The good, the Bad, & the NASTY

　　好东西、坏东西和令人讨厌的东西

　　Since we were very young, we have all been instructed to wash our hands before eating to avoid ingesting “germs.” Microorganisms (bacteria, protozoa, and yeast) seem to have a bad reputation when it comes to food! The nasty ones make headlines and the TV news, and some cause us a little discomfort. But this downside of germs is not the whole story! Beneficial microbes far outnumber the harmful (pathogenic) ones, and we depend on all their good deeds for our nutritional well-being and health! It’s high time the “good” bacteria get the credit they deserve!

　　The Intestinal Community

　　We all prefer to think of ourselves as individuals, but it would be much more accurate if we pictured ourselves as thriving communities! Right now, there are more microbes living on you than there are cells in your body! Of course, they are not actually part of you, but the bacteria alone add up to approximately 10 percent of your dry weight (you minus the water)! Microorganisms may be small, but their numbers make up for their size.

　　On every surface of your body there are tiny microbes going about their business. The surface of the digestive system takes the award for the largest population －by far －of microorganisms. Here, their numbers reach astronomical proportions. More individuals of the bacterial species Escherichia coli will inhabit your intestine during your lifetime than the number of people who have ever lived on the Earth! E. coli is but one of the 400 or so species that are common members of the intestinal community. A few yeast and protozoa species top off the list.

　　But wait! Not to worry! This may seem like a creepy, crawly mess, but the majority of your intestinal microorganisms are not nasty intruders. They are important and invited guests: your personal microbes. As well as helping you to digest and process your food and providing you with essential vitamins, they make it their business also to protect you against those nasty sorts. All of this, simply in exchange for food, warmth, and a space to multiply! (This kind of relationship in which everyone benefits is called mutualism.) As long as this arrangement is functioning properly, you remain largely unaware of your microbes and their business. But when it is not－that’s another story!

　　Moving Right Along

　　The microbes of your large intestine make their living by breaking down the discards of the small intestine, material that your human system has rejected as indigestible. Equipped with their own set of enzymes and metabolic pathways, they proceed to further tear apart the organic compounds that remain. Most of them get to work disassembling the plant carbohydrates. The majority of microbes of the large intestine are anaerobic, meaning that they live in the absence of oxygen. Instead of breathing in and out, they are able to derive all of their energy by splitting large carbohydrates into smaller fatty acid molecules and carbon dioxide, in a process known as fermentation.

　　Some of the fatty acids are reabsorbed through the intestinal walls, providing us with a bonus energy source. The rest help the bacteria to grow rapidly, rapidly enough to reproduce themselves as often as every twenty minutes! Synthesizing more of some of the B vitamins and vitamin K than they need for their own growth, they kindly make the surplus available to other creatures in their community, and to you, their host. Although you are unable to manufacture these vitamins yourself, you can count on your friendly microbes for an unfailing supply.

　　Scientists are just beginning to understand the complex relationships between the different microorganisms of this community and their interactions with a human host. It is a dynamic system, which constantly adjusts to a host’s changes in age and diet. Right after your birth, you begin to assemble your own select group of species. As your diet moves from mother’s to cow’s milk, and then to different solid foods, new species dominate the population.

　　The End of the Journey

　　Microbes that colonize the walls of the large intestine are survivors of a very difficult journey! From the mouth through the small intestine, they are attacked by digestive enzymes and strong acids. Those that complete the trip intact face more obstacles on their arrival. To grow, they must compete successfully with those already there for room and nutrients. Fortunately, the “friendly” bacteria are very adept at gluing themselves to most of the available wall space! Some of these friendly bacteria can produce acids and antibacterial compounds known as “bacteriocins,” which help to discourage invasion by the unsavory types. The sheer numbers of “good” bacteria usually prevent the “nasties” from gaining a “foothold” before passing on their way unnoticed.

　　This ability of friendly bacteria to control the populations of more dangerous ones has fueled a new interest in “probiotic” foods, products containing cultures of live bacteria, such as yogurt. Check the labels on your next container of yogurt to see what species might become your next guests!

　　And remember, although it is still a very good practice to wash your hands before eating, not all microbes are “germs.” Those that support and protect us deserve recognition.

　　小时候，我们都被告知饭前要洗手，防止把“细菌”吃下去。细菌（病毒、原生动物和酵母）同食物联系上时就都有了坏名声。报纸头条新闻和电视新闻对那些最有害的细菌进行报道，其中的一些还会令我们感到一点不舒服。但细菌的负面影响并不是它的全部。对人有益的细菌的数量远远大于有害细菌（致病菌）的数量。而且我们还要依靠这些细菌所带来的益处来获得平衡的营养和健康保障。现在该是给“好”细菌正名的时候了。

　　大肠内的生命群体

　　我们总是愿意把自己想象成为单一的个体，但我们如果把自己看成是一个有活力的集体的话就更准确了。现在，在你体内生存的细菌比细胞还要多。当然，这些细菌不真的是你身体的一部分。但把所有的细菌都加起来的重量约是你减去身体中水分的体重的10%。细菌体积也许很小，但它们的数量弥补了它们的体积。

　　在你身体的表面有许多细菌在忙着自己的事情，至今，在消化系统的表面细菌的数量还是最多的。在这里，它们的数量达到了天文数字。在你一生中，在你肠内定居的大肠杆菌个体的数量比在地球上居住过的所有人的数量还要多。大肠杆菌仅仅是大肠内四百多种常见的细菌中的一种，位居排行榜前列的还有一些酵母和原生动物。

　　等一等！不要担心。这看起来好像是一群爬来爬去的脏东西，但大肠内的大多数细菌并不是烦人的入侵者。它们是我们请来的重要客人：是属于你自己的细菌。它们不仅能够帮助你消化和处理食物，还能给你提供基本的维生素。它们的作用还表现在帮助你抵抗那些有害细菌。它们做这些工作仅仅是为了换取食物、温暖的环境和繁殖的场所（这种使双方都能受益的关系叫做共生关系）。只要这种安排运行正常的话，你基本上意识不到身体内部的细菌和它们的活动。但如果它们的功能发挥不正常，那就是另外一回事了。

　　继续前行

　　身体大肠内的细菌靠分解小肠内部的废弃物生活。这些东西由于不可消化，人体系统拒绝处理它们。这些细菌自己装备有一系列的酶和新陈代谢的通道。这样，它们能够继续把遗留的有机化合物进行分解。它们中的大多数的工作都是分解植物中的碳水化合物。大肠内部大部分的细菌是厌氧性的细菌，意思就是它们在没有氧气的状态下生活。它们不是呼出和呼入氧气，而是通过把大分子的碳水化合物分解成为小的脂肪酸分子和二氧化碳来获得能量。这一过程称为“发酵”。

　　一些脂肪酸通过大肠的肠壁被重新吸收，这会给我们提供额外的能源。剩余的脂肪酸帮助细菌迅速生长。其速度之快可以使它们在每20分钟内繁殖一次。因为它们合成的一些维生素B和维生素K比它们需要的多，所以它们非常慷慨地把多余的维生素供应给它们这个群体中其他的生物，也提供给你——它们的宿主。尽管你不能自己生产这些维生素，但你可以依靠这些对你非常友好的细菌来源源不断供应给你。

　　科学家们刚刚开始明白这一集体中不同的细菌之间的复杂关系，以及它们同人这个宿主之间的相互作用。这是一个动态的系统，随着宿主在饮食结构和年龄上的变化，这一系统也做出相应的调整。你一出生就开始在体内汇集你所选择的细菌的种类。当你的饮食结构从母乳变为牛奶，又变成不同的固体食物时，你的体内又会有新的细菌来占据主导地位了。

　　旅程的结束

　　积聚在大肠壁上的细菌是经历过艰难旅程后的幸存者。从口腔开始经过小肠，他们受到消化酶和强酸的袭击。那些在完成旅行后而安然无恙的细菌在到达时会遇到更多的障碍。要想生长，它们必须同已经住在那里的细菌争夺空间和营养。幸运的是，这些“友好的”细菌能够非常熟练地把自己粘贴到大肠壁上任何可利用的地方。这些友好的细菌中的一些可以产生酸和被称为“细菌素”的抗菌化合物。这些细菌素可以帮助抵御那些令人讨厌的细菌的侵袭。

　　那些友好的细菌能够控制更危险的细菌的数量，增加人们对“前生命期”食物的兴趣。这种食物含有培养菌，酸奶就是其中的一种。在你喝下一瓶酸奶的时候，检查一下标签，看一看哪种细菌将会成为你体内的下一批客人。

　　记住，尽管饭前洗手仍然是很好的习惯，并不是所有的细菌都是“病菌”。我们应当认可那些支持和保护我们的细菌。