2. 外语
  3. Antibiotics are drugs produced by certain microbes. Antibiotic substances are obtained from bacteria and fungi that live in the

Antibiotics are drugs produced by certain microbes. Antibiotic substances are obtained from bacteria and fungi that live in the

admin2009-07-11  101
问题 Antibiotics are drugs produced by certain microbes. Antibiotic substances are obtained from bacteria and fungi that live in the air, soil, and water. Most antibiotics are used by physicians to fight various diseases caused by harmful microbes. A few are used to treat certain cancers. Antibiotics are sometimes called" wonder drugs ’because they can cure many diseases that once were often fatal. The number of deaths that are caused by pneumonia and scarlet fever has declined drastically since people began using antibiotics.
2. There are more than 70 clinically useful antibiotics, Antibiotics fight pathogenic microbes and cancer cells by interfering with their normal cell processes. In most cases, this interference can occur in one of three ways: prevention of cell wall formation, disruption of the cell membrane, and disruption of chemical processes.
3. The contents of bacterial cells are enclosed in a membrane that is surrounded by a rigid wall that prevents the cells from splitting open. Penicillins and some other antibiotics destroy pathogenic microbes by hindering the formation of this wall. Human cells do not have nor need rigid cell walls and so are not damaged by these antibiotics.
4. Some antibiotics, including nystatin, disrupt the cell membrane of certain microbes. This membrane controls the movement of materials in and out of the cell. If the membrane is disrupted, vital nutrients may escape from the cell, or poisonous substances may enter and kill the cell. But the membranes of human cells are not affected because these antibiotics disrupt cell membranes that contain elements found only in microbial cells.
5. All cells produce proteins and nucleic acids, which are vital to the life of any organism. Some antibiotics fight disease by interfering with the chemical processes by which these substances are produced.For example, streptomycin prevent certain kinds of microbes from producing proteins, and rifampin interferes with the formation of nucleic acids. Human cells produce proteins and nucleic acids in much the same way that microbial cells do. But these processes differ enough so that some antibiotics interfere with chemical activities in microbial cells but not in human cells.
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答案B
解析 第二段段意为“现在有临床疗效的抗生素有70多种，抗生素通过干状病菌和癌细胞的正常功能与它们作斗争。
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