细菌和行为学

　　Gut instinct

　　肠道反应

　　Tantalising evidence that intestinal bacteria can influence mood

　　逗引的证据表明，肠道细菌可以影响人的情绪

　　Sep 3rd 2011 | from the print edition

　　A GOOD way to make yourself unpopular at dinner parties is to point out that a typical person is, from a microbiologist’s perspective, a walking, talking Petri dish. An extraordinary profusion of microscopic critters inhabit every crack and crevice of the typical human, so many that they probably outnumber the cells of the body upon and within which they dwell.

　　一个让你自己在晚宴上不受欢迎的好方式就是，从微生物学家的角度指出，一位有代表性的人就是一个会走动并会说话的皮氏培养皿。每位有代表性的人的裂纹和缝隙里生活着无数的微小生物，它们的数目如此之多可能超过它们所依靠并栖息的人体细胞。

　　Happily, these microbes are mostly harmless. Some of them, particularly those that live in the gut, are positively beneficial, helping with digestion and keeping the intestines in good working order. That is no surprise—bacteria as much as people have an interest in keeping their homes in sound condition. What is surprising is the small but growing body of evidence which suggests that bacteria dwelling in the gut can affect the brain, too, and thereby influence an individual’s mood and behaviour. The most recent paper on the topic, published this week in the Proceedings of the National Academy of Sciences, reports (like much of the research in this field) on results in mice.

　　幸运地是，这些微生物大部分都是无害的。其中一些，特别是那些生活在消化道里的微生物，实际上是有益的，不仅有助于消化还保持肠道良好的工作秩序。这毫不奇怪——细菌和人一样也喜欢保持家园状态完好。奇怪的是，虽然数量不多，但越来越多的证据表明，生活在肠道的细菌也可能影响大脑，继而影响个人的情绪和行为。本周有关该主题的论文刊登在《美国国家科学院院刊》的一篇文章报道（像这个领域的许多研究一样）了应用小鼠的实验结果。

　　The researchers, led by Javier Bravo of University College, Cork, split their rodent subjects into two groups. One lot were fed a special broth containing Lactobacillus rhamnosus, a gut-dwelling bacterium often found in yogurt and other dairy products. The others were fed an ordinary diet, not fortified with microbes.

　　科克大学学院的哈维尔?布拉沃（Javier Bravo）带领下的研究人员将这些啮齿目动物分成两组。一组以一种在酸奶和其它乳制品中经常发现的含乳酸杆菌属鼠李糖乳杆菌（学名为Lactobacillus rhamnosus）的特别液体培养基喂食。另一组则以不用微生物强化的一般食物喂食。

　　The team then subjected the mice to a battery of tests that are used routinely to measure the emotional states of rodents. Most (though not all) of these tests showed significant differences between the two groups of animals.

　　然后，研究组人员对小鼠进行了一套测试。这套测试经常用于衡量啮齿类动物的情感状态。大部分（虽然不是所有的）测试表明，两组动物之间有着显著的不同。

　　One test featured a maze that had both enclosed and open tunnels. The researchers found that the bacterially boosted mice ventured out into the open twice as often as the control mice, which they interpreted to mean that these rodents were more confident and less anxious than those not fed Lactobacillus.

　　一个测试是以封闭和敞开的隧道迷宫为特点。研究人员发现，喂食了李糖乳杆菌的小鼠更经常冒险进入敞开的隧道，而且次数是对照小鼠的两倍之多。他们的解释意味着这些啮齿类动物比那些没有喂食李糖乳杆菌的小鼠更有信心而且焦虑更少。

　　In another test the animals were made to swim in a container from which they could not escape. Bacteria-fed mice attempted to swim for longer than the others before they gave up and had to be rescued. Such persistence is usually interpreted by students of rodent behaviour as evidence of a more positive mood.

　　在另一组测试中，研究人员让这些动物在一个无法逃脱的容器内游泳。在这些小鼠放弃游泳并不得不被救出之前，李糖乳杆菌喂养的小鼠试图游泳的时间比其它小鼠更长。这种持久性经常被研究啮齿类动物行为的学生解释为更积极情绪的证据。

　　Direct measurements of the animals’ brains supported the behavioural results. Levels of corticosterone, a stress hormone, were markedly lower in the bacteria-fed mice than they were in the control group when both groups were exposed to stressful situations. The number of receptors for gamma-aminobutyric acid, a natural chemical messenger that helps dampen the activity of certain nerve cells, varied in statistically significant ways between the brains of the two groups, with more in some parts of the treated animals’ brains and fewer in others. Most intriguing of all, when Dr Bravo cut the animals’ vagus nerves—which transmit signals between the gut and the brain—the differences between the groups vanished.

　　对这些动物大脑的多次直接测量结果支持了行为学方面的效果。当两组小鼠进行应激实验时，喂食李糖乳杆菌的小鼠体内的应激激素皮质甾酮含量比那些对照组的小鼠体内要明显低地多。γ-氨基丁酸是有助于抑制特定神经细胞活动的自然化学信使。两组小鼠的大脑中γ-氨基丁酸感受器的数目以统计学上显著的方式改变。γ-氨基丁酸感受器在处理过的小鼠大脑的一些部分出现较多些，而在没有处理过的小鼠大脑内则少些。最引人注目的是，当布拉沃博士（Dr Bravo）切断这些动物的迷走神经---传输肠道和大脑之间的信号---两组小鼠间的差异突然消失了。

　　The idea that gut-dwelling microbes can affect an animal’s state of mind may strike some people as outlandish, and there are certainly loose ends still to be tied up. Beyond their evidence that the vagus nerve is crucial to the relationship, for example, Dr Bravo and his colleagues do not yet know the precise mechanisms at work. There is also an obvious follow-up question: whether a similar thing is going on in people. A few previous studies have hinted at the possibility. For example, bacterial treatments may help with the mental symptoms of illnesses such as irritable-bowel syndrome.

　　这个想法---生活在肠道里的微生物能够影响动物的精神状态---可能给一些人留下稀奇古怪的印象，而且肯定还有一些零星问题有待解决。他们手头的证据无法证明迷走神经对动物之间的关系至关重要，比如，布拉沃博士和其同事至今还不知道其起作用的明确机制。况且还有一个明显的后续问题：人类是否也有类似的情况。以前的几个研究已经暗示这种可能性。例如，细菌的治疗方法可能有助于改善诸如肠易激综合症疾病的精神症状。

　　All this is forcing a reassessment of people’s relationship with the bacteria that live on and in them, which have long been regarded mainly as a potential source of infections. An editorial in this week’s Nature raises the possibility that the widespread prescription of antibiotics—which kill useful bacteria as effectively as hostile ones—might be one factor behind rising rates of asthma, diabetes and irritable-bowel syndrome. If Dr Bravo’s results apply to people, too, then mood disorders may end up being added to this list.

　　所有这一切正在迫使人类重新评估自身与这些栖居于人体而且人类依靠它们维持生命的细菌之间的关系。长期以来，这些细菌主要被认为是潜在的感染源。本周《自然杂志》的社论提出了一种可能性，广泛使用的抗生素处方---抗生素杀死有益菌的有效性就像杀死不利的细菌一样---可能是哮喘，糖尿病和肠易激综合症比例上升的真正因素之一。假如布拉沃博士（Dr Bravo）的结果也应用于人类，那么情感障碍可能最终被添加到这个目录上。