新生児の腸内生態系、帝王切開で変化か　英研究

【東京ＡＦＰ＝時事】人の健康状態に生涯にわたって影響を及ぼす複雑な腸内生態系の形成に、出産の方法が大きくかかわっている可能性があることが最新の研究で明らかになった。（写真は資料写真）
　１９日に発表された研究によると、帝王切開で生まれる新生児は産道を通る自然分娩（ぶんべん）児に比べて、母親からもたらされる腸内細菌が少なく、病院内環境から取り込まれる細菌が多かったという。
　研究ではまた、出産時に母親が抗生物質を服用していた場合とそうでないとでは、新生児にみられるヒト常在細菌叢（そう、マイクロバイオーム）として知られる腸内環境に違いがあることも分かった。研究は７年にわたって行われたという。
　英科学誌ネイチャーに掲載された論文の執筆者らは、今回の研究を通じて、出産が腸内細菌によって構築されるヒト免疫系のその後を決定付ける時期だという証拠を得られる可能性もあると考えているという。
　英ロンドン大学ユニバーシティー・カレッジのナイジェル・フィールド臨床准教授は、今回の研究の発表会で「われわれの仮説は、出産の期間は免疫系にとっての感温期なのかもしれないというものだ。（中略）これにより、生涯にわたる免疫系が『確立』される」と説明した。
　なお、新生児の腸内細菌の違いは離乳までに消失した。そのため常在細菌叢の変化が、帝王切開児により多くみられる小児期の病気に関連している明確な証拠はまだ得られていない。
　■便と血液サンプルを精査
　英国の母親と新生児を対象とした研究では、便と血液のサンプル数万件が調べられた。サンプルは数週間後、数か月後にも継続して採取され、出生初期の新生児の腸内に存在する細菌の種類について、かつてないほど詳細な調査が行われた。
　その結果、自然分娩児は母親の腸内に生息する細菌を獲得している一方、帝王切開児は病院に由来する「日和見病原菌」が定着していることが判明した。日和見病原菌は直ちに害を及ぼすわけではないが、免疫系が低下したり細菌が体の他の部位に移動したりした場合に病気を引き起こす恐れがある。
　■細菌はどこから…
　今回の研究結果の一つは、母親の持つ細菌が新生児に定着する過程に関する従来説を覆すものとなった。これまでは、母親の膣（ちつ）管に由来する細菌が鍵となると考えられていたが、新生児の常在細菌叢を詳しく調査した結果、膣由来の細菌はほとんど存在しないことが明らかになったのだ。
　では、細菌はどこからもたらされたのか。
　論文の執筆者で、英バーミンガム大学のピーター・ブロックルハースト教授（女性健康学）は、「出産時、新生児の口が母親の肛門の近くを通過することと関係しているのだろう」と話す。
　「ほぼ確実にそこで新生児は細菌を取り込むと思われる。（中略）帝王切開では、新生児がその領域の近くに行くことはない」
　今回の研究について論文の執筆者らは、極めて複雑な分野に足を踏み入れたばかりで、抗生物質が新生児の常在細菌叢に与える影響など、数多くの疑問がまだ残っているとして注意を促している。【翻訳編集ＡＦＰＢＢＮｅｗｓ】
〔ＡＦＰ＝時事〕（2019/09/20-08:37）

The complex ecosystem of the human gut influences our health for life, and new research shows it may be crucially shaped by the way we come into the world.
A seven-year study published Thursday shows babies delivered by caesarean section have fewer gut microbes from their mother, and more bacteria from the hospital environment, than those delivered vaginally.
And it also found differences between the gut environment -- known as the microbiome -- of babies whose mothers received antibiotics during birth, compared to those who didn't.
The authors believe it could provide evidence that birth is a defining moment for the human immune system, which is shaped by our gut bacteria.
The hypothesis is that the moment of birth might be a thermostatic moment for the immune system... which 'sets' the immune system for life, said author Nigel Field, clinical associate professor at University College London, at a presentation of the research.
But the researchers caution that while the findings published in the journal Nature are ground-breaking, the implications are unclear.
Differences in the gut bacteria of newborns evened out by the time they were weaned, and there is no definitive evidence yet that the altered microbiome is tied to the childhood conditions more often seen in babies delivered by caesarean.
- 'Poo flying around' -
The study examined tens of thousands of faecal and blood samples collected from mothers and babies in Britain.
There was a lot of poo flying around, conceded author Peter Brocklehurst, professor of women's health at the University of Birmingham.
It examined in more detail than ever before what kind of bacteria were in babies' guts in the early days of life, with further sampling weeks and months later.
It found that babies born naturally acquired bacteria found in their mother's guts, but babies delivered by C-section were instead colonised by opportunistic pathogens from hospitals.
These pathogens are not immediately harmful, but can cause sickness if the immune system is weakened or the bacteria moves to other parts of the body.
The researchers cautioned that the study was not yet a reason to alter childbirth practices.
We are quite keen that we don't cause any upset or distress to women because we don't know what the consequences of these findings are, said Brocklehurst.
They could be associated with long-term outcomes for the baby, but we don't know that yet.
- 'Much more work' -
One of the study's findings overturns a previous theory about how a newborn's gut is colonised by the mother's bacteria.
It had been thought that bacteria from the mother's vaginal canal was key to the process, but examination of the newborn microbiome showed few of the bacteria present there were from the vagina.
So where do the bacteria come from?
It may not be a coincidence that you're born with your mouth close to your mother's anus, said Brocklehurst.
That's almost certainly where they pick up those bacteria... so of course if you're born by caesarean section, you don't go anywhere near that area.
The authors cautioned the work is only an initial foray into a highly complex field, with many questions remaining, including the role that antibiotics play in altering the newborn microbiome.
In Britain, all mothers delivering by caesarean are given antibiotics, making it hard to tease apart the effects due to the delivery method rather than the medication.
It is clear that delivery does play a role, the authors said, as evidenced by previous research in countries where antibiotics are not routine during C-sections. But further work is needed to separate out the different impacts.
The authors also want to examine how home births affect the microbiome, and to follow a larger cohort of babies over a longer period to look for links between delivery method and illnesses like asthma and diabetes.
Brocklehurst said that might involve as many as 40,000 infants.
We need to do much more work, he said.