SHAPE and SIZE OF BACTERIA Bacteria are classified by shape into three basic groups: cocci, bacilli, and spirochetes (Figure 2-1). The cocci are round, the bacilli are rods, and the spirochetes are spiral-shaped. Some bacteria are variable in shape and are said to be pleomorphic (heterogeneous shape). The shape of a bacterium is determined by its rigid cell wall. The microscopic appearance of a bacterium is one of the most important criteria used in its identification. In addition to their characteristic shapes, the arrangement of bacteria is important. For example, certain cocci occur in pairs (diplococci), some in chains (streptococci), and others in grapelike clusters (staphylococci). These arrangements are determined by the orientation and degree of attachment of the bacteria at the time of cell division. The arrangement of rods and spirochetes is medically less important and is not described in this introductory chapter. Bacteria range in size from about 0.2 to 5 um (Figure 2-2). The smallest bacteria (Mycoplasma) are about the same size as the largest viruses (poxviruses) and are the smallest organisms capable of existing outside a host. The longest bacteria are the size of some yeasts and human red blood cells (7 μm).
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