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  3. (1)We have all been instructed to wash our hands before eating to avoid ingesting "germs." Microorganisms(bacteria, protozoa, an

(1)We have all been instructed to wash our hands before eating to avoid ingesting "germs." Microorganisms(bacteria, protozoa, an

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问题     (1)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!
    (2)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.
    (3)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.
    (4)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!
    (5)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.
    (6)Some of the fatty acids are reabsorbed through the intestinal walls, providing us with a bonus energy source. The rest help me 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.
    (7)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.
    (8)Microbes that colonize the walls of the large intestine are survivors of a very difficult journey! From the mourn through the small intestine, they are attacked by digestive enzymes and strong acids. Those mat 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.
The majority of the microbes in the human body live _____.
选项 A、on the surface of our skin
B、on the surface of the digestive system
C、by breaking down indigestible materials
D、by disassembling me plant carbohydrates
答案B
解析 第3段第2句提到，在消化系统的表面细菌的数量是最多的，故B对。A与第3段的表述不符；C、D是文章对大肠中的细菌的描述，而不是大部分细菌的情况，属于以偏概全，故错误。
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