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Prescott's microbiology
Microorganisms are defined as those organisms and acellular biological entities too small to be seen clearly by the unaided eve (figure 1.1). They are generally 1 millimeter or less in diam¬eter. Although small size is an important characteristic of mi¬crobes, it alone is not sufficient to define them. Some cellular microbes, such as bread molds and filamentous photosynthetic microbes, are actually visible without microscopes. These mac¬roscopic microbes are often colonial, consisting of small aggre¬gations of cells. Some macroscopic microorganisms are multicellular. They are distinguished from other multicellular life forms such as plants and animals by their lack of highly dif¬ferentiated tissues. Most unicellular microbes are microscopic. However, there are interesting exceptions, as we describe in chapter 3. In summary, cellular microbes are usually smaller than 1 millimeter in diameter, often unicellular and, if multi¬cellular, lack differentiated tissues. The diversity of microorganisms has always presented a challenge to microbial taxonomists. The early descriptions of cellular microbes as either plants or animals were too simple. For instance, some microbes are motile like animals but also have cell walls and are photosynthetic like plants. Such mi¬crobes cannot be placed easily into either kingdom. An im¬portant breakthrough in microbial taxonomy arose from studies of their cellular architecture, when it was discovered that cells exhibited one of two possible "floor plans." Cells that came to be called prokaryotic cells (Greek pro, before, and karyon, nut or kernel; organisms with a primordial nucleus) have an open floor plan. That is, their contents are not divided.
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