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 ac…