Bacteria resemble plants in that they have rigid cell walls, although the walls are generally of nitrogen rather than cellulose. Some forms have flagella, while others are nonmotile. Bacteria have ribosomes and nucleic acids but lack mitochondria, plastids, and the central vacuole. Bacteria are distinguished from each other, in part, on the basis of their shapes. Rod-shaped bacteria are called bacilli, round forms are called cocci, and curved forms are called spirilli. Cocci can occur in pairs (diplococci), clusters (staphylococci), or chains (streptococci).
Figure 11-2 Types of bacteria.
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