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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.

Figure 2–1

Bacterial morphology. A: Cocci in clusters (e.g., Staphylococcus; A-1); in chains (e.g., Streptococcus; A-2); in pairs with pointed ends (e.g., Streptococcus pneumoniae; A-3); in pairs with kidney bean shape (e.g., Neisseria; A-4). B: Rods (bacilli): with square ends (e.g., Bacillus; B-1); with rounded ends (e.g., Salmonella; B-2); club-shaped (e.g., Corynebacterium; B-3); fusiform (e.g., Fusobacterium; B-4); comma-shaped (e.g., Vibrio; B-5). C: Spirochetes: relaxed coil (e.g., Borrelia; C-1); tightly coiled (e.g., Treponema; C-2). (Reproduced with permission from Joklik WK, Willett HP, Amos DB: Zinsser Microbiology, 20th ed. New York, NY: McGraw-Hill Education; 1992.)

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 µm (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).

Figure 2–2

Sizes of representative bacteria, viruses, yeasts, protozoa, and human red cells. The bacteria range in size from Mycoplasma, the smallest, to Bacillus anthracis, one of the largest. The viruses range from poliovirus, one of the smallest, to poxviruses. Yeasts, such as Candida albicans, are generally larger than bacteria. Protozoa have many different forms and a broad size range. HIV, human immunodeficiency virus. (Reproduced with permission from Joklik WK, Willett HP, Amos DB: Zinsser Microbiology, 20th ed. New York, NY: McGraw-Hill Education; 1992.)


The structure of a typical bacterium is illustrated in Figure 2–3, and the important features of each component are presented in Table 2–1.

Table 2–1Bacterial Structures

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