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 Molecules move so rapidly that within a tenth of a second the molecules around a bacterium have all been replaced by new ones; even bacteria without flagella are thus constantly exposed to a changing environment.

　　細(xì)菌細(xì)菌是極其微小的生物體。我們用英寸或厘米來測量自己的大小，而測量細(xì)菌卻 要用微米。 一微米等于千分之一毫米。 針頭直徑大約一毫米。?

　　棒狀細(xì)菌通常有 2～4 微米長，而圓形細(xì)菌的直徑一般只有 1 微米。因此，即使你把一個(gè)圓形細(xì)菌放大1000 倍，它也不過一個(gè)針頭那么大。??

　　可是如果把一個(gè)成年人放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(xì)菌時(shí)，你必須仔細(xì)觀察才能看見它們。 使用 100 倍的顯微鏡 時(shí)，你會發(fā)現(xiàn)細(xì)菌不過是隱約可見的小細(xì)棒或小點(diǎn)點(diǎn)，而它們的結(jié)構(gòu)你卻根本看不出來。 使用特殊的著色劑后，你會發(fā)現(xiàn)有的細(xì)菌上長著不少波狀的"毛發(fā)"即鞭毛，而有的細(xì)菌只有一 根鞭毛。? 鞭毛的旋轉(zhuǎn)可以推動細(xì)菌在水中行進(jìn)。??

　　不少細(xì)菌沒有鞭毛，因而不能自己行進(jìn)。 還有些細(xì)菌卻能通過某些鮮為人知的機(jī)制沿物體表面滑動。我們所熟知的世界在細(xì)菌眼中 完全是另一個(gè)樣子。??

　　對于細(xì)菌來說，水就同糖漿之于人類一樣稠密。細(xì)菌是如此的微小， 周圍化學(xué)分子的一舉一動都會對它們產(chǎn)生影響。在顯微鏡下，細(xì)菌，甚至包括那些沒有鞭 毛的細(xì)菌，經(jīng)常在水中跳來跳去。這是因?yàn)樗鼈兣c水分子相撞后，被彈向各個(gè)方向。分子 移動很迅速，僅 0.1 秒之隔，一個(gè)細(xì)菌周圍的分子就會完全更新。因此，即使是沒有鞭毛 的細(xì)菌也暴露在一個(gè)不斷變化的環(huán)境中。