枯草菌
Bacillus subtilis/ja
枯草菌（Bacillus subtilis）は、hay bacillusまたは、grass bacillusとして知られ、グラム陽性でカタラーゼ陽性の細菌であり、土壌や反芻動物、ヒト、海綿の消化管に生息する。バチルス属の一員として、枯草菌は棒状であり、強靭で保護的な内胞子を形成することができるため、極端な環境条件にも耐えることができる。枯草菌は歴史的に偏性好気性菌に分類されてきたが、通性嫌気性菌であるという証拠も存在する。枯草菌は最もよく研究されたグラム陽性菌であり、細菌の染色体複製と細胞分化を研究するモデル生物であると考えられている。分泌型酵素産生のチャンピオン菌のひとつであり、バイオテクノロジー企業によって工業的規模で利用されている。
| Bacillus subtilis/ja | ||
|---|---|---|
| ||
| [[Transmission electron microscopy/ja | TEM]]写真である。枯草菌細胞の断面の顕微鏡写真（スケールバー＝200nm） | |
Scientific classification 
| ||
| Domain: | Bacteria | |
| Phylum: | Bacillota | |
| Class: | Bacilli | |
| Order: | Bacillales | |
| Family: | Bacillaceae | |
| Genus: | Bacillus | |
| Species: | subtilis
| |
| Binomial name | ||
| subtilis (Ehrenberg 1835)
Cohn 1872 | ||
| Synonyms | ||
| ||
解説
枯草菌はグラム陽性の細菌で、棒状でカタラーゼ陽性である。当初はChristian Gottfried EhrenbergによってVibrio subtilisと命名され、1872年にFerdinand CohnによってBacillus subtilisと改名された（subtilisはラテン語で「細い、薄い、細長い」を意味する）。枯草菌の細胞は典型的な棒状で、長さ約4-10μm、直径0.25-1.0μm、定常期の細胞容積は約4.6fLである。
他のバチルス属のメンバーと同様に、温度や乾燥などの極端な環境条件に耐えるために内胞子を形成することができる。枯草菌は通性嫌気性菌であり、1998年までは偏性好気性菌と考えられていた。枯草菌は鞭毛虫が多く、液体中を素早く移動する能力を持つ。
枯草菌は遺伝子操作に非常に従順であることが証明され、実験室での研究、特に細胞分化の単純化された例である胞子形成のモデル生物として広く採用されるようになった。実験室のモデル生物としての人気という点では、枯草菌は、広く研究されているグラム陰性細菌である大腸菌と同等のグラム陽性細菌と見なされることが多い。
特徴
枯草菌のコロニー、形態学的、生理学的、生化学的特性を下表に示す。
| テストタイプ | テスト | 特徴 |
| コロニー特徴 | サイズ | ミディアム |
| タイプ | ラウンド | |
| 色 | 白っぽい | |
| 形 | 凸型 | |
| 形態学的特徴 | 形 | 棒状 |
| 生理的特徴 | 運動能力 | + |
| 6.5%NaClでの成長 | + | |
| 生化学的特性 | グラム染色 | + |
| オキシダーゼ | - | |
| カタラーゼ | + | |
| 酸化性-発酵性 | 発酵性 | |
| 運動能力 | - | |
| メチルレッド | - | |
| フォーゲス・プロスカウアー | + | |
| インドール | - | |
| H2S生成 | + | |
| ウレアーゼ | - | |
| 硝酸塩還元酵素 | + | |
| β-ガラクトシダーゼ | + | |
| 加水分解 | ゼラチン | + |
| エースクリン | + | |
| カゼイン | + | |
| トゥイーン40 | + | |
| トゥイーン60 | + | |
| トゥイーン80 | + | |
| 酸生成 | グリセロール | + |
| ガラクトース | + | |
| D-グルコース | + | |
| D-フルクトース | + | |
| D-マンノース | + | |
| マンニトール | + | |
| N-アセチルグルコサミン | + | |
| アミグダリン | + | |
| マルトース | + | |
| D-メリビオース | + | |
| D-トレハロース | + | |
| グリコーゲン | + | |
| D-トゥラノーズ | + |
注：＋＝プラス、-＝マイナス
生息地
この種は土壌の上層に多く生息し、枯草菌はヒトの正常な腸内常在菌であると考えられている。2009年の研究では、土壌に含まれる芽胞の密度（1グラムあたり約106個の芽胞）とヒトの糞便に含まれる芽胞の密度（1グラムあたり約104個の芽胞）を比較した。ヒトの腸内で発見された芽胞の数は、食物汚染による消費のみに起因するには多すぎた。いくつかのハチの生息地では、ミツバチの腸内細菌叢に枯草菌が現れる。枯草菌は海洋環境にも存在する。
枯草菌が腐生性であることを示す証拠がある。研究によると、この細菌は有機物の豊富な土壌で植物成長を示し、栄養分が枯渇すると胞子が形成されることが示されている。さらに、枯草菌は植物の根にバイオフィルムを形成することが示されており、腸内細菌叢によく見られる理由も説明できるかもしれない。おそらく、枯草菌のバイオフィルムを持つ植物を食べた動物は、消化管内でこの細菌の増殖を促進することができるのだろう。枯草菌のライフサイクル全体が消化管内で完結することが示されており、このことは、この細菌が植物の摂取によって腸内に入り、腸内で増殖する能力の結果として存在し続けるという考えに信憑性を与えている。
繁殖
枯草菌は対称的に分裂して2つの娘細胞を作る（二元分裂）か、非対称的に分裂して単一の内胞子を作り、数十年間生存し続けることができ、干ばつ、塩分、極端なpH、放射線、溶媒などの好ましくない環境条件に耐性を持つ。胞子嚢は栄養ストレスのある時に、加水分解を利用して形成され、条件が好転するまで生物が環境中に留まることを可能にする。胞子形成のプロセスの前に、細胞は鞭毛を産生することによって運動性になったり、環境からDNAを取り込んだり、抗生物質を産生したりする。これらの反応は、より好都合な環境を求めて栄養を探し求め、細胞が新たな有益な遺伝物質を利用できるようにする試み、あるいは単に競合を退治する試みと見なされる。
栄養欠乏などのストレス条件下では、枯草菌は胞子形成のプロセスを経る。この過程は非常によく研究されており、胞子形成を研究するためのモデル生物となっている。
胞子形成
枯草菌が胞子形成に入ると、シグマ因子シグマFが分泌される。この因子は胞子形成を促進する。胞子形成隔壁が形成され、染色体が前胞子の中にゆっくりと移動する。染色体の1コピーの3分の1が前胞体にあり、残りの3分の2が母細胞にあるとき、前胞体の染色体断片にはシグマFの遺伝子座があり、前胞体で発現が始まる。母細胞でのシグマFの発現を防ぐために、spoIIABがコードするアンチシグマ因子が発現する。前胞子に残存するアンチシグマ因子（そうでなければ胞子形成を阻害する）は、spoIIAAによってコードされるアンチシグマ因子によって阻害される。SpoIIAAはシグマ因子の遺伝子座の近くに位置するため、前胞子で常に発現している。spoIIAB遺伝子座はシグマF遺伝子座とspoIIAA遺伝子座の近くにはないので、母細胞でのみ発現し、その細胞での胞子形成を抑制し、前胞子での胞子形成を継続させる。母細胞に残存するspoIIAAはspoIIABを抑制するが、spoIIABは常に置換されるため、胞子形成を抑制し続ける。したがって、枯草菌の染色体の遺伝的非対称性とシグマF、spoIIAB、spoIIAAの発現が、枯草菌の胞子形成を決定している。
染色体の複製
枯草菌は細菌の染色体複製の研究に用いられるモデル生物である。単一環状染色体の複製は、一つの遺伝子座である起点（oriC）で始まる。） 複製は双方向に進行し、2つの複製フォークが染色体に沿って時計回りと反時計回りに進行する。染色体の複製は、フォークが染色体地図上で原点と反対側に位置する終端領域に達すると完了する。終端領域には、複製停止を促すいくつかの短いDNA配列（Ter部位）がある。特定のタンパク質がDNA複製のすべてのステップを仲介する。枯草菌と大腸菌の染色体DNA複製に関与するタンパク質を比較すると、類似点と相違点が明らかになる。複製の開始、伸長、終結を促進する基本的な構成要素はよく保存されているが、いくつかの重要な違いが見られる（例えば、一方の細菌にはもう一方の細菌に必須なタンパク質が欠けているなど）。これらの違いは、様々な細菌種がゲノムの複製を行うために採用してきた機構や戦略の多様性を強調している。
ゲノム
枯草菌には約4,100の遺伝子がある。このうち、必須であることが示されたのはわずか192個で、さらに79個が必須であると予測された。必須遺伝子の大部分は、細胞代謝の比較的少数の領域に分類され、約半分が情報処理に、5分の1が細胞外皮の合成と細胞の形と分裂の決定に、そして10分の1が細胞エネルギーに関係していた。
QB928株の全ゲノム配列は4,146,839塩基対、4,292遺伝子を持つ。QB928株は様々なマーカー[aroI(aroK)906 purE1 dal(alrA)1 trpC2]が存在するため、遺伝学的研究に広く用いられている。
2009年、枯草菌ゲノムにおいて、いくつかのノンコーディングRNAが明らかにされた。 マイクロアレイを用いた比較ゲノム解析により、枯草菌のメンバーはかなりのゲノム多様性を示すことが明らかになった。
FsrA is a small RNA found in Bacillus subtilis. It is an effector of the iron sparing response, and acts to down-regulate iron-containing proteins in times of poor iron bioavailability.
A promising fish probiotic, Bacillus subtilis strain WS1A, that possesses antimicrobial activity against Aeromonas veronii and suppressed motile Aeromonas septicemia in Labeo rohita. The de novo assembly resulted in an estimated chromosome size of 4,148,460 bp, with 4,288 open reading frames. B. subtilis strain WS1A genome contains many potential genes, such as those encoding proteins involved in the biosynthesis of riboflavin, vitamin B6, and amino acids (ilvD) and in carbon utilization (pta).
Transformation
Natural bacterial transformation involves the transfer of DNA from one bacterium to another through the surrounding medium. In B. subtilis the length of transferred DNA is greater than 1,271 kb (more than 1 million bases). The transferred DNA is likely double-stranded DNA and is often more than a third of the total chromosome length of 4,215 kb. It appears that about 7–9% of the recipient cells take up an entire chromosome.
In order for a recipient bacterium to bind, take up exogenous DNA from another bacterium of the same species and recombine it into its chromosome, it must enter a special physiological state called competence. Competence in B. subtilis is induced toward the end of logarithmic growth, especially under conditions of amino-acid limitation. Under these stressful conditions of semistarvation, cells typically have just one copy of their chromosome and likely have increased DNA damage. To test whether transformation is an adaptive function for B. subtilis to repair its DNA damage, experiments were conducted using UV light as the damaging agent. These experiments led to the conclusion that competence, with uptake of DNA, is specifically induced by DNA-damaging conditions, and that transformation functions as a process for recombinational repair of DNA damage.
While the natural competent state is common within laboratory B. subtilis and field isolates, some industrially relevant strains, e.g. B. subtilis (natto), are reluctant to DNA uptake due to the presence of restriction modification systems that degrade exogenous DNA. B. subtilis (natto) mutants, which are defective in a type I restriction modification system endonuclease, are able to act as recipients of conjugative plasmids in mating experiments, paving the way for further genetic engineering of this particular B. subtilis strain.
By adopting Green Chemistry in the use of less hazardous materials, while saving cost, researchers have been mimicking nature's methods of synthesizing chemicals that can be useful for the food and drug industry, by "piggybacking molecules on shorts strands of DNA" before they are zipped together during their complementary base pairing between the two strands. Each strand will carry a particular molecule of interest that will undergo a specific chemical reaction simultaneously when the two corresponding strands of DNA pairs hold together like a zipper, allowing another molecule of interest, to react with one another in controlled and isolated reaction between those molecules being carried into these DNA complementary attachments. By using this method with certain bacterias that naturally follow a process replication in a multi-step fashion, the researchers can simultaneously carry on the interactions of these added molecules to interact with enzymes and other molecules used for a secondary reaction by treating it like a capsule, which is similar to how the bacteria performs its own DNA replication processes.
Uses
20th century
Cultures of B. subtilis were popular worldwide, before the introduction of antibiotics, as an immunostimulatory agent to aid treatment of gastrointestinal and urinary tract diseases. It was used throughout the 1950s as an alternative medicine, which upon digestion has been found to significantly stimulate broad-spectrum immune activity including activation of secretion of specific antibodies IgM, IgG and IgA and release of CpG dinucleotides inducing interferon IFN-α/IFNγ producing activity of leukocytes and cytokines important in the development of cytotoxicity towards tumor cells. It was marketed throughout America and Europe from 1946 as an immunostimulatory aid in the treatment of gut and urinary tract diseases such as Rotavirus and Shigellosis. In 1966, the U.S. Army dumped bacillus subtilis onto the grates of New York City subway stations for five days in order to observe people's reactions when coated by a strange dust. Due to its ability to survive, it is thought to still be present there.
The antibiotic bacitracin was first isolated from a variety of Bacillus licheniformis named "Tracy I" in 1945, then considered part of the B. subtilis species. It is still commercially manufactured by growing the variety in a container of liquid growth medium. Over time, the bacteria synthesizes bacitracin and secretes the antibiotic into the medium. The bacitracin is then extracted from the medium using chemical processes.
Since the 1960s B. subtilis has had a history as a test species in spaceflight experimentation. Its endospores can survive up to 6 years in space if coated by dust particles protecting it from solar UV rays. It has been used as an extremophile survival indicator in outer space such as Exobiology Radiation Assembly, EXOSTACK, and EXPOSE orbital missions.
Wild-type natural isolates of B. subtilis are difficult to work with compared to laboratory strains that have undergone domestication processes of mutagenesis and selection. These strains often have improved capabilities of transformation (uptake and integration of environmental DNA), growth, and loss of abilities needed "in the wild". And, while dozens of different strains fitting this description exist, the strain designated '168' is the most widely used. Strain 168 is a tryptophan auxotroph isolated after X-ray mutagenesis of B. subtilis Marburg strain and is widely used in research due to its high transformation efficiency.
Bacillus globigii, a closely related but phylogenetically distinct species now known as Bacillus atrophaeus was used as a biowarfare simulant during Project SHAD (aka Project 112). Subsequent genomic analysis showed that the strains used in those studies were products of deliberate enrichment for strains that exhibited abnormally high rates of sporulation.
A strain of B. subtilis formerly known as Bacillus natto is used in the commercial production of the Japanese food nattō, as well as the similar Korean food cheonggukjang.
21st century
- As a model organism, B. subtilis is commonly used in laboratory studies directed at discovering the fundamental properties and characteristics of Gram-positive spore-forming bacteria. In particular, the basic principles and mechanisms underlying formation of the durable endospore have been deduced from studies of spore formation in B. subtilis.
- Its surface-binding properties play a role in safe radionuclide waste [e.g. thorium (IV) and plutonium (IV)] disposal.
- Due to its excellent fermentation properties, with high product yields (20 to 25 gram per litre) it is used to produce various enzymes, such as amylase and proteases.
- B. subtilis is used as a soil inoculant in horticulture and agriculture.
- It may provide some benefit to saffron growers by speeding corm growth and increasing stigma biomass yield.
- It is used as an "indicator organism" during gas sterilization procedures, to ensure a sterilization cycle has completed successfully. Specifically B. subtilis endospores are used to verify that a cycle has reached spore-destroying conditions.
- B. subtilis has been found to act as a useful bioproduct fungicide that prevents the growth of Monilinia vaccinii-corymbosi, a.k.a. the mummy berry fungus, without interfering with pollination or fruit qualities.
- Both metabolically active and non-metabolically active B. subtilis cells have been shown to reduce gold (III) to gold (I) and gold (0) when oxygen is present. This biotic reduction plays a role in gold cycling in geological systems and could potentially be used to recover solid gold from said systems.
Novel and artificial substrains
- Novel strains of B. subtilis that could use 4-fluorotryptophan (4FTrp) but not canonical tryptophan (Trp) for propagation were isolated. As Trp is only coded by a single codon, there is evidence that Trp can be displaced by 4FTrp in the genetic code. The experiments showed that the canonical genetic code can be mutable.
- Recombinant strains pBE2C1 and pBE2C1AB were used in production of polyhydroxyalkanoates (PHA), and malt waste can be used as their carbon source for lower-cost PHA production.
- It is used to produce hyaluronic acid, which is used in the joint-care sector in healthcare and cosmetics.
- Monsanto has isolated a gene from B. subtilis that expresses cold shock protein B and spliced it into their drought-tolerant corn hybrid MON 87460, which was approved for sale in the US in November 2011.
- A new strain has been modified to convert nectar into honey by secreting enzymes.
Safety
In other animals
Bacillus subtilis was reviewed by the US FDA Center for Veterinary Medicine and found to present no safety concerns when used in direct-fed microbial products, so the Association of American Feed Control Officials has listed it approved for use as an animal feed ingredient under Section 36.14 "Direct-fed Microorganisms". The Canadian Food Inspection Agency Animal Health and Production Feed Section has classified Bacillus culture dehydrated approved feed ingredients as a silage additive under Schedule IV-Part 2-Class 8.6 and assigned the International Feed Ingredient number IFN 8-19-119. On the other hand, several feed additives containing viable spores of B. subtilis have been positively evaluated by the European Food Safety Authority, regarding their safe use for weight gaining in animal production.
In humans
Bacillus subtilis spores can survive the extreme heat generated during cooking. Some B. subtilis strains are responsible for causing ropiness or rope spoilage – a sticky, stringy consistency caused by bacterial production of long-chain polysaccharides – in spoiled bread dough and baked goods. For a long time, bread ropiness was associated uniquely with B. subtilis species by biochemical tests. Molecular assays (randomly amplified polymorphic DNA PCR assay, denaturing gradient gel electrophoresis analysis, and sequencing of the V3 region of 16S ribosomal DNA) revealed greater Bacillus species variety in ropy breads, which all seems to have a positive amylase activity and high heat resistance.
B. subtilis CU1 (2 × 109 spores per day) was evaluated in a 16-week study (10 days administration of probiotic, followed by 18 days wash-out period per each month; repeated same procedure for total 4 months) to healthy subjects. B. subtilis CU1 was found to be safe and well tolerated in the subjects without any side effects.
Bacillus subtilis and substances derived from it have been evaluated by different authoritative bodies for their safe and beneficial use in food. In the United States, an opinion letter issued in the early 1960s by the Food and Drug Administration (FDA) designated some substances derived from microorganisms as generally recognized as safe (GRAS), including carbohydrase and protease enzymes from B. subtilis. The opinions were predicated on the use of nonpathogenic and nontoxicogenic strains of the respective organisms and on the use of current good manufacturing practices. The FDA stated that the enzymes derived from the B. subtilis strain were in common use in food prior to January 1, 1958, and that nontoxigenic and nonpathogenic strains of B. subtilis are widely available and have been safely used in a variety of food applications. This includes consumption of Japanese fermented soy bean, in the form of Natto, which is commonly consumed in Japan, and contains as many as 108 viable cells per gram. The fermented beans are recognized for their contribution to a healthy gut flora and vitamin K2 intake; during this long history of widespread use, natto has not been implicated in adverse events potentially attributable to the presence of B. subtilis. The natto product and the B. subtilis natto as its principal component are FOSHU (Foods for Specified Health Use) approved by the Japanese Ministry of Health, Labour, and Welfare as effective for preservation of health.
Bacillus subtilis has been granted "Qualified Presumption of Safety" status by the European Food Safety Authority.
こちらも参照
外部リンク
Media related to Bacillus subtilis/ja at Wikimedia Commons- SubtiWiki "up-to-date information for all genes of Bacillus subtilis"
- Bacillus subtilis Final Risk Assessment on EPA.gov. Archived from the original on 2015-09-09.
- Bacillus subtilis genome browser
- Type strain of Bacillus subtilis at BacDive - the Bacterial Diversity Metadatabase
