Members of the microbial world cover a tremendous range in their sizes, as is seen in figure 1.13. The smallest viruses are about 1 million times smaller than the largest eukaryotic cells. Even within a single group, wide variations exist. For example, Bacillus megaterium and Mycoplasma are both bacteria, but they differ enormously in size (see figure 1.13). The variation in size of bacteria was recently expanded when a bacterium longer than 0.5 mm was discovered (Perspective 1.1, p. 14). In fact, it is so big that it is visible to the naked eye. More recently, an even larger bacterium, round in shape, was discovered. Its volume is 70 times larger than the previous record holder. Likewise, a eukaryotic cell was recently discovered that is not much larger than a typical bacterium. These, however, are rare exceptions to the rule that eukaryotes are larger than prokaryotes, which in turn are larger than viruses.
As you might expect, the small size and broad size range of some members of the microbial world have required the use of measurements not commonly used in everyday life. The use of logarithms has proved to be enormously helpful, especially in designating the sizes of prokaryotes and viruses. A brief discussion of measurements and logarithms is given in appendix I.
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