ラクトバチルス・アシドフィルス
Lactobacillus acidophilus/ja
| Lactobacillus acidophilus/ja | |
|---|---|
| |
| "ラクトバチルス・アシドフィルス", 数字の目盛りは11 μm | |
Scientific classification 
| |
| Domain: | Bacteria |
| Phylum: | Bacillota |
| Class: | Bacilli |
| Order: | Lactobacillales |
| Family: | Lactobacillaceae |
| Genus: | Lactobacillus |
| Species: | acidophilus
|
| Binomial name | |
| acidophilus | |
ラクトバチルス・アシドフィルス（Lactobacillus acidophilus）（新ラテン語「酸を好む乳酸菌」）は、1900年に乳児の糞便から初めて分離された、棒状のグラム陽性菌、同種発酵、嫌気性菌の微生物である。この種は最も一般的にヒト、特に消化管、口腔、膣、および発酵乳やヨーグルトなどの様々な発酵食品に存在する。この種は低pHレベル（5.0以下）で最も増殖しやすく、至適増殖温度は37℃である。ある種のL. acidophilus株は強いプロバイオティクス効果を示し、酪農生産に商業的に利用されている。L.アシドフィルスのゲノムは解読されている。
L.アシドフィルスは黄色ブドウ球菌、大腸菌、サルモネラ・チフス菌、クロストリジウム・ペルフリンゲンスの増殖に対して拮抗作用を示す。この4菌のうち、最も影響を受けるのは黄色ブドウ球菌である。黄色ブドウ球菌と並んで、もう1つのグラム陽性菌であるC. perfringensは、グラム陰性菌である他の2つの菌よりも、L. アシドフィルス菌の影響を強く受けた。L.アシドフィルスは、ストレプトコッカス・ミュータンスによる口腔プラーク形成も減少させることが判明している。
歴史
ラクトバチルス・アシドフィルスは、1900年にエルンスト・モロ（Ernst Moro）によってヒトの消化管から初めて分離され、当初の名称はBacillus acidophilusであった。時を経て、生物の分類学的特徴を明らかにする方法には多くの変化があり、1929年にLactobacillusという属分けに至った。1つの分離株から複数の菌株が分離され、様々な名前が付けられると、原菌株を見つけるのが難しくなる。L.アシドフィルスに関する研究のほとんどは、ラクトバチルス・アシドフィルスNCFMという1つの菌株に焦点を当てたものであった。L.アシドフィルス菌NCFMに関する大量の情報が発見されたことで、米国食品医薬品局は、この微生物を飲料、乳製品、その他のプロバイオティクス食品の認可成分であると判断した。
生物学的および生化学的特徴
形態学的特徴
_Lactobacillus_acidophilus_(D%C3%B6derlein_bacillus).jpg)
ラクトバチルス・アシドフィルスは、大きさが2-10μmである、動かない棒状の（桿菌）グラム陽性生物である。アシドフィルスはリン脂質二重層膜を持ち、膜の外側にはペプチドグリカンからなる大きな細胞壁がある。L.アシドフィルスの細胞壁はテイコ酸と表面タンパク質が織り成すもので、陰イオン性多糖類と中性多糖類、そして細胞の外側を覆うS層がある。L.アシドフィルス菌のS層タンパク質は、粘液や他の細胞外タンパク質だけでなく、上皮細胞にも接着することが示されている。S-層は2つの構造ドメインからできている。C末端ドメインは細胞壁の固定を担い、N末端ドメインは細胞環境との相互作用とS層の自己形成を担っている。L.アシドフィルス属では、N-末端領域はアミノ酸変異が大きく、配列相同性も低い（31-72%）。L.アシドフィルス菌は、鞭毛やピルのような細胞外運動手段を持たないため、動かない微生物である。
_Lactobacillus_acidophilus_(D%C3%B6derlein_bacillus).jpg)
代謝
アシドフィルスはホモ発酵性嫌気性微生物であり、発酵の最終産物として乳酸のみを産生する。また、EMP経路（解糖系）によりヘキソース（ペントースではない）のみを発酵させることができる。L.アシドフィルスは、利用可能な栄養素が限られているため、宿主の中にいるときよりも牛乳の中で成長する時間が遅い。牛乳中のプロバイオティクスとして使用されているため、アメリカのJournal of Dairy Scienceが行った研究では、L. アシドフィルスの低い増殖速度を上げるために必要な栄養素を調べた。この研究では、グルコースとアミノ酸システイン、グルタミン酸、イソロイシン、ロイシン、リジン、メチオニン、フェニルアラニン、スレオニン、チロシン、バリン、アルギニンがL. アシドフィルスの増殖に必須な栄養素であることがわかった。グリシン、パントテン酸カルシウム、Mn2+は刺激性栄養素として作用する。この研究は、牛乳中のアシドフィルス菌の増殖率が低いことを説明するのに役立つ。消費率の高いアミノ酸を発酵乳に加えることは、この問題の解決策になりうる。
Genomics
The specialization of prokaryotic genomes is distinguishable when recognizing how the prokaryote replicates its DNA during replication. In L. acidophilus, replication begins at an origin called oriC and moves bi-directionally in the form of replication forks. The DNA is synthesized continuously on the leading strand and in discontinuous Okazaki fragments on the lagging strand with help from the DNA polymerase III enzyme. An RNA primer is needed to initiate the DNA synthesis on the leading and lagging strands. DNA polymerase III follows the RNA primer with the synthesis of DNA in the 5' to 3' direction. L. acidophilus consists of a small genome with a low guanine-cytosine content, approximately 30%. A study comparing 46 genomes of varying strains of L. acidophilus found the genome size ranged from 1.95 Mb to 2.09 Mb, with an average size of 1.98 Mb. The average number of coding sequences in the genome was 1780, with the strains isolated from fermented foods and commercial probiotics having more coding sequences on average than those isolated from humans. L. acidophilus has an open state pan-genome (all of the genes within a species), meaning that the pan-genome size increased as the number of genomes sequenced increased. The core-genome (the genes shared by all individuals of a species) consist of around 1117 genes in the case of L. acidophilus. Genetic analysis also revealed that all L. acidophilus strains contained at least 15 families of glycosyl hydrolases, which are the key enzymes in carbohydrate metabolism. Each of the 15 GH families were involved in metabolizing common carbohydrates, such as glucose, galactose, fructose, sucrose, starch, and maltose. Genes encoding antibiotic resistance by means of antibiotic efflux, antibiotic target alteration, and antibiotic target protection were present in all L. acidophilus strains, providing protection against 18 different classes of antibiotic across all strains. Fluoroquinolone, glycopeptide, lincosamide, macrolide and tetracycline were the five classes of antibiotic to which L. acidophilus displayed the highest level of tolerance, with more than 300 genes relevant to these classes.
Environment
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L. acidophilus grows naturally in the oral, intestinal, and vaginal cavities of mammals. Nearly all Lactobacillus species have special mechanisms for heat resistance which involves enhancing the activity of chaperones. Chaperones are highly conserved stress proteins that allow for enhanced resistance to elevated temperatures, ribosome stability, temperature sensing, and control of ribosomal function at high temperatures. This ability to function at high temperatures is extremely important to cell yield during the fermentation process, and genetic testing on L. acidophilus in order to increase its temperature tolerance is currently being done. When being considered as a probiotic, it is important for L. acidophilus to have traits suitable for life in the gastrointestinal tract. Tolerance of low pH and high toxicity levels are often required. These traits vary and are strain specific. Mechanisms by which these tolerances are expressed include differences in cell wall structure, along with other changes is protein expression. Changes in salt concentration have been shown to affect L. acidophilus viability, but only after exposure to higher salt concentrations. In another experiment highlighted by the American Dairy Science Association, viable cell counts only showed a significant reduction after exposure to NaCl concentrations of 7.5% or higher. Cells were also observed to distinctly elongate when grown in conditions of 10% NaCl concentration or higher. L. acidophilus is also very well suited for living in a dairy medium, as fermented milk is the ideal method of delivery for introducing L. acidophilus into a gut microbiome. The viability of L. acidophilus cells encapsulated by spray drying technology stored at refrigerated condition (4 °C) is higher than the viability of cells stored at room temperature (25 °C).
Quorum sensing
Quorum sensing among cells is the process among which cell signaling can lead to coordinated activities which can ultimately help bacteria control gene expression in a consecutive sequence. This is accomplished via detection of small autoinducers which are secreted in response to increasing cell-population density. In Lactobacillus acidophilus, which can be found in the gastrointestinal tract, quorum sensing is important for bacterial interaction when considering biofilm formation and toxin secretion. In L. acidophilus, along with many other bacteria, the luxS-mediated quorum sensing is involved in the regulation of behavior. In monoculture, the production of luxS increased during the exponential growth phase and started to plateau as it progressed to the stationary phase. Up-regulation of luxS can occur when L. acidophilus is placed in co-cultivation with another Lactobacillus species.
Vaginal microbiota
Lactobacillus acidophilus is part of the vaginal microbiota along with other species in the genus including Lactobacillus crispatus, Lactobacillus gasseri, Lactobacillus jensenii, and Lactobacillus iners. In experiments, L. acidophilus seemed to decrease Candida albicans’ ability to adhere to vaginal epithelial cells; however, L. acidophilus’ role in preventing yeast infections is unclear because this species of Lactobacilli has also been found not to have a very strong ability to adhere to (and thereby colonize) the vaginal cells.
Therapeutic uses
Research has shown that the presence of L. acidophilus can produce a variety of probiotic effects in humans, such as acting as a barrier against pathogens, assisting in lactose digestion, enhancing immune response, and reducing cholesterol level. L. acidophilus must exist in concentrations of 10^5 - 10^6 c.f.u (colony-forming units) per mL in order for these effects to be seen. A study conducted at the Wake Forest School of Medicine examined the effects of L. acidophilus on the structure and composition of the gut microbiome of mice with respect to the age of the mice. The research established the importance of the interactions between microbes within a gut microbial environment on the overall health of the organism, and the data showed that mice supplemented with L. acidophilus had reduced proteobacteria levels, and increased levels of other probiotic bacteria when compared to other mice of similar age. Another study conducted at Maranatha Christian University studied the impact of L. acidophilus cell free supernatants (a liquid medium containing the metabolites produced by microbial growth) on the growth pattern Salmonella typhi, the microbe assiciated with Typhoid fever. The study showed that the presence of L. acidophilus metabolites significantly inhibited the growth curves displayed by S. typhi, supporting the idea that L. acidophilus presence has a positive impact on the species makeup of a gut microbial community, providing the organism with intestinal health benefits. The innate immune system of L. acidophilus also produces antimicrobial peptides. The group of short peptides found there have shown antimicrobial properties such as their strength against viruses and other cell types, including cancer cells. There is also some evidence supporting the use of a symbiotic gel (containing L. acidophilus) in treating gastrointestinal symptoms in patients who had received a hemodialysis treatment. This gel also reduced the occurrence of vomit, heartburn, and stomachaches. Further study concerning this subject is needed to draw firm conclusions.
Dairy industry usage
As stated in a journal from the American Dairy Science Association, "Lactobacillus acidophilus is a commercial strain and probiotic that is widely used in the dairy industry to obtain high-quality fermentation products." Increased levels of beneficial bacteria, and decreased levels of pathogenic bacteria within the intestine due to the consumption of fermented milk containing strains of L. acidophilus has a range of probiotic effects. Reduced serum cholesterol levels, stimulated immune response, and improved lactic acid digestion are all probiotic effects associated with intestinal L. acidophilus presence. L. acidophilus was also effective in reducing Streptococcus mutans levels in saliva, as well as decreasing risk factors associated with the development of nonalcoholic fatty liver disease. The strain of L. acidophilus that has been most widely researched, and is most widely used as an antibiotic and is referred to as NCFM.
Side effects
Although probiotics are generally safe, when they are used by oral administration there is a small risk of passage of viable bacteria from the gastrointestinal tract to the blood stream (bacteremia), which can cause adverse health consequences. Some people, such as those with a compromised immune system, short bowel syndrome, central venous catheters, cardiac valve disease and premature infants, may be at higher risk for adverse events.
こちらも参照
外部リンク
- Lactobacillus at MedlinePlus
- Lactobacillus acidophilus at University of Maryland Medical Center
- Lactobacillus acidophilus from the U. of Wisconsin
- Global analysis of carbohydrate utilization by Lactobacillus acidophilus using cDNA microarrays
- Safety and protective effect of Lactobacillus acidophilus and Lactobacillus casei used as probiotic agent in vivo
- Pictures and research on Lactobacillus acidophilus Archived 2017-07-01 at the Wayback Machine
- Type strain of Lactobacillus acidophilus at BacDive – the Bacterial Diversity Metadatabase
- Lactobacillus acidophilus - Benefits & Probiotic Uses at Pocket Reviewer