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Bacteria are extremely small living things. While we measure our own sizes in inches or centimeters, bacterial size is measured in microns. One micron is a thousandth of a millimeter: a pinhead is about a millimeter across. Rod-shaped bacteria are usually from two to four microns long, while rounded ones are generally one micron in diameter. Thus if you enlarged a rounded bacterium a thousand times, it would be just about the size of a pinhead. An adult human magnified by the same amount would be over a mile (1.6 kilometers)tall.
Even with an ordinary microscope, you must look closely to see bacteria. Using a magnification of 100 times, one finds that bacteria are barely visible as tiny rods or dots. One cannot make out anything of their structure. Using special stains, one can see that some bacteria have attached to them wavy-looking "hairs" called flagella. Others have only one flagellum. The flagella rotate, pushing the bacteria through the water. Many bacteria lack flagella and cannot move about by their own power, while others can glide along over surfaces by some little-understood mechanism.
From the bacterial point of view, the world is a very different place from what it is to humans. To a bacterium water is as thick as molasses is to us. Bacteria are so small that they are influenced by the movements of the chemical molecules around them. Bacteria under the microscope, even those with no flagella, often bounce about in the water. This is because they collide with the water molecules and are pushed this way and that. Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.
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細菌是極其微小的生物體。我們用英寸或釐米來測量自己的大小，而測量細菌卻要用微米。一微米等於千分之一毫米。針頭直徑大約一毫米。棒狀細菌通常有2～4 微米長，而圓形細菌的直徑一般只有 1 微米。因此，即使你把一個圓形細菌放大1000 倍，它也不過一個針頭那麼大。可是如果把一個成年人放大1000 倍，就會變成 1 英里(或 1.6 公里)多高。
用一般的顯微鏡觀察細菌時，你必須仔細觀察才能看見它們。使用 100 倍的顯微鏡時，你會發現細菌不過是隱約可見的小細棒或小點點，而它們的結構你卻根本看不出來。使用特殊的.着色劑後，你會發現有的細菌上長着不少波狀的"毛髮"即鞭毛，而有的細菌只有一根鞭毛。鞭毛的旋轉可以推動細菌在水中行進。不少細菌沒有鞭毛，因而不能自己行進。還有些細菌卻能通過某些鮮爲人知的機制沿物體表面滑動。
我們所熟知的世界在細菌眼中 完全是另一個樣子。對於細菌來說，水就同糖漿之於人類一樣稠密。細菌是如此的微小，周圍化學分子的一舉一動都會對它們產生影響。在顯微鏡下，細菌，甚至包括那些沒有鞭毛的細菌，經常在水中跳來跳去。 這是因爲它們與水分子相撞後，被彈向各個方向。分子移動很迅速，僅 0.1 秒之隔，一個細菌周圍的分子就會完全更新。因此，即使是沒有鞭毛的細菌也暴露在一個不斷變化的環境中。
