
中国科学院微生物研究所,中国微生物学会
文章信息
- 董雷, 韩嘉瑞, 李帅, 岳凌翔, 李文均. 2023
- DONG Lei, HAN Jiarui, LI Shuai, YUE Lingxiang, LI Wenjun.
- 链霉菌最新研究进展
- The latest research progress of streptomycetes
- 微生物学报, 63(5): 1815-1832
- Acta Microbiologica Sinica, 63(5): 1815-1832
-
文章历史
- 收稿日期:2023-02-22
2. 中国科学院新疆生态与地理研究所 荒漠与绿洲生态国家重点实验室, 新疆 乌鲁木齐 830011
2. State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, Xinjiang, China
放线菌代表了自然界中物种多样性最高的微生物类群之一,同时也是极具开发价值的类群[1],受到人们的长期关注。由放线菌生产的化合物占当今临床使用的抗生素药物的三分之二,同时还包括大量的抗癌化合物、免疫抑制剂、驱虫剂、除草剂和抗病毒化合物[2-4]。链霉菌是放线菌门中最为庞大且极具代表性的分支,具备强大的生态适应能力,广泛分布于陆地、水体生态系统以及高等真核生物等宿主相关生境中[5]。
自该类群首次被发现和报道的近80年来,链霉菌研究取得了长足的发展。尤其是针对链霉菌天然产物资源的开发及应用研究[6],为医药、农业和工业提供了丰富的、至关重要的研究材料和产品实体。链霉菌系统分类学与以培养导向的多样性研究同样取得了重要进展[7],人们对于放线菌的起源、进化和生理特征的认知进一步拓展。特别是基因组学时代的到来,极大地促进了现代分类学体系的发展。近期Madhaiyan等[8]也基于链霉菌科的基因组系统学分析,重新划分并生效发表了6个链霉菌科新属。本文基于链霉菌系统分类学、资源多样性以及天然产物挖掘这3个方向,综述了链霉菌的相关研究近况,包括技术革新带来的链霉菌研究方法与研究策略的更新,也对目前研究中存在的机遇与挑战进行了梳理,以期对链霉菌未来的研究方向提供依据和参考。
1 链霉菌系统分类学研究进展 1.1 链霉菌的系统发育体系Waksman和Henrici于1943年(批准名录1980)首次提出并描述了链霉菌科(Streptomycetaceae)[9]。Stackebrandt等[10]在1997年基于放线菌纲成员的16S rRNA基因序列进行系统发育分析,并建议将链霉菌科提升为放线菌纲中的一个亚目,命名为链霉菌亚目(Streptomycineae)。2012年发布的《伯杰氏系统细菌学手册》(Bergey’s Manual of Systematic Bacteriology)中Kämpfer等[11]将链霉菌亚目正式划分为链霉菌目(Streptomycetales)。
链霉菌具有鲜明的生理生化特征,它们为好氧细菌、革兰氏染色阳性,不抗酸和乙醇,易形成广泛分枝的基内菌丝且菌丝一般不碎裂。在链霉菌成熟时,气生菌丝形成由多个分生孢子组成的链状结构,这也是链霉菌名称的由来。基内菌丝体细胞壁中的特征性氨基酸为L, L-或meso-两种DAP (2, 6-二氨基庚二酸),孢子细胞壁为L, L-DAP。全细胞水解物富含半乳糖或鼠李糖。链霉菌科成员主要脂肪酸成分均包含饱和iso-脂肪酸和anteiso-脂肪酸。主要的甲基萘醌为MK-9 (H6, H8)。主要极性脂质为二磷脂酰甘油(diphosphatidyl glycerol, DPG)、磷脂酰乙醇胺(phosphatidyl ethanolamine, PE)、磷脂酰肌醇(phosphatidylinositol, PI)和磷脂酰肌醇甘露糖(phosphatidylinositol mannose, PIMs)。DNA的G+C含量为66–74 mol%[12]。
国际原核微生物系统学委员会(International Committee on Systematics of Prokaryotes, ICSP)目前认可的链霉菌仅1个科,即链霉菌科,模式属为链霉菌属[9]。同时,链霉菌科中包含有效发表且正确命名的共10个属,包括别样链霉菌属(Allostreptomyces)、恩布利菌属(Embleya)、北里孢菌属(Kitasatospora)、弯曲嗜酸菌属(Streptacidiphilus)、链霉菌属(Streptomyces)和黄英菌属(Yinghuangia);以及Madhaiyan等[8]最新提议的6个新属中的4个,分别是:红树林放线孢菌属(Mangrovactinospora)、彼得肯普弗菌属(Peterkaempfera)、弯曲抗生素菌属(Streptantibioticu)以及李文均菌属(Wenjunlia),另外2个提议的新属,放线嗜酸菌属(Actinacidiphila)和灰褐嗜酸菌属(Phaeacidiphilus)目前处于生效发表但未正确命名(图 1)。截至2023年2月20日,链霉菌科在原核生物标准命名名录(List of Prokaryotic names with Standing in Nomenclature, LPSN, https://lpsn.dsmz.de/family/streptomycetaceae)中收录的有效发表并正确命名的物种数为732个。
1.2 链霉菌的现代分类方法及技术
链霉菌的物种分类,最初主要是基于传统分类(traditional classification),主要指以形态特征(morphological characteristics)、培养特征(cultural characteristics)及生理生化特性(physiological characteristics)等表观分类学指征(phenotypic information)[9]。传统分类法中的形态特征常用作分类系统中科以上单元的划分,而形态结构特征结合生理生化特性可以用于科以下单元的分类。后来基于传统分类并与单基因系统发育分析相结合发展形成了“多相分类(polyphasic taxonomy)”的现代分类体系,但是这种研究并没有在链霉菌中提供很好的系统发育分析[13-14]。Labeda等[15]研究表明16S rRNA基因序列可以用来证明链霉菌科内成员的物种多样性,并确定了130个具有统计学意义的分支和额外的单种谱系。近年来,链霉菌的系统发育分辨率的显著提高要得益于利用atpD、gyrB、rpoB、recA和trpB这些看家基因的多位点序列分析技术(multi-locus sequence analysis, MLSA)[16]。MLSA方法基于多位点序列分型(multilocus sequence typing, MLST)方法发展而来,具有快速简便、重复性好和高区分力的特点[17]。MLSA方法会将具有保守功能蛋白质的部分基因序列,即看家基因,进行编码用于生成系统发育树,并推断系统发育关系。该分析方法支持了北里孢菌属和弯曲嗜酸菌属间的系统发育差异,以及链霉菌属中的9个种转入北里孢菌属。对于北里孢菌属更进一步的修订是将该属的描述修正,并把原来链霉菌属的生靛链霉菌(Streptomyces indigoferus)和杀黄胞菌素链霉菌(Streptomyces xanthocidicus)重分类为北里孢菌属成员[18]。
基于基因组指标的分类学方法如今已成为一种主流的分类学研究手段,其可用于划分链霉菌相关的科、属和种[13, 19-23]。目前,由基因组序列衍生而来的分析方法,如数字DNA-DNA杂交(digital DNA-DNA hybridization, dDDH)、平均核苷酸一致性(average nucleotide identity, ANI)和平均氨基酸一致性(average amino acid identity, AAI),通常用于种和属的水平分类学描述[24-25]。此外,当MLSA只利用较少看家基因时,其分类结果并不十分可靠,系统发育基因组学(phylogenomics)分析则会获得更可信的分类学结果。系统发育基因组学是面向一组具有相关关系物种的基因组分析,是进化与基因组学的交汇点[26]。基因组中包含了更多的物种进化信息,基于基因组构建系统发育,相较于基于单一保守基因的系统发育,可以更真实地反映类群之间的进化关系。
1.3 链霉菌系统分类学的最新进展系统分类学一直是一个活跃且动态发展的基础学科,链霉菌系统分类学格局也在现代分类方法及技术的发展下改变。2020年,Salam等[27]基于已有的基因组序列,利用16S rRNA基因序列系统发育分析,对具有纯培养物的放线菌门高级分类单元做了分类地位梳理。该研究提议将链霉菌科的别样链霉菌属(Allostreptomyces)划分出去,并建立一个属于链霉菌的新科,命名为别样链霉菌科(Allostreptomycetaceae)。2021年,Volpiano等[22]通过对链霉菌属中的热自养链霉菌(Streptomyces thermoautotrophicus)[28]的重分类,提议将其从原有的属划分出去,并建立一个属于链霉菌的新科,命名为碳放线孢菌科(Carbonactinosporaceae)。然而,这2个对链霉菌新科的提议尚未被ICSP认可。
近期,Madhaiyan等[8]利用基因组序列数据阐明链霉菌科分类单元之间的进化关系,并对需要重新分类的分类单元进行适当的分类排序。为了研究链霉菌科内的分类学关系,该研究首先重建了包含别样链霉菌属(Allostreptomyces)、恩布利菌属(Embleya)、北里孢菌属(Kitasatospora)、弯曲嗜酸菌属(Streptacidiphilus)、链霉菌属(Streptomyces)和黄英菌属(Yinghuangia)成员的大多数模式菌株的16S rRNA基因系统发育树。该研究发现了链霉菌科中的19株模式菌株与同科其他成员16S rRNA基因相似性较低(< 97.3%),而后利用保守通用蛋白进行系统基因组学分析,结果反映了其较高的遗传多样性并确定了6个不同的属级别分支(图 1蓝色标注),其中2个分支从弯曲嗜酸菌属分离,另外4个分支从链霉菌属中分离。这些新属同弯曲嗜酸菌属和链霉菌属成员的AAI值为63.9%–71.3%,即与属间比较阈值一致。该研究[8]又利用软件工具PhyloPhlAn 3.0[29]对其进行全基因组系统发育重建。其中AAI和保守蛋白百分率(percentage of conserved proteins, POCP)分析结果表明,这些系统发育具有明显差异的类群可能归属于6个新属(表 1,图 1),分别命名为放线嗜酸菌属(Actinacidiphila)、红树林放线孢菌属(Mangrovactinospora)、彼得肯普弗菌属(Peterkaempfera)、灰褐嗜酸菌属(Phaeacidiphilus)、弯曲抗生素菌属(Streptantibioticu)以及李文均菌属(Wenjunlia)。
No. | Original name | New name | Current status of classification status |
1 | Streptomyces acididurans | Actinacidiphila acididurans | Synonym |
2 | Streptomyces alni | Actinacidiphila alni | Synonym |
3 | Streptomyces bryophytorum | Actinacidiphila bryophytorum | Synonym |
4 | Streptomyces epipremni | Actinacidiphila epipremni | Synonym |
5 | Streptomyces glaucinigra | Actinacidiphila glaucinigra | Synonym |
6 | Streptomyces guanduensis | Actinacidiphila guanduensis | Synonym |
7 | Streptomyces oryziradicis | Actinacidiphila oryziradicis | Synonym |
8 | Streptomyces paucisporea | Actinacidiphila paucisporea | Synonym |
9 | Streptomyces rubida | Actinacidiphila rubida | Synonym |
10 | Streptomyces soli | Actinacidiphila soli | Synonym |
11 | Streptomyces yanglinensis | Actinacidiphila yanglinensis | Synonym |
12 | Streptomyces yeochonensis | Actinacidiphila yeochonensis | Synonym |
13 | Streptacidiphilus bronchialis | Peterkaempfera bronchialis | Correct name |
14 | Streptacidiphilus griseoplana | Peterkaempfera griseoplana | Correct name |
15 | Streptacidiphilus oryzae | Phaeacidiphilus oryzae | Synonym |
16 | Streptomyces gilvigrisea | Mangrovactinospora gilvigrisea | Correct name |
17 | Streptomyces tyrosinilytica | Wenjunlia tyrosinilytica | Correct name |
18 | Streptomyces vitaminophila | Wenjunlia vitaminophila | Correct name |
19 | Streptomyces cattleyicolor | Streptantibioticus cattleyicolor | Correct name |
该研究[8]从弯曲嗜酸菌属中划成了3个谱系,其包括狭义弯曲嗜酸菌属(Streptacidiphilus sensu stricto),即被再分类的弯曲嗜酸菌属,以及2个新谱系,分别为彼得肯普弗菌属(Peterkaempfera)和灰褐嗜酸菌属(Phaeacidiphilus)。
弯曲嗜酸菌属经过再分类后,其包括弯曲嗜酸菌属中的安眠岛弯曲嗜酸菌(Streptacidiphilus anmyonensis)[30]、产黑素弯曲嗜酸菌(Streptacidiphilus melanogenes)[30]、皱褶弯曲嗜酸菌(Streptacidiphilus rugosus)[30]、白色弯曲嗜酸菌(Streptacidiphilus albus)[31]、碳弯曲嗜酸菌(Streptacidiphilus carbonis)[31]、中性弯曲嗜酸菌(Streptacidiphilus neutrinimicus)[31]、江西弯曲嗜酸菌(Streptacidiphilus jiangxiensis)[32]和居松树弯曲嗜酸菌(Streptacidiphilus pinicola)[33]共8个物种水平成员,模式种为白色弯曲嗜酸菌(Streptacidiphilus albus)。
1.3.1 彼得肯普弗菌属再分类得到的其中一个新谱系为彼得肯普弗菌属,该谱系由新分类的灰平弯曲嗜酸菌(Streptacidiphilus griseoplanus)[34]和支气管弯曲嗜酸菌(Streptacidiphilus bronchialis)[34]组成。它们的16S rRNA基因序列相似性为98.1%,在系统发育分析中形成了较高自引支持度(bootstrap support)的聚类。这2个模式菌株AAI值为81.3%,但与该属的其他成员的AAI值为67.8%–70.0% (平均为68.41%),即低于不同属间比较的AAI阈值(约70%–74%)[35-36]。将这2个种的模式菌株与北里孢菌属成员进行比较时,得到的AAI平均值略高,为70.7% (69.2%–72.8%)。Li等[20]的系统基因组分析结果表明,支气管弯曲嗜酸菌模式菌株相比于弯曲嗜酸菌属与北里孢菌属的进化关系更近,同时提出其可能是一个新属。这些数据和研究支持将这2个种重新划分为链霉菌科的一个新属,并以灰平彼得肯普弗菌(Peterkaempfera griseoplana)为模式种,命名为彼得肯普弗菌属(Peterkaempfera)。
1.3.2 灰褐嗜酸菌属第二个新谱系为灰褐嗜酸菌属,该谱系由稻田弯曲嗜酸菌(Streptacidiphilus oryzae)组成[37]。在对其16S rRNA基因进行系统发育分析时,发现其系统发育关系最接近的是灰平弯曲嗜酸菌,相似性为97.4%。稻田弯曲嗜酸菌与其他狭义弯曲嗜酸菌属成员的AAI值为67.3%–68.6% (平均为67.8%)[8],即低于不同属间比较的AAI阈值(约70%–74%)。将该菌株与弯曲嗜酸菌属的其他成员相比,该物种的POCP值为46.4%–51.2% (平均为48.8%),这也表明其不应划分在弯曲嗜酸菌属内。因此,这些多相分类的数据支持这个种被重新划分为链霉菌科的一个新属,命名为灰褐嗜酸菌属(Phaeacidiphilus),原稻田弯曲嗜酸菌(Streptacidiphilus oryzae)再分类为稻田灰褐嗜酸菌(Phaeacidiphilus oryzae),为灰褐嗜酸菌属模式种。
1.3.3 弯曲抗生素菌属卡特兰链霉菌(Streptomyces cattleya)[38]已经在天然产物领域被广泛且深入的研究,其能够产生较高应用价值的噻烯霉素(thienamycin)和氟化化合物[38-41],但在分类学领域尚未被正式描述为一个物种[16]。该物种与链霉菌属标准菌株间的AAI值为60.0%–71.1% (平均为70.0%),与链霉菌属的模式种白色链霉菌(Streptomyces albus)的POCP值为52.7%,略高于先前研究提出的属间POCP阈值(50%),许多研究表明这个阈值过于严格[42-44]。这些已有的系统基因组学分析、AAI和POCP数据支持该物种被重新划分为链霉菌科的一个新属,命名为弯曲抗生素菌属(Streptantibioticu),原卡特兰链霉菌(Streptomyces cattleya)再分类为卡特兰弯曲抗生素菌(Streptantibioticu cattleya),为弯曲抗生素菌属模式种。
1.3.4 放线嗜酸菌属耐酸链霉菌(Streptomyces acididurans)[45]、桤木链霉菌(Streptomyces alni)[46]、苔藓链霉菌(Streptomyces bryophytorum)[47]、浅绿灰链霉菌(Streptomyces glauciniger)[48]、关杜河链霉菌(Streptomyces guanduensis)[49]、暗红链霉菌(Streptomyces rubidus)[49]、阳陵链霉菌(Streptomyces yanglinensis)[49]、寡孢链霉菌(Streptomyces paucisporeus)[49]、稻根链霉菌(Streptomyces oryziradicis)[50]、土壤链霉菌(Streptomyces soli)[51]和骊川链霉菌(Streptomyces yeochonensis)[52]这11个种的标准菌株在基于保守蛋白的系统基因组学分析中被划分到一个单独的谱系。该谱系内的种与卡特兰链霉菌和狭义链霉菌属成员的16S rRNA基因序列相似度都比较低,分别为96.3%–97.4%和96.1%–97.3%。对这11个种的AAI分析结果显示,该谱系内物种间AAI值在70.6%–87.2% (平均为74.9%),与链霉菌属的AAI值为67.2%–71.3% (平均为69.0%)。由此可见,这些AAI值与不同属间比较的AAI阈值(约70%–74%)一致。这11个标准菌株间的POCP值为41.8%–71.2% (平均为54.2%),其与链霉菌属的POCP值为38.9%–62.7% (平均为47.3%)。这些分析数据支持该谱系被重新划分为链霉菌科的一个新属,并以骊川放线嗜酸菌(Actinacidiphila yeochonensis)为模式种,命名为放线嗜酸菌属(Actinacidiphila)。
1.3.5 红树林放线孢菌属黄灰链霉菌(Streptomyces gilvigriseus)[53]的模式菌株被划分到了一个单独的谱系,且这一谱系只包含这一个种。与这个谱系亲缘最近的是青兰港链霉菌(Streptomyces qinglanensis)[54],两者16S rRNA基因序列相似度较低,为96.2% (两者ANI、dDDH和AAI值分别为74.8%、20.8%和64.1%)。AAI分析结果表明,黄灰链霉菌与链霉菌属的标准菌株间的AAI值为63.9%–64.9% (平均为64.5%),即低于不同属间比较的AAI阈值(约70%–74%)。黄灰链霉菌与链霉菌属所有标准菌株的POCP值不高于55.5% (平均为44.0%),再次表明该谱系可代表一个独立的属。系统发育分析结合较低的AAI值,支持这个谱系被重新划分为链霉菌科的一个新属,并以黄灰红树林放线孢菌(Mangrovactinospora gilvigriseus)为模式种,命名为红树林放线孢菌属(Mangrovactinospora)。
1.3.6 李文均菌属与黄灰链霉菌类似,嗜维生素链霉菌(Streptomyces vitaminophilus)[55-56]和解酪氨酸链霉菌(Streptomyces tyrosinilyticus)[57]的标准菌株也在基于保守蛋白的系统基因组学分析中被划分至链霉菌属之外。在核心基因组系统发育树中,与该谱系系统发育关系密切的卡特兰链霉菌和黄灰链霉菌的平均AAI值分别为69.6%和65.0%,即低于不同属间比较的AAI阈值(约70%–74%)。该谱系内的嗜维生素链霉菌与解酪氨基酸链霉菌的AAI值为73.6%,但是彼此间相比较得出的POCP值为49.5%。根据AAI和系统基因组学数据将它们划分为同一个属更为合理。嗜维生素链霉菌最初被命名为嗜维生素孢囊放线菌(Actinosporangium vitaminophilum)[56],而后Goodfellow等[55]根据其生理特性和化学分类特征将其划入链霉菌属。Krasil’nikov和Yuan (1980年批准名录)[58]提出的孢囊放线菌属(Actinosporangium)目前仍然是有效发表的状态,但是该属的模式种紫色孢囊放线菌(Actinosporangium violaceum)已被划分至链霉菌属[55]。目前该谱系被重新划分为链霉菌科的一个新属,并以嗜维生素李文均菌(Wenjunlia vitaminophila)作为模式种,命名为李文均菌属(Wenjunlia)。
综上研究,基因组系统发育分析、16S rRNA基因序列分析和AAI分析等数据均充分表明链霉菌科中的这6个类群需要在科水平或属水平上进行重分类。目前,部分重分类的提议已被ICSP认可,对于链霉菌分类体系的认知也需要与时俱进。
2 链霉菌分离培养策略 2.1 链霉菌的分离培养研究方法近10年来,基因组测序技术、宏基因组学和系统发育分析方法的进步极大地改变了人们对微生物多样性以及生命之树的看法与见解[59-60]。尽管基因组数据有助于许多关于微生物重要且新颖的研究,但未培养谱系的生物学和生理学功能仍需有效的纯培养物才能得以验证,进而正确理解它们的生态学功能。链霉菌中的大多数化合物也是通过菌株的纯培养物发酵并萃取分离获得。获取可培养的链霉菌资源是深入研究及应用链霉菌强大代谢潜能的重要前提。
分离培养菌株资源的第一步就是对样品进行预处理,正确的预处理能够选择性抑制非目标类群的生长,提高标类群的分离比例和分离效率。干热或室温干燥的方式通常用于抑制和清除土壤中的以革兰氏阴性菌为主的大多数细菌,进而大幅提高放线菌的分离比例[61-62]。针对土壤样品常用的物理预处理方法是超声处理法,这种方法既有助于链霉菌等放线菌类群的细胞或孢子从土壤颗粒上脱落又能够有效杀死常见细菌类群,进而提高链霉菌的分离比例和分离效率[63-65]。
在分离培养研究中,分离培养基的有效性尤为重要。成熟的分离培养策略通常会将多种选择性培养基组合起来,以获取更加丰富的分离菌株数量和种类的多样性。分离培养基的有效性与目标菌株类群的生态特性密切相关,链霉菌的成功培养与碳氮源、矿物质、维生素、水分以及必要的生长因子密切相关[66]。例如,淀粉酪蛋白琼脂(starch casein agar, SCA)在分离链霉菌属和小单孢菌属(Micromonospora)等类群表现出较好的选择性效果[67-69]。培养基一般根据营养物质含量的高低分为富营养培养基和寡营养培养基。已有研究表明,寡营养培养基在链霉菌等放线菌类群的分离上表现得更为高效[3, 70-71]。寡营养培养基能够为菌株创造一个低有机质的生长环境,这对极端生境来源链霉菌等放线菌类群分离更具选择性[71]。链霉菌本身包含着不同的链霉菌类群,不同链霉菌类群对于营养物质的需求也有一定的差异。为了提高菌株分离效果,研究人员也会向分离培养基中添加一定量的抑制剂。在针对链霉菌等放线菌类群的分离培养研究中,在分离平版上与目标类群形成竞争关系的部分革兰氏阳性菌、革兰氏阴性菌以及一些真菌类群会被视为污染物[66]。抗生素等化学物质会被用于抑制或清除“污染物”,进而保证目标类群的分离比例和分离效率[66, 72]。链霉菌分离培养中最常使用的抑制剂分别是放线菌酮(cycloheximide)、制霉菌素(nystatin)和萘啶酮酸(nalidixic acid)[68, 73-78]。
培养条件的设置也是一个重要因素。影响培养条件的主要因素是培养温度和培养时间[79-81]。在针对某个生境的分离培养研究中,掌握样地的温度参数是极为必要的,这对于培养过程中培养温度的设置具有重要参考意义。成熟的分离培养策略通常会设置2个或更多培养温度,以更全面地研究可培养链霉菌的多样性。一般来讲,链霉菌等大多数放线菌类群的最适生长温度是25–30 ℃,这也被视为一般培养温度[82]。极端生境会存在许多有特殊温度偏好的微生物,所以要根据研究对象更灵活地选择一些非常规的培养温度,这样可能会获得更多或更稀有的链霉菌类群[64, 83]。另一个因素就是培养时间,设置特定的培养时间可以分离出不同生长速度的链霉菌类群,但是培养时间的设置要考虑诸多因素,例如分离培养基种类、培养温度和目标类群的生理特征等。
上述几点是分离培养研究中重要的影响因素,研究人员为了达到研究目标往往会将不同影响因素有机结合起来形成一套分离培养体系,这不仅能够高效地得到目标类群,也能减少不必要的成本投入。近期,Li等[7]在对古尔班通古特沙漠样品进行分离培养实验时提出了2套分离培养体系,即常规细菌分离培养规程(conventional culture procedure, CCP)和链霉菌分离培养规程(streptomycetes culture procedure, SCP)。CCP与SCP主要是在分离培养基和抑制剂两个方面做了不同的组合(表 2)。研究结果表明,CCP策略共分离得到细菌978株(占总数61.5%),SCP策略共分离得到细菌611株(占总数39.5%)。然而,在对于链霉菌类群的分离表现上,CCP策略共分离得到链霉菌106株(占链霉菌总数22.6%),SCP策略共分离得到链霉菌363株(占链霉菌总数77.4%)。SCP策略不仅能够分离更多数量的链霉菌菌株,其分离得到的链霉菌具有更高的多样性。尽管CCP策略能够获得更多数量的菌株,但是把研究目标聚焦到链霉菌类群时,SCP策略的效果要远好于CCP策略。这表明根据研究目标建立合适的分离培养体系是一项极其重要的工作。
Training strategies | Isolation medium | Inhibitor (final concentration) |
CCP | R2A, MM, 0.2×TSA, 0.1×NA | 制霉菌素Nystatin (50 mg/L) |
SCP | R2A, MM, AIA, SCA | 制霉菌素、萘啶酮酸、放线菌酮、重铬酸钾 Nystatin (50 mg/L), naphthyridone acid (25 mg/L), cycloheximide (25 mg/L), potassium dichromate (25 mg/L) |
2.2 链霉菌培养与免培养的关系
在当下基因组数据暴发式增长的时代下,细菌和古菌多样性已经有了更深入、更全面的研究[84],这也让人们窥见了巨大的未知微生物多样性,同时产生了一个对微生物学领域具有深远影响的思维范式,即只有不到1%的微生物是可培养的[85-86]。然而,Martiny[87]挑战了这一观点,其研究表明不同环境中的许多微生物类群可以用已知的培养技术培养且远超过1%。Lagier等[88]基于多培养条件、基质辅助激光解吸电离飞行时间质谱(matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, MALDI-TOF MS)和16S rRNA基因序列鉴定技术提出了培养组学(culturomics)概念。通过这一方法培养获得了宏基因组分析未发现的物种,并将人类肠道细菌物种数量增加了至少一倍。Li等[7]对于链霉菌类群的分离培养研究表明,分离培养得到的链霉菌多样性甚至高于免培养的链霉菌多样性。因此,人们不能将“不可培养”视为“永远不能培养”,只是目前缺乏关于未培养微生物的关键生物学信息,尚未找到合适的培养方法。
菌株的分离培养是一项需要巨大的人力、时间和经费投入的研究工作,在分离培养新颖或稀有类群时,这项投入会变得更大。Thrash[89]也据此做了一项关于培养与免培养的成本效益分析,并指出分离培养的研究要基于科学性与合理性。在测序技术支持下,研究人员可以充分利用免培养技术指导分离培养研究,例如,运用宏基因组学技术提供的未培养微生物功能预测结果设计针对目标类群的选择性分离培养策略。
3 后组学时代的链霉菌资源勘探 3.1 链霉菌基因组资源链霉菌的生态分布极其广泛,这种极强的生态适应能力要归因于其次级代谢能力的进化。链霉菌次级代谢能力的进化主要由垂直遗传和水平基因转移这2个因素驱动,这种成熟的交换生物合成基因的机制是链霉菌被视为天然产物丰富宝库的原因。来自生态系统的选择性压力可能诱导链霉菌中巩固自身生态位必要的生物合成基因簇向染色体核心区迁移,这些基因簇可以通过垂直遗传更稳定地保持下去,因此在不同链霉菌菌种之间显示出更高的保守性[90-92]。不仅如此,一些研究指出微生物生物合成核心基因的基因系统发育和物种系统发育之间具有很强的异质性[93-95]。因此在勘探链霉菌天然产物资源时不仅要考虑研究对象的系统发育关系,也要考虑同种不同菌株代谢潜能的差异。
1940–1970年之间的近30年被称为“抗生素发现的黄金时代”,在此期间研究人员从土壤链霉菌中发现了大量沿用至今的抗生素,包括丰富多样的多烯大环内酯、芳香聚酮和糖苷类化合物[96-97]。随着基因组测序技术的出现与发展,生物研究也进入了基因组时代,研究人员发现微生物的代谢潜能远超预期。例如,在天蓝色链霉菌(Streptomyces coelicolor)[98]的模式菌株A3(2)基因序列信息公布之前的半个世纪,研究人员已经将其作为模式生物做了大量研究并分离出了十余种不同构型的天然产物。然而这还未超过其次级代谢编码潜能的一半,通过对天蓝色链霉菌基因组资源挖掘后,研究人员又发现了7种不同构型的天然产物。
相比于经典的化学分析,基因组挖掘具有独特的优势[99]。首先,基因组资源的研究向人们揭示了隐秘生物合成途径的概念,这是指那些已在基因组中被识别,但其同源的天然产物并没有在实验室或工业条件下合成的途径。其次,基因组学研究能够将已发现的分子与其生物合成基因连接起来,使得异源表达和批量生产成为可能,这对于“微生物智造”的产业转化尤为重要。
3.2 链霉菌代谢潜能挖掘微生物次级代谢产物的挖掘一直是热点研究领域。然而近些年来新颖天然产物药物的发掘效率显著下降,许多研究的结果都是重复发掘[100]。在工业或实验室筛选过程中,微生物通常在具有充足营养资源的隔离环境中培养,这与它们复杂且多变的原生栖息环境成了鲜明的对比。同时,目前在实验室环境下激活其生物合成基因簇表达的方法有限,链霉菌的全部代谢潜能尚未能得到完全激发。这些在基因组水平上呈现的代谢潜能如果不能在实验室或者工业生产中得到表达,那么其应用意义将大打折扣。
微生物次级代谢产物被认为是生态系统中资源竞争的重要媒介,对于微生物的生存以及巩固其生态位极其重要[101-102]。因此,微生物的相互作用对相关的生物合成基因簇的激活起着关键作用[103-105]。挖掘微生物代谢潜能同时增加新颖天然产物的发掘需要找到激活生物合成基因簇表达的触发因子[106-107]。如今的实验通常会一些手段来模拟这些自然发生的“化学—生态”关系,这样隐秘生物合成基因簇可能在实验条件下被激活。Onaka等[108]发现将链霉菌菌株S-522与产霉菌酸的肺冢村菌(Tsukamurella pulmonis)共培养可激活抗生素alchivemycin A的隐匿生物合成途径。Sung等[109]将来自海洋的链霉菌与耐甲氧西林的金黄色葡萄球菌(Staphylococcus aureus)等多种人类病原体进行共培养时发现多种抗生素的产量增加且生物活性增强,这表明将链霉菌与具备多重耐药性的细菌共培养可能会成为一种有效的、可针对性挖掘特定病原体抑制活性新化合物的策略。
微生物代谢网络调控的触发因子是多种多样的,这些因子包括细胞与细胞间的物理互作[108, 110]、营养耗竭率变化[111]、化合物前体的酶促反应[112]、水平基因转移[113]和微生物小分子互作[105, 108, 114]。因此,现在针对单个菌株代谢组分的调控方法主要有:改变培养基的组分[115]、诱导抗生素耐药性[116]和微生物共培养[117-118]。然而,对于许多相互作用机制的研究并不明晰,尤其是分子的相互作用机制仍存在许多未知。
如今,如成像质谱等检测技术的发展使微生物之间相互作用产生的分子交换更直观地呈现给研究人员[105, 119]。检测技术的发展也引导了分析技术的发展,现在对于微生物代谢潜能的分析主要运用的是组学研究方式。对于基因组数据有antiSMASH[120]、PRISM[121]等工具提供生物合成基因簇的识别和注释等分析,对于代谢组的质谱数据有GNPS[122]、Qemistree[123]等工具提供化合物预测和聚类等分析。这些方法学的进步有助于阐明上述相互作用所涉及触发因子,进而更全面地挖掘微生物代谢潜能。
4 总结与展望链霉菌作为最具代表性的放线菌之一,是研究人员长期关注的重要功能类群。目前ICSP认证且正确命名的链霉菌成员包含1个科,10个属以及732个物种,还有大量的新物种有待被鉴定。菌株资源是微生物学研究的基础,链霉菌分离培养又是链霉菌资源研究的重中之重。Li等[7]研究揭示了根据目标类群建立合适的分离培养体系的重要性。该研究的培养和免培养多样性结果对比也预示着在新的研究理念和技术支持下将“不可培养”变为“可培养”是极有可能的,提示人们多维度的探索对于充分认识微生物多样性的必要性。
在后组学时代,多组学的联用是研究的大趋势[124],多维度的剖析也有助于更深层次地去全面认知链霉菌。同时,设计出更加科学、高通量的评价及应用技术方法和科学装置,是有效挖掘链霉菌代谢潜能宝库的重要发展方向。随着科学研究方法和技术水平的不断进步,链霉菌整体研究在未来会有更进一步的发展,为人类的健康和生活提供重要保障。
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