮層[13]。真菌毒素 作為異源生物在植物體內被識別需要經歷3 個化學修飾階段,第一個階段是還原、氧化和乙酰化,這個階段可能導致激活衍生型分子的毒性變高;第二個階段是酶轉化活性基團如進行結合、糖苷化、硫基化作用,形成親水性化合物,更好地消除真菌毒素、減弱毒性;第三個階段是隔離真菌毒素,使毒素區(qū)域化,如將真菌毒素包裹在植物細胞液泡內,或結合在植物細胞壁上[14]。
植物代謝物質,如一些植物揮發(fā)性物質也可以減少毒素積累,甚至會促進植物降解真菌毒素。丁香、山百里香等植物揮發(fā)油可以抑制黃曲霉毒素(aflatoxin B1,AFB1)的積累[15]。3 種藥用植物(Thy mus daenensis、Satureja khozistanica和Satureja macrosiphonia)揮發(fā)油、乙醇提取物可以抑制曲霉屬(Aspergillus)的生長,但水萃取物可以不同程度降解AFB1[16-18]。玉米烯酮是一種鐮刀菌毒素, Berthiller等[19]使用玉米烯酮溶液處理擬南芥(Arabidopsis thaliana)植物,液相色譜-質譜聯(lián)用檢測后發(fā)現有17 種代謝物質,主要有糖苷、丙糖苷、玉米烯酮雙已糖和已糖-戊糖二糖,α-和β-玉米烯酮等。真菌毒素的降解并不完全,會形成共軛真菌毒素并積累在植物體內或表面。
真菌毒素的發(fā)生、生物代謝還不是很清楚,并且用于分析真菌毒素的儀器大都不能檢測到共軛的真菌毒素。還有一個令人頭疼的問題是減毒后的共軛真菌毒素被人類或動物消化后可以重新激活,也有人把這種真菌毒素稱為隱蔽型真菌毒素[20]。以DON和ZON為例,在糧食儲存和啤酒發(fā)酵過程中,會容易形成共軛的DON和ZON,危害人類健康。
2.1 DON
Tran和Smith在澳大利亞多個地區(qū)對DON進行的一項調查中顯示。2009—2011年污染樣品中都檢測到以共軛形式存在的隱蔽型DON,污染率分別達到61%、87%、68%。共軛DON對作物存在潛在影響[21]。啤酒是比較受人們歡迎的飲品,麥芽是其基本的原料之一,在釀造過程中特別容易污染鐮刀菌(Fusarium)毒素,如DON、3-Acetyldeoxynivalenol(3ADON)、D3G。Varga等[22]發(fā)現啤酒所含D3G和DON平均質量濃度分別是6.9 μg/L和8.4 μg/L。
2.2 ZON
1990年,Gareis等[23]在谷物樣品中發(fā)現玉米烯酮的糖苷式Zearalenone-glycoside,用含有zearalenone-4-p-D-glucopyranoside的飼料飼喂豬14 d后,在消化過程中,糖苷被水解掉,又會形成ZON和α-Zearalenol。這種 不容易被檢測到,但對人體或動物存在潛在危害的真菌毒素被Gareis為“masked mycotoxin”。Vendla等[24]發(fā)現以谷物為基礎的產品中發(fā)現大量毒素,如Deoxynivalenol、Zearalenone,還存在它們的代謝物質D3G、3ADON l、Zearalenol-4-glucopyranoside、α-Zearalenol、β-Searalenol、α-Zearalenol-4-glucopyranoside、β-Zearalenol-4-glucopyranoside和Zearalenone-4-sulfate。其中,豐富度最高的是Zearalenone-4-sulfate。麥麩中有較高的毒素含量,其中DON 254 ng/g、Zearalenone-4-sulfate 6 ng/g和ZON 44 ng/g。
3.1 乳酸菌的抑菌作用
生物防治病原菌是抑制食品腐敗提高食品保質期的有效方法,乳酸菌作為益生菌可以抑制食品中的腐敗菌,提高食品保質期[25]。乳酸菌抑制腐敗菌生長的作用機理分為幾個原因:乳酸菌產生細菌素(蛋白類物質抗菌多肽)、乳酸、乙酸、苯基乳酸、脂肪酸、酚類化合物[26],許多乳酸菌可以產生黃素蛋白氧化酶類但不產生過氧化氫酶,在氧氣存在的條件下產生H2O2,使具有強氧化作用的H2O2可以破壞細菌細胞蛋白。乳酸菌(Lactobacillus paracasei ssp. tolerans)對真菌和霉菌(如Fusarium proliferatum M5689、M5991、Fusarium graminearum R4053)有一定的抑制作用[27]。Magnusson從環(huán)境中分離37株抑制真菌活性的乳酸菌,能夠抑制A. fumigatus、Aspergillus nidulans、Penicillium commune和Fusarium sporotrichioides的生長[28]。表1顯示了乳酸菌能夠抑制的一些病原菌或腐敗菌類型。
表1 乳酸菌抑制菌種Table1 Strains against lactic acid bacteria
3.2 乳酸菌對真菌毒素的結合及降解作用
乳酸菌細胞壁與真菌毒素可以通過氫鍵、離子鍵、疏水作用力進行結合,減弱樣品毒性[36]。Wu等[37]發(fā)現酵母和乳酸菌作為黃曲霉霉素的生物吸附劑可以阻止其進入人類和動物腸道。乳酸菌(Lactobacillus casei、Lb. plantarum和Lactobacillus fermentum)等一些傳統(tǒng)的益生菌可以結合黃曲霉毒素,起到對食品進行解毒的作用。被鐮刀菌毒素污染的玉米青貯中可以分離到8 株乳酸桿菌屬(Lactobacilli)和3 株明串珠菌屬(Leuconostoc),研究發(fā)現這幾株菌可以將ZON轉化成 α-Zearalenol,但是不能轉化DON和伏馬菌素(fumonisins),許多菌可以和這些毒素結合[38]。Fazeli等[38]從酸面包和奶制品中分離得到乳酸菌(Lb. casei、Lb. plantarum和Lb. fermentum),發(fā)現所有菌株都有能力去除AFB1,Lb. fermentum和Lb. plantarum可以快速去除AFB1,去除率達到61%~65%。根據以上結果,乳酸菌與AFB1可以有效結合,但是,Niderkorn等發(fā)現[39]結合不是很緊密,經過離心后,會有AFB1脫離菌體,不同菌體與不用毒素的結合能力不同,可能與毒素結構和菌體表面結構不同。Franco等[40]發(fā)現不管是活體乳酸菌細胞還是熱滅活的乳酸菌細胞,乳酸菌在體外可以去除DON,pH值和培養(yǎng)基濃度都不會影響去除能力,并且發(fā)現熱滅活的細胞有更強的去除真菌毒素的能力,DON減少率35%~67%,活體細胞只達到16%~56%。Niderkorn等[41]發(fā)現乳酸菌與毒素結合反應涉及到乳酸菌中的肽聚糖和TCA循環(huán),特別是細胞壁肽聚糖中部分肽結構在與伏馬菌素B1和B2結合的過程中起到明顯作用,熱激或酸處理后的細胞結合真菌毒素的能力增強,可能是熱激和酸處理后細胞壁表面結合位點暴露。乳酸菌在植物加工或貯存過程中,是可以減弱真菌毒素污染的良好菌劑,也可以提高食物風味,可以作為糧食儲存保護劑或修復劑[42]。在啤酒發(fā)酵過程中,麥芽是不可缺少的原料,在麥芽浸泡過程中,乳酸菌數量達到108CFU/g,其中異型發(fā)酵明串珠菌屬占優(yōu)勢地位[43-44],乳酸菌在麥芽發(fā)酵過程中,可以產生細菌素,抑制腐敗菌生長[45]。
研究發(fā)現乳酸菌可以有效降解真菌毒素,在酸面團發(fā)酵中,乳酸菌與酵母混合接菌可以降解赭曲霉素(ochratoxin A)[46]。被毒素污染的玉米與乳酸菌共培養(yǎng)處理組相比于未用乳酸菌處理組,對SNO細胞(人類食道癌細胞系)毒性較弱[47]。Roig從腸道分離得到9 株乳酸菌(Bif i dobacterium longum、Bif i dobacterium bif i dum、Bifidobacterium breve、Bifidobacterium adolescentes、Lactobacillus rhamnosus、Lactobacillus casei-casei、Streptococcus termofilus、Lactobacillus ruminis、Lb. casei)和22株釀酒酵母(Saccharomyces cerevisiae),所有菌株在體外培養(yǎng)基中降解范圍為5%~99%,在小麥粉食物系統(tǒng)中Enniations(ENs)的降解1.3%~49.2%[48]。
食品安全是食品行業(yè)和消費者越來越關注的問題,乳酸菌可以抑制病原菌的生長、吸附或降解真菌毒素。因此,今后應加強乳酸菌與病原菌拮抗機理的研究,探討乳酸菌在食品生產加工過程微生物群落中生態(tài)作用,使乳酸菌菌劑可以應用于食物防腐工業(yè)中。乳酸菌會產生一些代謝產物,如細菌素、有機酸等,對病原菌具有良好的抑制作用,如果能大規(guī)模生產使用,在糧食儲存、食品加工過程中有廣闊的應用前景。但是以上研究顯示,乳酸菌只能部分降解或結合真菌毒素,并沒有完全消除真菌毒素的危害,對于乳酸菌是否能完全徹底的降解真菌毒素還需要進一步研究。
3.3 其他降解菌對真菌毒素的降解作用
研究發(fā)現許多從土壤、水體等自然環(huán)境中獲得真菌毒素降解菌[49-50],也有許多研究發(fā)現從人體或動物胃腸道獲得的微生物可以抑制真菌生長或降解真菌毒素。Kusumaningtyas等[51]研究將釀酒酵母(S. cerevisiae)和寡孢根霉菌(Rhizopus oligosporus)分別接種和組合接種被黃曲霉毒素污染的雞飼料中,在處理第5天后,AFB1達到最大降解率,R. oligosporus接種的處理組降解AFB1活性最高。被赫曲霉素污染的大麥固體發(fā)酵物,在接種白腐真菌Pleurotus ostreatus 4周后,OTA降解到23%,OTB降解到3%,在降解過程中OTB水解為Ochratoxin β[52]。Zuo等[53]發(fā)現Lb. casei、枯草芽孢桿菌(B. subtilis)和異常畢赤酵母(Pichia anomala)混合培養(yǎng)可以有效地降解AFB1,在12 h內可去除59.3%,在48 h內可去除87.04%。非病原菌嗜吡啶紅球菌(Rhodococcus pyridinivorans)K408與ZON孵育5 d后可以達到87.21%的降解率[54]。
以上研究發(fā)現微生物對真菌毒素具有良好的降解或轉化能力,可能是微生物產生一些物質(如酶類)作用于腐敗真菌,使菌體生長減弱,或作用于真菌毒素,使毒素降解或使毒性減弱。Meca等[55]研究發(fā)現釀酒酵母(S. c erevisiae LO9、YE5、A34和A17)混合粗酶液,加到白僵菌毒素(beauvericin)標準溶液中,毒素降解率為83%~100%,以污染白僵菌毒素的玉米粉為基質,降解率為66%~91%。細菌和黑酵母降解伏馬菌素的途徑,首先是伏馬菌素羧酸脂酶水解兩個三羧酸側鏈,第二步是在黑色酵母中胺氧化酶作用水解后的伏馬菌素B1脫氨基作用,或是在細菌中氨基轉移酶作用轉氨基。Hartinger等[56-60]發(fā)現鞘脂單胞菌(Sphingopyxis sp. MTA144)編碼氨基轉移酶FumⅠ,FumⅠ能夠催化水解了的伏馬菌素B1脫氨基作用,FumⅠ和伏馬菌素羧酸脂酶Fum D聯(lián)合作用,使伏馬菌素解毒成為可能。
圖1 黃曲霉毒素BB1水解過程Fig.1 Hydrolysis of aflatoxin B1
AFB1通過環(huán)氧化物水解酶水解為AFB1-8-9-二氫二醇(AFB1-8,9-dihydrodiol)(圖1),Wang等[61]對來自白腐真菌Phanerochaete sordid YK-624的錳過氧化物酶可以有效消除AFB1,5 nkat的MnP與AFB1共培養(yǎng)48 h AFB1的最大消除達到86.0%,錳過氧化物酶將ABF1環(huán)氧化成AFB18,9-epoxide,再水解為AFB1-8,9-二氫二醇(AFB1-8,9-dihydrodio l)。羧肽酶A 、胰蛋白酶、α-糜蛋白酶和組織蛋白酶C能在體外條件下水解赭曲霉毒素OTA為弱毒性的OTα[62-63](圖2)。
圖2 赭曲霉毒素A水解過程Fig.2 Hydrolysis of ochratoxin A
不同種類的微生物對真菌毒素的降解途徑不同,以微生物降解DON為例,革蘭氏陽性菌和革蘭氏陰性菌對DON具有不同的降解作用,革蘭氏陽 性菌可以吸收DON,將DON作為碳源利用,而革蘭氏陰性菌則不能吸收利用,需要在含有DON培養(yǎng)一段時間后才能起到降解DON的作用。革蘭氏陽性菌與革蘭氏陰性菌存在不同DON降解途徑,在這兩種途徑中有不同的降解酶基因表達調控機制,但Sato等[64]發(fā)現兩種菌的降解產物均為3-Epi-deoxynivalenol。
圖3 脫氧雪腐鐮刀菌烯醇結構式Fig.3 Structure of deoxynivalenol
DON毒性主要是中間環(huán)氧鍵的存在(圖3)[65]。許多胃腸道微生物可以將DON還原去環(huán)氧基團,一些來自動物消化系統(tǒng)的微生物代謝可以降解DON為弱毒性的Deepoxy-deoxynivalenol (DOM-1)[66]。Gratz等[67]發(fā)現人類糞便微生物可以解毒DON,形成DOM-1。Yu等[68]從雞腸道中分離得到DON降解細菌,將DON轉化成DOM-1。許多研究都是胃腸道微生物在厭氧環(huán)境下通過還原去環(huán)氧化作用解毒DON[69-71]。Islam等[72]用農田土壤制備培養(yǎng)基,在有氧條件下可以降解DON、去環(huán)氧化。
圖4 脫氧雪腐鐮刀菌烯醇乙?;苌锝Y構式Fig.4 Structure of acetylated derivatives of deoxynivalenol
DON存在下常常伴隨著兩個乙酰化衍生物:3-ADON和15-ADON(圖4)。Pinton等[73]通過對細胞增殖、胃腸道屏障功能和腸道形態(tài)結構等觀察研究,比較體內體外條件下DON、3-ADON、15-ADON毒性,發(fā)現15-DON因為能激活MAPK途徑使其具有更高的毒性。Pinton等[74]對DON、3-DON、15-DON毒性進行了分析,它們的毒性大小依次為3-ADON≤DON<15-ADON。但是DON雖然被轉化和降解,但是降解并不徹底,其衍生物依然有弱毒性,危害人類健康。
3.4 微生物對真菌毒素降解的不完全性
真 菌毒素的降解取決于環(huán)境條件,如:微生物的種類、數量、化合物結構等。往往微生物對真菌毒素具有一定 的降解作用都具有不完全性,雖然降解率較高,但降解產物具有不徹底性,只能減弱毒性,被人體或動物吸收后,經過胃腸道微生物作用和修飾,降解產物又會回復到原來的毒性較強的化合物,很少會轉變成無機礦物質、H2O、CO2、。
對于真菌毒素的檢測存在許多種方法薄層層析法、酶聯(lián)免疫分析方法、氣相色譜法、高效液相色譜法等。這幾種方法各有優(yōu)缺點。由于真菌毒素有不同的化學式和物理化學性質,特殊的提取、凈化、合適的檢 測器,只能有效的檢測一種或一類真菌毒素。Tanaka等[76]論述了水稻真菌毒素的檢測方法,脫氧雪腐鐮刀菌烯醇(deoxynivalenol,DON)、鐮刀菌烯酮和雪腐鐮刀菌醇一般使用氣相色譜-質譜聯(lián)用檢測,單 端孢霉烯族化合物使用氣相色譜-電子捕獲檢測(gas chromatography and electron capture detection,GC-ECD)確定。雜色曲霉素(sterigmatocystin,ST)是曲霉屬或其他一些真菌產生的致癌性化合物,氣相色譜-質譜聯(lián)用(gas chromatography-mass spectrometry,GC-MS)、液相色譜-質譜聯(lián)用(liquid chromatography-mass spectrometry(LC-MS)、LC-MS/MS)和液相紫外檢測(liquid ultraviolet detection,LC-UV) 都可以用于STE的檢測,但是LC-UV的檢測效果比較好。但是對隱蔽型真菌毒素的檢測沒有做具體論述。
多種方法并用,可以更有效地檢測難以檢測的隱蔽型真菌毒素。LC-MS/MS由于其普遍性、選擇性、敏感性的優(yōu)點,近幾年應用比較廣泛。Vendl等[77]用LC-MS/MS從谷物食物中檢測到DON和ZON還有8 種它們的代謝產物如:Deoxynivalenol-3-glucoside (D3G)、3-Acetyl-deoxynivalenol(3ADON)、Zearalenone-4-glucoside(Z4G)、α-Zearalenol (α-ZOL)、β-Zearalenol(β-ZOL)、α-Zearalenol-4-glucoside (α-ZG)、β-Zearalenol-4-glucoside(β-ZG)、Zearalenone-4-sulfate(Z4S)。Nakagawa等[78]利用高分辨率LC-軌道阱質譜檢測到單端孢霉烯族化合物A型的糖基化衍生物:Neosolaniol-glucoside(NESGlc)和Diacetoxyscirpenolglucoside(DASGlc)、單端孢霉烯族化合物B型化合物的糖基化衍生物D3G。Vidal等[79]論述了生物傳感器在檢測真菌毒素方面的應用,但是多種毒素檢測、生物標志物和蒙面真菌毒素的檢測方面還需進一步研究。
雖然在檢測共軛真菌毒素技術上得到提高,但是對于共軛真菌毒素的完全解毒還需要進一步研究。通過糖基化作用來“偽裝”的真菌毒素,如DON的糖基化DON-3-Glc可以水解又回到DON,研究發(fā)現DON-3-Glc可以抵抗酸性環(huán)境,因此在胃液的酸性環(huán)境中水解DON-3-Glc是非常困難的。Berthiller等[80]發(fā)現人類胞質中的β-葡糖苷酶對D3G水解效果較差,但是纖維素酶和纖維二糖酶對其的最大水解率分別達到13%和73%,這表明D3G被瘤胃動物吸收后,在瘤胃纖維素分解菌的作用下使D3G回到DON。同時他們又發(fā)現胃腸道細菌如Bifidobacterium adolescentis、Enterococcus durans、Enterococcus mundtii、Lb. plantarum 8 h對D3G的水解率分別為:17%~25%、14%~27%、38%、62%。雖然植物將DON解毒為D3G,但是在腸道微生物作用下D3G又水解為DON。
轉基因植物中過表達幾種植物或細菌基因可以加強植物修復系統(tǒng),在汞污染的修復過程中,Heaton等[81]將植物轉基因表達細菌的merA基因或merB基因,這兩種基因來自細菌的mer操縱子,merA基因編碼NADPH依 賴的汞離子還原酶,可以轉化離子汞(Hg(Ⅱ))轉化成(Hg(0)),merB基因編碼有機汞裂合酶,可以降解MeHg為甲烷和Hg(Ⅱ),有效的修復汞污染。以此借鑒,細菌中水解酶基因轉移到植物中,植物可以更好地降解土壤以及吸收后的真菌毒素,使真菌毒素鈍化,利用轉基因植物導入細菌或真菌編碼水解或還原真菌毒素基因的方法,希望可以有效緩解被黃曲霉毒素污染現狀。對于環(huán)境中防治真菌毒素,高溫度、高濕度會黃曲霉大量繁殖,RNA沉默技術減少黃曲霉生長和其毒素的合成,從而減弱毒性[82]。
真菌毒素廣泛存在于糧食、飼料、食品中,會對人類和動物健康造成威脅。許多研究發(fā)現,外界環(huán)境可以影響真菌毒素的合成,甚至可以調節(jié)毒力因子基因的表達,一些產毒素真菌還會受其宿主植物信號分子的影響,從而調控真菌毒素基因表達。糧食作物在加工或儲存過程中可能會有效地減弱真菌毒素的毒性,但作用并不完全。乳酸菌作為益生菌可以調節(jié)人類胃腸道菌群,提高食品或飼料的口感、營養(yǎng)。其細胞壁特殊肽段對真菌毒素具有吸附作用,還有研究表明乳酸菌可以對真菌毒素起到降解作用。許多環(huán)境微生物對真菌毒素具有降解作用,降解的機理有水解作用、還原作用、乙?;饔玫龋墙到庾饔貌⒉粡氐?,形成真菌毒素的中間衍生物,沒有完全降解成H2O和CO2等,依然對環(huán)境有害。因此對于隱蔽型真菌毒素的降解還需進一步研究,歸結為以下幾點。1)來自宿主植物的信號分子可以減少真菌毒素積累,植物揮發(fā)油發(fā)現可以促進降解真菌毒素,這就為減少真菌毒素污染提供了新的思路;2)研究發(fā)現來自環(huán)境和腸道微生物具有降解真菌毒素的功能,新型、安全的真菌毒素降解菌還需要進一步篩選;3)在以往的檢測中,往往忽略了隱蔽型真菌毒素的存在及其危害性,因此需進一步提高檢測技術,檢測糧食或飼料中隱蔽型真菌毒素;4)大部分植物還是微生物對真菌毒素只是起到吸附、不完全降解或減弱毒性的作用,并沒有將真菌毒素完全降解成H2O或CO2,隱蔽型真菌毒素的完全降解還需深入研究。
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Advances in the Formation and Degradation of Masked Mycotoxins
WANG Yan-su1, ZHOU Jia-yu1, XIE Xing-guang1, DING Cheng-long2, DAI Chuan-chao1,*
(1. Jiangsu Engineering and Technology Research Center for Industrialization of Microbial Resources, College of Life Science, Nanjing Normal University, Nanjing 210023, China; 2. Institute of Animal Science, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China)
Mycotoxin-producing fungi are ubiquitous in moldy ce reals and animal feeds and pose a serious threat to human and animal health. Mycotoxins can be partly detoxified through incomplete degradation by plants or microbes into masked mycotoxins which, however, still exist in plants or the environment. This paper is focused on the following aspects of mycotoxins: distribution and hazards, incomplete degradation during the growth and postharvest storage of plants, and the formation of masked mycotoxins by microbial conversion or degradation as well as their further degradation. Hopefully this paper can provide guidelines for future studies on complete degradation of mycotoxins.
mycotoxins degradation; phytotransformation; microbial degradation; masked mycotoxins
Q939.97
A
1002-6630(2014)21-0326-08
10.7506/spkx1002-6630-201421062
2013-12-08
江蘇省農業(yè)自主創(chuàng)新項目(CX(12)1002)
王彥蘇(1989—),女,碩士研究生,主要從事微生物學研究。E-mail:wangyansu1990@163.com
*通信作者:戴傳超(1970—),男,教授,博士,主要從事微生物生態(tài)學研究。E-mail:daichuanchao@njnu.edu.cn